Solids Removal

Removal of solids produced by aquaculture may be attempted for making the water suitable for reuse, or to meet discharge guidelines. Solids account for a high percentage of the biochemical oxygen demand (BOD) in aquaculture effluents; they tend to be highly variable in size and break up easily if subjected to mechanical forces. Solids are formed from uneaten feed and fecal material. Their density tends to be very close to that of water.

Removal is achieved by filtration with particulate (eg, sand) or microscreen (eg, stainless steel) filters, or by sedimentation (25). The high organic content of the particulates make them highly biodegradable, and filters need to be backwashed frequently to prevent clogging by a mat of biological slime. Screen filters are common in aquaculture and a number of filters have been developed that incorporate some form of automated cleaning (eg, drum or disk filters). Simple settling tanks or ponds with overflow rates between 40 and 80 m d_1 can result in total suspended solids removal rates of 65 to 85% (25). Aquaculture settling ponds traditionally have been operated with infrequent sludge removal, relying on biological decomposition to prevent excessive accumulation of sludge.

Recent concerns with the possible release of nutrients, especially phosphorus, from aquaculture operations to natural waters have caused a réévaluation of techniques available for solids removal. Given the technical difficulties and expense associated with treating aquaculture effluents, a combination of nutritional and engineering approaches are being studied to reduce the solids (and nutrient) content in effluents.

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