Development of mixed denitrification (MD)-SANI process

The successful seawater toilet flushing practice in Hong Kong has enabled us to develop anovel Sulfate reduction, Autotrophic denitrification and Nitrification Integrated (SANIRTM)process for saline sewage treatment with minimal sludge production. Based on SANIRTMprocess, this study is focused on developing a new process for co-treatment of simple wet fluegas desulfurization (WFGD) wastes (alkaline absorption of sulfur dioxide in coal buriningpower plant) with freshwater sewage in inland areas, as well as conducting series of tests toclearly specify the sulfur bio-transformation during this sulfur cycle-based wastewatertreatment process.;A lab-scale trial of the Mixed Denitrification--SANI process was conducted with syntheticfreshwater sewage under different dosing schemes of sulfite and/or sulfate in order to confirmthe feasibility of co-treatment of simple WFGD with freshwater sewage. The results indicatedthat biological reduction of mixed sulfite with sulfate generates sulfide, thiosulfate andorganic residues. These electron donors induced three simultaneous denitrifying reactions:autotrophic denitrification (AD) on sulfide, AD on thiosulfate, and heterotrophicdenitrification (HD), namely the mixed denitrification (MD) in the anoxic bioreactor. Thismultiple-denitrification process increased the denitrification activity from 0.10 (sulfide-basedAD) to 0.55 kg NO3--N/kg VSS/d because of such efficient MD. Hence, the start-up of thisbioreactor or refered to as the anoxic up-flow sludge bed (AnUSB) was completed within twoweeks. The results also show a high biomass-specific organic removal rate (0.26 kg COD/kgVSS/d) and a low sludge yield (0.03 kg VSS/kg COD). Besides, the whole process not only provided sufficient alkalinity for AD and the nitrification reaction, but also recovered 35% ofalkalinity for operation of simple WFGD. Therefore, the MD--SANI process is potentiallyfeasible in co-treatment of SWFGD wastes with freshwater sewage.;The major SRB species in the sulfate and sulfite co-reducing anaerobic bioreactor of theprocess were dominated by Desulfobulbus and Desulfomicribium according to 454-pyrosequencing analysis of the SRUSB sludge. Thauera- and Thiobicillus-like species werethe major denitrifying bacteria in the AnUSB, which possibly carried out the AD and HD respectively for nitrogen removal through MD. Different sulfur compounds as electronacceptors were confirmed to have a significant effect on the SRB group composition inSRUSB. Desulfomicrobium-like species are dominant in sulfite reduction and Desulfobulbus-like species are dominant in sulfate reduction.;Thiosulfate was found to be the major intermediate during the sulfate/sulfite reduction in theanaerobic reactor. It induced a fast AD reaction, revealing that thiosulfate (S2O32-) can play animportant role in the MD--SANI process. Thiosulfate is produced from the co-reduction ofsulfate and sulfite through biological pathway at an optimum pH of 7.5 for organics removal. S2O32- may disproportionate to sulfide and sulfate during both S2O3 2- reduction and oxidation,which was further confirmed by the presence of Desulfovibrio-like species. These findingsobtained from this research improve the understanding of the role of thiosulfate in organic andnitrogen removal in the MD--SANI process.