Disinfection

Clean wastewater effluent discharge to receiving waters

Figure 16.3 Conventional wastewater treatment processing scheme.

sludge or biosolids. They typically yield about 0.5 to 0.7 kg of biosolids (dry weight) per kilogram of BOD removed. Thus, they convert a large problem (the volume of wastewater to be treated) into a much smaller problem, but one that still needs an ultimate solution.

The dominant biochemical mechanism in the vast majority of secondary wastewater treatment processes is that of respiration, which is distinctly different from that of the fermentation reactions that come into play more often in the sludge processing systems (e.g., anaerobic digestion) used to process wastewater residuals. Both respiration and fermentation depend on a balanced set of redox reactions (i.e., coupling oxidation plus reduction steps), which are linked, respectively, to electron donor and acceptor compound conversions.

With respiration, both reduced biodegradable organics (i.e., carbonaceous BOD) and reduced, energy-rich inorganics (e.g., ammonia, nitrite, sulfide, ferrous iron) in the raw wastewater may be oxidized to secure energetic electrons, compounds referred to as electron donors. Conversely, the electrons will then be ''respired'' or consumed by an electron acceptor compound. In most conventional wastewater treatment, dissolved oxygen is preferred as the electron acceptor, so that the zero-valence gas-phase diatomic oxygen (O2) species is reduced to water.

From a thermodynamic point of view, oxygen is the most favorable electron acceptor, and aerobic or facultative cells always opt for this aerobic respiration pathway if sufficient oxygen (i.e., greater than a few tenths of a milligram per liter) is present, largely ignoring other electron acceptors that might be available. Even in the absence of oxygen, respiration may be continued, following a pathway referred to as anoxic respiration, using a variety of alternative electron acceptors, such as nitrate, nitrite, sulfate, and oxidized iron and manganese. Whether aerobic or anoxic, the fact that respiration relies solely on inorganic electron acceptors is a notable feature for respiration and a key reason that fermentation is not used widely for wastewater treatment. Indeed, using anaerobic reactors rather than aerobic reactors would incur a far higher level of odor emission in these plants, brought about by volatile organic emissions generated reductively through fermentation.

Although characterized as biological processes, secondary treatment systems also involve a complex array of chemical transformations (e.g., acid-base, redox, chelation, sorption) and physical transfers (e.g., gas-liquid exchange, heat transfer). Furthermore, secondary biological operations involve biochemical mechanisms that couple removal and production pathways. Contaminants are removed via catabolic oxidization and anabolic assimilation while producing a concentrated waste product called sludge or bioso-lids, containing residual particulate solids at concentrations on the order of 1% or higher (i.e., >10,000 ppm).

Notwithstanding their mutual reliance on aerobic respiration, there are considerable differences between the four main options for secondary wastewater processing in terms of their involved engineering and biological details, and there are also dramatic variations in their necessary footprint areas relative to high vs. low waste loading rates. Of course, the more heavily loaded, mechanically intensive attached- and suspended-growth systems would commensurately require the smallest land areas; the more natural slow-rate and lowly loaded design options (i.e., constructed wetlands and lagoons) would require far larger land areas. Figure 16.4 compares the typical organic loading rates (i.e., representing

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