Sludge filterability and dewaterability in a membrane bioreactor for municipal wastewater treatment

The physical properties of excess sludge wasted from a large pilot scale membrane bioreactor (MBR) have been routinely monitored over almost two years. A statistical analysis highlighted the significant impact of temperature on the capillary suction time and sludge filterability, due to the increase of organic matter in the liquid phase. Suspended solids have resulted to be the most important component affecting sludge filterability, although the impact of colloids and solutes increased when temperature decrease, thus confirming the generally worse characteristics of sludge in such conditions. Conditioning and dewatering test have been performed on a pilot scale fixed volume recessed plate filter press. Six different chemicals were used for sludge pre-conditioning and, for each additive, three dosages were tested in the range 5–25 gpolymer kgMLSS− 1. After about sixty filtration trials at three different pressure values (7, 11 and 15 bar), the kind of polymer seem to be the most important factor influencing the final cake-dryness, with less evident impact for dosage and operational pressure. Finally, when performed on the aerobically digested excess sludge wasted from a conventional activated sludge plant, the filtration tests show no differences with the MBR sludge.