Trials using submerged biological filters, made of moulded polyethylene and aerated by compressed air, are described. The tests included operation in an activated sludge type plant with sludge recycle as a single stage filter, and as the second, nitrification, stage of a 2-stage system. Observations on the type of biological growth formed and on the need to have good hydraulic conditions to prevent clogging are made. Some design criteria are given.

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