HSSF CWs require good mechanical pretreatment with suspended solids being the major target. Excessive sus pended solids may cause filtration bed clogging and subsequent surface flow. Small systems for domestic sew age with low flows usually use a three chamber septic tank. Pretreatment in systems designed for municipal sewage mostly comprise screens and Imhoff tank. When stormwater runoff is also treated (combined sewer sys tem), a grit chamber is included (Figures 3 and 4). If wastewater contains oils and/or grease, the grease filter should be a part of the pretreatment. Various types of wastewater may require different types of pretreatment; for example, landfill leachate treatment systems usually include aerated lagoons, systems for the treatment of concentrated wastewaters from agricultural operations commonly include facultative lagoons.
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