Enhanced Denitrifying Phosphorus Removing Of Low Strengh Municipal Wastewater Treatment

Eutrophication is a common environmental problem that the current world isencountering. Development of phosphorus and nitrogen removal processes with highefficiency and low power consumption can effectively reduce effluent nitrogen,phosphorus and other nutrients discharging to water bodies and thus slowdowneutrophication, if not eliminate it; Since carbon is normally insufficient forconventional BNR processes treating low strength municipal wastewater, it is urgentto develop high efficient carbon utilization BNR processes too.The parallel AN/AO (anaerobic-anoxic/ anaerobic-aerobic) process wasproposed on basis of existing processes which used denitrifying phosphorus removingbacteria (DPB); its main body can be partitioned into six square compartments, whichcan be operated in different modes. The process is simple, with small lay-out area andeconomic competitive investment. DPB is enriched in the process as much as possibleto guarantee phosphorus and nitrogen removal with high efficiency and low powerconsumption. There are two main configurations and three operating modes of theprocess, and are designed with the principle of enriching DPB with longer anaerobichydraulic retention time and higher proportion of anoxic ratio in the system.Major optimization results of the parallel AN/AO process were as follows:1) Optimized parameters for configurationâ… are: Influent flow ratio (AN/AO) is12:8; SRT are 15 days; total nominal HRT lasts for six hours; internal retum ratio is200%; external return is 25%. In optimal operating conditions, averageconcentration of effluent COD, NH₃-N, TN, and TP are 49.1 mgL^-1,0.4 mgL^-1,14.8mgL^-1,and 0.8 mgL^-1 with removal efficiency of 74%, 99%, 68%, and 83%,respectively.2) Main optimized parameters for configurationâ…¡are: external return rate is100%; internal return rate (from the anaerobic zone to the anoxic zone) is 100%; andthe intemal anoxic-anaerobic return point is locating in anoxic reactor 1 or in anoxicreactor 3. In optimal operating conditions (SRT and HRT assumpted to be 15d and 6hrespectively), average concentration of effluent COD, NH₃-N, TN, and TP are 45.9mgL^-1, 0.2 mgL^-1, 12.7 mgL^-1, and 0.9 mgL^-1 with removal efficiency of 74%, 99%, 74%, and 80% respectively.On basis of operation data with real wastewater, synthetic wastewater was usedto quantitatively study DPB ratio and anoxic phosphorus removal rate in the parallelAN/AO process, internal reasons for "deactivation" and "resurrection" of PAOs,types of carbon source, initial phosphorus concentration on aerobic/anoxicphosphorus removal etc. The results showed that:1) Activity of DPB in the parallel AN/AO process was about 80% that of aerobicPAOs.2) Residual nitrate concentration in the anaerobic tank and residual dissolvedorganic carbon (DOC) concentration in the anoxic tank were directly related to"deactivation" and "resurrection" phenomenon of PAO. Residual nitrate-Nconcentration in the anaerobic tank as low as 2.3 mgL^-1 and DOC concentrations inthe anoxic tank as low as 3.0 mgL^-1 can achieve desired results.3) Glucose and acetate or their mixtures can be good carbon sources for aerobic/anoxic phosphorus removal: in same conditions, anaerobic phosphorus removal ofsodium acetate, mixture of sodium acetate and glucose (with COD contribution ratio1:1), and glucose were about 0.08gP (COD)^-1, 0.07gP (COD)^-1 and 0.04gP (COD)^-1respectively.4) Anoxic phosphorus removal efficiency was very low when the carbon sourcewas lactic acid or mixture of propionic acid and glucose, and there was no anaerobicphosphorus release at all; when the carbon source was propionic acid, anaerobicphosphorus release was normal but phosphorus removal was as low as 68% withintwo hours.5) When the carbon source was mixture of sodium acetate and propionate acid,mixture of sodium acetate and mixture of lactic acid and glucose, COD synthesisefficiencies were 77%, 78% and 76% respectively; anaerobic COD synthesis was lesscomplete when the carbon source was lactic acid, mixture of glucose and propionicacid, and propionic acid with COD synthesis efficiencies of 33%, 36% and 51%respectively.6) With same TP influent concentrations, the ranking in order of phosphorusremoval efficiencies should be 6h＞2h＞4h in terms of anaerobic retention time;while with the same anaerobic retention time, the ranking in order of phosphorus removal efficiencies should be 12 mgL^-1＞6 mgL^-1 in terms of initial influent TPconcentration.Finally, cloning and sequencing of polymer ase chain reaction-denaturinggradient gel electrophoresis (PCR-DGGE) products, FAMEs (Fatty Acid MethylEster) and quinone fingerprint technologies were used complementarily to analyzestructure of microbial community in the parallel AN/AO process; results showedthat:1) Overall microbial community structures were quite similar; however, somebacteria were different in quantity among different reactors of the process. Meanwhile,all the functional zones i.e. the anaerobic zone, the anoxic zone and the aerobic zone,had a small amount of specific microbial population.2) Dominant organisms in the parallel AN/AO process were actinomycosis suchas Rhodococcus sp.,α-proteobacterium such as Bdellovibrio sp., andβ-proteobacterium such as Thiothrix sp. When oxygen was used as electron acceptor,dominant PAOs were Dechloromonas sp., etc.; when nitrate was used as electronacceptor, dominant PAOs were Bdellovibrio sp., etc. It assumed that that microbialpopulation appeared only in the anaerobic-anoxic phosphorus removal operationmode, such as Bdellovibrio sp. were DPBs.3) When pH was changing in the range of 6.5-8.5, the microbial communitystructure of PAOs did not change much.4) There were similarities and dissimilarities features in structure of microbialcommunities among reactors of the parallel AN/AO process when analyzing withFAMEs, quinone fingerprints and SEM technologies, which can complement thePCR-DGGE results.