A pilot–scale investigation was carried out to evaluate the parameters affecting biofilm accumulation and sludge production. The behaviour of a cross flow (CF) plastic medium filter (4.3 m of Plasdek B 27) was followed over an eight month period. Four hydraulic loadings, from 2.5 to 3.6 m3/m2h, (organic loadings from 1.5 to 6.2 Kg BOD5/m3d) were experimented. Medium temperature, BOD5 influent and hydraulic loading were determinants for biofilm accumulation and sludge production. The specific oxygen uptake rate (SOUR) of waste sludge was found related to inlet BOD5. Furthermore a relationship between median retention time (MRT) and filter biomass was obtained; this relationship gave also a good capacity to interpret MRT for low–rate filters.

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