The successful seawater toilet flushing practice in Hong Kong has enabled us to develop a novel S̲ulfate reduction, A̲utotrophic denitrification and N̲itrification I̲ntegrated (SANI^®) process for saline sewage treatment with minimal sludge production. Based on SANI^® process, this study is focused on developing a new process for co-treatment of simple wet flue gas desulfurization (WFGD) wastes (alkaline absorption of sulfur dioxide in coal burining power plant) with freshwater sewage in inland areas, as well as conducting series of tests to clearly specify the sulfur bio-transformation during this sulfur cycle-based wastewater treatment process.

A lab-scale trial of the Mixed Denitrification–SANI process was conducted with synthetic freshwater sewage under different dosing sch...[ Read more ]

The successful seawater toilet flushing practice in Hong Kong has enabled us to develop a novel S̲ulfate reduction, A̲utotrophic denitrification and N̲itrification I̲ntegrated (SANI^®) process for saline sewage treatment with minimal sludge production. Based on SANI^® process, this study is focused on developing a new process for co-treatment of simple wet flue gas desulfurization (WFGD) wastes (alkaline absorption of sulfur dioxide in coal burining power plant) with freshwater sewage in inland areas, as well as conducting series of tests to clearly specify the sulfur bio-transformation during this sulfur cycle-based wastewater treatment process.

A lab-scale trial of the Mixed Denitrification–SANI process was conducted with synthetic freshwater sewage under different dosing schemes of sulfite and/or sulfate in order to confirm the feasibility of co-treatment of simple WFGD with freshwater sewage. The results indicated that biological reduction of mixed sulfite with sulfate generates sulfide, thiosulfate and organic residues. These electron donors induced three simultaneous denitrifying reactions: autotrophic denitrification (AD) on sulfide, AD on thiosulfate, and heterotrophic denitrification (HD), namely the mixed denitrification (MD) in the anoxic bioreactor. This multiple-denitrification process increased the denitrification activity from 0.10 (sulfide-based AD) to 0.55 kg NO₃^--N/kg VSS/d because of such efficient MD. Hence, the start-up of this bioreactor or refered to as the anoxic up-flow sludge bed (AnUSB) was completed within two weeks. The results also show a high biomass-specific organic removal rate (0.26 kg COD/kg VSS/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% of alkalinity for operation of simple WFGD. Therefore, the MD–SANI process is potentially feasible in co-treatment of SWFGD wastes with freshwater sewage.

The major SRB species in the sulfate and sulfite co-reducing anaerobic bioreactor of the process were dominated by Desulfobulbus and Desulfomicribium according to 454- pyrosequencing analysis of the SRUSB sludge. Thauera- and Thiobicillus-like species were the major denitrifying bacteria in the AnUSB, which possibly carried out the AD and HD respectively for nitrogen removal through MD. Different sulfur compounds as electron acceptors were confirmed to have a significant effect on the SRB group composition in SRUSB. 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 the anaerobic reactor. It induced a fast AD reaction, revealing that thiosulfate (S₂O₃^2-) can play an important role in the MD–SANI process. Thiosulfate is produced from the co-reduction of sulfate and sulfite through biological pathway at an optimum pH of 7.5 for organics removal. S₂O₃^2- 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 findings obtained from this research improve the understanding of the role of thiosulfate in organic and nitrogen removal in the MD–SANI process.