Research On The Performance Of Anoxic/Aerobic Moving Bed Biofilm Reactors For Domestic Wastewater Treatment

Urbanization and increasing urban population leadto increase the wastewatergeneration while reducing the available land area to build new treatment facilities. Eventhe activated sludge process modified numerous times in order to produce higher qualityeffluent, this process can present some shortcomings (sludge bulking, sludge rising, andhigher hydraulic retention time (HRT) requirement) which drastically reduced theefficiency of this process leads to higher reactors volumes, finally end up in large footprint especially when treating wastewater for nutrients such as nitrogen and phosphorus,because the removal takes more time compared to other organic matter.Therefore thereis necessity of finding new treatment method which can produce higher quality effluentwhile having minimum foot print.Moving Bed Biofilm Reactor (MBBR) wasintroduced in order to face those challenges.The idea of the MBBR process is acontinuous flow processwhich combine the two different processes (attached andsuspended biomass) without theirs disadvantageby adding biofilm small high densitypolyethylene (HDPE) carrier elements with a large surface area and a density littlelighter density than waterinto the reactors for biofilm attachment. The small carrierelements kept in movement along with a water stream inside the reactor and themovement can be caused by aeration in an aerobic reactor and by a mechanical stirrer inan anaerobic or anoxic reactor, thus improving the processing performance of thereactor.The main objective of this study is to present the continuous up-flow Kaldnes (k1)MBBRs which is consists of combined cylindrical anoxic/aerobic MBBRs in in theform of overlapping cylinders by placed the anoxic reactor inside the aerobic reactorwith anoxic/aerobic volume ratio equal to1/6under fully nitrification-denitrificationprocess as an alternative and more economy successful method for simultaneousorganic and nutrients removal from domestic wastewater. Nitrogen removal is themajor concern in the design of those MBBRs which were used to treated4m3/d ofdomestic wastewater in Chongqing city at Southwest China.The treatment must besatisfactory to meet with grade B of discharge standard of pollutants for municipalwastewater treatment plant in China (GB/T18918-2002).The anoxic MBBR wasdesigned according to the hanging coefficient (g MLSS/carrier) for the carrier withsafety factor equal to4in order to reduce its required space and achieve the partially denitrification (nitrate oxidation to nitrite) and anammox process which about50%fromammonium is directly oxidized to dinitrogen gas using nitrite as the electron acceptor.The aerobic MBBR was designed to achieve the fully nitrification processes andorganic carbon removal. The phosphorus will be removed according to both anoxic andaerobic processes. The values of the main design parameters of the system were7/1forgas/water ratio,100%for nitrate recycle ratio, and6.2hours for hydrulic retention time.The reactors was operated under fully nitrification-denitrification processes which mustmeet stringent TN limits without sludge returning into the system and only an internalrecycling was performed from aerobic to anoxic reactor.Our MBBRs was built in the DADUKOU wastewater treatment plant which islocated in DADUKOU district at Chongqing city in Southwest China.The system wasconsisted of square primary settling tank (made of PVC1m×1m×1m), steel anoxicreactor (D=0.6m and H=0.9m), and steel aerobic reactor (D=1.2m and H=2m) followedby square final clarifier (made of PVC1m×1m×1m).Kaldnes (K1) media was used as acarrier in both reactors at a media fill ratio equal to50%. The Kaldnes (K1) carrierelements are made of polyethylene (density0.93g/cm3) and shaped like small cylinders(about25mm in diameter and10mm long) with a cross inside to provide sites foractive bacteria attachment in a suspended growth medium. The effective specificgrowth area is500m2/m3at100%filling ratio.The biofilm carrier elements are keptsuspended in the water by air from the diffusors in aerobic reactor and by means ofpropeller mixer in anoxic reactor. The propeller mixer consist of central,2-blade doublestirrer of25cm diameter and with blades placed at20and40cm below top-water level,the stirrer speed was100rpm.The carrier elements are retained by means of small sizedsieve(about2mm opening). Aeration system consisted of4fine bubble membranediffuser (4aeration dishes220mm in diameter) distributed equally on the perimeter ofthe reactor and fixed at the height0.3m from the bottom of the reactor.The domesticwastewater reached to the primary settling tank from the preliminary treatment part inDADUKOU wastewater treatment plant which is responsible for removal of wastewaterconstituents such as rags, sticks, floatable, grit, and grease that may cause operationalproblems with the treatment operations. The anoxic MBBR was continuously receivedthe domestic wastewater from the primary settling tank in the start-up phase and fromboth the primary settling tank and the final clarifier in steady state phase.The experiment tests started in early May2012to the end of March2013,approximately lasted one year. Under normal temperature (T≥150C) we investigated the performance of the both anoxic and aerobic MBBRs under startup phase by operatedboth reactors as a Sequencing Batch Moving Bed Biofilm Reactor (SBMBBR) withoutnitrate recycle for4weeks.During this period the biofilm growth in both anoxic andaerobic MBBRs and the increase of biomass attached on the carriers was observedtogether with a decrease in the biomass concentration in the activated sludge. Therotational speed of100rpm and air flux of1.17m3/h (gas/water ratio of7/1) weresuitable for biofilm growth and did not caused any detachment of the biomass from thecarriers. At the end of4thweek the concentration value for MLSSTotal(MLSSSuspension+MLSSAttach) in both anoxic and aerobic MBBR reach to2459mg/L and2914.2mg/Lrespectively and the pilot plant was operated under continuous operation mode athydraulic retention time (HRT) of6.2hours, with nitrate recycle ratio equal to100%and gas/water ratio equal to7/1for1week, getting prepared from the main start-up.During start-up phase the pH was found to be strongly dependent on the dissolvedoxygen concentration in both anoxic and aerobic MBBRs and the relationship betweenthem is an inverse relationship. Dissolved oxygen in the reactors is directly related tothe activities of microorganisms state.The more stable period of the PH and DO in theanoxic MBBR was from17thday-35thday, whereas the pH value was maintainedbetween7.5-7.74and the DO between0.13mg/L-0.17mg/L,while the more stableperiod in aerobic MBBR was29thday-35thday, whereas the pH value was maintainedbetween7.44-7.45and the DO between4.34mg/L-4.51mg/L.Both anoxic andaerobic MBBRs achieved to the steady performance state after passing17days fromstarting-up,whereas anoxic and aerobic MBBRs efficiency in COD removal were stablein range54.432%and95.67%respectively, while anoxic and aerobic MBBRsefficiency in NH4+-N removal were stable in range14.14%and98.53%respectively.Under normal temperature (T≥150C) we examined influence of the main designparameters of the MBBRs (Gas/water ratio, Nitrate recycle ratio, and Hydrulicretention time) on the process performance of the total system and found the optimalvalues of those parameters which give the best results meet with reuse Standards(GB/T18918-2002-B) by operated the MBBRs under continuous flow for11weeks.At the end of5thweekthe effect of gas/water ratio on biological nutrients removal fromdomestic wastewater was investigated by operated the reactors under5differentgas/water ratio ranging from5/1to24/1(5/1,7/1,10/1,14/1,and24/1) by exchanged thevalue of this parameter every one week with internal recycle ratio and hydraulicresidence time (HRT) equal to100%of inflow rate and6.2hours respectively.The experiment results showed that optimum value of the gas/water ratio for simultaneousorganic carbon and nutrients removal was equal to7/1. In this gas/water ratiotheaverage COD, NH4+-N, TN, and TP effluent results (18.1mg/L for COD,0.53mg/L forNH4+-N,14.38mg/L for TN, and0.34mg/L for TP) could meet with grade A ofdischarge standard of pollutants for municipal wastewater treatment plant in China(GB/T18918-2002) with average removal efficiencies of92.67%,98.83%,71.37%and90.49%for COD, NH4+-N,TN and TP respectively, while the average dissolvedoxygen concentration (DO) in aerobic and anoxic MBBRs were4.49mg/L and0.16mg/L respectively. At the end of10thweek the effect of nitrate recycle ratio onbiological nutrients removal from domestic wastewater was investigated by operatedthe reactors under3different nitrate recycle ratio ranging from50%to150%(50%,100%, and150%) by exchanged the value of this parameter every one week.During this operation mode, the MBBRs was operated under the optimal value ofgas/water ratio which equal to7/1and hydraulic retention time (HRT) equal to6.2hours.The results illustrated that optimum value of the nitrate recycle ratio forsimultaneous organic carbon and nutrients removal was equal to100%. In this nitraterecycle ratio the average COD, NH4+-N, TN, and TP effluent results (17.63mg/L forCOD,0.49mg/L for NH4+-N,13.25mg/L for TN, and0.30mg/L for TP) could meetwith grade A of discharge standard of pollutants for municipal wastewater treatmentplant in China (GB/T18918-2002) with average removal efficiencies of92.16%,98.84%,71.23%and91%for COD, NH4+-N,TN and TP respectively, while theaverage dissolved oxygen concentration (DO) in aerobic and anoxic MBBRs were4.35mg/L and0.19mg/L respectively.Finally at the end of13thweek the effect of hydraulicretention time on biological nutrients removal from domestic wastewater wasinvestigated by operated the reactors under3different hydraulic retention time (HRT)ranging from4.95to8.25hrs.(4.95hrs.,6.2hrs., and8.25hrs.) by exchanged the valueof this parameter every one week. During this operation mode, the MBBRs wasoperated under the optimal value of both gas/water ratio which equal to7/1and nitraterecycle ratio which equal to100%. The obtained results showed that optimum value ofthe hydraulic retention time (HRT) for simultaneous organic carbon and nutrientsremoval was equal to6.2hours. In this HRT the average COD, NH4+-N, TN, and TPeffluent results (16.2mg/L for COD,0.69mg/L for NH4+-N,12.92mg/L for TN, and0.31mg/L for TP) could meet with grade A of discharge standard of pollutants formunicipal wastewater treatment plant in China (GB/T18918-2002) with average removal efficiencies of93.15%,98.06%,71.67%and90.88%for COD, NH4+-N,TNand TP respectively, while the average dissolved oxygen concentration (DO) in aerobicand anoxic MBBRs were4.23mg/L and0.14mg/L respectively.Under normal temperature (T≥150C) from the end of16thweek we evaluated theremoval rate and removal efficiencies for both anoxic and aerobic MBBRs for COD,NH4+-N, TN, and TP at different loading rate by operated the MBBRs under continuousflow with nitrate recycle for10weeks.During this period the continuous flow combinedcylindrical anoxic/aerobic MBBRs with low anoxic/aerobic volume ratio (anoxicvolume equal about16%from the aerobic volume) by fully nitrification-denitrificationprocesses was succeeded to achieved good nitrogen and phosphorus removal (where133.77(g/m3.d) TN and15..3(g/m3.d) TP were removed with removal efficiency equalto71.25%for TN and90.51%for TP)with high COD and NH4+-N removal (where862.39(g/m3.d) COD and154.33(g/m3.d) NH4+-N were removed with removalefficiency equal to91.61%for COD and97.89%for NH4+-N).Aerobic MBBR wassucceeded to achieved fully nitrification process (NH4+-N oxidation to NO3--N), where96.17%from the NH4+-N loading rate in aerobic MBBR (81.33(g/m3.d) NH4+-N) wasremoved by the nitrification process.Anoxic MBBR was succeeded to achieved bothpartially denitrification process (NO3--N oxidation to NO2--N) and anamoxprocess(anoxic NH4+-N oxidation), where61.23(g/m3.d) NO3--N was removed withdenitrification efficiency of66.76%and73(g/m3.d) NH4+-N was removed with removalefficiency of45.74%.Under low temperature (T <150C) the continuous flow combined cylindricalanoxic/aerobic MBBRs with low anoxic/aerobic volume ratio (anoxic volume equalabout16%from the aerobic volume) by fully nitrification-denitrification processes wasfailed to achieve good nitrogen and ammonium removal (efficiency equal to60.22%forNH4+-N and27.56%for TN),whereas the NH4+-N and TN effluent results could notmeet with grade B of discharge standard of pollutants for municipal wastewatertreatment plant in China (GB/T18918-2002),while the total system was succeeded toachieved good COD and TP removal (efficiency equal to88.48%for COD and89.08%for TP) with COD and TP effluent results could meet with grade B of reuse standard(GB/T18918-2002).Under low temperature (T <150C) the improve of the system efficiency byoperating the MBBRs under intermittent flow and aeration with increase the hydraulicretention time (HRT) by4times was succeeded to achieve good COD, NH4+-N, TN, and TP removal (efficiency equal to95.17%for COD,92.48%for NH4+-N,78.84%for TN, and95.04%for TP)with effluent results could meet with grade B of reusestandard (GB/T18918-2002).