CHEMICAL SCIENCES, ENGINEERING AND TECHNOLOGY RESOURCES - SAMPLE CHAPTERSCHEMISTRY 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
2. Wastewater Analysis
3. Wastewater Composition
4. Wastewater Quantities
The chemistry of wastewater reflects our activities in this life. Industrial, agricultural, and municipal activities are represented by the wastewater produced in each. Due to its value and scarcity, wastewater is treated, discharged to a receiving stream, and withdrawn for reuse by the downstream population. Consequently, the chemical and bacteriological composition must be monitored to ensure the public health. In addition, the oxygen consuming material in the wastewater must be minimized to protect the receiving stream from low dissolved oxygen conditions which can be deleterious to desirable aquatic species. Nutrients, such as nitrogen and phosphorus, should be removed to prevent eutrophication and siltation. Microbiological contaminants and other pollutants should be removed to protect downstream users, including contact users (e.g. boaters and swimmers). Typical municipal wastewater contains about 220 mg L-1 of both suspended solids and BOD. The organic composition of wastewater is approximately 50 percent proteins, 40 percent carbohydrates, 10 percent fats and oils, and trace amounts of priority pollutants and surfactants. The microbiological composition of wastewater includes 105-108 CFU coliform organisms, 103-104 CFU fecal streptococci, 101-103 protozoan cysts, and 101-102 virus particles. This chapter discusses the water quality parameters concerning wastewater composition, generation, and treatment.
Wastewater is the liquid end-product, or by-product, of municipal, agricultural, and industrial activity. As such, the chemical composition of wastewater naturally reflects the origin from which it came. In fact, the chemistry of wastewater reflects to a very high degree the chemistry of life. Just as wastewater chemistry reflects life’s chemistry, so too, does wastewater microbiology reflect the microbiology of life. It is perhaps the microbiology of wastewater that presents the greatest concern to humanity from a public health standpoint.
The term ‘wastewater,’ however implies that it is a waste product to be discarded in an environmentally sound manner. This could not be farther from the truth. In fact, the world’s available fresh water supply is about 3 percent of that total water supply. Only 20 percent of this amount is available for use in drinking water supplies. The remainder of the world water is salt water, which is costly to desalinate for drinking water purposes. Consequently, the water we use for drinking, washing, bathing, etc. ultimately ends up back in the stream, river, lake, or groundwater where it will be withdrawn, treated, and used again (see Water Quality). While it is not necessarily a positive mental image, the drinking water we are using today was the wastewater discharged by another community yesterday, or the day before, or the day before. Consequently, the chemical and microbiological composition of that wastewater must be monitored in order to safeguard downstream users. Current water use standards are enacted with this in mind. Table 1 provides a list of wastewater contaminants that are of concern, and the reasons that they are of concern.
Table 1. Important contaminants of concern in 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.
2. Wastewater Analysis
Analysis of wastewater typically concentrates on the water quality parameters that affect the receiving stream. For instance, if the receiving stream for a wastewater discharge is a lake, the nutrients, nitrogen and phosphorus, in the wastewater may be the primary concern. Nutrients discharged to a lake or river can cause eutrophication, a condition that degrades water quality by increasing algae growth, depleting dissolved oxygen concentrations, and increasing sedimentation (see Eutrophication and Algal Blooms). If the receiving stream is a high quality river, the primary concern may be the oxygen consuming organics in the wastewater. Oxygen depletion in the river could lead to deterioration in the quality and diversity of fish species, and severe oxygen depletion will cause fish kills. The biochemical oxygen demand and chemical oxygen demand tests both measure the oxygen consuming organics in a wastewater sample. These tests do not identify individual components in the wastewater, but rather provide an indication of what effect the wastewater might have if discharged to a receiving stream where the dissolved oxygen concentration is impacted. Oxygen is sparingly soluble in water (e.g. at 25°C the equilibrium saturation concentration of oxygen in water is only 8.24 mg L-1). The quality of the fish habitat begins to decrease when the dissolved oxygen concentration drops below 4 or 5 mg L-1. Consequently, even if the receiving stream is at saturation (which is unlikely), that leaves only 3 or 4 mg L-1 of oxygen to be used for assimilation of the wastewater discharge.
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