CHEMICAL SCIENCES, ENGINEERING AND TECHNOLOGY RESOURCES - SAMPLE CHAPTERS

CHEMISTRY OF WASTEWATER

Timothy G. Ellis,

Department of Civil, Construction and Environmental Engineering, Iowa State University, Ames, Iowa, USA

Keywords: wastewater, biochemical oxygen demand, chemical oxygen demand, suspended solids, turbidity, nitrogen, phosphorus, nitrification, denitrification, surfactant, endocrine disruptors, Ascaris, nitrosomonas, nitrobacter, Schistosomiasis, hypoxic zone, Giardia, Cryptosporidium, olfactometer

Contents

1. Introduction

2. Wastewater Analysis

3. Wastewater Composition

4. Wastewater Quantities

5. Conclusion

Related Chapters

Glossary

Bibliography

Bibliographical Sketch


2.2 Chemical Oxygen Demand

An alternative to the BOD test for determining the oxygen consuming potential of a wastewater sample is the chemical oxygen demand (COD) test. As the name implies, the carbonaceous oxygen demand is oxidized chemically in the COD test. Like BOD, the units for COD are in miligrams of oxygen per liter (mg L-1). The advantage of this test is that it is quick and reproducible. The disadvantage is that not all of the measured COD can be degraded biologically. Therefore, there is still a need to ascertain what the biodegradable portion of the oxygen demand is, since that is how the performance of biological wastewater treatment systems (e.g. activated sludge, trickling filters, anaerobic digesters, rotating biological contactors, oxidation ponds, and lagoons) will be evaluated. In addition, the BOD (not COD) is the component that is expected to induce an oxygen demand in the receiving stream. There are also some interferences in the COD test. For instance, low molecular weight fatty acids and aromatic hydrocarbons may not be well oxidized during the test and inorganic ions (chloride and nitrite for instance) may be oxidized. The addition of certain catalysts during the test can eliminate most of these interferences.

The real advantage of the COD test is that it is a measure of the energetics of the system. If one wants to keep track of a biological reaction, they must know what the energetics of the reaction are (i.e. what are the electron donors and electron acceptors for the system). Microorganisms degrade pollutants in biological treatment systems to our benefit, but also so that they can grow and reproduce. They require two major things: carbon and energy for growth. They also need lesser quantities of macro nutrients, nitrogen, phosphorus, and sulfur in addition to trace amounts of micronutrients. If we can keep track of the flow of carbon and the flow of energy in a system, we can understand the nature and extent of the biochemical reactions occurring. If we can keep track of, and manipulate, the biochemical reactions, we can engineer biological treatment systems to our advantage. The two main items to monitor are carbon and energy. Unfortunately, carbon is difficult to keep track of since upon mineralization of an organic compound, it is solubilized in water or released as a gas in the form of carbon dioxide (CO2). The best way to track changes in carbon species is through 14C labeling, but this obviously requires sophisticated laboratory procedures and equipment (e.g. radiolabbelled carbon compounds and liquid scintillation counting). Energy equivalents, or specifically electrons, are much easier to measure. When we measure the amount of oxygen consumed in a reaction, we are in essence measuring the electrons transferred from the organic compound to the terminal electron acceptor (i.e. oxygen). Performing a mass balance on electrons is much simpler than on carbon, since all we need to do to balance electrons is measure the chemical oxygen demand of our starting and end products and measure the amount of oxygen consumed during the reaction (or methane produced if it is an anaerobic or methane producing reaction).

For instance, consider the reaction in equation 2 in which glucose is converted to carbon dioxide and water by a stoichiometric amount of oxygen (1.067 miligrams of oxygen per miligram of glucose). If we know the concentration of glucose both before and after biological treatment and calculate the biomass concentration before and after, we will know how much oxygen was consumed during the conversion of glucose to biomass and end products. This way we have a much better indication of what the reactions are than if we simply measure the total carbon concentration before and after treatment. In addition, the analysis of COD is much more practical than the analysis for specific pollutants.

The advantages of the COD test are that it is relatively fast and the results are reproducible. The disadvantage is that it measures everything, including non-biodegradable organic matter, that can be oxidized by potassium dichromate. In addition, the test produces a small amount of hazardous waste that must be disposed of properly. Thus, the measured oxygen demand in the COD test may differ due to sampling and analytical error, and possible interferences in the COD procedure. The measured BOD may differ for the same reasons in addition to the fact that a portion of the compound will be incorporated into new biomass. A list of the theoretical oxygen demand of a variety of substances that can be found in municipal and industrial wastewater is provided in a book by Pitter and Chudoba published in 1990. The list of chemicals is organized by compound type (e.g. hydrocarbons, alcohols/phenols, aldehydes, ketones, quinones, organic and amino acids, esters, ethers, amines, amides, nitriles, halogen and nitro- derivatives, heterocyclic compounds, saccharides, alkyl benzene sulfonates and alkyl sulfates, dyes, and miscellaneous substances). From this extensive list of chemicals, two points should be noted. First, there is a large number of compounds that can be present in a wastewater samples. Second, the oxygen demand that could be exerted from a known quantity of a specific pollutant can be quickly determined.

2.3 Solids

Solids are an important constituent to measure in any wastewater sample. Solids in a wastewater effluent represent a pollutant load in addition to a potential sediment load on a receiving stream. There are two main categories of solids: suspended and dissolved. Typically, the division between the two is somewhat arbitrary. For instance, whatever is filtered out of a sample by a 0.45-1.2 :m filter is considered suspended solids. Dissolved solids are constituents that pass through the filter. Within the dissolved and suspended solids classification, the solids can be further characterized as volatile or fixed solids. Volatile solids will combust at a temperature of 550°C and fixed solids will not. The implication is that the volatile solids represent the organic portion of the solids. In a biological treatment system, the volatile solids are often associated with the biomass and can be used as a measure of the microorganism population. The difficulty with this assumption is that often the organic solids in the wastewater fed to the system are incorporated into total solids retained in the system. Determining the volatile portion of these solids does not guarantee that the solids are part of the active degrading population. Therefore, care must be exercised in correlating the volatile solids concentration with the microbial population.

The dissolved solids in the sample include the organic pollutants in solution (carbohydrates, proteins, fats, oils, surfactants, volatile acids, urea, ammonia, trace pollutants) as well as inorganic compounds, some of which were present in the source drinking water. Table 2 shows the relationship between suspended, dissolved, fixed, and volatile solids.

Table 2. Solids matrix showing the relationship between volatile, fixed, dissolved, and suspended solids. Rows can be added from the bottom to get the constituents on the top (e.g. FSS+VSS=TSS) and analogously columns can be added from the right (e.g. TDS+TSS=TS).

2.4 Nitrogen

As indicated in equations 5 and 6, nitrogen in the form of ammonia can create a significant oxygen demand on a receiving stream. In fact, when a fish kill results from a manure spill, the most common cause of the fish kill is the depletion of oxygen (from the high oxygen demand from the organic material and ammonia). Ammonia by itself, however, is toxic, especially to spawning fish. The other concerns with nitrogen discharges include the problem of eutrophication or nutrient enrichment (see Eutrophication and Algal Blooms). In addition, nitrates from agricultural runoff and the oxidation of ammonia are problematic in drinking water. Nitrates can cause methemoglobinemia, or blue baby syndrome, in which the nitrates in the infants bottle or nursing mother’s milk cause suffocation in the infant. This is due to the fact that infants lack the enzyme (nitrate reductase) to break down nitrate. Consequently, nitrate interferes with the blood’s ability to absorb and release oxygen, and as a result, the baby turns blue.

2.5 Phosphorus

The limiting nutrient requirements of species that contribute to eutrophication, namely algae, are usually nitrogen and phosphorus. In freshwater systems, the limiting nutrient (the nutrient which limits the growth of the organism) is usually phosphorus. In brackish or saltwater environments, nitrogen usually limits growth. In ecosystems with excess nutrients, excessive algae growth occurs. Algal growth is problematic since it adds to the sediment load on a river, lake, or estuary. In some cases, the excess algae production leads to serious oxygen depletion. During the day, photosynthetic algae produce oxygen, which add to the overall oxygen balance. During the night, however, algae become oxygen consumers. In addition, when algae die they add to the problem of oxygen depletion, since the added organic matter poses a significant contribution of BOD (see Eutrophication and Algal Blooms). In fact, each year from late Spring to early Fall in the Gulf of Mexico a dead zone appears in which the dissolved oxygen concentrations fall to below the level acceptable for aquatic life. The main culprit for this dead zone is the over enrichment of nutrients and sediment from the Mississippi River. The nutrients contribute to algae growth, and the sediment blocks sunlight thereby limiting reoxygenation through photosynthesis. The size of the dead zone is estimated at 18 000 km2. During this seasonal hypoxia, gulf fisherman are forced to travel greater distances out to sea to avoid the dead zones. Often fishermen can have good catches at the boundary of the hypoxic zone by netting the fish that are either escaping the oxygen depleted waters or are feeding off the phytoplankton enriched waters.

Most likely, phosphorus is the limiting nutrient for the hypoxia problem in the Gulf of Mexico. Annual phosphorus inputs to the Mississippi are 136 000 metric tons (while nitrogen inputs are approximately 11 times this amount). In general, the phosphorus requirement of microorganisms is approximately 20 percent of the nitrogen requirement. Therefore, since the phosphorus input to the Gulf is less than 10 percent of the nitrogen, decreasing the nitrogen load would have little impact unless the phosphorus input was lowered in proportion. In contrast, lowering the phosphorus input would directly decrease the amount of algal (and other) biomass that grow on the nutrients.

Phosphorus is typically measured in three forms: orthophosphate, polyphosphate, and organic phosphate. Polyphosphate, or phosphoric acid (H3PO4), is the most predominant form of phosphorus present in domestic and industrial wastewaters. The concentrations of orthophosphate have decreased dramatically in the U.S. due to mandatory or voluntary bans on phosphate detergents in most states. Most detergents sold in the U.S. are currently phosphate free. Polyphosphates (e.g. Na3(PO3)6, Na4P2O7 ) are added at water treatment plants to increase the stability of the water to prevent scaling from calcium and magnesium. Polyphosphates in drinking water are eventually hydrolyzed to phosphates by the time they reach the wastewater treatment plant. Polyphosphates from detergents (if any), however, will still be present. In addition, in the aerobic basin of biological nutrient removal wastewater treatment systems, intracellular phosphate will be stored as polyphosphate granules. Organic phosphate is bound within organic molecules (e.g. protein, DNA, RNA, phospholipids, etc.). The organic phosphate is usually released by the biological breakdown of the organic constituents during wastewater treatment. Another class of phosphates, organophosphates (e.g. chlorpyrifos or Dursban™), are used as pesticides and are very resistant to biodegradation (see Chemistry of Organic Pollutants). These compounds have been suspected to contribute to effluent toxicity in some wastewater treatment plants.

2.6 Bacteriological

Wastewater is alive with microorganisms. These can be viruses, bacteria, protozoa, and helminths. Our main concern with living organisms in wastewater is their potential for causing disease (i.e. pathogens). Each of these four classes has pathogenic members. Table 3 lists many of the common pathogens of public health concern. A London physician, John Snow, is credited with making the first association between wastewater and disease. In 1854 there was a cholera epidemic, and Snow was able to show that 59 of the 77 people affected used a specific water pump on Broad Street. There was also a workhouse in the same vicinity in which no one was affected, and this workhouse had its own, separate well. From this observation, Snow was able to conclude that the water pump was infected. The source of infection turned out to be the drain of an infected person that was within three feet of the pump.

Table 3. Infectious agents potentially present in raw domestic wastewater. The helminths listed are those with a worldwide distribution.

Adapted from Metcalf and Eddy (1991) Wastewater Engineering. Treatment Disposal Reuse, G. Tchobanoglous and F.L. Burton (Eds.), 1820 pp. New York: McGraw-Hill.

With the advent of chlorination of drinking water supplies, countless lives have been saved from the ravages of waterborne disease. Despite these advances, waterborne outbreaks of disease still occur. For instance, in 1993 a Cryptosporidium outbreak in Milwaukee, Wisconsin caused 403 000 cases of severe diarrhea and over 100 deaths. The source of the Cryptosporidium was runoff from an animal feedlot. Due to improper operation of the water treatment plant, specifically coagulation and filtration, the chlorine resistant cysts passed through the plant and into the distribution system. Giardia and Cryptosporidium are two of the prevalent protozoan pathogens that are a major concern due to their resistance to disinfection. They require few organisms to cause infection, and they are fairly widespread in natural water sources. The primary line of defense is water filtration, followed by disinfection. To insure adequate kill of organisms, a time dose relationship is employed.

Helminths, and in particular Ascaris, are also prevalent worldwide with an incidence from 700 million to 1 billion. Symptoms include pneumonia, nausea, abdominal pain, and malnourishment; although, 85 percent of infections are symptomless. Each female worm can produce 200 000 eggs per day. Like protozoa, worms are effectively removed by filtration. In soil, Ascaris eggs can survive for periods up to seven years. Another helminth, Schistosomiasis, infects 200 million people worldwide and causes 200 000 deaths each year. Symptoms include enlargement of the liver, diarrhea, and anemia. Dams and irrigation projects in developing countries have caused ideal conditions for the spread of Schistosomiasis since part of the life cycle of the helminth is spent in an intermediate host snail. The snail emits the free swimming larvae, cercaria, which attach to human skin and penetrate to the blood stream.

Not all organisms that exist in nature are pathogenic. Table 4 lists the types and numbers of organisms found in a typical garden soil sample at various depths. From this table the ubiquitous nature of the various types of microorganisms can be seen. In nature, there exists a vast genetic library that is available to provide the necessary enzymes for the biodegradation pathways for the wide array of pollutants that exist in wastewater. Many biological wastewater treatment systems do not require an external seed due to the naturally occurring organisms already present in the wastewater. Table 5 lists the numbers and types of organisms present in untreated wastewater. Due to their prevalence and ready detectability, coliform bacteria are typically used as indicator organisms of bacterial contamination. Coliforms are aerobic and facultative anaerobic, gram negative, non-spore forming, rod-shaped organisms that ferment lactose within 48 hours at 35˚C. This functional definition includes the bacteria Escherichia coli, Kleibsiella, and Citrobacter. There are high levels of coliforms in human and animal feces (109 CFU per capita per day). A subset of coliforms, fecal coliforms, in particular are enteric, i.e, they are from the intestines of warm blooded animals and humans. Fecal coliforms (FC) can ferment lactose at 44.5˚C. About 2–8 percent of the fecal coliform organisms are pathogenic, causing gastroenteritis and travelers diarrhea.

Table 4. Distribution of microorganisms at various soil depths in number of colony forming units (CFU) per gram of soil.

Adapted from Csuros M., Csuros C. (1999). Microbiological Examination of Water and Wastewater, 324 pp. Boca Raton, Florida, USA: CRC Press.

Table 5. Types and numbers of microorganisms typically found in untreated domestic wastewater.

Adapted from Metcalf and Eddy (1991) Wastewater Engineering. Treatment Disposal Reuse, G. Tchobanoglous and F.L. Burton (Eds.), 1820 pp. New York: McGraw-Hill.

Another group of bacteria, fecal streptococci (FS), are also used as indicator organisms. This classification includes enteric bacteria such as Streptococcus faecalis, Streptococcus bovis, and Streptococcus equinus. If both fecal coliform and fecal streptococci are measured, the origin of the contamination can be traced (see Table 6). For instance, from Table 6, it can be seen that a fecal to streptococci coliform (FC/FS) ratio of approximately 4.4 indicates that the contamination is of human origin. A FC/FS ratio less than 1.0 indicates contamination from animal origin. A FC/FS ratio between 1.0 and 4.4 suggests that the contamination is from a mixture of human and animal sources.

Table 6. Estimated per capita contribution of indicator microorganisms from human beings and some animals.

Adapted from Metcalf and Eddy (1991) Wastewater Engineering. Treatment Disposal Reuse, G. Tchobanoglous and F.L. Burton (Eds.), 1820 pp. New York: McGraw-Hill.

2.1 Biochemical Oxygen Demand

3. Wastewater Composition


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