Performance of an ECSB reactor for high-rate anaerobic treatment of cheese industry wastewater: effect of pre-acidification on process efficiency and calcium precipitation

An external circulation sludge bed (ECSB) reactor was studied at full-scale (1,000 m 3 ) during anaerobic treatment of cheese and other dairy products industry wastewater (CWW). The latter was characterized by a high calcium content, therefore the study focused on the potential negative impact that calcium may have in the long-term. The degree of CWW acidification (25 and 40%) on ECSB reactor performance was evaluated over a wide range of organic loading rates from 5 up to 18 kg m 3 d -1 , while process efficiency and calcium precipitation were examined in detail. Independently of the operating conditions, the volatile suspended solids content of the anaerobic granular sludge, as well as its calcium content, remained stable along the ECSB reactor operation, indicating that there was no calcium build up in the biomass. The results of this study demonstrate that the ECSB design seems to be particularly suitable to treat calcium-rich wastewater that is probably due to the fact that in this system CaCO 3 precipitates in the bulk liquid of the external circulation tank and not the biomass present in the main reactor, and that the CaCO 3 crystals are washed-out from it due to the high upflow velocity applied to the system (5 m h -1 ).