Stonecutters Island Sewage Treatment Works has one of the world largest chemically enhanced primary treatment (CEPT) facility that treats on average 1.8 million m³ of sewage every day. High removal efficiency of 70 % and 80 % are obtained for biological oxygen demand (BOD) and suspended solid (SS), respectively, with a 2-fold higher overflow rate of 60 m/d compared to conventional primary treatment. Despite of the high effectiveness of CEPT, it is not effective in removing ammonia and soluble BOD. Partial Nitrification-Anammox (PN/A) process is a promising nitrogen removal technology to be utilized in downstream treatment. However, BOD:N ratio of 2.0 – 2.5 in CEPT effluent could potentially deteriorate the autotrophic PN/A process. On the other hand, SANI process is developed fo...[ Read more ]

Stonecutters Island Sewage Treatment Works has one of the world largest chemically enhanced primary treatment (CEPT) facility that treats on average 1.8 million m³ of sewage every day. High removal efficiency of 70 % and 80 % are obtained for biological oxygen demand (BOD) and suspended solid (SS), respectively, with a 2-fold higher overflow rate of 60 m/d compared to conventional primary treatment. Despite of the high effectiveness of CEPT, it is not effective in removing ammonia and soluble BOD. Partial Nitrification-Anammox (PN/A) process is a promising nitrogen removal technology to be utilized in downstream treatment. However, BOD:N ratio of 2.0 – 2.5 in CEPT effluent could potentially deteriorate the autotrophic PN/A process. On the other hand, SANI process is developed for simultaneous organic and nitrogen removal in sulfate-containing saline sewage. Nevertheless, SANI process would encounter hampered denitrification capacity when treating low BOD:N ratio CEPT effluent. Given the constraints of the above two processes, the aim of this MPhil study is to develop a novel Sulfidogenesis Anammox (SMOX) process, that integrates PN/A cycle into SANI process, to improve the process stability and nitrogen removal efficiency.

Firstly, a high-rate anaerobic sulfidogenic moving bed biofilm reactor (MBBR) was established for organic removal without internal recirculation. The effects of organic loading rate (OLR) and hydraulic retention time (HRT) on organic removal efficiency were investigated. Organic removal efficiency improved with OLR from 0.44 to 0.89 kg total organic carbon (TOC)/m³/d, but would deteriorate after reaching the hypothetic maximum sulfidogenic capacity of 0.92 kg TOC/m³/d. On the other hand, sulfate reducing bacteria (SRB) community remained stable when HRT was lowered from 2 hr to 0.67 hr. However, further lowering HRT to 0.5 hr shrank the SRB community from 2718 to 1863 operational taxonomic unit (OTU) counts in biomass. Besides, MBBR configuration had intrinsic advantages that sludge flotation and sludge clogging could be prevented. Afterwards, MBBR-based PN-SANI system was developed to treat synthetic CEPT effluent. PN-SANI system was able to achieve an overall organic and total inorganic nitrogen (TIN) removal efficiency of 79.4 ± 6.2 % and 32.9 ± 6.0 %, respectively. In order to improve the denitrification capacity of PN-SANI system, partial nitrification was aimed to be established inside nitrifying reactor for potential Anammox activity. With both residual ammonia concentration (RAC) (13.4 ± 3.2 mg N/L) and DO-to-RAC ratio (0.17 ± 0.05 mg O₂/mg N) control, a high nitrite-to-NOx ratio (66.2 ± 10.9 %) was obtained under relatively high DO concentration (2.3 ± 0.4 mg O₂/L). To promote the transformation of nitrifying reactor into a partial nitrification-anammox (PN/A) reactor, nitrifying reactor was separated from PN-SANI system and fed with organic-free wastewater. Immediately, high nitrite accumulation ratio of 82.0 ± 8.6 % was accomplished under DO-to-RAC ratio of 0.19 ± 0.06 mg O₂/mg N. Subsequently, enrichment of anaerobic ammonia oxidizing bacteria (AnAOB) was boosted by elevating the temperature to 29.4 ± 0.6 ºC. Remarkably, the abundance of marine AnAOB species, Candidatus Scalindua, increased substantially from 172 to 2802 OTU counts under mainstream condition. Lastly, the transformed PN/A reactor was integrated into the novel SMOX system. Installation of Anammox pathway alongside with SOAD was found to be successful, and SMOX process was proved to be feasible. With same synthetic CEPT effluent, an overall organic and TIN removal efficiency of 78.0 ± 6.2 % and 57.8 ± 11.2 % were achieved, respectively. A 3.5-fold higher TIN removal rate was obtained by SMOX system compared to prior PN-SANI system. The average TIN removal rate of SOAD in R2 and TIN removal rate of Anammox in R3 were 0.13 ± 0.05 kg N/m³/d and 0.29 ± 0.09 kg N/m³/d, respectively. For our SMOX system, the total TIN removal showed clear dependency on DO concentration and nitrate production rate of NOB and was crucial for optimizing the total TIN removal. These two factors controlled the balance between nitrite and nitrate availability for Anammox and SOAD, thus, maximizing the TIN removal via both pathways.