Comparison of the anaerobic digestion of activated and aerobic granular sludges under brackish conditions

The presence of salts in wastewaters is common in coastal areas, where leakages of salty water occur into the sewage systems, but also when the industrial process involves the use of salts or seawater. Aerobic granular systems can be suitable to treat brackish wastewaters as an alternative to activated sludge ones since the high biomass concentrations make up for the decrease of the activity due to salt presence. These granular systems have also as advantage the lower amount of sludge generated in comparison with the activated sludge ones but, in both cases, the excess of sludge produced needs to be post-treated. Anaerobic digestion is normally used to reduce the concentration of solids and to produce biogas. To the present, the effect of saline conditions in anaerobic digestion was preferentially studied in systems treating wastewaters but scarcely revised when treating sewage sludge.The aim of this work is to determine the feasibility, under brackish conditions, of the anaerobic biodegradability of the aerobic granular sludge (AGS) and to compare the results with the anaerobic digestion of flocculent activated sludge (FLAS). The results showed that the biodegradabilities of AGS (32%) and FLAS (27%) were similar, which indicates that the aggregation state of the substrate did not limit the process. Brackish conditions led to a concentration of sodium and free sulphide (FS) inside the reactor in the range of 2.1-5.2gL^-1 and 38-93mgL^-1 respectively, that are within the inhibitory levels reported for the anaerobic treatment of wastewaters. However, the biomethane potential and biodegradability obtained in this work indicated that neither sodium nor free sulphide had an inhibitory effect at these concentrations. The content of H₂S in the biogas was relevant (1.5-3.8%) and its pre-treatment is needed if the biogas is going to have any further use, i.e., in energy production.