Biochemical operations only alter and destroy materials that microorganisms act upon, i.e., those that are subject to biodégradation or biotransformation. If soluble pollutants are resistant to microbial attack, they are discharged from a biochemical operation in the same concentration that they enter it, unless they are acted on by chemical or physical mechanisms such as sorption or volatilization (see Chapter 22). Insoluble pollutants entering a suspended growth biochemical operation become intermixed with the bioniass and, for all practical purposes, are inseparable from it. Consequently, engineers consider this mixture of biomass and insoluble pollutants as an entity, calling it mixed liquor suspended solids (MLSS). If insoluble pollutants are biodegradable, their mass is reduced. On the other hand, if they are nonbiodegradable, their only means of escape from the system is through MLSS wastage and their mass discharge rate in the wasted MLSS must equal their mass input rate to the system. Attached growth processes usually have little impact on nonbiodegradable insoluble pollutants, although in some cases those pollutants are flocculated and settled along with the biomass discharged from the operation.
When wastewater treatment engineers design biochemical operations they use natural cycles to accomplish in a short time what nature would require a long time to accomplish, often with environmental damage. For example, if biodegradable organic matter were discharged to a stream, the bacteria in that stream would use it as a source of carbon and energy (electrons) for growth (see Chapter 1). In the process, they would incorporate part of the carbon into new cell material and the rest would be oxidized to carbon dioxide to provide the energy for that synthesis. The electrons removed during the oxidation would be transferred to oxygen in the stream, but if the supply of oxygen were insufficient, the dissolved oxygen (DO) concentration would be depleted, killing fish and causing other adverse effects. On the other hand, in a well designed biochemical operation, microbial growth is allowed to occur in an environment where the appropriate amount of oxygen can be supplied, thereby destroying the organic matter and allowing the treated wastewater to be discharged without environmental harm.
The two major cycles employed in biochemical operations are the carbon and nitrogen cycles. Actually, most biochemical operations only use half of the carbon cycle, i.e.. the oxidation of organic carbon, releasing carbon dioxide. While some biochemical operations use algae and plants to fix carbon dioxide and release oxygen, thereby using the other half of the carbon cycle, they are not as widely applied and will not be covered in this book. However, almost all of the nitrogen cycle is used, as illustrated in Figure 2.1. In domestic wastewaters, most nitrogen is in the form of ammonia (NIT) and organic nitrogen, whereas industrial wastewaters sometimes con-lain nitrate (NO; ) nitrogen as well. Organic nitrogen is in the form of amino groups (NH ). which are released as ammonia — in the process called ammonification — as .lie organic matter containing them undergoes biodégradation. The form in which bacteria incorporate nitrogen during growth is as ammonia. If an industrial wastewater has insufficient ammonia or organic nitrogen to meet the growth needs of the bacteria, but contains nitrate or nitrite (NO. ) nitrogen, they will be converted to ammonia through assimilative reduction for use in cell synthesis. On the other hand.
if a wastewater contains ammonia-N in excess of that needed for cell synthesis, nitrification can occur, where the excess ammonia-N is oxidized to nitrate-N, going through the intermediate, nitrite. Discharge of nitrate to a receiving water is preferable to discharge of ammonia because nitrification in the receiving water can deplete the DO, just as degradation of organic matter can. In some cases, however, discharge of nitrate can have a deleterious effect on the receiving water, and thus some effluent standards limit its concentration. In that case, biochemical operations that use deni-trification to convert nitrate and nitrite to nitrogen gas must be used to reduce the amount of nitrogen in the effluent. The only step in the nitrogen cycle not normally found in biochemical operations is nitrogen fixation, in which nitrogen gas is converted to a form that can be used by plants, animals, and microorganisms.
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