Because photosynthesis is driven by sunlight, the dissolved oxygen levels are highest during the day and drop off at night. Photosynthetic algae release oxygen into the water and at the same time consume carbon dioxide produced by the respiration of bacteria. It is the shallowest of the ponds, ensuring that sunlight penetrates the full depth for photosynthesis to occur. An aerobic pond is commonly referred to as a maturation, polishing, or finishing pond because it is usually the last step in a series of ponds and provides the final level of treatment. The aerobic and anaerobic organisms work together to achieve BOD reductions of up to 75%.Īnaerobic and facultative ponds are designed for BOD removal, while aerobic ponds are designed for pathogen removal (see also pathogens and contaminants). Settleable solids accumulate and are digested on the bottom of the pond. The lower layer is deprived of oxygen and becomes anoxic or anaerobic. The top layer of the pond receives oxygen from natural diffusion, wind mixing and algae-driven photosynthesis. In a series of WSPs, the effluent from the anaerobic pond is transferred to the facultative pond, where further BOD is removed. Anaerobic bacteria convert organic carbon into methane and through this process, remove up to 60% of the BOD. Solids and BOD removal occurs by sedimentation and through subsequent anaerobic digestion inside the accumulated sludge (see also anaerobic digestion general). The entire depth of this fairly deep man-made lake is anaerobic. The anaerobic pond is the primary treatment stage and reduces the organic load in the wastewater. For the most effective treatment, WSPs should be linked in a series of three or more with effluent being transferred from the anaerobic pond to the facultative pond and, finally, to the aerobic pond. They are semi-centralised treatment systems combined after wastewater has been collected from toilets (see also wastewater collection and user interface). Waste stabilization ponds are large man-made basins in which greywater, blackwater or faecal sludge can be treated to an effluent of relatively high quality and apt for the reuse in agriculture (e.g.
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