Anaerobic Membrane Bioreactors (ANMBR) for Sludge Digestion: TWAS Versus Mixed Sludge

A comparison of the treatment of thickened waste activated sludge (TWAS) and mixed sludge (primary sludge (PS) and TWAS) with anaerobic membrane bioreactors (AnMBR) was conducted. The comparison was conducted at an SRT of 30 days and an HRT of 15 days that resulted in average reactor TS concentrations of 2% for the TWAS feed and 3.4% for the mixed sludge feed. The volatile solids reduction (VSr), chemical oxygen demand reduction (CODr) and methane yield with the mixed sludge feed were respectively 16%, 18% and 1%, greater than with the TWAS feed. The average daily fluxes were 35 and 20 L/m2·h (LMH) at corresponding average transmembrane pressures of 29 and 20 kPa, when TWAS and mixed sludge were treated respectively. The reduced flux with mixed sludge (31%) corresponded to a 40% increase (18 g/L to 30 g/L) of TS in the AnMBR. The average fouling index (FI) was 20% higher when treating mixed sludge as compared to TWAS. A cleaning strategy that included four sequential steps: (1) a hot water (55–60 °C) flush; (2) an abrasive clean using 25 mm diameter manufacturer-supplied sponge balls; (3) an alkali clean (NaOH (aq) solution, pH=10, 55–60 °C) and (4) an acid clean (citric acid, C6O7H8 (aq) solution, pH=2, 55–60 °C) was evaluated. The water flush and acid clean proved to be the most effective components of the sequence.