The start-up and saline adaptation of mesophilic anaerobic sequencing batch reactor treating sludge from recirculating aquaculture systems

Abstract

Treatment of sludge from aquaculture is a matter of special importance and there is a need for salt-tolerant biological wastewater treatment to coincide with the development of brackish/marine aquaculture. The aims of the current study were to determine the ability of anaerobic sequencing batch reactor (ASBR) to anaerobically digest sludge from fresh-water recirculating aquaculture systems and the ability of adaptation to low saline conditions. The mesophilic ASBR were evaluated with loading rates between 0.12 and 0.41g chemical oxygen demand (COD)/day at a 20-day hydraulic retention time (HRT) for start-up and with organic loading rates (OLR) of 0.39–0.41gCOD/Lday at a 20-day HRT for saline adaptation. The average removal rate of total chemical oxygen demand (TCOD), total suspended solids (TSS) and volatile suspended solids (VSS) of the ASBR were above 97%, 96% and 91% during the stabilization period of the experimental reactors. The average daily gas production of ASBR was between 0.013 and 0.022L/g TCOD from day 118. A sludge-mass reduction of up to 94±2.3%, TCOD reduction of 44±13% and VSS/SS of 39–70% were demonstrated for the reactor performance during the gas production period. However, the process of gas production was obviously inhibited, presumably by salt, and unstable due to the dissolved COD (DCOD), total ammonium nitrogen (TAN) and alkalinity of the effluents of the experimental reactors and TSS and sludge volume index (SVI) observed within the reactors. The daily gas production was observed to decrease during the saline adaptation period and stopped when the salinity of the effluents was higher than 8.7ppt until the end of the experiment.