Science and Industry Updates
TweetYears ago, high purity water was used only in limited applications. Today, deionized (Dl) water has become an essential ingredient in hundreds of applications including: medical, laboratory, pharmaceutical, cosmetics, electronics manufacturing, food processing, plating, countless industrial processes, and even the final rinse at the local car wash. THE DEIONIZATION PROCESS The vast majority of dissolved […]
Years ago, high purity water was used only in limited applications. Today, deionized (Dl) water has become an essential ingredient in hundreds of applications including: medical, laboratory, pharmaceutical, cosmetics, electronics manufacturing, food processing, plating, countless industrial processes, and even the final rinse at the local car wash.
THE DEIONIZATION PROCESS
The vast majority of dissolved impurities in modern water supplies are ions such as calcium, sodium, chlorides, etc. The deionization process removes ions from water via ion exchange. Positively charged ions (cations) and negatively charged ions (anions) are exchanged for hydrogen (H+) and hydroxyl (OH-) ions, respectively, due to the resin’s greater affinity for other ions. The ion exchange process occurs on the binding sites of the resin beads. Once depleted of exchange capacity, the resin bed is regenerated with concentrated acid and caustic which strips away accumulated ions through physical displacement, leaving hydrogen or hydroxyl ions in their place.
Deionizers exist in four basic forms: disposable cartridges, portable exchange tanks, automatic units, and continuous units. A two-bed system employs separate cation and anion resin beds. Mixed-bed deionizers utilize both resins in the same vessel. The highest quality water is produced by mixed-bed deionizers, while two-bed deionizers have a larger capacity. Continuous deionizers, mainly used in labs for polishing, do not require regeneration.
TESTING Dl WATER QUALITY
Water quality from deionizers varies with the type of resins used, feed water quality, flow, efficiency of regeneration, remaining capacity, etc. Because of these variables, it is critical in many Dl water applications to know the precise quality. Resistivity/ conductivity is the most convenient method for testing Dl water quality. Deionized pure water is a poor electrical conductor, having a resistivity of 18.2 million ohm-cm (18.2 megohm) and conductivity of 0.055 microsiemens. It is the amount of ionized substances (or salts) dissolved in the water which determines water’s ability to conduct electricity. Therefore, resistivity and its inverse, conductivity, are good general purpose quality parameters.
Because temperature dramatically affects the conductivity of water, conductivity measurements are internationally referenced to 25°C to allow for comparisons of different samples. With typical water supplies, temperature changes the conductivity an average of 2%/°C, which is relatively easy to compensate. Deionized water, however, is much more challenging to accurately measure since temperature effects can approach 10%/°C! Accurate automatic temperature compensation, therefore, is the “heart’ of any respectable instrument.
RECOMMENDED MYRON L METERS
Portable instruments are typically used to measure Dl water quality at points of use, pinpoint problems in a Dl system confirm monitor readings, and test the feed water to the system. The handheld Myron L meters have been the first choice of Dl water professionals for many years. For two-bed Dl systems, there are several usable models with displays in either microsiemens or ppm (parts per million) of total dissolved solids. The most versatile instruments for Dl water is the 4P or 6PFCE Ultrameter II™, which can measure both ultrapure mixedbed quality water and unpurified water. It should be noted that once Dl water leaves the piping, its resistivity will drop because the water absorbs dissolved carbon dioxide from the air. Measuring of ultrapure water with a hand-held instrument requires not only the right instrument, but the right technique to obtain accurate, repeatable readings. Myron L meters offer the accuracy and precision necessary for ultrapure water measurements.
Inline Monitor/controllers are generally used in the more demanding Dl water applications. Increased accuracy is realized since the degrading effect of carbon dioxide on high purity water is avoided by use of an in-line sensor (cell). This same degradation of ultrapure water is the reason there are no resistivity calibration standard solutions (as with conductivity instruments). Electronic sensor substitutes are normally used to calibrate resistivity Monitor/controllers.
Myron L Meters carries a variety of inline instruments, including resistivity Monitor/controllers designed specifically for Dl water. Seven resistivity ranges are available to suit any Dl water application: 0-20 megohm, 0-10 megohm, 0-5 megohm, 0-2 megohm, 0-1 megohm, 0-500 kilohm, and 0-200 kilohm. Temperature compensation is automatic and achieved via a dual thermistor circuit. Monitor/controller models contain an internal adjustable set point, piezo alarm connectors and a heavy-duty 10 amp relay circuit which can be used to control an alarm, valves, pump, etc. Available options include 4-20 milliamp output, 3 sensor input, 3 range capability and temperature. Internal electronic sensor substitutes are standard on all Monitor/controllers.
Sensors are available constructed in either 316 stainless steel or titanium. All sensors are provided with a 3/4″ MNPT polypropylene bushing and 10 ft./3 meters of cable. Optional PVDF or stainless steel bushings can be ordered, as well as longer cable lengths up to 100 ft./30 meters.
The following table briefly covers recommended Myron L meters for Dl water applications.
Tweet Reverse Osmosis RO Meter – RO-1: 0-1250 ppm with color band RO Meters The choice of professionals for years, this compact instrument has been designed specifically to demonstrate and test Point of Use (POU) reverse osmosis or distillation systems. By measuring electrical conductivity, it will quickly determine the parts per million/Total Dissolved Solids […]
TweetINTRODUCTION AND OVERVIEW The most popular germicide used in water treatment is chlorine, which kills bacteria by way of its power as an oxidizing agent. Chlorine is used not only as a primary disinfectant at the beginning of the treatment process, but also at the end to establish a residual level of disinfection during distribution […]
INTRODUCTION AND OVERVIEW
The most popular germicide used in water treatment is chlorine, which kills bacteria by way of its power as an oxidizing agent. Chlorine is used not only as a primary disinfectant at the beginning of the treatment process, but also at the end to establish a residual level of disinfection during distribution as a guard against future contamination.
The most popular field test instruments and test systems for judging the level of residual chlorine, also called Free Available Chlorine (FAC), are based on colorimetric methods whereby dyeing agents are added to the sample being tested. These additives react to FAC causing a color change in the test sample. While they may detect the presence of FAC, they do not directly measure the electrochemical characteristic of FAC responsible for its disinfecting power: Oxidation-Reduction Potential (ORP). They give an incomplete and sometimes misleading picture of sanitizing strength. These methods have gained industry-wide acceptance. Unfortunately, so have the weaknesses and inaccuracies inherent to them.
The Free Chlorine Equivalent (FCE) feature avoids these pitfalls by directly measuring ORP, the germ-killing property of chlorine and other oxidizing germicides. It displays both the ORP reading (in millivolts DC) for the sample being tested as well as an equivalent free chlorine concentration in familiar ppm (parts per million). It accounts for the very significant effect of changing pH on chlorine sanitizing power; can be used for other types of oxidizing germicides and; will track the effect of additives, such as cyanuric acid, that degrade chlorine effectivity without changing the actual concentration of free available chlorine present.
NaOCl, common household bleach (5.3% NaOCl by weight) is the most popular chlorinating agent in use today. When added to water it hydrolyzes as:
NaOCl + H2O HOCl + Na+ + OH-
Sodium hypochlorite Water Hypochlorous acid Sodium ion Hydroxide
Additionally, some of the HOCl dissociates into H+ and OCl-:
HOCl H+ + OCl-
Hypochlorous acid hydrogen ion hypochlorite ion
Both HOCl and OCl- are oxidants and effective germicides, particularly against bacteria and viruses, with some effectivity against protozoa and endospores. HOCl is the stronger and more effective of the two species.
When chlorine is added to water, not all of it is available to act against future contaminates. Some is deactivated by sunlight. Some is consumed by reactions with other chemicals in the water or by out-gassing as Cl2. More commonly, it is used up directly by disinfection of the pathogens already
present in the water or by combining with ammonia (NH3) and ammonium (NH4+) (byproducts of living bacteria) to form various chloramine compounds.
Chlorine Demand is the amount of chlorine in solution that is used up or inactivated after a period of time and therefore not available as a germicide.
Free Available Chlorine
Free Available Chlorine (FAC) is any residual chlorine that is available, after the chlorine demand is met, to react with new sources of bacteria or other contaminants. According to White’s Handbook of Chlorination and Alternative Disinfectants, 5th edition, this is the sum of the all of the chemical species that contain a chlorine atom in the 0 or +1 oxidation state and are not combined with ammonia or other organic nitrogen. Some species of FAC that might be present are:
• Molecular chlorine: Cl2
• Hypochlorous acid: HOCl
• Hypochlorite: OCl-
• Trichoride: Cl3- a complex formed by molecular chlorine and the chloride ions (Cl-)
In most applications the two most common species of free chlorine will be HOCl and OCl-. Much of the Cl2 will hydrolyze into HOCl that, depending on pH, will stay in the form of HOCl or partially dissociate into OCl-. Cl3- is very unstable and only trace amounts will be present. In fact, in most of the literature describing chlorination and the monitoring of chlorine residuals, free chlorine is considered to be the sum of HOCl and OCl-.
Chlorine dioxide, ClO2, is another chlorine derivative used in some public water supplies as a disinfectant. It is 10 times more soluble in water than chlorine and doesn’t hydrolyze into HOCl or dissociate into OCl-. In the absence of oxidizable substances and in the presence of hydroxyl ions, ClO2 will dissolve in water then decompose slowly forming chlorite ions (ClO2-) and chlorate ions (ClO3-), both of which are oxidants fitting White’s definition of free chlorine.
All other things being stable (temperature, pH, etc.), ORP values are related to FAC concentration levels. As the concentration of FAC in solution rises or falls, regardless of the species (HOCl, OCl-, ClO2-, ClO3- or Cl3-), the ORP value does, as well.
Combined and Total Chlorine
The term Combined Chlorine usually refers to residual chlorine that has combined with NH3 or NH4+ to form monochloramine (NH2Cl),
dichloramine (NHCl2) or trichloramine (NCl3). Combined chlorine is noteworthy here because chloramines are oxidizers and are used as germicides, though their reduction potential and therefore, disinfecting power is lower than other species of chlorine, such as HOCl, OCl- or ClO2.
Total chlorine is the sum of FAC and Combined Chlorine. An advantage of ORP-based systems is
that the aggregate ORP value of the water being tested includes the ORP levels contributed by all oxidizers, including chloramines. Therefore, ORP- based measurements automatically take into account Total Chlorine and can readily be used to judge total sanitizing strength.
Chlorine as an Oxidizing Germicide
Both HOCl and OCl- are oxidants and as such their effectivity as germicides can be determined using
ORP measurements. The cytoplasm and proteins in the cell walls of many harmful microbes are negatively charged (they have extra electrons). Any oxidant that comes into contact with the organism will gain electrons at the expense of the proteins, denaturing those proteins and killing the organism.
When enough chlorine is added to water to reach an ORP value of 650mV to 700mV, bacteria such as E. coli and Salmonella can be killed after only 30 seconds of exposure. Many yeast species and fungi can be killed with exposure of only a few minutes. Even ORP values of 350mV to 500mV indicate effective levels of chlorination with satisfactory microbe kill levels, although exposure times are required to be in the minutes rather than seconds.
The Importance of pH
pH significantly changes relative effectiveness of chlorine as a disinfectant. Different species of chlorine ions are more prevalent at different pH levels. Under typical water treatment conditions in the pH range 6–9, HOCl and OCl- are the main chlorine species. Depending on pH level, the ratio of these two free chlorine species changes.
Figure 1 – Distribution of Free Chlorine Species in Aqueous Solutions
Figure 1 shows that chlorine hydrolysis into HOCl is almost complete at pH ≤ 4. Dissociation of HOCl into OCl- begins around 5.5 pH and increases dramatically thereafter2. This is important because HOCl and OCl- do not have the same effectivity as disinfectants. HOCl can be 80-100% more effective as a disinfectant than OCl-. Optimum disinfection occurs at pH 5 to 6.5 where HOCl is the prevailing species of free chlorine present. As pH rises above that level, the ratio shifts towards being primarily OCl-. At pH 7.5 the ratio is about even. When the pH value rises to 8 or higher, OCl- is the dominant species. Therefore, assuming the concentration of Cl2 species is constant, the higher the pH of the solution rises above 5.5, the lower the oxidation capability and disinfecting power of the FAC.
The bottom line is knowing the concentration of FAC ions in a solution without taking pH into account can give an incomplete and sometimes incorrect picture of disinfecting power.
WHY A CHANGE IS NEEDED
The Problem with Colorimetric Testing
First and foremost, colorimetric tests only report how much chlorine is present, and as we saw in the previous section, knowing “how much” is not at all the same as knowing “how effective”.
Colorimeters and DPD kits add a reagent or several reagents to the water being tested that causes a color change representing the amount of FAC in water. In fact, they fundamentally change the chemistry of the water just to get an easy measurement.
The most obvious change is related to pH. The typical reagent/dye used in the process forces the pH of the sample to a specific level, usually 6.5 pH and thus radically alters the HOCl to OCl- ratio. If the original sample was at a pH of 7.4 to 7.6 (suggested levels for pools and spas) about 50% of the FAC present would be in the form of HOCl. At a pH of 6.5 this ratio rises to nearly 90%. While the actual concentration of FAC may be correct, a fact entirely overlooked is that the FAC in the source water includes about 40% of the much weaker sanitizing OCl-.
If that were the only change being made to the chemistry of the sample, it would be severe enough.
Figure 2 shows the result of a comparison test made using a UV spectrophotometer on two samples of water. Both were taken from a master water sample containing 5 ppm Cl2 prepared using a closed system that ensured no other oxidants or interferants were present. One was processed using a colorimeter reagent according to its operator’s manual instructions. The other was left untreated.
Figure 2: Chemical Alteration of Chlorinated Water by Colorimeter Additives
UV spectrophotometric analysis shows how dramatically the chemistry of the sample was changed by the addition of the colorimeter’s coloring reagent.
• The shift in the center of the spike indicates that the species of chlorine present has been altered. What was OCl- is now some other chemical species.
• The amplitude of the spike demonstrates how severely the amount of chlorine has been amplified.
• The absorption spectra where OCl- used to be is significantly depressed.
Because the area of UV absorption spectra where any OCl- would appear is so depressed, it is clear that a radical alteration is taking place above and
beyond simply changing the pH. The “ppm” value reported for the chlorine content of the water seems to be converted to a single species whose
concentration is significantly higher than the original OCl- content.
Even assuming a linear relationship between this altered chemistry and the original FAC content of the water that might be factored into the final colorimetric measurement, by completely divorcing the measurement from the pH of the source water, any direct correlation to the reduction potential of the FAC present and, therefore, real disinfecting power, is lost.
ORP = DISINFECTING POWER
What is ORP?
ORP is the acronym for Oxidation Reduction (REDOX) Potential. It is a differential measurement of the mV potentials built up between two electrodes exposed to solutions containing oxidants and/or reductants. ORP describes the net magnitude and direction of the flow of electrons between pairs of chemical species, called REDOX pairs. In REDOX reactions, one chemical of the pair loses electrons while the other chemical gains electrons. The chemicals that acquire electrons are called the oxidants (HOCl, OCl-, ClO2, bromine, hydrogen peroxide, etc.). The chemicals that give up electrons are called the reductants (Li, Mg2+, Fe2+, Cr, etc.).
Oxidants acquire electrons through the process of reduction, i.e., they are reduced. Reductants lose their electrons through the process of oxidation, i.e., they become oxidized.
How is ORP Measured?
ORP sensors are basically two electrochemical half- cells: A measurement electrode in contact with the solution being measured and a reference electrode in contact with an isolated reservoir of highly concentrated salt solution. When the solution being measured has a high concentration of oxidizers, it accepts more electrons than it looses and the measurement electrode develops a higher electrical potential than the stable potential of the reference electrode. A voltmeter in line with the two electrodes will display this difference in electrical potential (reported in mV). Once the entire system reaches equilibrium, the resulting net potential difference represents the Oxidation Reduction Potential (ORP). A positive reading indicates an oxidizing solution, and a negative reading indicates a reducing solution. More positive or negative values mean the oxidants or reductants present are stronger, they are present in higher concentrations or both.
What Does ORP Measure?
Measuring ORP is the most direct way to determine the efficacy of oxidizing disinfectants in aqueous solutions. It measures the actual chemical mechanism by which oxidizers, like chlorine, kill bacteria and viruses. The higher the ORP value, the stronger the aggregate residual oxidizing power of the solution, the more aggressively the oxidants in it will take electrons from the cells of microbes and, therefore, the more efficiently and effectively any source of new microbial contamination will be neutralized.
Also, because ORP measures the total reduction potential of a solution, ORP measures the total efficacy of all oxidizing sanitizers in solution: hypochlorous acid, hypochlorite, monochloramine, dichloramine, hypobromous acid, ozone, peracetic acid, bromochlorodimethylhydantoin, etc.
Can ORP Replace Free Chlorine Measurements?
When correlated with known disinfection control methods, measurements and bacterial plate counts, this type of measurement gives an accurate picture of the residual chlorine sanitizing activity reported as an empirical number that is not subject to visual interpretations. Solutions with certain ORP levels kill microbes at a certain rate. Period!
ORP was first studied at Harvard University in the 1930s as a method for measuring and monitoring microbial disinfection. It has been advocated as the best way to judge residual disinfecting power of chlorinated water by water quality experts since the 1960s. ORP has long been used in bathing waters as the only means for automatic chemical dosing. The World Health Organization (WHO) suggests an initial ORP value of between 680-720 mV for safe bathing water3 and ~800 mV for safe drinking water.
For the purpose of pretreatment screening to detect chlorine levels prior to contact with chlorine- sensitive RO membranes, some manufacturers of RO membranes and other water quality treatment equipment will also specify an ORP tolerance value for prescreening and influent control.
There are, however, applications where reporting residual disinfecting power in terms of FAC concentrations is preferred and sometimes required. While ORP measurements do not directly measure the concentration of FAC, they can be correlated to free chlorine levels in ppm. Variables such as pH and temperature must be accounted for or controlled. Interfering chemicals that might be present, such as other oxidants or reductants, must also be accounted for, or better yet, removed.
Once all these factors are known or controlled, ORP values can be linked to concentrations either by way of laboratory experimentation or via mathematical formulas like the Nernst Equation, an equation that describes the relationship between the electrode potential of a specific chemical in a solution and its concentration. In either case this is an often complex and laborious process … until now.
FCE = HANDHELD ORP ACCURACY
ORP Relevance in a Handheld Instrument
The Myron L Company has developed an innovative method for using ORP-based measurement to directly monitor the disinfecting power of free chlorine and report the result in both familiar ppm units as well as straight ORP mV values.
Myron L Company’s FCE function utilizes the accurate and reliable electronic design of Myron L Company’s instruments combined with simple one- button operation to make ORP-based chlorine measurement available in an easy-to-use, handheld field instrument. Other handheld instruments may measure ORP, but only a Myron L Company instrument equipped with FCE quickly correlates ORP and pH with FAC concentration. The FCE function also includes a predictive algorithm that extrapolates a final, stable ORP value of a solution without waiting out the long response time of the typical ORP sensor.
When the FCE function is active, the instrument display alternates between the Predicative ORP reading (mV) and the Free Chlorine Equivalent (FCE) concentration (ppm). Together these features combine to make ORP-based free chlorine measurement relevant in a handheld field instrument.
FCE – How and Why it Works
The Myron L Company FCE feature cross-references ORP values with pH levels to automatically arrive at a concentration value for FAC that reflects the effect of pH on the ratio of HOCl to OCl-. This correlation is derived from a series of experiments in which exact amounts of chlorine (in the form of laboratory grade bleach: 5% NaOCl; 95% H2O) were added to deionized water in a closed system, thus controlling and excluding possible interferants. By using both a pH measurement and an ORP measurement, FCE can determine the relative contributions of HOCl and OCl- to the final ORP value and factors them into a final concentration calculation.
Figure 3 – Sample Experimental Data Relating FAC ppm to ORP and pH
Similar experiments were performed using water to which precise amounts of calcium chloride (CaCl2) and sodium bicarbonate (NaHCO3) were added to slightly buffer the water. This allows the FC feature to correlate low ORP values to the typically low FAC concentrations of tap water after it has been in the municipal water system for several days.
FCE – pH Included, Not Ignored
Unlike other FAC test methods that ignore the effect of pH on sanitizing power by artificially forcing the pH of their test sample to a single value, Myron L Company’s FCE includes pH in its concentration calculation. This capability gives Myron L Company’s FCE the ability to compensate for the effect of the changing ratio of chlorine species as pH
changes, resulting in a FAC concentration value germane to the actual sanitizing power of the source water. OCl- is measured as OCl-, and HOCl is measured as HOCl. Those users who are primarily concerned with or who prefer free chlorine concentration levels have a reliable measurement that gives consistent and comparable results, reading to reading, without having to rigorously control or artificially manipulate the sample’s pH.
In addition, because the FCE function displays both the FAC concentration and a predicted, stable ORP value, the user can, by comparing these two values from successive measurements, track how ORP (and disinfecting power) falls as pH rises and how ORP rises as pH is lowered when concentration is constant.
FCE – Chemistry Measured, Not Altered
Both DPD kits and colorimeters may tell the user the FAC concentration of the sample in the test tube, but since the chemistry of that sample is quite different from the source water being analyzed, the results are imprecisely related to actual disinfection power.
DPD kits and colorimeters only imply true disinfecting power; they do not measure it, and that is, after all, the whole point of the exercise.
The Myron L Company FCE method avoids these pitfalls and inaccuracies. FCE measures the real, unaltered chemistry of source water, including moment-to-moment changes in that chemistry.
The following controlled study shows exactly how differently the two methods respond, particularly at the high end where the effects of changes in pH are the greatest. In this study measurements were made with a digital colorimeter and a Myron L Company Ultrameter II 6P equipped with FCE. The solutions tested were made with various known concentrations of NaOCl in deionized water. The water was heated to above 80°C to remove any CO2 and, therefore, avoid interference from REDOX reactions between HOCL, OCl- and carbonates (HCO3).
Table 1 – Comparison of FCE to Digital Colorimeter
In this study as the pH rises and the ratio of OCl- to HOCL rises dramatically, the FCE is able to accurately track the changing concentration of FAC. The colorimeter’s results do not.
FCE – Handheld ORP Accuracy Without the ORP Delay
One of the challenges in implementing an ORP- based free chlorine measurement in a handheld field instrument is the sometimes lengthy response time of ORP sensors. It is not uncommon for an ORP sensor to take 12 to 15 minutes to arrive at a valid stable reading. In extreme cases, such as an older sensor in poor condition and measuring a complex solution with a very low ionic strength, the ORP measurement can take up to an hour to fully stabilize. Obviously, for a handheld instrument these are unacceptably long times.
The Myron L Company FCE function includes a pioneering feature that dramatically reduces the wait for stable ORP readings. This unique feature determines an extrapolated, final, stabilized ORP reading within 1 to 2 minutes rather than the typical 15 minutes or hours for other ORP systems.
The Predictive ORP feature’s calculations are based on a model of sensor behavior developed through a series of experiments that measured the response time of a representative sample of ORP sensors over a range of controlled chlorine concentrations. The results of this set of experiments revealed that the shape of the curve is very similar for various ORP levels differing only in the initial starting point and the final stabilized reading.
Figure 4 – Example of ORP Sensor Response Study
Using a proprietary curve-matching algorithm, the Predictive ORP feature determines what point along the typical sensor response curve a measurement occurred and extrapolates an appropriate final reading. This extrapolated value is used to calculate the FCE ppm value without having to wait for the sensor to stabilize and is also reported directly to the instrument’s display.
FCE – FLEXIBILITY FOR THE REAL WORLD
Another advantage of an ORP-based measurement such as the Myron L Company FCE feature is that, within the limits of its range, it can be used to measure the disinfection effectivity of ANY oxidizing germicide. Myron L Company FCE measurement can be used with non-bleach oxidants, such as chloramines or even non-chlorine oxidants, such as peracetic acid, bromine or iodine.
The Predictive ORP value displayed when the FCE function is active is directly relevant for monitoring and controlling the sanitizing effectivity of oxidizing sanitizers besides HOCl and OCl-.
While the concentration values reported by the FCE function will not be absolutely correct for non-FAC oxidants, since they are based on a HOCl / OCl- model, FCE can still be an effective tool for monitoring relative changes in concentration levels. For absolute accuracy a correlative study should be performed to relate concentration levels of the oxidant in question to the ORP values displayed by the Predictive ORP feature and ppm values output by the FCE.
FCE = Effective Chloramine Control
A perfect example of the Myron L Company FCE ‘s flexibility is the use of chloramines as a germicide.
Chloramines are formed when chlorine (Cl2) and ammonia (NH3) come into contact, forming three different inorganic chemicals: monochloramine (NH2Cl), dichloramine (NHCl2) and trichloramine (NCl3). In some applications chloramines are considered an unavoidable side effect of the chlorination process; however, because they are also oxidants, there are other applications where they are used as the primary disinfectant in water treatment.
Chloramines are effective at killing bacteria and other microorganisms, but because their relative ORP levels are lower compared to HOCl or OCl- at the same concentrations, the disinfection process is slower.
Table 2 – Electrode Potentials of Chloramines vs. HOCl
On the plus side, chloramines last longer than HOCl and OCl- (as long as 23 days in some cases), impart a less strong flavor or smell and their sanitizing
strength is not appreciably affected by changing pH.
Most importantly, chloramine-based water treatment methods produce much fewer hazardous byproducts. The US EPA limits the total concentration of the four chief hazardous byproducts of chlorination (chloroform, bromoform, bromodichloromethane, and dibromochloromethane), referred to as total trihalomethanes (TTHM), to 80 parts per billion in treated water. To avoid exceeding theses standards, many municipal water districts prefer using chloramine rather than chlorine.
The following table shows typical ORP values for various concentrations of monochloramine (NH2Cl).
Table 3 – ORP Values for NH2Cl in Pure Water
When ORP levels of NH2Cl approach and exceed 500 mV, effective sanitization occurs with exposure times of 20 to 30 minutes. This is more than adequate for municipal water treatment.
Since Myron L Company FCE function bases its measurement on ORP, it presents an empirical, easy to interpret measurement, both in terms of the Predictive ORP display and the FAC equivalent concentration, allowing the user to monitor falling chloramine concentration as disinfection proceeds.
FCE and Cyanuric Acid (CYA) – Don’t Guess. Know!
Outdoor pool maintenance is a prime application where ORP-based chlorine measurement should be preferred. The use of chlorine to sanitize pools runs afoul of the fact that much of the chlorine added to an outdoor pool is deactivated by exposure to UV radiation in sunlight.
Cyanuric Acid (CYA) is often added to the pool water to “protect” FAC. In a typical pool at 7.6 pH about 50% of the FAC is OCl-, which reacts to UV radiation that passes through the Earth’s ozone layer (290nm). CYA combines with FAC to form N-chlorinated-cyanurates, which only react to UV radiation (215nm to 235nm) removed by the Earth’s ozone layer. Since N-chlorinated-cyanurates are also oxidizers, they also act as germicides. Unfortunately, they have much lower reduction potentials and, therefore, a much lower strength as a germicide.
Figure 5 – Effect of Cyanuric Acid on Chlorine ORP Values
Pool maintenance websites that advocate the use of cyanuric acid (not all of them do) often recommend levels of 40 to 80 ppm. Figure 5 shows how severely ORP and disinfecting power are affected by CYA.
The addition of only 20 ppm CYA decreases the pool water’s ORP 120 mV, reducing the effectivity of FAC from 1.5 ppm to an effectivity that is equivalent to only 0.3 ppm. Adding 40 ppm, or worse, 80 ppm, reduces sanitizing strength even more severely. This is definitely a case where more is not better.
A 1972 study on water chlorination showed that water treated with enough chlorine to kill 100% of the E. coli present in 3 minutes or less required almost 6 times as much chlorine be added for the same effect when 50 ppm of CYA was added.
Cyanuric acid beneficially affects pool chlorination by greatly reducing that portion of chlorine demand related to loss due to UV. Unfortunately, if you are using a colorimeter or DPD kit, it will tell you that your FAC concentration is unchanged and significantly misrepresent sanitizing strength
The Myron L Company FCE function’s ability to react to changes in ORP makes it an ideal tool for keeping track of how CYA affects residual sanitizing power when added to a chlorinated pool. The Predicative ORP display provides a direct and effective way to monitor changes in ORP values as CYA concentration increases. Because the FCE ppm
display reacts to changes in ORP and pH, it will reflect changes in the sanitizing strength as an “equivalent” or “effective” FAC concentration.
Judging the true effectivity of chlorine as an oxidizing germicide requires more than just knowing how much chlorine is present. Changing pH or the addition of additives like cyanuric acid can radically alter the effectivity of the chlorine present. To accurately measure the effects of these issues requires a test method based on the precise measurement of ORP (the chemical characteristic directly responsible for killing microbes like bacteria and viruses) and cross-referenced to pH.
The Myron L Company FCE is the first measurement function that allows handheld, field instruments to integrate ORP and pH measurements into a system for monitoring the residual disinfecting power of free available chlorine in aqueous solutions.
• It provides an empirical measurement that does not require interpretation.
• It is not affected by water color or turbidity.
• It measures the true chemistry of the water, unaltered.
• It accounts for changes in pH.
• It reports the effects of CYA on disinfecting power.
• It can be used to monitor non-chlorine oxidants.
Myron L Meters features the Myron L FCE function in several instruments that read free chlorine – the Ultrameter II 6P, the Ultrameter III 9P, the PoolPro PS9 and PS6 models, and the
new Ultrapen PT4, soon to be released.
TweetAll Myron L meters are factory calibrated with NIST traceable Standard Solutions having specific conductivity/ppm values. MyronL Standard Solutions are made under strictly controlled conditions using reagent grade salts. These salts are mixed with deionized water having a resistivity of at least 5 megohms-cm purity. Myron L Standard Solutions have an accuracy of +1% based […]
All Myron L meters are factory calibrated with NIST traceable Standard Solutions having specific conductivity/ppm values. MyronL Standard Solutions are made under strictly controlled conditions using reagent grade salts. These salts are mixed with deionized water having a resistivity of at least 5 megohms-cm purity.
Myron L Standard Solutions have an accuracy of +1% based on values published in the International Critical Tables and traceable to the National Institute of Standards and Technology. NIST certificates , while not available on Ultrapens, are available on most other Myron L meters and solutions. Check the product page for the NIST certificate option. See example below:
Regular use of these solutions is recommended to ensure specified instrument accuracy. Frequency of conductivity recalibration depends upon use, but once every month should be sufficient for an instrument used daily. pH models, depending upon use, should be recalibrated with pH 7 Buffer every 1-2 weeks, and checked with pH 4 and/or 10 Buffers at similar intervals. pH Sensor Storage Solution is recommended for keeping the pH sensor hydrated. Myron L solutions are available in quart/1 ltr., gallon/3,8 ltr. and 2 oz./59 ml plastic bottles, ready to use.
Below is the official NIST traceability policy from the website of the National Institute of Standards and Technology.
The mission of NIST is to promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life. To help meet the measurement and standards needs of U.S. industry and the nation, NIST provides calibrations, standard reference materials, standard reference data, test methods, proficiency evaluation materials1, measurement quality assurance programs, and laboratory accreditation services that assist a customer in establishing traceability of measurement results.
Metrological traceability requires the establishment of an unbroken chain of calibrations to specified references. NIST assures the traceability of measurement results that NIST itself provides, either directly or through an official NIST program or collaboration. Other organizations are responsible for establishing the traceability of their own results to those of NIST or other specified references. NIST has adopted this policy statement to document the NIST role with respect to traceability.
Statement of Policy
To support the conduct of its mission and to ensure that the use of its name, products, and services is consistent with its authority and responsibility, NIST adopts for its own use and recommends for use by others the definition of metrological traceability2 provided in the most recent version of the International Vocabulary of Metrology: “property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.” (International Vocabulary of Metrology – Basic and General concepts and Associated Terms (VIM), definition 2.41, see Reference ).
To support the conduct of its mission and to ensure that the use of its name, products, and services is consistent with its authority and responsibility, NIST:
1. Adopts for its own use and recommends for use by others the definition of traceability provided in the most recent version of the International vocabulary of metrology – Basic and general concepts and associated terms (VIM): “property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty..” .
2. Establishes metrological traceability of the results of its own measurements and of results provided to customers in NISTcalibration and measurement certificates, operating in accordance with the NIST Quality System for Measurement Services.
3. Asserts that providing support for a claim of metrological traceability of the result of a measurement is the responsibility of theprovider of that result, whether that provider is NIST or another organization; and that assessing the validity of such a claim is the responsibility of the user of that result.
4. Communicates, especially where claims expressing or implying the contrary are made, that NIST does not define, specify,assure, or certify metrological traceability of the results of measurements except those that NIST itself provides, either directly or through an official NIST program or collaboration. (See also NIST Administrative Manual, Subchapter 5.03, NIST Policy on Use of its Name in Advertising.)
5. Collaborates on development of standard definitions, interpretations, and recommended practices with organizations that have authority and responsibility for variously defining, specifying, assuring, or certifying metrological traceability.
6. Develops and disseminates technical information on traceability and conducts coordinated outreach programs on issues of traceability and related requirements.
7. Assigns responsibility for oversight of implementation of the NIST policy on metrological traceability to the NIST Measurement Services Advisory Group (MSAG).
1 Underlined terms are defined in III Glossary of Terms in the Supplementary Materials section following.
2 The full term, “metrological traceability” is preferred when there is a risk of confusion with other meanings of the abbreviated term “traceability”, which is sometimes used to refer to the “history” or “trace” of an item. The abbreviated term is also used in this document to improve readability, since it is clear that “metrological traceability” is meant in every case.
The National Institute of Standards and Technology (NIST) is an agency of the U.S. Department of Commerce. Original found here: http://www.nist.gov/traceability/nist_traceability_policy_external.cfm
TweetWHAT IS ORP? Oxidation Reduction Potential or Redox is the activity or strength of oxidizers and reducers in relation to their concentration. Oxidizers accept electrons, reducers lose electrons. Examples of oxidizers are: chlorine, hydrogen peroxide, bromine, ozone, and chlorine dioxide. Examples of reducers are sodium sulfite, sodium bisulfate and hydrogen sulfide. Like acidity and alkalinity, […]
WHAT IS ORP?
Oxidation Reduction Potential or Redox is the activity or strength of oxidizers and reducers in relation to their concentration. Oxidizers accept electrons, reducers lose electrons. Examples of oxidizers are: chlorine, hydrogen peroxide, bromine, ozone, and chlorine dioxide. Examples of reducers are sodium sulfite, sodium bisulfate and hydrogen sulfide. Like acidity and alkalinity, the increase of one is at the expense of the other.
A single voltage is called the Oxidation-Reduction Potential, where a positive voltage shows a solution attracting electrons (oxidizing agent). For instance, chlorinated water will show a positive ORP value whereas sodium sulfite (a reducing agent) loses electrons and will show a negative ORP value.
ORP is measured in millivolts (mV), with no correction for solution temperature. Like pH, it is not a measurement of concentration directly, but of activity level. In a solution of only one active component, ORP indicates concentration. As with pH, a very dilute solution will take time to accumulate a measurable charge.
An ORP sensor uses a small platinum surface to accumulate charge without reacting chemically. That charge is measured relative to the solution, so the solution “ground” voltage comes from the reference junction – the same type used by a pH sensor.
HISTORY OF ORP
ORP electrodes were first studied at Harvard University in 1936. These studies showed a strong correlation of ORP and bacterial activity. These tests were confirmed by studies on drinking water and swimming pools in other areas of the world. In 1971 ORP (700 mV) was adopted by the World Health Organization (WHO) as a standard for drinking water. In 1982 the German Standards Agency adopted the ORP (750 mV) for public pools and in 1988 the National Swimming Pool Institute adopted ORP (650 mV) for public spas.
WHERE IS ORP USED?
As you can tell by the previous paragraphs, ORP is used for drinking water, swimming pools and spas. However, ORP is also used for cooling tower disinfection, groundwater remediation, bleaching, cyanide destruction, chrome reductions, metal etching, fruit and vegetable disinfection and dechlorination.
In test after test on poliovirus, E. coli, and other organisms, a direct correlation between ORP and the rate of inactivation was determined. It is, therefore, possible to select an individual ORP value, expressed in millivolts, at which a predictable level of disinfection will be achieved and sustained regardless of variations in either oxidant demand or oxidant concentration. Thus, individual ORP targets, expressed in millivolts, can be determined for each application, which will result in completely reliable disinfection of pathogens, oxidation of organics, etc. Any level of oxidation for any purpose can be related to a single ORP number which, if maintained, will provide utterly consistent results at the lowest possible dosage.
WHY USE ORP?
ORP is a convenient measure of the oxidizer’s or reducer’s ability to perform a chemical task. ORP is not only valid over a wide pH range, but it is also a rugged electrochemical test, which can easily be accomplished using in-line and handheld instrumentation. It is by far a more consistent and reliable measurement than say chlorine alone.
LIMITATIONS FOR ORP
As with all testing, ORP has certain limitations. The speed of response is directly related to the exchange current density which is derived from concentration, the oxidation reduction system, and the electrode. If the ORP of a sample is similar to the ORP of the electrode, the speed will be diminished.
Carryover is also a possible problem when checking strong oxidizers or reducers, and rinsing well will help greatly.
Although a better indicator of bactericidal activity, ORP cannot be used as a direct indicator of the residual of an oxidizer due to the effect of pH and temperature on the reading. ORP can be correlated to a system by checking the oxidizer or reducer in a steady state system with a wet test, and measuring pH. If the system stays within the confines of this steady state parameter (usually maintained by in- line or continuous control), a good correlation can be made. The best recommendation for ORP is to use wet tests, and over three test periods correlate the ORP values to those test parameters.
FREE CHLORINE CONVERSION USING ORP
The most ubiquitous and cost-effective sanitizing agent used in disinfection systems is chlorine. When chlorine is used as the sanitizer, free chlorine measurements are required to ensure residual levels high enough for ongoing bactericidal activity. Myron L meters accurately convert ORP measurements to free chlorine based on the understanding of the concentrations of the forms of free chlorine at a given pH and temperature. The conversion is accurate when chlorine is the only oxidizing/reducing agent in solution and pH is stable between 5 and 9. This pH range fits most applications because pH is usually maintained such that the most effective form of free chlorine, hypochlorous acid, exists in the greatest concentration with respect to other variables such as human tolerance.
MYRON L METERS
Myron L offers a variety of handheld instruments and in-line Monitor/controllers that may be used to measure, monitor and/or control ORP. The latest is the Ultrapen PT3, ORP/Redox and Temperature Pen. The Ultrameter III™ 9PTKA, Ultrameter II™ 6PFCE, PoolPro™ PS6FCE and PS9TK, and D-6 Digital Dialysate Meter™ are multi-parameter handheld instruments with ORP and FCE free chlorine measuring capabilities. These instruments also have the capability to measure conductivity, TDS, resistivity, pH, mineral/salt concentration and temperature, making them the preferred instruments for all water treatment professionals. The 720 Series II Monitor/controllers are an excellent choice for continuous in-line measurements.
For additional information, visit us at MyronLMeters.com.
Tweet In water remote sensing, or ‘ocean color‘, people extract water quality parameters from satellite imagery. Probably the most interesting parameter is chlorophyll-a concentration, which can be related to the phytoplankton (algae, cyanobacteria) biomass in the water. Applications are quite diverse, ranging from harmful algal […]
In water remote sensing, or ‘ocean color‘, people extract water quality parameters from satellite imagery. Probably the most interesting parameter is chlorophyll-a concentration, which can be related to the phytoplankton (algae, cyanobacteria) biomass in the water. Applications are quite diverse, ranging from harmful algal blooms early warning systems, eutrophication assessment to climate change research – to name a few.
The same algorithms are used on spectra taken directly in the field to calibrate/validate remotely sensed concentrations. However, spectrometers used for these field measurements are quite expensive (>20k USD) and often not trivial to operate. This hampers the use of ‘water color’ by a wider audience, e.g. water managers, fisheries and especially the public that might want to know what the water quality in their pond is.
Spectra from cheap, open source spectrometers can potentially bridge this gap.
From my experience with the foldable spectrometer and the desktop kit I see three main issues: sensitivity, spectral resolution and calibration.
- Sensitivity Water appears often very dark, especially if only little scattering substance is present. In contrast, the reflection of the direct sun light on the water surface is extremely bright. Thus, to resolve the ‘true’ water color, the instrument has to be relatively sensitive.
- Spectral Calibration and Calibration Pigment absorption, other optically active substances and the water itself cause the water’s color. Pigments in phytoplankton, such as the main photosynthetic pigment chlorophyll-a, have very sharp absorption features. In order to resolve those peaks, the spectral resolution has has to be sufficiently high and the calibration sufficiently accurate.
These days I had the opportunity to take the desktop kit out on a fieldwork campaign on Lake Peipsi and Lake Vortsjarv in Estonia. Fortunately, this little Baltic country has one of the best 3G-networks worldwide and so I could use the spectral workbench to upload my spectra straight from the boats – pretty cool! However, an offline version of the workbench is essential (couldn’t get the local webserver running so far). The spectra are available here: Lake Peipsi (R_sky, R_water), Lake Vortsjarv (R_sky, R_water).
- Preparations / Calibration The initial wavelength calibration with fluorescent light line peaks is pretty brilliant. Still, at least a rudimentary intensity calibration is necessary to interpret the shape of the spectra. I can use our calibration lab for this purpose and give some feedback to the community on how the spectral response curve of at least my desktop kit’s webcam looks like. For public use, I’m thinking of using daylight spectra as a reference, as the shape is rather stable (not the intensity, due to atmospheric conditions). Maybe we can find a better solution for that.
- Measurements We need at least two measurements in order to run a spectral unmixing algorithm: upwelling radiance (light from the water) and downwelling irradiance (complete skylight). As we most likely won’t be able to build a cheap spectrometer that has even vaguely defined entry optics (e.g. 9deg field of view for the radiance and a perfect cosine response for the irradiance), some improvisation is needed. For the irradiance measurement I’m thinking of using a white table-tennis ball on top of the slit as a diffusor. The radiance measurement is mainly hampered by water surface reflections. To avoid those, I’d like to measure either just underneath the water surface (–> how to make the spectrometer water tight) or to use a sun shade (such as for camera lenses). For now, a black bucket with a hole in the bottom should do the job.
- Postprocessing The current procedure to extract a spectrum from the webcam-video is pretty smart and straightforward but probably not optimal if sensitivity is a priority. Currently, as I understand, only one row of the ‘stitched spectral image’ is used to extract the spectrum (‘set sample row’ in the workbench). Skylight, as well as the water leaving radiance are stable on the timescales of a measurement. Therefore I’d suggest to average over all rows to improve the signal to noise ratio. If that is not enough, one could think about extracting not only one line from the webcam’s video stream but e.g. ten and save the average in the stitched spectral image. In a last desperate step, one could use the whole webcam image and correct for the curvature caused by the DVD, however, I don’t think this will be necessary.
- This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.
Research by philippg
TweetIntroduction Oceans contain over 97.2% of the planet’s water. Because of the high salinity of ocean water and the significant costs associated with seawater desalination most of the global water supply has traditionally come from fresh water sources – groundwater aquifers, rivers and lakes. Today, however, changing climate patterns combined with population growth pressures and […]
Oceans contain over 97.2% of the planet’s water. Because of the high salinity of ocean water and the significant costs associated with seawater desalination most of the global water supply has traditionally come from fresh water sources – groundwater aquifers, rivers and lakes. Today, however, changing climate patterns combined with population growth pressures and limited availability of new and inexpensive fresh water supplies are shifting the water industry’s attention: in an emerging trend, the world is reaching to the ocean for fresh water.
Until recently, desalination was limited to desert-climate dominated regions. Technological advances and an associated decrease in water production costs over the past decade have expanded its use in coastal areas traditionally supplied with fresh water resources. Today, desalination plants provide approximately 1% of the world’s drinking water supply and the commissioned and installed desalination capacity has been increasing exponentially over the past 10 years.
Higher Productivity Membrane Elements
A key factor that has contributed to the dramatic decrease of seawater desalination costs over the past 10 years is the advancement of the SWRO membrane technology. Today’s high-productivity membrane elements are designed with several features that yield more fresh water per membrane element than any time in the recent history of this technology: higher surface area, enhanced permeability and denser membrane packing. Increasing active membrane leaf surface area and permeability allows it to gain significant productivity using the same size (diameter) membrane element. Active surface area of the membrane elements is typically increased by membrane production process automation, by denser membrane leaf packing and by adding membrane leaves within the same element.
The total active surface area in a membrane element is also be increased by increasing membrane size/diameter. Although 8-inch SWRO membrane elements are still the “standard” size most widely used in full-scale applications, larger 16-inch and 18-inch size SWRO membrane elements have become commercially available over the past three years and have already found full-scale implementation in several SWRO projects worldwide (Bergman & Lozier, 2010).
In the second half of the 1990s, the typical 8-inch SWRO membrane element had a standard productivity of 5,000 to 6,000 gallons per day (gpd) at salt rejection of 99.6%. In 2003, several membrane manufacturers introduced high-productivity seawater membrane elements that are capable of producing 7,500 gpd at salt rejection of 99.75%. Just one year later, even higher productivity (9,000 gpd at 99.7% rejection) seawater membrane elements were released on the market. Over the past three years SWRO membrane elements combining productivity of 10,000 to 12,000 gpd and high-salinity rejection have become commercially available and are now gaining wider project implementation.
The newest membrane elements provide flexibility and choice and allow users to trade productivity and pressure/power costs. The same water product quality goals can be achieved in one of two general approaches: (1) reducing the system footprint/construction costs by designing the system at higher productivity, or (2) reducing the system’s overall power demand by using more membrane elements, designing the system at lower flux and recovery, and taking advantage of newest energy recovery technologies which further minimize energy use if the system is operated at lower (35% to 45%) recoveries.
Innovative hybrid membrane configuration combining SWRO elements of different productivity and rejection within the same vessel which are sequenced to optimize the use of energy introduced with the feed water to the desalination vessels is also finding wider implementation. In addition, a number of novel membrane SWRO train configurations have been developed over the past five years aiming to gain optimum energy use and to reduce capital costs for production of high-quality desalinated water.
Design and Equipment Enhancements for Lower Energy Use
Energy is one of the largest expenditures associated with seawater desalination. Figure 1 shows a distribution of the energy use within a typical seawater desalination plant. As shown on this figure, the SWRO system typically uses over 70% of the total plant energy.
Figure 1 – Energy Use Breakdown of Typical SWRO Desalination Plant
Increased High Pressure Pump Efficiency
One approach for reducing total RO system energy use which is widely applied throughout the desalination industry today is to incorporate larger, higher efficiency centrifugal pumps which serve multiple RO trains. This trend stems from the fact that the efficiency of multistage centrifugal pumps increases with their size (pumping capacity). For example, under a typical configuration where individual pump is dedicated to each desalination plant RO train, high pressure pump efficiency is usually in a range of 80% to 83%. However, if the RO system configuration is such that a single high pressure pump is designed to service two RO trains of the same size, the efficiency of the high pressure pumps could be increased to up to 85%.
Proven design that takes this principle to the practical limit of centrifugal pump efficiency (≈ 90%) is implemented at the 86 MGD Ashkelon seawater desalination plant in Israel, where two duty horizontally split high pressure pumps are designed to deliver feed seawater to 16 SWRO trains at guaranteed long term efficiency of 88%. Continuous plant operational track record over the past 5 years shows that the actual efficiency level of these pumps under this configuration is close to 90%.
A current trend for smaller desalination facilities (plants with fresh water production capacity of 250,000 gpd or less) is touse positive displacement (multiple-piston)high-pressure pumps and energy recovery devices, which often are combined into a single unit. These systems are configured to take advantage of the high efficiency of the positive displacement technology which practically can reach 94% to 97%.
Improved Energy Recovery
Advances in the technology and equipment allowing the recovery and reuse of the energy applied for seawater desalination have resulted in a reduction of 80% of the energy used for water production over the last 20 years. Today, the energy needed to produce fresh water from seawater for one household per year (~2,000 kW/yr) is less than that used by the same household’s refrigerator.
While five years ago, the majority of the existing seawater desalination plants used Pelton Wheel-based technology to recover energy from the SWRO concentrate, today the pressure exchanger-based energy recovery systems dominate in most desalination facility designs. The key feature of this technology is that the energy of the SWRO system concentrate is directly applied to pistons that pump intake seawater into the system. Pressure-exchanger technology typically yields 5% to 15% higher energy recovery savings than the Pelton-Wheel-based systems.
Figure 2 depicts the configuration of a typical pressure exchanger-based energy recovery system. After membrane separation, most of the energy applied for desalination is contained in the concentrated stream (brine) that also contains the salts removed from the seawater. This energy-bearing stream (shown with red arrows on Figure 2) is applied to the back side of pistons of cylindrical isobaric chambers, also known as pressure exchangers (shown as yellow cylinders on Figure 2). These pistons pump approximately 45% to 50% of the total volume of seawater fed into the RO membranes for salt separation. Since a small amount of energy (4 to 6%) is lost during the energy transfer from the concentrate to the feed water, this energy is added back to feed flow by small booster pumps to cover for the energy loss. The remainder (45% to 50%) of the feed flow is handled by high-pressure centrifugal pumps. Harnessing, transferring and reusing the energy applied for salt separation at very high efficiency (94% to 96%) by the pressure exchangers allows a dramatic reduction of the overall amount of electric power used for seawater desalination.In most applications, a separate energy recovery system is dedicated to each individual SWRO train. However, some recent designs include configurations where two or more RO trains are serviced by a single energy recovery unit.
Figure 2 – Pressure Exchanger Energy Recovery System
While the quest to lower energy use continues, there are physical limitations to how low the energy demand could go using RO desalination. The main limiting factors are the osmotic pressure that would need to be overcome to separate the salts from the seawater and the amount of water that could be recovered from a cubic meter of seawater before the membrane separation process is hindered by salt scaling on the membrane surface and the service systems. This theoretical limit for the entire seawater desalination plant is approximately 4.5 kWh/kgal.
Seawater Desalination Cost Trends
Advances in seawater RO desalination technology during the past two decades, combined with transition to construction of large capacity plants, and enhanced competition by using the Build-Own-Operate-Transfer (BOOT) method of project delivery have resulted in an overall downward cost trend. While the costs of production of desalinated water have benefited from the most recent advances in desalination technology, the cost spread among individual desalination projects observed over the past three years is fairly significant.
Most recently commissioned large seawater desalination projects worldwide produce desalinated water at an all-inclusive cost of US$3.0 to US$5.5/kgal. However, the traditionally active desalination markets in Israel and Northern Africa (i.e., Algeria) have yielded desalination projects with exceptionally low water production costs (110 MGD SWRO Plant in Sorek, Israel – US$2.00/kgal; 87 MGD Hadera Desalination Plant, Israel – US$2.27/kgal; 132 MGD Magtaa SWRO Plant in Algeria – US$2.12/kgal).
On the other end of the cost spectrum, some of the most recent seawater desalination projects in Australia had been associated with the highest desalination costs observed over the past 10 years – i.e., the Gold Coast SWRO Plant in Queensland at US$10.95/kgal; the Sydney Water Desalination Plant at US$8.67/kgal; and the Melbourne’s Victorian Desalination Plant at US$9.54/kgal.
While this extreme cost disparity has a number of site-specific reasons, the key differences associated with the lowest and highest-cost projects are related to five main factors: (1) desalination site location; (2) environmental considerations; (3) phasing strategy; (4) labor market pressures; (5) method of project delivery and risk allocation between owner and private contractor responsible for project implementation.
The desalination projects with highest and lowest costs have a very distinctive difference in terms of project phasing strategy. While the large high-cost projects incorporate single intake and discharge tunnel structures built for the ultimate desalination plant capacity (which often equals two times the capacity of the first project phase), the desalination projects on the low end of the cost spectrum use multi-pipe intake systems constructed mainly from high density polyethylene (HDPE) that have capacity commensurate with the production capacity of the desalination plant. Additional multiple intake pipes and structures are installed as needed at the time of plant expansion for these facilities.
While the single-phase construction of desalination plant intake and outfall structures dramatically reduces the environmental and public controversy associated with the plant capacity expansion at a later date, this “ease-of-implementation” benefit typically comes with an overall cost penalty. The notion that the larger costs associated with building complex intake and outfall concrete tunnels in one phase will somehow be offset by economies of scale usually does not yield the expected overall project cost savings. The main reason is the fact that the cost of 100 linear feet of deep concrete intake or discharge tunnel is over four times higher than the cost of the same capacity intake or discharge constructed from multiple HDPE pipes located on the ocean bottom, while the economy of scale from one-stage construction is usually less than 30%.
Labor market differences can have a profound impact on the cost of construction of desalination projects. The overlapping schedules of the series of large desalination projects in Australia have created temporary shortage of skilled labor, which in turn has resulted in a significant increase in unit labor costs. Since labor expenditures are usually 30% to 50% of the total desalination plant construction costs, a unit labor rate increase of 20% to as high as up to 100%, could trigger sometimes unexpected and not frequently observed project cost increases.
Without exception, the lowest cost desalination projects to date have been delivered under turnkey BOOT contracts where private sector developers share risks with the public sector based to their ability to control and mitigate the respective project related risks.
On the other hand, the most costly desalination projects worldwide have been completed under an “alliance”[(a type of design-build-operate (DBO)] model where the public utility retains the ownership over the project assets but expects the DBO team to take practically all project-related risks. In this case, DBO contractors take upon project risks over which they have limited or no control, by delivering very conservative designs, incorporating high contingency margins in the price of their construction, operation and maintenance services, and by insuring these project risks at very high premiums. As a result, the projects delivered under such structure carry very high contingencies and upfront insurance and performance security payments which ultimately reflect on the overall increase of the cost of water production.
While under a typical BOOT project, the insurance and contingency costs are usually well below 20% of the total capital costs, projects with disproportionate transfer of risk to the private contractor result in built-in insurance and contingency premiums which exceed well over 30% of the total project capital costs. As a result, most often benefits gained from using state-of-the-art technologies, equipment and design, are negated by overly burdensome insurance and contingency expenditures and high cost of project funding.
Seawater Desalination Challenges in US
Water Production Costs
Currently, the cost of desalinating seawater in the US is relatively high compared to that of traditional low-cost water sources (groundwater and river water) and to production costs for water reclamation and reuse for irrigation and industrial use. Indeed, the cost of traditional local groundwater water supplies in some parts of the US are as low as US$0.50/kgal to US$0.90/kgal. However, the quantity of such low-cost sources in coastal urban centers of California, Texas, Florida, South Carolina and other parts of the US exposed to recent long-term drought pressures is very limited.
The generally lower costs for production of reclaimed water and for implementation of water conservation measures have often been used as an argument against the wider use of seawater desalination. This argument however, is fatally flawed by the fact that water conservation and reuse do not create new sources of drinking water – they are merely a rational tool to maximize the beneficial use of the available water supply resources. Under conditions of prolonged drought when the available water resources cannot be replenished at the rate of their use, aggressive reuse and conservation can help but may not completely alleviate the need for new water resources and water rationing.
Typically, seawater desalination cost benefits extend beyond the production of new water supplies. If seawater desalination is replacing the use of over-pumped coastal or inland groundwater aquifers, or is eliminating further stress on environmentally sensitive estuary and river habitats, than the higher costs of this water supply alternative would also be offset by its environmental benefits. Similarly, seawater desalination provides additional benefits in the time of drought where traditional water supplies may not be reliable and their scarcity may increase their otherwise relatively low costs.
Salt separation from seawater requires a significant amount of energy to overcome the naturally occurring osmotic pressure exerted on the reverse osmosis membranes. This in turns makes seawater desalination several times more energy intensive than conventional treatment of fresh water resources. Table 1 presents the energy use associated with various water supply alternatives. The table does not incorporate the costs associated with raw water treatment and product water delivery.
Table 1 – Energy Use of Various Water Supply Alternatives
While energy use for seawater desalination is projected to decrease by 10 to 20 % in the next 5 years as a result of technological advances discussed previously, the total energy demand for conventional water treatment would likely increase by 15 to 20 % in the same time frame because of the energy demand associated with the additional treatment (such as micro- or ultra-filtration, ozonation, UV disinfection, etc.) which would be needed in order to meet the most recent regulatory requirements for production of safe drinking water in the USA.
A number of the seawater desalination projects under consideration in California and Florida are proposed to be collocated with power generation plants which currently use seawater for production of electricity. Under the collocation configuration the desalination plant does not have a separate intake and discharge to the ocean and both the desalination plant intake and desalination plant discharge are connected to the exiting power plant discharge outfall or canal.
Collocation yields a number of benefits mainly because it avoids construction and permits for new intake and concentrate discharge facilities, and because of the energy cost savings associated with the desalination of warmer source water. However this intake configuration alternative has been considered undesirable by some environmental groups due to the potential loss of marine organisms caused by the impingement of marine organisms against the screens of the power plant intake and their entrainment inside the power plant conveyance and cooling system and subsequently inside the desalination plant.
Based on recently introduced regulatory requirements, the 21 once-through cooling plants along the California coast are required to prepare comprehensive plans for discontinuation of their use of open intakes and switching to air-cooling towers or to water close-circulation cooling towers in order to reduce impingement and entrainment of marine organisms.
Opponents of collocated seawater desalination plants have often present the argument that if the power plant changes its cooling system in the future, seawater desalination under collocated configuration at the particular location would no longer be available. This argument however, is unfounded in reality, because even if the host power plants abandon once-through cooling in the future, the desalination projects will still retain the main cost-benefits of collocation – avoidance of the need to construct a new intake and outfall. The cost savings from the use of the existing power plant intake and outfall facilities would be over 25%, resulting in a significant net benefit with or without the power plant in operation.
Recent studies of wedge-wire screens in Santa Cruz, California indicate that this type of open intake may prove to be a viable alternative for dramatic reduction in impingement and entrainment of marine organisms. Typically, wedge-wire screens are designed to be placed in a water body where significant prevailing ambient cross flow current velocities (³ 1 ft/s) exist. This cross high flow velocity allows organisms that would otherwise be impinged on the wedge-wire intake to be carried away with the flow.
A 2-mm cylindrical wedge wire screen intake is also planned to be tested for one year at the West Basin Municipal Water District’s Ocean Water Desalination Demonstration Facility in Redondo Beach. This demonstration facility is currently under operation.
Various sub-surface intake technologies (i.e., beach wells, horizontally directionally drilled and slant wells and innovative infiltration gallery configurations) have been heavily promoted by the California Coastal Commission and local environmental groups as a viable alternative to power plant collocation and construction of new open intakes along the California coast. Ongoing long-term studies of subsurface intakes in Long Beach and Dana Point, California are expected to provide comprehensive data that would allow completing a scientifically-based analysis of the viability and performance benefits of alternative subsurface intakes.
Seawater desalination plants along the US coastline would produce concentrate of salinity that is approximately 1.5 to 2 times higher than the salinity of the ambient seawater (i.e., in a range of 52 ppm to 67 ppm). While most marine organisms can adapt to this increase in salinity, some aquatic species such as abalone, sea urchins, sand dollars, sea bass and top smelt, are less tolerant to high salinity concentrations. Therefore, thorough assessment of the environmental impact of the discharge of concentrate and of any other byproducts of the seawater treatment process is a critical part of the evaluation of project viability.
At seawater desalination projects that are proposed to be collocated with power plants, the desalination plant discharge is planned to be diluted with the cooling water of the power plant to salinity levels that typically do not have significant impact on aquatic life. The magnitude and significance of impact, however, mainly depend on the type of marine organisms inhabiting the area of the discharge and on the hydrodynamic conditions of the ocean in this area, such as currents, tide, wind and wave action, which determine the time of exposure of the marine organisms to various salinity conditions.
Extensive salinity tolerance studies completed over the past several years at the Carlsbad seawater desalination demonstration facility in California indicate that after concentrate dilution with power plant cooling seawater down to 40 ppm or less, the combined discharge does not exhibit chronic toxicity on sensitive test marine species. Recent acute toxicity studies at this facility further show that sensitive marine species can event tolerate salinity of 50 ppm or more over a short period of time (2 days or less).
Some seawater desalination projects are planning to use deep injection wells to discharge the high-salinity seawater concentrate generated during the reverse osmosis separation process. However, the full-scale experience with this concentrate disposal method to date is very limited.
A third disposal alternative, besides injection wells and co-disposal with power plant cooling water, currently under consideration for implementation at a number of seawater desalination projects in the US, is the discharge of the concentrate through existing wastewater treatment plant ocean outfall. International experience with such co-located discharges is fairly limited. However, this technology may have a number of merits similar to these derived from the collocation of desalination and power generation plants.
Proving that concentrate discharge from a seawater desalination plant is environmentally safe requires thorough engineering analysis including: hydrodynamic modeling of the discharge; whole effluent toxicity testing; salinity tolerance analysis of the marine species endogenous to the area of discharge; and reliable intake water quality characterization that provides basis for assessment of concentrate’s make up and compliance with the numeric effluent quality standards applicable to the point of discharge. Comprehensive pilot testing of the proposed seawater desalination system is very beneficial for the project environmental impact analysis.
Summary and Conclusions
Over the past decade seawater desalination has experienced an accelerated growth driven by advances in membrane technology and environmental science. While conventional technologies, such as sedimentation and filtration have seen modest advancement since their initial use for potable water treatment several centuries ago, new more efficient seawater desalination membranes and membrane technologies, and equipment improvements are released every several years. Although, no major technology breakthroughs are expected to bring the cost of seawater desalination further down dramatically in the next several years, the steady trend of reduction of desalinated water production costs coupled with increasing costs of water treatment driven by more stringent regulatory requirements, are expected to accelerate the current trend of increased reliance on the ocean as an attractive and competitive water source. This trend is forecasted to continue in the future and to further establish ocean water desalination as a reliable drought-proof alternative for many coastal communities in the United States and worldwide.
Although seawater desalination projects in the US face a number of environmental challenges, these challenges can be successfully addressed by carefully selecting the project site, by implementing state-of-the art intake and concentrate discharge technologies and by incorporating energy efficient and environmentally sound equipment and systems.
Nikolay Voutchkov, PE, BCEE Water Globe Consulting, LLC, Stamford, CT. Editor of Desalination Technology: Health and Environmental Impacts (2010) IWA Publishing.
Water Desalination and European Research
Nanofiltration for Brackish Desalination
GWI – Global Water Intelligence: Market-leading Analysis of the International Water Industry, 10, pp.9, 2009.
Bergman R.A. and J.C. Lozier.“Large-Diameter Membrane Elements and Their Increasing Global Use”. IDA Journal. pp. 16, First Quarter 2010.
Global Water Intelligence
International Desalination Ascociation
Joseph Cotruvo, Nikolay Voutchkov, John Fawell, Pierre Payment, David Cunliffe, Sabine Lattemann Desalination Technology: Health and Environmental Impacts (2010) IWA Publishing.
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TweetJohn Roach, Contributing Writer, NBC News Elevated levels of methane and other stray gases have been found in drinking water near natural gas wells in Pennsylvania’s gas-rich Marcellus shale region, according to new research. In the case of methane, concentrations were six times higher in some drinking water found within one kilometer of drilling operations. […]
John Roach, Contributing Writer, NBC News
Elevated levels of methane and other stray gases have been found in drinking water near natural gas wells in Pennsylvania’s gas-rich Marcellus shale region, according to new research. In the case of methane, concentrations were six times higher in some drinking water found within one kilometer of drilling operations.
TweetMyron L Meters provides introductory as well as professional level material on water quality and water treatment. Did you know that Myron L Meters carries a meter specifically for testing RO systems (see below)? Water purification systems have purified brackish water and sea water for the military, businesses and farms in many different locations on […]
Myron L Meters provides introductory as well as professional level material on water quality and water treatment. Did you know that Myron L Meters carries a meter specifically for testing RO systems (see below)?
Water purification systems have purified brackish water and sea water for the military, businesses and farms in many different locations on planet Earth. Reverse osmosis water purification will create clean drinkable water when used on your drinking water.
Reverse osmosis will generally remove salt, manganese, iron, fluoride, lead, and calcium (Binnie et. al., 2002). Most mineral constituents of water are physically larger than water molecules and they are trapped by the semi-permeable membrane and removed from drinking water when filtered through an RO system (AllAboutWater.org, 2004). Meanwhile, consumers are concerned about the removal of minerals from their drinking water.
Reverse Osmosis (RO) removed 90-99.99% of all the contaminants including minerals from the drinking water supply (see Figure 1). RO removes minerals because they have larger molecules than water. The subject of minerals and RO created controversy and disagreement among water and health professionals. The World Health Organization (WHO) made clarification that majority of healthy minerals are needed for human body is from food or dietary supplementary sources and not from drinking tap water. In addition, minerals found in water can be harmful to human health. The evidence is strong that calcium and magnesium are essential elements for human body (WQA, 2011). However, its a weak argument to suggest that we should make up this deficiency through water consumption (WQA, 2011). Tap water presents a variety of inorganic minerals which the human body has difficulty absorbing (Misner, 2004). Their presence is suspect in a wide array of degenerative diseases, such as hardening of the arteries, arthritis, kidney stones, gall stones, glaucoma, cataracts, hearing loss, emphysema, diabetes, and obesity. What minerals are available, especially in “hard” tap water, are poorly absorbed, or rejected by cellular tissue sites, and, if not evacuated, their presence may cause arterial obstruction, and internal damage (Dennison, 193; Muehling, 1994; Banik, 1989).
Figure 1. Reverse Osmosis Membrane (Source:DOI-BUR, 2009)
Organic Minerals vs. Inorganic Minerals
There are two types of minerals in water, organic and inorganic. Human physiology has a biological affinity for organic minerals. Most organic minerals for our body functions come from dietary plant foods (Misner, 2004). A growing plant converts the inorganic minerals from the soils to a useful organic mineral (Misner, 2004). When an organic mineral (from a plant food) enters the stomach it must attach itself to a specific protein-molecule (chelation) in order to be absorbed, then it gains access to the tissue sites where it is needed (Misner, 2004). Once a plant mineral is divested within the body, it is utilized as a coenzyme for composing body fluids, forming blood and bone cells, and the maintaining of healthy nerve transmission (Balch & Balch 1990).
Reverse Osmosis has Little Effect on Water pH
Water pH levels will automatically change when water is ingested and comes into contact with the food in your stomach (Wise, 2011). Even on an empty stomach, your stomach acid alone is already several times more acidic than RO water (pH 6-8) with a pH level of 2 (Wise, 2011). The human body regulates pH levels constantly to find balance and equilibrium (see Figure 2). Therefore under normal conditions it will always maintain a neutral 7.4 pH balance (Wise, 2011). The healthy body is very robust and it will restore homeostatic pH fairly quickly and easily (Wise 2011). Soft drinks and sports drinks typically have a pH level of 2.5, orange juice has a 3 pH and coffee has a 4 pH level and we drink these beverages all the time without problems (Wise, 2011).
Figure 2. Comparison of pH Levels (Source: Wise, 2011)
Water filtered or treated by reverse osmosis is generally pure, clean, and healthy. A reverse osmosis treatment system is currently the only technology that can remove most of the emerging contaminants (i.e., prescription drugs and perchlorate) including other contaminants (i.e., arsenic, cyanide, and fluoride) that are difficult to remove by other treatment methods. No more ingesting of harmful inorganic minerals means the body will no longer be stressed with trying to absorb something that wasn’t supposed to be there in the first place (Wise, 2011). Consumers should not be concerned about the removal of minerals by RO system. As the WHO (2009) and WQA (2011) pointed out, the human body obtains the vast majority of minerals from food or supplements, not from drinking water.
One of the downsides to the reverse osmosis process is that it is so effective in removing particles, it will also remove minerals from your water that may be beneficial. The body needs certain minerals, such as calcium and magnesium, to function properly. In addition, some people believe minerals such as this actually add flavor to the water, so that will be missing if you filter the water. Some find a certain acidic taste to water that has been purified by reverse osmosis. A reverse osmosis system also wastes a certain amount of water. For every gallon of purified water, three or four gallons have to be processed. If water is scarce or expensive in your area, this is a strong consideration.
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The choice of professionals for years, this compact instrument has been designed specifically to demonstrate and test Point of Use (POU) reverse osmosis or distillation systems. By measuring electrical conductivity, it will quickly determine the parts per million/Total Dissolved Solids (ppm/TDS) of any drinking water.
With a single ‘before and after’ test, this handy device effectively demonstrates how your RO or distillation system eliminates harmful dissolved solids. It will also service test systems, including membrane evaluation programs. Find out more about the RO-1 meter HERE.
TweetWastewater is treated in 3 phases: primary (solid removal), secondary (bacterial decomposition), and tertiary (extra filtration). fig. 1 Origins of Sewage Sewage is generated by residential and industrial establishments. It includes household waste liquid from toilets, baths, showers, kitchens, sinks, and so forth that is disposed of via sewers. In many areas, sewage also includes […]
Wastewater is treated in 3 phases: primary (solid removal), secondary (bacterial decomposition), and tertiary (extra filtration).
Origins of Sewage
Sewage is generated by residential and industrial establishments. It includes household waste liquid from toilets, baths, showers, kitchens, sinks, and so forth that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry and commerce. The separation and draining of household waste into greywater and blackwater is becoming more common in the developed world. Greywater is water generated from domestic activities such as laundry, dishwashing, and bathing, and can be reused more readily. Blackwater comes from toilets and contains human waste.
Sewage may include stormwater runoff. Sewerage systems capable of handling storm water are known as combined sewer systems. This design was common when urban sewerage systems were first developed, in the late 19th and early 20th centuries. Combined sewers require much larger and more expensive treatment facilities than sanitary sewers. Heavy volumes of storm runoff may overwhelm the sewage treatment system, causing a spill or overflow. Sanitary sewers are typically much smaller than combined sewers, and they are not designed to transport stormwater. Backups of raw sewage can occur if excessive infiltration/inflow (dilution by stormwater and/or groundwater) is allowed into a sanitary sewer system. Communities that have urbanized in the mid-20th century or later generally have built separate systems for sewage (sanitary sewers) and stormwater, because precipitation causes widely varying flows, reducing sewage treatment plant efficiency.
As rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. (See urban runoff.) Some jurisdictions require stormwater to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for stormwater include retention basins, wetlands, buried vaults with various kinds of media filters, and vortex separators (to remove coarse solids).
Sewage treatment is done in three stages: primary, secondary and tertiary treatment (Figure 1).
In primary treatment, sewage is stored in a basin where solids (sludge) can settle to the bottom and oil and lighter substances can rise to the top. These layers are then removed and then the remaining liquid can be sent to secondary treatment. Sewage sludge is treated in a separate process called sludge digestion.
Secondary treatment removes dissolved and suspended biological matter, often using microorganisms in a controlled environment. Most secondary treatment systems use aerobic bacteria, which consume the organic components of the sewage (sugar, fat, and so on). Some systems use fixed film systems, where the bacteria grow on filters, and the water passes through them. Suspended growth systems use “activated” sludge, where decomposing bacteria are mixed directly into the sewage. Because oxygen is critical to bacterial growth, the sewage is often mixed with air to facilitate decomposition.
Tertiary treatment (sometimes called “effluent polishing”) is used to further clean water when it is being discharged into a sensitive ecosystem. Several methods can be used to further disinfect sewage beyond primary and secondary treatment. Sand filtration, where water is passed through a sand filter, can be used to remove particulate matter. Wastewater may still have high levels of nutrients such as nitrogen and phosphorus. These can disrupt the nutrient balance of aquatic ecosystems and cause algae blooms and excessive weed growth. Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate accumulate organisms that store phosphate in their tissue. When the biomass accumulated in these bacteria is separated from the treated water, these biosolids have a high fertilizer value. Nitrogen can also be removed using nitrifying bacteria. Lagooning is another method for removing nutrients and waste from sewage. Water is stored in a lagoon and native plants, bacteria, algae, and small zooplankton filter nutrients and small particles from the water.
Sludge Digestion & Disposal
Sewage sludge scraped off the bottom of the settling tank during primary treatment is treated separately from wastewater. Sludge can be disposed of in several ways. First, it can be digested using bacteria; bacterial digestion can sometimes produce methane biogas, which can be used to generate electricity. Sludge can also be incinerated, or condensed, heated to disinfect it, and reused as fertilizer.
When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Sewage sludge scraped off the bottom of the settling tank during primary treatment is treated separately from wastewater. Sludge can be disposed of in several ways. First, it can be digested using bacteria; bacterial digestion can sometimes produce methane biogas, which can be used to generate electricity. Sludge can also be incinerated, or condensed, heated to disinfect it, and reused as fertilizer.
Typically, sludges are thickened (dewatered) to reduce the volumes transported off-site for disposal. There is no process which completely eliminates the need to dispose of biosolids. There is, however, an additional step some cities are taking to superheat sludge and convert it into small pelletized granules that are high in nitrogen and other organic materials. In New York City, for example, several sewage treatment plants have dewatering facilities that use large centrifuges along with the addition of chemicals such as polymer to further remove liquid from the sludge. The removed fluid, called “centrate,” is typically reintroduced into the wastewater process. The product which is left is called “cake,” and that is picked up by companies which turn it into fertilizer pellets. This product is then sold to local farmers and turf farms as a soil amendment or fertilizer, reducing the amount of space required to dispose of sludge in landfills. Much sludge originating from commercial or industrial areas is contaminated with toxic materials that are released into the sewers from the industrial processes. Elevated concentrations of such materials may make the sludge unsuitable for agricultural use and it may then have to be incinerated or disposed of to landfill.
Notably, throughout the development of excreta, wastewater, wastewater sludge and biosolids management – from the least developed to the most developed countries – there are inevitable public concerns about how best to manage this “waste” that is also a resource. Putting biosolids to their best uses in each local situation is the goal of most of the programs discussed in the following reports. That is the goal of many sanitation and water quality experts. But the general public has other goals: avoiding the waste and the odors it can produce.There is a natural aversion to fecal matter and anything associated with it. Conflicts arise when experts propose recycling this “waste,” usually in a treated and tested form commonly called “biosolids,” back to soils in communities.
Managing excreta and wastewater sludge to produce recyclable biosolids involves many technical challenges. But equally significant are these social, cultural, and political challenges. Funding is required to build infrastructure – and, around the world, the public is the source of funding, either through taxes or sewer usage fees. In order for proper sanitation to be built and operated, complex community sanitation agencies with support from state, provincial, and national governments are needed.
Wastewater quality indicators are laboratory tests to assess suitability of wastewater for disposal or re-use. Tests selected and desired test results vary with the intended use or discharge location. Tests measure physical, chemical, and biological characteristics of the wastewater.
Aquatic organisms cannot survive outside of specific temperature ranges. Irrigation runoff and water cooling of power stations may elevate temperatures above the acceptable range for some species. Temperature may be measured with a calibrated thermometer.
Solid material in wastewater may be dissolved, suspended, or settleable. Total dissolved solids or TDS (sometimes called filtrable residue) is measured as the mass of residue remaining when a measured volume of filtered water is evaporated. The mass of dried solids remaining on the filter is called total suspended solids (TSS) or nonfiltrable residue. Settleable solids are measured as the visible volume accumulated at the bottom of an Imhoff cone after water has settled for one hour. Turbidity is a measure of the light scattering ability of suspended matter in the water. Salinity measures water density or conductivity changes caused by dissolved materials.
Virtually any chemical may be found in water, but routine testing is commonly limited to a few chemical elements of unique significance.
Water ionizes into hydronium (H3O) cations and hydroxyl (OH) anions. The concentration of ionized hydrogen (as protonated water) is expressed as pH.
Most aquatic habitats are occupied by fish or other animals requiring certain minimum dissolved oxygen concentrations to survive. Dissolved oxygen concentrations may be measured directly in wastewater, but the amount of oxygen potentially required by other chemicals in the wastewater is termed an oxygen demand. Dissolved or suspended oxidizable organic material in wastewater will be used as a food source. Finely divided material is readily available to microorganisms whose populations will increase to digest the amount of food available. Digestion of this food requires oxygen, so the oxygen content of the water will ultimately be decreased by the amount required to digest the dissolved or suspended food. Oxygen concentrations may fall below the minimum required by aquatic animals if the rate of oxygen utilization exceeds replacement by atmospheric oxygen.
The reaction for biochemical oxidation may be written as:
Oxidizable material + bacteria + nutrient + O2 → CO2 + H2O + oxidized inorganics such as NO3 or SO4
Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows:
S– + 2 O2 → SO4–
NO2- + ½ O2 → NO3-
Since all natural waterways contain bacteria and nutrient, almost any waste compounds introduced into such waterways will initiate biochemical reactions (such as shown above). Those biochemical reactions create what is measured in the laboratory as the biochemical oxygen demand (BOD).
Oxidizable chemicals (such as reducing chemicals) introduced into a natural water will similarly initiate chemical reactions (such as shown above). Those chemical reactions create what is measured in the laboratory as the chemical oxygen demand (COD).
Both the BOD and COD tests are a measure of the relative oxygen-depletion effect of a waste contaminant. Both have been widely adopted as a measure of pollution effect. The BOD test measures the oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand of biogradable pollutants plus the oxygen demand of non-biodegradable oxidizable pollutants.
The so-called 5-day BOD measures the amount of oxygen consumed by biochemical oxidation of waste contaminants in a 5-day period. The total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion is called the Ultimate BOD. The Ultimate BOD is too time consuming, so the 5-day BOD has almost universally been adopted as a measure of relative pollution effect.
There are also many different COD tests. Perhaps, the most common is the 4-hour COD.
There is no generalized correlation between the 5-day BOD and the Ultimate BOD. Likewise, there is no generalized correlation between BOD and COD. It is possible to develop such correlations for a specific waste contaminant in a specific wastewater stream, but such correlations cannot be generalized for use with any other waste contaminants or wastewater streams.
The laboratory test procedures for the determining the above oxygen demands are detailed in the following sections of the “Standard Methods For the Examination Of Water and Wastewater” available at www.standardmethods.org:
5-day BOD and Ultimate BOD: Sections 5210B and 5210C
COD: Section 5220
Nitrogen is an important nutrient for plant and animal growth. Atmospheric nitrogen is less biologically available than dissolved nitrogen in the form of ammonia and nitrates. Availability of dissolved nitrogen may contribute to algal blooms. Ammonia and organic forms of nitrogen are often measured as Total Kjeldahl Nitrogen, and analysis for inorganic forms of nitrogen may be performed for more accurate estimates of total nitrogen content.
Chlorine has been widely used for bleaching, as a disinfectant, and for biofouling prevention in water cooling systems. Remaining concentrations of oxidizing hypochlorous acid and hypochlorite ions may be measured as chlorine residual to estimate effectiveness of disinfection or to demonstrate safety for discharge to aquatic ecosystems.
Water may be tested by a bioassay comparing survival of an aquatic test species in the wastewater in comparison to water from some other source. Water may also be evaluated to determine the approximate biological population of the wastewater. Pathogenic micro-organisms using water as a means of moving from one host to another may be present in sewage. Coliform index measures the population of an organism commonly found in the intestines of warm-blooded animals as an indicator of the possible presence of other intestinal pathogens.
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