Physicochemical processes of wastewater treatment are most evident in the primary treatment facilities of a wastewater treatment plant. The adjective, primary may have had its undisputed claim on preeminence in the majority of wastewater treatment plants; this is no longer the case, because the environmental regulations and discharge standards of wastewater effluents tighten in many developed nations. Nevertheless, primary treatments in many wastewater treatment facilities in the U.S. or elsewhere are the most common wastewater treatment processes and contain many forms of physicochemical processing that may be categorized into a small number of basic operations called unit operations. Physicochemical processes in the primary treatment of food and agricultural wastewater generally comprise a set of unit operations that are intended to remove particulates and other coarse materials from the wastewater stream prior to the secondary treatment processes (mostly biological processes). The removed solids are fed into either aerobic digesters or anaerobic digesters for further volume reduction (Fig. 3.1). In primary treatment, only physicochemical processes are used to separate suspended solids and greases from wastewater. Primary treatment of food and agricultural wastewater usually include screening, flotation, flocculation, sedimentation, and sometimes, granular sand filtration. In a typical wastewater treatment facility for a food processing plant, wastewater is normally held in a tank for several hours, allowing the particles to settle to the bottom and the greases to float to the top. The solids drawn off the bottom and skimmed off the top receive further treatment as sludge. The clarified wastewater flows on to the next stage of wastewater treatment. The exact lineup and sequence of unit op-

Figure 3.1. A flow chart of a typical wastewater treatment scheme.

erations largely depend on the characteristics of wastewater streams, objectives of treatment, and local environmental laws and regulations.

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