AEROBIC DOMESTIC WASTE-WATER TREATMENT IN A PILOT-PLANT WITH COMPLETE SLUDGE RETENTION BY CROSS-FLOW FILTRATION

An aerobic wastewater treatment pilot plant with cross-flow filtration was operated for more than 300 days to examine whether reduced sludge production and stable treatment performance can be achieved when sludge is completely retained. The volumetric loads ranged between 0.9 and 2.0 g COD . l(-1). day(-1). Technical observations were: the oxygen transfer rate became poor at high sludge concentrations; membrane capacities declined but could be mostly sufficiently restored by cleaning. Sludge was hardly produced when the mixed liquor suspended solid (MLSS) concentration had increased to 40-50 g . l(-1). Then, the sludge load was only 0.021 g COD . g MLSS(-1). day(-1) and only 6% of the carbon supplied was assimilated. Non-volatile compounds hardly accumulated as the fraction of inorganic compounds in sludge increased from 21.6 to 23.5% during the last 200 days, whereas the carbon, phosphor and kjeldahl nitrogen contents were stable. After 300 days the content of polluting trace elements, such as mercury, lead and cadmium, were similar to that of a conventional treatment plant supplied with this wastewater. Carbon and kjeldahl nitrogen removal was always quite satisfactory. Carbon was always removed for more than 90% and kjeldahl nitrogen that was not assimilated was completely nitrified at all times. The nitrification capacity at 30 degrees C was constantly around 0.2 mmol . g MLSS(-1). h(-1), which shows that the viability of the nitrifying population did not cease. In addition, up to 40% of nitrogen supplied was lost as a result of denitrification. Hence stable treatment performance and a very low sludge production can be achieved when complete sludge retention is applied at high hydraulic loads.