Biological treatment of high nitrogen-laden wastewater : lab to pilot scale studies and model-based evaluation of nitritation process

Stricter effluent standards have been implemented in order to reduce the receiving water pollution in China. The removal of nitrogen present in wastewaters is essential, since it could lead to severe environmental and health risks, such as eutrophication. Conventional biological nitrogen removal (BNR) process usually consists of oxidation of ammonia to nitrate (nitrification) and subsequent reduction of nitrate to nitrogen gas (denitrification) using biodegradable COD as the electron donor. However, the conventional BNR process is not suitable to treat high nitrogen-laden wastewater due to high oxygen demand, high alkalinity consumption, high carbon source additions and slow reaction rates. This study focuses on the feasibility exploration of nitritation/denitritation in membrane bioreactor (MBR). Moreover, a novel mathematical model was developed base on lab and pilot study. Firstly, a pilot membrane biological reactor (MBR) plant showed a good performance to treat the sludge reject water. The effluent ammonia concentration was below 10 mg N/L when the HRT was about 12 hours with TKN loading of 0.73 kg N/m3. The ASM1 model after calibration accurately describes the COD removal and nitrification processes in the system. Moreover, a lab scale sequencing batch reactor (SBR) was applied to treat synthetic wastewater and landfill leachate for the nitritation investigation. The experimental results showed the SBR was suitable for realizing nitritation for treating high nitrogen-laden wastewater. In the nitritation process, the ammonium loading and nitritation rates can reach 3 and 2 kg NH4+-N/m3/d, respectively, which are 3-5 times faster than the conventional nitrification processes. On the basis of the experimental result, it can be concluded that this nitrite build-up was achieved by the out-competition of NOB due to inhibition by free ammonium (FA) and/or free nitrous acid (FNA). The bath inhibition tests showed that the inhibition constant of FA and FNA for AOB were 50 mg NH3-N/L and 0.4 mg HNO2-N/L, respectively. Furthermore, the feasibility of landfill leachate nitritation/denitritation in MBR has been investigated. The ammonia removal efficiency is up to be 80~90% with the loading rate of 0.47 kg NH4+-N/m3/d. Nitritation was taken place easily by FNA or FA inhibition. The nitrite accumulation degree could be above 80% when FA concentration was above 5 mg/L. In spite of the low C/N ratio of 2 in raw leachate, total nitrogen removal efficiency could be 70~80% and the carbon source requirement was only 2.2-2.6 kgCOD/kgN, which was much lower than the carbon source requirement for complete nitrification and denitrification. However, when the influent C/N ratio decreased to 1:1, total nitrogen removal efficiency was poor, only bout 30%, and the effluent nitrite accumulated. It indicates that an external carbon source must be added in order to improve the total nitrogen removal efficiency when the organic matter in leachate was insufficient. Finally, based on the experiment results, a mathematical model of the nitritation/denitritation was successfully developed, calibrated and validated. This novel model separately considered the inhibitory effects on catabolism and anabolism of AOB and NOB and integrated most physical-chemical processes (such as CO2 stripping), pH variation and the difference of maintenance and decay for the first time. As a result, this model was capable of accurately predicting the main outcomes of the system (ammonium, nitrite, nitrate, and pH). This mathematical model has served as a tool for the upgrading an existing landfill leachate treatment plant. The simulation result showed that the nitritation/denitritation process in the low nitrogen MBR system can be realized by controlling the FNA concentration. The nitrogen loading can be doubled without adding any capital investment