Activated sludge process improvements for the treatment of a municipal landfill leachate | Amelioration du procede par boues activees pour epurer un lixiviat de decharge d'ordures menageres

The main goal was to improve the common activated sludge process using three different means (FeCl3, preselectors and the PACT) for the treatment of a municipal landfill leachate. This waste water threatens water quality mainly by its high COD (1,000 to 6,000 mg O2 per l and sometimes more) and Kjeldahl nitrogen content (often between 0.5 and 1 g TKN per l). Our goal was achieved by a long experimental work (165 days), on four miniaturized activated sludge plants. First of all, we assessed the yields of the leachate purification by the activated sludge alone. The COD removal was above 75 per cent when the biological load was less than 0.15 g COD per g MLSS per day and when the biodegradability (DBO5/DCO) of the percolate was above 0.4. In these conditions, the COD level at the outlet was just below the discharge standard of 450. The nitrate standard, however, could not be reached (max 45 mg N-NO3 per l) because the 80 per cent nitrification yield led to approximately 130 mg N-NO3 per l in the effluent. This first step also enabled us to measure precisely the two parameters of Mc Carty's model. In a second step we had to cope with a zoogloeal bulking problem. First we reduced the bulking by adding FeCl3, whereas a long term progressive elimination of the Zoogloea sp. filaments was achieved by means of a small anoxic basin (preselector) added upstream of the aerated tank. FeCl3 slightly improved COD removal but impeded nitrification in a reversible way. During phase 3, we studied the PACT (Powdered Activated Carbon Treatment). By adding powdered activated carbon to the sludge we recorded a significant 3 per cent increase of the COD removal. The activated carbon content (1.35 g per l reactor) and consumption (0.16 g per l treated leachate) were low. We would have expected further improvement if the moderate leachate biodegradability (DBO5/DCO = 0.42) had not suddenly increased (0.49) just before the second phase. However, we confirmed the 3 advantages of the preselectors which are : the bulking inhibition, the denitrification of 30 per cent of the leachate nitrogen, the removal of about 68 per cent of the COD. Thanks to the preselectors all discharge standards were respected. Our study results led to recommendations for the design of a waste water treatment plant for the leachate investigated.