products formed during the biological degradation of organic compounds and microbial cellular constituents (Metcalf & Eddy, Inc., 1991). The suspended solids are mainly organic in nature, consisting of biological solids produced during secondary treatment and other solids that escaped treatment and separation.

Stabilization ponds use algae to provide oxygen to the system. This system is considered a low-rate biological process. Mechanically aerated lagoon systems sometimes are used to provide secondary-level treatment. Stabilization ponds are capable of providing considerable nitrogen removal under certain conditions (e.g., high temperature and pH and long detention times) and are effective in removing microorganisms from wastewater. Well-designed and well-operated pond systems are capable of achieving a 6-log (99.9999 percent) reduction of bacteria, a 3-log (99.9 percent) reduction of helminths, and a 4-log (99.99 percent) reduction of viruses and cysts (Mara and Cairncross, 1989). Algae produced during pond treatment may present soil clogging problems during recharge.

Tertiary Treatment

The treatment of wastewater beyond the secondary or biological stage is sometimes called tertiary treatment. The term normally implies the removal of nutrients such as phosphorus and nitrogen, and a high percentage of suspended solids. However, the term tertiary treatment is now being replaced in most cases by the term advanced wastewater treatment—which refers to any physical, chemical, or biological treatment process used to accomplish a degree of treatment greater than that achieved by secondary treatment.

Advanced Wastewater Treatment

Advanced wastewater treatment processes are designed to remove suspended solids and dissolved substances, either organic or inorganic in nature. Advanced wastewater treatment processes generally are used when a high-quality reclaimed water is necessary, such as for direct injection into potable aquifers. Commonly used processes and their principal removal functions are given in Table 2.6. The major advanced wastewater processes associated with ground water recharge are coagulation-sedimentation, filtration, nitrification, denitrification, phoshorus removal, carbon adsorption, and reverse osmosis.

Coagulation-Sedimentation

Chemical coagulation with lime, alum, or ferric chloride followed by sedimentation removes suspended solids, heavy metals, trace substances, phosphorus, and turbidity. Vital inactivation under alkaline pH conditions can be accom-



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