Wastewater Treatment

Clean water is a vital commodity, and usage is now so extensive that waste-waters must be repurified to avoid destruction of aquatic ecosystems and because, often, the water will be reused. Industrial wastewater may require specialized treatment that depends on the contaminants, while treatment of domestic waste (sewage) involves more general procedures.

A great deal of water is used for cooling purposes. Such water, if discharged back to freshwater systems, could lead to so-called thermal pollution by increasing lake and stream temperatures. Impurities are introduced only if additives are present to reduce fouling of heat exchangers (a common practice) or if substances are dissolved from them. Trace metals extracted as corrosion products would be possible examples of the latter case. The chemistry of thermal pollution lies chiefly in the effects of temperature on physicochemical and biological processes. For example, warm water will reduce the solubility of O2, thus lowering its capacity to support life and to oxidize impurities. Large amounts of artificially warmed water, which will alter the relative volumes of the epilimnion and hypolimnion in a lake, may maintain the density gradient when normal conditions would lead to turnover (Section 9.1.2). Since fish and other aquatic organisms often prefer very restricted temperature conditions, the volumes of the habitats available for different species, and therefore the population balance of the water system, may be changed greatly. Dedicated cooling ponds from which the water may be recirculated or cooling towers which transfer the heat to the air, for example by evaporation as the hot water is sprayed through the tower, are alternatives to direct discharge. Use of the discharge water to heat buildings, greenhouses, and the like is attractive if such facilities are nearby.

Treatment of domestic wastewater (i.e., sewage) is important, and some steps used in common processes are described briefly below. Depending on the final water quality desired, treatment may be at the primary, secondary, or tertiary level. A typical treatment scheme is shown in Figure 11-5.

11.5.2.1 Primary Sewage Treatment

Domestic wastewater normally contains about 0.1 wt % impurities, composed mostly of a variety of organic materials. Many of these can be removed by filtration or sedimentation. Large particles (grit, paper, etc.) are removed by coarse mechanical screening followed by settling. Much suspended and fine particulate material remains, which is usually removed by sedimentation processes. Floating material such as grease can be skimmed off the top. In well-designed settling tanks, nearly 90% of the total solids and perhaps 40% of the organic matter can be removed. To remove the smaller particles, however,

Raw sewage

Removes large particles

Removes small particles

Removes most organics, some nutrients

Large particles Small/suspended particles Dissolved organics Dissolved inorganics Nutrients Heavy metals

Removes large particles

Removes small particles

Removes most organics, some nutrients

1

Bacterial treatment

Sludge

Sludge

Sludge

To disposal or tertiary treatment

FIGURE 11-5 Typical steps in primary and secondary sewage treatment.

assistance is needed and coagulation may be employed. This process, using aluminum sulfate, ferrous sulfate, or similar compounds, was discussed for drinking water purification in Section 11.5.1. The process is very similar when applied to wastewater.

In practice, sewage waters often contain considerable bicarbonate ion that can act as a base; CaCO3 may be part of the precipitate. Lime, Ca(OH)2, alone will cause CaCO3 to precipitate and has some coagulant action. An added value to using calcium, iron, or aluminum in this process is that much of the phosphorus present can be removed as the insoluble phosphate salts. Up to 95% removal of phosphate is possible.

The solid produced in this process, which may contain toxic industrial wastes, grease, bacteria, and other substances, must be disposed of. It is often buried in landfill, or sometimes dumped at sea, although this mode of disposal is being phased out in many countries. The coarse materials removed in the preliminary steps must be disposed of in a similar way.

The treatment processes just described are called primary treatment, and a great deal of sewage receives no further processing. A large amount of dissolved material is left behind. Much of this is organic, but there are also nitrate, some phosphate, and metal ions. Secondary treatment removes the organic materials. This is important with respect to the overall quality of the discharged water, since a high organic content will use up the dissolved oxygen in the water, cause reducing conditions and the formation of various undesirable reduction products. Aquatic life also suffers. A practical measure of the reducing impurities in water is the so-called biological oxygen demand (BOD) (Section 7.2.5).

11.5.2.2 Secondary Sewage Treatment

The most widely used method of secondary treatment is a biochemical one, the activated sludge process. Bacterial action decomposes the organic matter in the sewage, producing a bacterial sludge that can be separated by sedimentation for disposal. This sludge is about 1% solids, and amounts to about 700 million gallons per day in the United States. Disposal is a major problem. As of 1993, over one-third (around 2.5 million tons dry weight) was used as fertilizer (often referred to as biosolids, which has a more attractive sound than sewage sludge), around 40% was landfilled, and under 20% incinerated. An even larger fraction of sewage sludge is used as fertilizer in Europe. Use as a fertilizer is attractive: there is significant N, P, and S content; but perhaps more significantly the practice builds up the organic content of soils, which in turn generally supports the natural soil biota and improves soil quality. Most is used in farmland, but other major uses are forest restoration and rejuvenation of mining areas. There are restrictions on the areas where sludge can be used to avoid excessive runoff, and other regulations limit the buildup of toxic materials from repeated applications. It is the possibility of introducing toxic materials that give rise to the greatest objections to this use. There are three categories of concern.

1. Pathogens. Various harmful organisms remain in the sludge, although they are maintained at acceptable levels and have not been directly implicated in disease. Various treatments are available to reduce these undesirable components to below detectable levels, but they are not used in most facilities.

2. Undecomposed toxic organic compounds. Highly stable materials such as dioxins and PCBs (Section 8.7) may be present, especially if the sewage contains industrial wastes. Numerous tests have shown that levels in most sewage sludge are small.

3. Toxic heavy metals. These known components of the wastes accumulate in soils in which they are used; take-up by plants can introduce them into the food supply, thus limiting long-term application of biosolids to agricultural land. These limits are indicated in Table 11-5. It is notable that European and U.S. standards are quite different. U.S. standards are based on risk estimates to humans based on various pathways for possible ingestion by humans, while

TABLE 11-5

Allowable Limits of Metals in Soils Treated with Sewage Sludge, (mg/kg)

TABLE 11-5

Allowable Limits of Metals in Soils Treated with Sewage Sludge, (mg/kg)

Standards

Cd

Cu

Cr

Hg

Ni

Pb

Zn

United States

20

750

1500

8

210

150

1400

European

3

140

150

1.5

75

300

300

European standards are based on possible direct ecological effects on the biota of the soils.

Alternate sludge processing methods involve further bacterial digestion. Anaerobic digestion of the sludge converts carbon compounds chiefly to CH4 and CO2, while nitrate, sulfate, and phosphate are converted at least in part to NH3, H2S, and PH3. About 70% of the gaseous products is methane. This can be burned to heat the reaction tanks, power pumps, and so on, and in fact such degradation of organic wastes to methane is a possible source of energy (Section 16.4). The other gaseous products (except CO2) must be trapped or reacted to less noxious forms.

Aerobic digestion of the sludge with no additional source of nutrients results in autoxidation as the bacterial cells use up their own cell material. The organic materials are converted to CO2 or carbonates, nitrogen, sulfur, and phosphorus to the oxo anions, and the solution ultimately produced is potentially useful as a liquid fertilizer. This approach to activated sludge disposal is comparatively new.

An alternative approach to sewage treatment is the use of lagoons, shallow open ponds in which the water can stand for several days to weeks while the biological activity of bacteria and algae under sunlight destroys most of the organic waste. Sludge must be removed from the lagoon periodically and disposed of as in other treatments. The final water still contains phosphorus and nitrogen nutrients. This approach depends on climate and available land areas.

11.5.2.3 Tertiary Sewage Treatment

Final cleanup of the water effluent from the secondary treatment is called tertiary treatment. Several types of materials must be considered in this respect. It should be kept in mind that while there are many tertiary treatment approaches, they are expensive and not widely used. Even secondary treatment is often not applied in many sewage disposal systems. The specific details of waste treatment vary considerably with different sewage handling facilities.

a. Colloidal Organic Matter

The remains of the activated sludge that do not settle out in the sedimentation tanks are returned to as colloidal organic matter. Different types of filter can be used, and it is common at this point to use an Al2(SO4)3 coagulation process as described earlier as the key step in aiding the removal of these materials.

b. Phosphate

A phosphate concentration of 1 mg/liter is adequate to support extensive algal growth (blooms). Typical raw wastes contain about 25 mg of phosphate per liter. Thus, a large reduction is necessary to ensure that these effluents do not contribute to eutrophication of lakes. Removal of phosphorus by precipitation of an insoluble salt has been mentioned in connection with primary waste treatment. In fact, this precipitation may be done in the primary, secondary, or tertiary step. Lime is commonly used, with hydroxyapatite being the product:

5Ca(OH)2 + 3HPO4~ ^ Ca5OH(PO4)3 + 3H2O + 6OH~ (11-16)

Lime is cheap, and the efficiency of phosphate removal is theoretically very high, although colloid formation by the hydroxyapatite, slow precipitation, and other problems often reduce this efficiency in practice. Polyphosphates, if present, form soluble calcium complexes (Section 10.5). Other precipitants are MgSO4 (giving MgNH4PO4), FeCl3 (giving FePO4), and Al2(SO4)3 (giving AlPO4). Phosphate precipitation may occur as part of a coagulation process, especially if iron or aluminum salts are used. An alternate process to precipitation is adsorption, using activated alumina (i.e., acid-washed and dried Al2O3). The alumina can be regenerated by washing with NaOH.

c. Nitrate

The activated sludge process converts some nitrogen to organic forms that are removed with the sludge; much, however, remains behind. Converting this to nitrate is possible if excess air is used, but this method of operation, more expensive than efficiently removing carbon compounds, is usually not used. The nitrogen remaining in the effluents then is largely some form of ammonia: NH3 or NH4. This can be stripped from solution by a stream of air if the solution is basic (the pH can be adjusted to be greater than 11 by the addition of lime, which can simultaneously remove phosphate). The chief problem lies in disposing of the ammonia in the air stream. It can be adsorbed by an acidic reagent, but at further cost. If allowed to escape, it is a potential local air pollutant, and eventually enters the runoff water through rain. Alternatively, aerobic bacterial nitrification of ammonia to nitrate, followed by denitrifica-tion by other bacteria in the presence of a reducing carbon compound (e.g., methanol) can be employed:

nitrifying

bacteria

denitrifying

bacteria

Purely chemical processes for nitrogen removal are not available; suitable insoluble salts do not exist for a precipitation process, but reduction to N2 or N2O (which can be captured) by Fe(II) has been suggested for possible development.

d. Dissolved Organic Material

Comparatively little dissolved organic material remains after secondary treatment, but what remains is significant in terms of taste, odor, and toxicity. Chemical oxidation is a possibility, with a variety of oxidants being proposed, including chlorine, hydrogen peroxide, and ozone. Adsorption on activated carbon is effective and perhaps more practical.

e. Dissolved Inorganic Ions

Besides nitrate and phosphate ions, a significant quantity of other inorganic substances is present. Although many of these are not harmful, their accumulation eventually renders water unsuitable for reuse. In addition, toxic heavy metal ions may be present, although most of these precipitate under basic conditions. A variety of methods can be used to remove these substances, although in general the methods are expensive and rarely used in present tertiary wastewater treatments. When high-quality water is scarce, use of one or more of these methods may be practical as it is with brackish or seawater in arid areas, and for some of them the primary application is in desalinization plants to produce drinking water. Of main interest for desalination are distillation, electrodialysis, reverse osmosis, ion exchange, and freezing.

Distillation Distillation is an old technique, and quite simple if the waters do not contain volatile impurities such as ammonia. Although expensive, it has been used for many years to supply fresh water from seawater. Fossil fuel has normally served as the heat source, but solar distillation, the use of solar energy to evaporate the water has low cost potential in suitable areas.

Electrodialysis This is a newer technique. Water is passed between membranes that are permeable to positive ions on one side of the water stream and to negative ions on the other. An electrical potential is applied across the system so that the ions migrate, the positive ions passing out through the cation-permeable membrane (which prevents anions from flowing in), while the negative ions move out in the opposite direction through the anion-permeable, cation-impermeable membrane. The result is a stream of deionized water between the membranes, and this stream can then be separated as clean water. This process is critically dependent on the composition of the water; large ions, for example, do not pass through the membranes. It has been applied to tertiary treatment on a pilot-plant scale.

Reverse Osmosis Sometimes called hyperfiltration, reverse osmosis is another newer process for water purification which is widely used. If pure water is separated from an aqueous solution by a membrane (e.g., cellulose acetate) that is permeable to water but not to the solute, then the solvent will flow from the pure water side into the solution. If the apparatus is designed appropriately, a hydrostatic pressure that will counteract the tendency of water to flow into the solution side of the membrane can be developed across the membrane. Without this hydrostatic pressure, liquid level in the solution compartment rises, corresponding to the osmotic pressure of the solution. However, if a hydrostatic pressure exceeding the value of the osmotic pressure is applied to the solution, water will flow from the solution to the clean water side of the membrane. This is the principle of reverse osmosis. The process is in use to obtain pure water from seawater in the Middle East, the Caribbean, on ships, and for some other pure water supplies. In practical systems, a very thin supported membrane is used; very small pores permit the water molecules but not the hydrated metal ions to pass.

Ion Exchange Frequently used for small-scale water purification, ion exchange is often found in home water softening devices. Ion exchange materials are usually organic resins, but some inorganic materials can also function in this way. The bulk of the resin carries a charge (e.g., a group such as—OSO-or a quaternary amine N+), where an ion of opposite charge is held by Coulombic forces to produce electrical neutrality. An ion of a given charge can be replaced by another, and if those present initially are H+ on the cation exchanger and OH~ on the anion exchanger, ionic impurities can be removed completely if water is passed through one and then the other. The exchangers are then regenerated by acid and base solutions, respectively. In household water softening, removing the Ca2+ ions, replacing them with Na+ ions, is often adequate. Regeneration can then use an NaCl solution.

Partial Freezing This is another method that can be used to produce useable water from a saline or contaminated source. Typically, when a dilute aqueous solution freezes, the first material to solidify is the solvent, water. Ice is formed, while the unfrozen solution becomes more concentrated. This ice can be separated and melted as clean water, and the remaining concentrated solution discarded.

11.6 ANAEROBIC SYSTEMS

The solubility of molecular oxygen in water is sufficient to support aerobic organisms and oxidizing conditions in much of the natural water systems that we encounter. However, inefficient mixing often permits bottom waters of lakes and oceans, and especially the waters in the sediments that underlie them, to become depleted in oxygen as biological processes use it up. This has been mentioned in several places (e.g., Section 9.1.2). Marshes and other systems with large amounts of decaying organic matter, soils with high organic contents and poor drainage, coal deposits, and landfill sites also lead to groundwaters that are anaerobic.

Normal respiration produces energy by the conversion of organic carbon compounds, the composition of which can be approximated by the empirical formula (CH2O)n, to CO2 using oxygen as the oxidizing agent:

This produces the maximum energy of any respiration process available to organisms; the free energy change involved in this process being about —500 kj per "mole" of CH2O. However, several alternative oxidizing agents are available to appropriate anaerobic organisms.

One of these alternatives is the nitrate ion:

5 (CH2O)n+ 4NO— + 4H+ ^ 5CO2 + 7H2O + 2N2 (11-20)

Use of nitrate in this way is a common process in soils, but nitrate is rarely abundant in sediments. In actuality, the microorganisms that carry out this process do so in several steps: nitrate ^ nitrite ^ nitrogen oxides ^ nitrogen. Some organisms utilize only some of these steps, and intermediates such as nitrous oxide may be released. The total free energy change is smaller than if oxygen were used, —480 kj per "mole" of organic material, which means that nitrate-based metabolism is less efficient. As part of the decay process, nitrogen contained in the organic matter is converted to ammonia (recall that biological material contains about 16 nitrogen atoms for every 100 carbon atoms; Section 10.5). Under aerobic conditions, bacteria will oxidize the ammonia that is not assimilated, but under anaerobic conditions it too may be released. Reduction of nitrite to ammonia is somewhat more favorable thermodynamic-ally than reduction to nitrogen, but does not seem to be an important step biochemically. Reduction of nitrogen to ammonia is of course an important biochemical process, but apparently not one that is used for energy production.

Sulfate is a second substrate available to anaerobic organisms, particularly in marine environments where sulfate is relatively abundant in seawater.

2 (CH2O)„+ SO2~ + 2H+ ^ 2CO2 + 2H2O + H2S (11-21)

It is less energy efficient than the nitrate process, with a free energy change of only —103 kj per "mole." The H2S produced may be released to the atmosphere, but alternatively it may be precipitated as metal sulfides, since many transition metal and heavier metal sulfides are insoluble. For example, iron, which is relatively abundant, is likely to be present under reducing conditions as Fe2+ and can then generate ferrous sulfide deposits. Under other (aerobic) conditions, oxidation of sulfide or even of the Fe2+ can be energy sources for other organisms. In some cases, elemental sulfur can be a by-product.

Methane-producing organisms are likely to predominate when nitrate and sulfate concentrations are low. The process is

n with only 93 kj of free energy available per "mole." Methane is frequently produced in marshy areas (swamp gas) and in landfills. Methane can also be used as an energy source for some bacteria. A similar process can generate hydrogen

n with an energy release of about 26.8 kj.

11.7 IRRIGATION WATERS

Much agriculture depends on irrigation. Many of the most fertile soils occur in arid regions. Both surface water and groundwater sources are heavily used. Irrigation, however, is not simply adding enough water to the soil to maintain plant growth; it introduces some serious problems both for the water being used and for the soil itself. These problems relate to the mineral content of the water, which increases through the irrigation process. Inevitably, some water used for irrigation evaporates. The salt content, made up in part from the initial mineral content of the water, but increased by the salt leached from the soil as the water flows through the irrigation system, may reach a level at which further evaporation leads to deposition of additional salts in the soil rather than their removal. An essential feature of long-term irrigation is the need to use enough water to dissolve and carry away excess salts. In some areas, drainage systems are placed below the root level to facilitate this; an example is the San Joaquin Valley in California.

Irrigation water quality depends on at least four factors.

1. Salinity, the total salt content of the water. With a high soluble salt content, the soil may become unsuited to plants with low salt tolerance, and the returned irrigation water may be unfit for further use. Salt deposits from successive irrigations may carry deposited salt to lower depths, where it will accumulate in the root zone if adequate leaching does not take place.

2. Sodium content relative to the magnesium and calcium ion content (called the sodium absorption ratio, SAR). Because of ion exchange reactions with clay minerals in the soil, a high sodium ion content relative to the others can cause breakdown of the soil particles, leading to impermeability to water (waterlogging) and to hardening of the dry soil, even if the total salt concentration is low (Sections 9.7 and 2.3.1).

3. Alkalinity, the carbonate and bicarbonate content of the water (Section 9.2.2). This property influences soil pH, but also may result in precipitation of poorly soluble calcium and magnesium carbonates as the water evaporates, leading to an increased SAR for the remaining water.

4. Toxic elements, Some compounds that can be damaging to particular crops—for example, borates in parts of the western United States—may be present in the source irrigation water. These substances can accumulate in the soil over time, even if the initial levels are not harmful.

If enough excess water is used so that salts, carbonates, or toxic materials do not accumulate through evaporation, this excess mineral-laden water becomes runoff, entering either surface water systems or aquifers and perhaps degrading their qualities also. Buildup of toxic elements to hazardous levels can occur. Examples in the United States are boron and selenium (Sections 10.6.8 and 10.6.12), both of which have exceeded acceptable levels in some California irrigation waters. Selenium-rich water from irrigation uses in the San Joaquin Valley draining into the Kesterson Wildlife Refuge in California had serious effects on the aquatic life and wildfowl (it is teratogenic) until the input was stopped.

Additional Reading

Baker, L. A., ed., Environmental Chemistry of Lakes and Reservoirs, ACS Advances in Chemistry

Series 237. American Chemical Society, Washington, DC, 1994. Drever, J. L., The Geochemistry of Natural Waters. Prentice-Hall, Englewood Cliffs, NJ, 1982. Forster, B. A., The Acid Rain Debate. Iowa State University Press, Ames, 1993. Howells, G., Acid Rain and Acid Waters, 2nd ed. Ellis Horwood, New York, 1995. Laws, E. A., Aquatic Pollution: An Introductory Text, 2nd ed. Wiley, New York, 1993. Libes, S. M., An Introduction to Marine Biogeochemistry. Wiley, New York, 1992. Sopper, W. E., Municipal Sludge Use in Land Reclamation. Lewis Publishers, Boca Raton, FL,

1993.

EXERCISES

11.1. Describe the changes in pH as typical river water evaporates. What solid products would you expect to deposit?

11.2. What are the five most abundant inorganic materials in seawater? How do these compare with elemental abundance in the environment as a whole?

11.3. Give equations for the reactions of silica in water; of alumina in water. Do these reactions occur to a significant extent in the environment?

11.4. Write some model equations to illustrate how the ocean pH might be determined by reactions involving alumina and silica.

11.5. What are the usual components of acid rain? What are the sources of these acids? What is the relative importance of the different sources?

11.6. Describe the usual geographic patterns of pH of precipitation in the United States and in Europe. Explain the reasons for these patterns.

11.7. We have mentioned acid precipitation in North America and Europe. Is it a problem in other parts of the world? Where else would you anticipate problems from acid precipitation? Explain why.

11.8. List the steps that are usually employed in the treatment of drinking water.

11.9. Use an equation to explain why the disinfectant power of chlorine is pH dependent.

1.10. What are the available practical methods for disinfecting drinking water? Give advantages and disadvantages of each.

1.11. Give equations showing the mechanism of formation of chloroform from acetophenone, C6H5C(O)CH3, as a by-product of water chlorin-ation.

1.12. Describe the chemistry involved in the process of flocculation in water treatment. Include equations.

1.13. Describe the processes of primary, secondary, and tertiary sewage treatment.

1.14. Indicate some practical methods for disposing of the sludge from sewage treatment. Compare the advantages and disadvantages of each.

1.15. Give the equations of the reactions that take place in anaerobic metabolic processes. Under what environmental conditions is each expected to be most important?

1.16. Compare the energy available from oxidation of one gram of organic material with O2, NO- and SO4~ as oxidizing agents.

1.17. Explain how irrigation can lead to deterioration of the agricultural quality of soils.

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