Combined UASB-MBR For Shortcut Nitrification Coupled With Simultaneous Methanogenesis And Denitrification

The energy crisis and environment deterioration are two of the main problems the world is facing today.Hence,the simultaneous bio-energy production and nutrient removal has been given much attention in the wastewater treatment techniques during the last decade.The aim of this study is to establish a feasible method of treating municipal wastewater in an UASB coupled with an aerobic MBR.The pre-UASB was designed to retain a high carbon level for denitrification via nitrite and for methanogenesis and to proved the low C/N ratio effluent feeding to the MBR to perform the partial nitrification step while the MBR performed the nitrification step and improved the effluent quality.Therefore,the focus of this work was to investigate simultaneous carbon and nitrogen removal by shortcut nitrification and denitrification with a methane production process treating low strength synthetic wastewater in a combined UASB-MBR system.Firstly,a series of experiments was carried out to find out the impact factors and feasibility on a simultaneous methanogenesis and shortcut/complete denitrification process using the adapted anaerobic mixed methanogenic culture.The results suggested that the obvious inhibition of NO_X^--N addition on carbon removal rate started to be observed when the initial NO₂^--N concentration was higher than 15 mg/L and the initial NO₃^--N concentration was higher than 60 mg/L.Whereas,the less carbon requirement for denitrification and faster denitrification rate via NO₂^--N than NO₃^--N still allowed methanogenesis to proceed in the same bioreactor treating wastewater with a lower NO₂^--N concentration,but with a higher TOC/TN ratio under the optimal temperature of 30-35℃and pH of 7.0-8.0.Under the same optimal conditions,a long-term inhibition assay in an up-flow anaerobic sludge blanket(UASB) treating the synthetic NO₂^--N concentration wastewater indicated that the stable carbon and nitrogen removal efficiencies with an acceptable biogas production rate only could be obtained at the low NO₂^--N concentration although the varieties of dominant bacterial community composition were observed in the PCR-DGGE analysis results.At the high NO₂^--N concentration,the higher N₂,but lower CH₄ production was observed eventhough the carbon and nitrogen removal efficiencies maintaining at high level.It is indicated that the technique of simultaneous methane production and nitrogen removal only can be used to treat a low nitrogen strength wastewater. Secondly,a combined bioenergy production and nitrogen removal system consisting of an UASB and MBR was developed to treat synthetic municipal wastewater.The results indicated that the simultaneous methanogenesis and denitrification for treating a low strength synthetic municipal wastewater was technically feasible with more than 98.0%total TOC removal,98.0%NH₄⁺-N removal and 48.1-82.8%TN removal as the recycling ratio increased from 50%to 800%.In this combined UASB-MBR system,the methane production was not affected obviously by the denitrification process.A slight reduction in the methane percentage up to a recycling ratio of 400%was observed due to an increased nitrogen production.However,at the highest recycling ratio of 800%,a rapid decrease in the methane percentage occurred.It was probably due to high DO concentration was introduced into the UASB,which restrained the activity of methanogens and induced more organic substances being converted to CO₂ instead of methane according to the anoxic or aerobic biodegradation theories.Hence,the results indicated that the recycle ratio should be controlled at the range of 200-400%considering the nitrogen concentration and the discharged criterion.At the same time,the effect of aeration rate in the MBR on the combined system performance was investigated.Optimal aeration rate range(2.5-5.0 L/min) was suggested based on the methane production and ratio as well as stable TOC(＞98%) and TN removal efficiencies(＞80%).At the low aeration rate,high methane production and ratio were achieved,but TN removal performance was restrained by the nitrification efficiencies.When the aeration rate was above 2.5 L/min,TN removal performance arrived to around 80%and could not be affected by the nitrification and denitrification efficiencies.However,the methane production decreased slightly follwong the increase in the DO concentration.Hence,in the combined UASB-MBR system,simultaneous methane production and denitrification was technically feasible as an energy efficiently treating way to achieve high effluent quality,minimal biosolids generation,and optimal nitrogen removal efficiencies by controlling the recycling ratio and aeration rate.Thirdly,the NO₂^--N accumulation was considered as the key factor to achieve simultaneous methanogenesis and denitrification using a combined UASB and MBR treating municipal wastewater.Based on analysis results,it was suggested that the internal recirculation pattern,higher pH and lower C/N ratio were the important factors leading to partial nitrification in the combined UASB-MBR.Lastly,membrane fouling tendency,foultant composition,fouling mechanism and the fouling control methods were investigated and researched.The higher fouling rate present in the MBR treating anaerobic digestion effluent.The membrane foulants consisted of sludge flocs,colloidal particles and solutes,especially,the small particles in sludge suspension,those diameter are similar with the membrane pore size,had a strong deposit tendency on the membrane surface.At the same time,the polysaccharide was observed as the main EPS on the membrane surface while the protein is the main contain in the suspernant EPS.Intermittent suction and air sparing were two kinds of effective cleaning strategies to decrease the membrane fouling rate.Under the intermittent suction mode,extending idle time benefited for reducing membrane fouling.This impact was more significant under higher flux. The results of air sparing on membrane fouling tendency showed that small or large aeration intensity had a negative influence on membrane permeability.Low aeration could not remove the membrane foulants from membrane surface effectively.However,the larger aeration intensity resulted in a severe breakup of sludge floes,and promoted the release of colloidal and soluble components from the microbial floes to the bulk solution due to microbial floc breakage,thus caused a rapid loss in membrane permeability.