Study On Coupling Anaerobic Baffled Reactor And Membrane-Aerated Biofilm Reactor

Nitrogen pollution is a key eutrophication factor in receiving water, so the removal of nitrogen has ever been a hot topic in the field of wastewater treatment for years. Generally, the performance of nitrogen removal process not only requires the coupling of large quantities of special microorganisms, but also depends on very strict operation conditions. It accounts for the difficulty in nitrogen removal in high-strength wastewaters with nitrogenous organic pollutants, and this has been particularly emphasized as an important issue in the worldwide wastewater treatment. Recently, the aerobic and anaerobic combination technology has been widely used for municipal and industrial wastewater treatment in the world, which still has problems such as system complexity, large footprint, high operating costs, and requirement of extra water recycling equipments within the process etc.In this study, an anaerobic baffled reactor (ABR) and membrane-aerated biofilm reactor (MABR) were combined for nitrogen removal from synthetic wastewater. Two processes were separately started up. The aerating membrane module was installed into a compartment of anaerobic baffled bioreactor to form the Hybrid MAB-ABR (HMABR) and its construction was based on the consistent anaerobic condition of the outside anaerobic biofilm on the aerated membrane and anaerobic baffled reactor. In this hybrid process, dependent on the biological phase-separation of acidate bacteria and methanogens bacteria in the ABR and the unique stratification of the aerating membrane biofilm into aerobic nitrifying bacteria and anaerobic denitrifying bacteria, the possible coupling was developed, rather than the competition and inhibition in different pollution removal processes. Thus, the improved simultaneous removal of carbon and nitrogen for high-strength nitrogenous organic pollutants was realized in a single reactor.(1) ABR was started up with anaerobic granule sludge as inoculation under low-loading start-up conditions. After the initial start-up phase of 15 days, pseudo-steady state of the ABR was reached. The loading rate was increased by increasing influent organic carbon concentration for two times, and the COD removal efficiency was still kept above 90%. When the influent COD concentration was 1800 mg/L, the effluent VFA concentrations of the three compartments were kept steady, which were 673 mg/L, 148 mg/L and 24 mg/L in average, and the total volumes of biogas produced in these three compartments were 1.13 L/d, 2.57 L/d and 0.71 L/d, respectively. The experiment results show the good performance of pollution removal in the ABR. By the chemical analysis of water quality and the microscope observations of granules surface, the biological phase-separation of acidate bacteria and methanogens bacteria was validated in the ABR. (2) Tubular carbon-membrane, wrapped up with non-woven materials as support media for biofilm was applied as membrane module for the aerating membrane by its virtue of better bacteria adhering capacity than other materials. Either air or pure oxygen was pumped into membrane lumen of the aerating membrane to examine the operation performance of the MABR. When pseudo-steady states were reached, simultaneous nitrogen and carbon removal were realized with highest removal efficiencies of 83.6 % and 81.6 % using air as aerating gas, while 82.4 % and 84.2 % using pure oxygen as aerating gas, respectively. The biomass unique stratification structure was also formed due to the gradient of oxygen concentration. The region near the carbon membrane shell side was favorable for aerobic autotrophic bacteria due to sufficient oxygen supply and organic carbon-depletion conditions; whereas the region near bulk liquid was favorable for growth of heterotrophic denitrifying bacteria. With the increased loading rate, the excess growth of biomass acts as a diffusive barrier for COD and ammonia, and eventually deteriorates nitrogen removal efficiency. Afterward, the air aerating membrane module and anaerobic baffled reactor were coupled to form HMABR for the simultaneous removal of nitrogenous and carbonaceous organic pollutants.(3) The HMABR has excellent COD removal performance, the average effluent concentration and removal efficiency was 51 mg/L and 96.8 %, respectively. When organic loading rate was increased by 50 %, the effluent COD concentration was still below the level of 60 mg/L, indicating its good capability of counteracting influent organic loading fluctuation. The HMABR also demonstrated good nitrogen removal performance with the average TN removal efficiency of 80.1 % during the steady state. At the same time, due to the decreased COD concentration and increased nitrate concentration in the third compartment after installing the membrane module, the biogas volume and methane content in the third compartment were decreased, resulting in the steady and excellent effluent quality. The experiment results show that the biofilm in the hybrid reactor kept the unique configuration of inner aerobic, outer anaerobic. In this case, when the inner aerobic nitrifying bacteria were inactive, the nitrogen removal performance deteriorated correspondingly.(4) A mathematical simulation model of MAB was founded by employing the simulation program of AQUASIM 2.0 to validate that the aerobic autotrophic bacteria were mainly distributed in the inner biofilm, while anaerobic heterotrophic bacteria were mainly distributed in the outer biofilm. Sensitivity analysis results indicated that values of the maximum specific growth rate, oxygen saturation constant and the stoichiometric parameters were the most significant in simulating concentration change of inlet substrates, while other constants were of minor significance in terms of predictive quality of the model. In most runs, the modeling results indicated that the C/N ratio and biofilm thickness were important factors for the biofilm operational regime. When C/N ratio was too low, the amount of COD was not sufficient for heterotrophic bacteria, causing deterioration of nitrogen removal efficiency due to lack of COD. On the other hand, too high C/N ratio resulted in growth inhibition of autotrophic bacteria. Moreover, the high biofilm thickness would be a diffusive barrier for COD and ammonia, and too thin biofilm thickness reduced the aerobic or anaerobic region available for bacteria, which also eventually deteriorated nitrogen removal efficiency. The mathematical modeling has the potential to be used for optimizing configuration design, and practical application of HMABR process.