Anaerobic biodegradation and dewaterability of aerobic granular sludge

Abstract

BACKGROUND Although a growing number of full‐scale wastewater treatment plants have already been constructed and operated with aerobic granular sludge (AGS), only limited information is available about further post‐treatment, in particular about sludge stabilization and dewaterability. The aim of the present study was to investigate the biodegradation and methane yield of AGS by the use of anaerobic laboratory‐scale reactors operated under mesophilic conditions and hydraulic retention times of 25 and 40 days. RESULTS The methane yield of AGS was ca 260 mL gVSS−1 (volatile suspended solids) and thus slightly increased compared to that of suspended activated sludge (SAS; 240 mL gVSS−1). A clear difference between the methane yield was found for separated pure granules (500 μm), which was ca 50% higher compared to that for SAS. VSS removal of AGS during anaerobic degradation was ca 52%. Dewaterability of AGS after anaerobic digestion was slightly lower compared to SAS. Extracellular polymeric substance (EPS) extraction and fluorescence analysis showed tryptophan contents which were almost twice as high compared to the EPS extracted from SAS. CONCLUSIONS Overall, the anaerobic digestion of AGS was found to be a suitable stabilization strategy with the benefit of recovering energy in the form of methane. Further tests are needed to validate the decreased dewatering behaviour with full‐scale applications. The presented approach for tryptophan measurement allows the transfer of qualitative results from a fluorescence analysis into quantitative values and could be further adapted for identifying relevant EPS constituents.