Evaluation of the Sulfate Reduction-Autotrophic Denitrification-Nitrification Integrated (SANI) Process for Saline Wastewater Treatment

Hong Kong has practiced seawater toilet flushing since 1950s. It saves 22% of fresh water but inevitably results in high sulfate-laden saline sewage, causing corrosion of pressure sewers and odor (mainly H2S) at sewage treatment plants. The former has been solved by applying anti-corrosion pipes and dosing with super-oxygenated liquids. However, the latter cannot be solved at low cost because there are two major odor sources: 1) primary treatment and 2) sludge treatment. At the same time, current sewage treatment plants in Hong Kong produce 1000 tonnes of dried sludge per day, which has to be incinerated in the near future since the landfill capacity will be surpassed by 2017. In order to solve these problems cost-effectively as well as to maximize the benefits of the seawater toilet flushing practice, we have recently developed a novel biological nitrogen removal process for saline sewage treatment, which is named Sulfate reduction, Autotrophic denitrification and Nitrification Integrated (SANIRTM) process. The key features of this novel process include: elimination of primary treatment and sludge production as well as oxygen demand in organic matter removal. This novel process uses sulfate in the saline sewage originating from seawater toilet flusing to realize biological sulfate reduction (BSR) for effective removal of organic matter under an anaerobic condition. The produced sulfide dissolved completely due to production of sufficient alkalinity, providing adequate electron donors for subsequent autotrophic denitrification. Since all the three major biomasses, sulfate-reducing bacteria (SRB), autotrophs for denitrification and nitrification produce little sludge, total sludge production can thus be minimized significantly. A 500-day lab-scale system has demonstrated that no purposeful withdrawal of excess sludge was needed. In order to verify these results and further understand this feature, a steady-state model was developed in this research based on the mass balances of chemical oxygen demand (COD), nitrogen, sulfur and charge and the stoichiometries of the sulfate reduction, autotrophic denitrification and nitrification. The model predictions agreed well with the measured data on COD, nitrate and sulfate removals, sulfide production, effluent Total Suspended Solids (TSS) as well as the mass balances of COD, sulfur and nitrogen in the system. The model also well explained the performance of the SANIRTM lab-scale system in the sludge production and the COD and nitrogen removals under various operating conditions.;In order to further demonstrate the SANIRTM process in treating real saline sewage, a pilot-scale study was conducted with 10 m3/day of 6-mm screened saline sewage at the Tung Chung Sewage Pumping Station. The SANIRTM pilot plant consisted of a sulfate reduction up-flow sludge bed (SRUSB), an anoxic bioreactor for autotrophic denitrification and an aerobic bioreactor for nitrification. The plant was operated at a steady state for 225 days, during which the average removal efficiencies of both COD and TSS were 87% and no excess sludge was purposefully withdrawn. The total nitrogen (TN) removal efficiency was found to be 55% only, which was attributed to a very high fraction (26.5%) of inert soluble organic nitrogen in the incoming sewage, which mainly originated from the wastewater of the Hong Kong International Airport. Furthermore, a tracer test of the SRUSB revealed 5% shortcircuit flow and 34.6% dead zones in this key reactor of the plant, indicating a good possibility to maximize the treatment capacity of the process for full-scale saline sewage treatment through reactor design optimization. Compared with conventional biological nitrogen removal processes, the SANIRTM process eliminates 90% sludge waste, saves 35% energy and reduces 36% greenhouse gas (GHG) emission. This research work has confirmed that the SANIRTM process not only helps to eliminate the major odor sources originating from primary treatment and sludge treatment, but also promotes saline water supply as an economic and sustainable solution for water scarcity and sewage treatment in water-scarce costal areas. (Abstract shortened by UMI.)...