|Parameters / Analytes|
The previous page described some of the techniques and equipment used in water and wastewater analysis. This page discusses what things we want to measure, what their significance is, and what methods are used for each of them. Click on the parameter in the left frame to view information here.
While the pH measures the concentration of hydrogen or hydroxide ions,
it may not measure the total amount of acid or base in the solution. This
is because most acids and bases do not dissociate completely in water.
That is, they only release a portion of their hydrogen or hydroxide ions.
A strong acid, like hydrochloric acid, HCl, releases essentially all of its H+ in water. The concentration of H+ is the same as the total concentration of the acid. A weak acid, like acetic acid (the acid in vinegar), may release only a few percent of the hydrogen that it has available.
If you are trying to neutralize an acid by adding a base, like sodium hydroxide, the amount you would need to neutralize a strong acid could be calculated directly from the pH of the acid solution. But for a weak acid, the pH does not tell the whole story; the total amount of base needed would be a lot more. This is because as the OH- from the base reacts with the H+ in solution to form water, more H+ will break loose from the undissociated portion of the acid to take its place. The neutralization will not be complete until all of the weak acid has dissociated. To measure the total acidity, also called base-neutralizing capacity (BNC) of a water sample, it has to be titrated with base. That is, a solution of a base whose concentration is known must be added to the water sample slowly until the neutralization is complete. By measuring the volume of the base added, you can figure out the original concentration of acid.
In a similar way, the acid-neutralizing capacity (ANC), or alkalinity of a water sample has to be determined by titrating it with a solution of a strong acid of known concentration.
For a more technical explanation of pH and alkalinity, look at this "mini-tutorial", which includes formulas, reactions, examples, and titration curves.
Significance: Although there are some microorganisms which can function at extreme pH's, most living things require pH's close to neutrality. Many enzymes and other proteins are denatured by pH's which differ much from pH7, which disrupts the functioning of the organism and may kill it. Besides the harm to aquatic life in natural waters, pH imbalances can inhibit-- or completely wipe out-- biological processes in wastewater treatment plants, resulting in incomplete treatment and pollution of the receiving waters. Low (acidic) pH's also cause corrosion in sewers systems and increase the release of toxic and foul-smelling hydrogen sulfide gas. (This gas has been responsible for the deaths of numerous sewer workers.) Low pH's also increase the release of metals, some toxic, from soils and sediments. Alkalinity is an important parameter because it measures the water's ability to resist acidification, for instance, to acid rain. In wastewater treatment, some processes produce acidity. If there is not enough alkalinity to neutralize it, the pH of the process can drop and cause it to become inhibited. Alkalinity can be supplemented by chemical addition to avoid this.
Measurement: There are indicator solutions which change color in different pH ranges, and these can be used for approximate estimation of pH in solutions which contain high enough concentrations of pH-determining ions. "pH paper", impregnated with such indicators, are commonplace in testing laboratories. For accurate measurements and use in dilute solutions, electrochemical measurement (a "pH meter") is required. Alkalinity and acidity are determined by titration with strong base or acid, respectively, using either indicators or a pH meter to mark the endpoint.
(D.O.): Like solids and liquids, gases can dissolve in
water. And, like solids and liquids, different gases vary greatly in their
solubilities, i.e, how much can dissolve in water. A solution containing
the maximum concentration that the water can hold is said to be saturated.
Oxygen gas, the element which exists in the form of O2 molecules,
is not very water soluble. A saturated solution at room temperature and
normal pressure contains only about 9 parts per million of D.O. by weight
( 9 mg/L). Lower temperatures or higher pressures increase the solubility,
and visa versa.
Significance: Dissolved oxygen is essential for fish to breathe. Many microbial forms require it, as well. The oxygen bound in the water molecule (H2O) is not available for this purpose, and is in the wrong "oxidation state", anyway. The low solubility of oxygen in water means that it does not take much oxygen-consuming material to deplete the D.O. As mentioned before, the biodegradation products of bacteria which do not require oxygen are foul-smelling, toxic, and/or flammable. Sufficient D.O. is essential for the proper operation of many wastewater treatment processes. Activated sludge tanks often have their D.O. monitored continuously. Low D.O.'s may be set to trigger an alarm or activate a control loop which will increase the supply of air to the tank.
Measurement: D.O. can be measured by a fairly tricky wet chemical procedure known as the Winkler titration. The D.O. is first trapped, or "fixed", as an orange-colored oxide of manganese. This is then dissolved with sulfuric acid in the presence of iodide ion, which is converted to iodine by the oxidized manganese. The iodine is titrated using standard sodium thiosulfate. The original dissolved oxygen concentration is calculated from the volume of thiosulfate solution needed.
Measurements of D.O. can be made more conveniently with electrochemical instrumentation. "D.O. meters" are subject to fewer interferences than the Winkler titration. They are portable and can be calibrated directly by using the oxygen in the air.
(Click to enlarge)
Oxygen Demand: The biochemical oxygen demand, abbreviated as BOD, is a test for measuring the amount of biodegradable organic material present in a sample of water. The results are expressed in terms of mg/L of D.O. which microorganisms, principally bacteria, will consume while degrading these materials. As this method is a fairly long-term bioassay test (5 days), a more rapid (2 hour) test is often used to estimate the BOD; it is known as the COD, or chemical oxygen demand test. An even more rapid test, known as the TOC, or total organic carbon test takes only a few minutes, but requires expensive instrumentation. In the United States, most regulatory agencies specify the BOD test for permit reporting, especially for biological treatment plants.
Significance: For reasons discussed earlier, the depletion of oxygen in receiving waters has historically been regarded as one of the most important negative effects of water pollution. Preventing these substances from being discharged into our waterways is a key purpose of wastewater treatment. Monitoring BOD removal through a treatment plant is necessary to verify proper operation. However, because the test takes too long to be useful for short-term control of the plant, the chemical or instrumental surrogate tests are often used as guides.
Measurement: The BOD test is performed in a specially designed bottle with a flared cap which forms a water seal to keep out air. The bottle is filled completely with sample, which must be near neutral pH and free of toxic materials. After an initial measurement of the D.O., the bottle is sealed and stored in a dark incubator at 20C for five days. The D.O. is measured again after this incubation period. The difference is the BOD. (The bottles are kept in the dark because algae which may be present in the sample will produce oxygen when exposed to light.) Since most wastewaters have BOD's which are much higher than the limited solubility of oxygen in water, it is necessary to make a series of dilutions containing varying amounts of sample in a nutrient-containing, aerated "dilution water." The measured BOD's are then multiplied by the appropriate dilution factors. A variation of this test, called the carbonaceous BOD, adds an inhibitor which prevents the oxidation of ammonia, so that the test is a truer measure of the amount of biodegradable organic material present. Samples which do not contain enough bacteria to carry out the BOD test can be "seeded" by adding some from another source. Examples of samples which would need seeding are industrial wastewaters which may have been at high temperatures or high or low pH, or samples which have been disinfected. (If there is residual disinfectant present, it must be neutralized before testing.)
The COD test is done by heating a portion of sample in an acidic
chromate solution, which oxidizes organic matter chemically. The amount
of chromate remaining (measured by a titration), or the amount of reduced
chromium produced (measured spectrophotometrically), is translated into
an oxygen demand value. Biodegradability, toxins, and bacteria are not
important, and the test is complete in about two hours. The figure will
be higher than the BOD.
The TOC is done instrumentally. The organic carbon is oxidized to carbon dioxide by burning or by chemical oxidation in solution. The carbon dioxide gas is swept out and measured by infrared spectrometry or by redissolving it in water and measuring the pH change (the gas is acidic.) Both COD and TOC can often be correlated with BOD for a specific wastewater sample, but each wastewater is different. As a rough guide, the COD of a raw domestic wastewater is about 2.5 times the 5-day BOD.
Water, a liquid, can contain quite a bit of solid material, both in
dissolved and suspended forms. The term "dissolved" implies
that the individual molecules of a substance are mixed in among the water
molecules. In practice, solids are classified as "dissolved" if they pass
through a standard glass-fiber filter with about one micrometer pore size.
Solids captured on the filter are, by definition, "suspended" solids..
Solids which settle out of a water sample on standing for a period of an hour are defined as "settlable." .
Solids are also further classified as "fixed" or "volatile." Fixed solids are basically the ash left over after burning the dried solids; volatile solids are those that are lost in this procedure. The sum of the two is referred to as "total". (This can be confusing, as the word "total" is also used in describing the sum of suspended and dissolved solids.) Volatile solids are often used as an estimate of the organic matter present.
Significance: Solids in wastewater contribute to sediment formation; volatile solids may be associated with oxygen demand.
Measurement: Total solids (TS) are determined by drying a known amount of a sample at a temperature of 103 to 105 C in a tared (pre-weighed) vessel, such as a porcelain dish, cooling in a dry atmosphere (in a container known as a desiccator), weighing on an analytical balance, subtracting the tare weight, and dividing by the original amount of sample. Results can be expressed in mg/L if the sample was originally measured out by volume; or percent by weight, if the sample was originally weighed. If the sample is then burned in a furnace at about 500 C, cooled, and weighed, the fixed (FS) or volatile solids (VS) can be determined. .
If the original sample is filtered through a tared glass-fiber filter, which is then dried, the weight of the material captured on the filter is used to figure the total suspended solids (TSS). Burning the filter in the furnace allows measurement of volatile suspended solids (VSS) or fixed suspended solids (FSS).
The dissolved solids (DS) can be estimated from the difference
between the total solids and the total suspended solids, but the official
method calls for drying the filtrate (the liquid which passes through the
filter) in a dish at 180C. (And, of course, there are TDS, FDS and VDS).
An estimate of total suspended solids can be obtained by an optical/instrumental measurement known as turbidity. The sample is placed in a glass tube; a beam of light is shined through it, and the light scattered at right angles to the beam is measured photometrically. In the same way that COD can be correlated with BOD, turbidity can be correlated with TSS; but the correlation will hold only for the particular sample from which it was derived.
Similarly, an estimate of dissolved solids is often made by measuring the water's electrical conductivity. Pure water does not conduct electricity. If substances which dissociate into electrically charged ions are dissolved in the water, they will conduct a current, roughly proportional to the amount of dissolved substances. Conductivity can be used to track sewage pollution. Note, however, that many organic materials dissolve in water without producing ions. So, while a salt solution may have a high electrical conductivity, a concentrated solution of sugar would go undetected by this method.
Nutrients are usually thought of as compounds of nitrogen or phosphorus,
although certainly other elements, such as iron, magnesium, and potassium
are also necessary for bacterial and plant growth.
Nitrogen occurs primarily in the oxidized forms of nitrates (NO3-) and nitrites (NO2-) or the reduced forms of ammonia (NH3) or "organic nitrogen"-- where the nitrogen is part of an organic compound such as an amino acid, a protein, a nucleic acid, or one of many other compounds. All of these can be used as nutrients, although the organic nitrogen first needs to decompose to a simpler form .
Phosphorus is biologically important in the form of phosphate, the most highly oxidized state of the element. The most biologically available form is dissolved orthophosphate, (PO4-3). (In solution, there are up to three hydrogens attached to the molecule, each one decreasing the negative charge of the ion by one. How many hydrogens are attached depends on the pH.) There are also condensed forms of phosphate, with more than one phosphorus atom per ion, such as pyrophosphate and polyphosphates. There are also organic phosphates, and all of these forms can be either dissolved or particulate (i.e., insoluble). The sum of all the forms is known as total phosphorus.
Significance: These nutrients are important in natural waters because, in excess, they can cause nuisance growth of algae or aquatic weeds. In wastewater treatment, a deficiency of nutrients can limit the effectiveness of biological treatment processes. In some plants treating industrial wastewaters, ammonia or phosphoric acid must be added as a supplement.
Measurement: Ammonia can be measured colorimetrically, by the Nessler or phenate methods, after distillation from an alkaline solution to separate it from interferences. It can also be determined by an electrode method, sometimes without distillation, since there are fewer interferences. Organically-bound, reduced nitrogen can be determined by the same methods after a digestion (the Kjeldahl digestion) which converts the nitrogen in those compounds to ammonia. The combination of ammonia and organic nitrogen is known as "Total Kjeldahl Nitrogen," or TKN. (TKN analysis is used for measuring protein content of animal feeds, as well.) Nitrite is determined colorimetrically. Nitrate can also be determined this way; the most popular way is by first reducing nitrate to nitrite chemically using cadmium, then analyzing the nitrite. There is an electrode method for nitrate, but it is not considered too accurate. Finally, ammonia (as the positively charged ammonium ion, NH4+), nitrate, and nitrite can be measured by ion chromatography, as well.
Phosphate can be measured by ion chromatography, also. Greater sensitivity, at lower cost, is obtained by colorimetric methods which measure dissolved orthophosphate. Some insoluble phosphates and condensed phosphates-- so called "acid-hydrolyzable phosphate"-- can be included by heating the sample with acid to convert these forms to orthophosphate. If the organic phosphate is to be included, to measure "total phosphate", then the sample must be digested with acid and an oxidizing agent, to convert everything to the orthophosphate form.
The pure element exists as the molecule, Cl2, which is a gas
or a liquid at normal temperatures, depending on the pressure. When dissolved
in water, most of it reacts to form hypochlorous acid (HOCl) and hydrochloric
acid (HCl) which make the water more acidic. The HOCl dissociates,
to some extent, to form H+ and OCl-, called hypochlorite
ion. (The HCl dissociates completely.) If there is enough alkalinity to
react with the hydrogen ions produced and keep the pH around neutral, most
of the chlorine will be in the form of hypochlorous acid and hypochlorite
ion. Disinfection can be done using solutions of sodium hypochlorite, which
produce the same substances in solution. Hypochlorite ion is not considered
as strong a disinfectant as HOCl, so the pH can affect the disinfectant
efficiency. Dissolved chlorine, hypochlorous acid, and hypochlorite ion,
taken together, are all known as "free chlorine". Free chlorine can react
with ammonia in solution to form compounds called chloramines, which
are weaker disinfectants than free chlorine, but have the advantage of
not being used up by side reactions to the extent that free chlorine is.
Free chlorine (and chloramines) also react with organic nitrogen compounds
to form organic chloramines, which are even weaker disinfectants. The chloramines
are termed "combined chlorine," and the sum of the free and combined forms
are called "total chlorine." (Note that a large enough amount of chlorine
can oxidize ammonia to nitrogen gas; this can be used as a chemical means
of destroying ammonia.)
Significance: Chlorine is the most commonly used disinfecting agent for drinking water and wastewater. It is coming into some disfavor because of toxic and carcinogenic byproducts, such as chloroform, which are formed when it reacts with organic matter present in the water. Unless reduced to chloride, chlorine itself is toxic to aquatic life in receiving waters. Pure chlorine liquid or gas is also a storage and transportation hazard because of the possibility of accidental releases to the atmosphere. Some treatment plants are switching to hypochlorite solution because it is safer to handle. Others are eliminating it entirely and using UV light or ozone for disinfection.
Measurement: There are several choices for chlorine measurement, some of which can distinguish between free chlorine and the various chloramines. There are titrations involving visual, color-indicator endpoints, as well as electrochemically measured endpoints. Some of them can be used to differentiate among the various forms of chlorine depending on whether iodide ion is added to the testing mixture. The indicator known as DPD (full name, N,N-diethylparaphenylenediamine) can be used to measure free or total chlorine both colorimetrically or as a titration indicator. "Amperometric titration" is a sensitive electrochemical method.
Oil and Grease
is the name given to a class of materials which can be
extracted from water using certain organic solvents. They can be of biological
origin (animal fat, vegetable oil); they can be "mineral" (petroleum hydrocarbons);
or they can be synthetic organic compounds. Fats and greases from restaurants
and food processing industries can clog sewers, causing blockages and backups.
Petroleum products can be toxic and flammable, and can coat surfaces and
interfere with biodegradation by microorganisms in wastewater treatment
plants. They are mostly biodegradable, especially biological oils and greases,
but are a problem due to forming a separate phase from the water.
Measurement: The major method of analysis is liquid-liquid extraction. Currently, the chlorofluorocarbon known as CFC-113 is used, but is due to be phased out in favor of the hydrocarbon, hexane, because of the damage done by CFC's to the stratospheric ozone layer. In the procedure, the sample is acidified, and then shaken several times with the solvent. The solvent portions are combined and evaporated, and the residue is measured by weight. In a CFC solution, the concentration of the oil/grease can also be measured by infrared spectrophotometry without having to evaporate the solvent. To determine petroleum hydrocarbons alone, the extract solution can be treated with the material, silica gel, which absorbs the more polar biological compounds. A newer method, solid phase extraction, passes the water sample through a small column or filter containing solid sorbent material which absorbs the oil and grease. It is then desorbed from the sorbent using a solvent and analyzed as above.
|(Click the picture to see a
flame atomic absorption spectrophotometer in use.)
Cyanide is the name of an ion composed of carbon and nitrogen, CN-.
It is used in the mining and metal finishing and plating industries-- usually
as the sodium or potassium salts, NaCN or KCN-- because of its ability
to bind very strongly to metals to form water-soluble complex ions. This
same property makes it highly toxic to living things because it prevents
the normal activity of biologically important, metal-containing molecules.
It is, however, biodegradable by some bacteria in low concentrations; and
they can become acclimated to higher concentrations if given enough time.
For unacclimated microorganisms in a wastewater treatment plant, however,
a cyanide "dump" by an industry can lead to inhibition or even death, which
can cause a severe "plant upset."
Measurement: Cyanides are usually measured by a sensitive colorimetric/ spectrophotometric procedure which can detect levels down to about 5 parts per billion in water. Since much of the cyanide in a sample is likely to be bound to metal ions, a digestion/distillation procedure is necessary to measure "total" cyanide. Cyanide can also be measured by ion chromatography or an electrode method, though the latter is not considered too accurate.
Compounds: An organic compound is any compound which
contains carbon, with the exception of carbon monoxide and carbon dioxide,
carbonates, or cyanides. Organic compounds contain chains and/or rings
of connected carbon atoms, often with other elements attached. There are
millions of possible compounds, with many useful properties. Many are biologically
active, since all living things are made up of organic molecules. Industries
use and produce thousands of organic compounds in manufacturing such items
as plastics, synthetic fibers, rubber, pharmaceuticals, pesticides, and
petroleum products. Some of the compounds are starting materials; some
are solvents; some are byproducts. The US EPA lists 116 of them as toxic
"priority pollutants"; many states have longer lists. One of the major
groupings is volatile organic compounds (VOC's), many of which are chlorine-containing
solvents. There are also petroleum hydrocarbons and starting materials
for plastics, dyes, and pharmaceuticals. The "semi-volatile" group include
solvents, PAH's (polycyclic aromatic hydrocarbons, like naphthalene and
anthracene which are coal tar constituents), as well as pesticides (especially
chlorinated pesticides) and PCB's (polychlorinated biphenyls, which were
formerly used in electrical transformers and other products).
Measurement: Most of these are analyzed routinely by gas chromatography (GC), often followed by mass spectrometry (MS) for identification. HPLC is also used for some analytes. A technique which is becoming available for field measurements for some of these compounds is immunoassay, sometimes called ELISA, for "enzyme-linked immunosorbent assay." This method, which produces a color reaction related to the concentration of the target compound, or family of compounds, is portable, relatively inexpensive and does not require a great deal of training. It is in use more for surveying hazardous waste sites, however, than for water analysis.
microorganisms: Sewage contains large numbers of microbes
which can cause illness in humans, including viruses, bacteria, fungi,
protozoa and worms (and their eggs or ova). They originate from
people who are either infected or are carriers. While many of these can
be measured directly by microscopic techniques (some after concentration),
the analyses most commonly performed are for so-called "indicator organisms."
These organisms, while not too harmful themselves, are fairly easy to test
for and are chosen because they indicate that more serious pathogens are
likely to be present. For instance, wastewater treatment plants are often
required to test their effluents for the group known as "fecal coliforms,"
which include the species E. coli, indicative of contamination by
material from the intestines of warm-blooded animals. Water supplies test
for a more inclusive group called "total coliforms", and in some cases,
for general bacterial contamination (heterotrophic plate count,
Measurement: The two most commonly used methods of analysis for indicator organisms are the multiple tube fermentation technique and the membrane filter procedure.
In the first method, a number of tubes containing specific growth media are innoculated with different amounts of the sample and incubated for a particular time at a prescribed temperature. The appearance of colors, fluorescence, or gas formation indicates the presence of bacteria belonging to the target group. The number of organisms per 100 mL in the original sample is estimated from most probable number (MPN) tables, which list the values of MPN for different combinations of positive and negative results in tubes which contained different initial volumes of the sample. Often, positive results must be confirmed by further innoculation of small amounts of material from the positive tubes into tubes containing a different media, which can extend the test to several days.
The second technique involves filtering a known volume of sample through a membrane filter (made of a material such as cellulose acetate) which has a small enough pore size to retain the bacteria. The filter is then placed in a dish of sterile nutrient media, either soaked into an absorbent pad or in a gel such as agar, and sealed. The dish is incubated for the prescribed time and temperature. The media contain a colored indicator which will identify the target bacteria. Each bacterium in the original sample will result in a colony after incubation, which is large enough to see without a great deal of magnification. The concentration in the sample can be determined by direct count of the colonies, knowing the volume of sample used. In some cases, these colonies require further confirmation.
Detection and enumeration of HTP or of specific pathogenic bacteria, such as Salmonella, E. coli, or Enterococcus can be done by similar methods, but utilizing specific growth media for each type. Viruses are usually measured by concentration, followed by addition to cultures of cells which they infect and counting the number of plaques formed due to cell destruction. Pathogenic protozoa and ova of multicelled organisms are determined by concentration and direct counting under the microscope, often with the aid of fluorescent staining compounds.
Besides, direct observation, identification of pathogenic microorganisms can be done by standard techniques used in clinical laboratories involving observing reactions in a battery of different indicating media. Some newer methods use chromatography to identify patterns of compounds which serve as "fingerprints" for certain bacteria; DNA analysis is another recent innovation. Most wastewater treatment plants, however, confine their testing to simply counting the numbers indicator bacteria.
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