Abstract
Integrated pond systems consisting of facultative and high-rate ponds operating in series can produce an effluent low in dissolved organic material, nutrients, and hazardous bacteria. However, the effluent is rich in algae, and cannot meet stringent water-quality criteria on suspended solids. The algae, therefore, must be removed before final discharge or reuse of the effluent. This study reports on the efficiency of slow sand filtration in removing algae from high-rate pond effluents. When agricultural sandy soil with an effective grain size of 0.08 mm was used as the filter medium, an average filtration rate of about 1.3 m 3/m 2 .d was obtained. If filtration was preceded by sedimentation, the duration of a filtration run was about 100 h. At the end of each run, the filter was cleaned by scraping off the top few centimeters of the filter bed. The filtered effluent consistently had a biochemical oxygen demand (BOD) value less than 20 mg/L and undetectable fecal coliforms. The filter media, being rich in organic matter and having moisture-retaining properties, can be used as a soil conditioner.
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