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 fl...[ Read more ]

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 (SANI^®) 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 SANI^® lab-scale system in the sludge production and the COD and nitrogen removals under various operating conditions.

In order to further demonstrate the SANI^® 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 SANI^® 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 SANI^® process eliminates 90% sludge waste, saves 35% energy and reduces 36% greenhouse gas (GHG) emission. This research work has confirmed that the SANI^® 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.

A new steady-state model was further developed in this research for evaluating the SANI^® pilot plant. The model comprised: 1) a COD-based anaerobic hydrolysis kinetics to determine the removal of biodegradable COD and anaerobic hydrolysis rate in BSR under different hydraulic retention times (HRTs) and sludge retention times (SRTs), 2) elements (C, H, O, N, P, S), COD and charge mass balances for prediction of the concentrations of alkalinity (H₂CO₃^* alkalinity+H₂S alkalinity), COD, sulfate, sulfide, nitrate and free saline ammonia in the SRUSB, the anoxic autotrophic denitrifying reactor as well as the aerobic autotrophic nitrifying reactor of the plant, and 3) an inorganic carbon (HCO₃^-) and sulfide (H₂S/HS^-) mixed weak acid/base chemistry for pH prediction. Through characterization of the sewage organic matter and determination of the anaerobic hydrolysis kinetic rate and other relevant parameters, the steady-state model was validated successfully for application in the SANI^® process. The model predictions agreed well with the experimental data of the pilot-scale trial, demonstrating that the model developed from this research can explain the causes and conditions for the minimal sludge production in the SANI^® pilot plant.

An integrated biological kinetic model was finally developed and applied to simulate the SANI^® pilot plant. This kinetic model allows for five organic types, i.e. volatile fatty acids (VFA), fermentable biodegradable soluble organics (FBSO), biodegradable particulate organics (BPO), unbiodegradable particulate organics (UPO) and unbiodegradable soluble organics (USO), with different compositions of the incoming saline sewage. The kinetic model predictions (restricted to the steady state conditions) conformed favorably to the experimental measurements and the steady state model predictions, which validated the kinetic model. Based on the system optimization by this kinetic model, the SANI^® pilot plant showed the potential to futher increase the volumetric loading rate and reduce the HRT simultaneously. The optimum values of recycle rateio from aerobic to anoxic bioreactor can be selected between 2.5 and 2.75, such as 2.5 in the operation of SANI^® pilot plant.