Research On Domestic Wastewater Treatment Using A Step-feed Anaerobic-multiple Anoxic/oxic-membrane Process

With the development of industry and agriculture,water eutrophication induced by nitrogen and phosphorus becomes more and more serious,which does great harm to the ecosystem and to human health as well.Therefore,it is urgent to remove nitrogen and phosphorus from the wastewater before discharging wastewater into water bodies. Biological nutrients removal(BNR)is regarded as one of the most economical, effective and promising solutions to this problem.Under this background,this study was conducted aiming at solving the problems existed in most of the BNR processes.Adopting the new concepts,such as biological reaction dynamic theory,activated sludge dynamic loading theory,and the simultaneous nitrification and denitrification theory,and the advance membrane separation technology,an innovated step-feed anaerobic-multiple anoxic/oxic-membrane bioreactor[An-M(A/O)-MBR],was developed for nutrients removal.The An-M(A/O)-MBR process was composed of an anaerobic reactor、a multiple phases of anoxic and aerobic(A/O)zones in sequence followed by a continuous aerated MBR.Characteristics of the An-M(A/O)-MBR process were as following;(1)When step-feed strategy was adopted,on the one hand,the low organic load rate(OLR),condition could be maintained along the system flow,which was favorable for nitrification.On the other hand,the influent received at multiple oxic/anoxic tanks could be used as external carbon source for denitrification and thus save the cost of external carbon source;(2)More combinations of oxic and anoxic tanks in series enabled nitrification stream flow from oxic zone of one pass to anoxic zone of next pass straightly,which could decrease the negative effect of nitrate accumulation on nitrification.As a result,high nitrogen removal was achieved without internal nitrate recycling;(3)The conventional second sedimentation tank was replaced by membrane separation in the An-M(A/O)-MBR process,which enhanced the MLSS. concentration,resulting the decrease of OLR and the improvement of the biological reaction rate.Additionally,high-quality effluent could be obtained through the membrane filtration.Comparing to other BNR processes,the An-M(A/O)-MBR process has many advantages,such as eliminating the internal nitrate recycling and the additional external carbon source,high nitrogen removal efficiency,low capital construction investment and operation cost,and flexible manipulation and maintenance.In this paper,the feasibility of the step-feed An-M(A/O)-MBR process for domestic wastewater treatment was discussed theoretically.Then,to validate the theoretic analysis,process performances under different conditions were investigated on a lab-scale system.The operation mechanism of each reactor and the optimal operational parameters were evaluated during the operation.Major investigations included;(1)preliminary performance study on the single-point feed An-M(A/O)-MBR process;(2)detailed analysis on the performance of the step-feed An-M(A/O)-MBR process under different operation conditions;(3)characteristics of the activated sludge during the operation;(4)characteristics of membrane fouling and cleaning methods;(5) dynamic simulation of the step-feed An-M(A/O)-MBR process.The major findings of this study were as following;1.Under the single-point influent feed condition,the An-M(A/O)-MBR system exhibited high and stable performance on COD and total phosphorus(TP)removal. When the COD and TP concentration fluctuated at 340.5～395.6mg/L and 3.92～5.67 mg/L,the average removal efficiency of COD and TP was 98.01% and 74.80%, respectively.As for total nitrogen(TN)removal,without the internal nitrate recycling, the An-M(A/O)-MBR system could achieve the same performance level(60.22%)as that of other conventional BNR processes,so the operational cost could be deceased. Although the nitrate was the major component in the effluent,a fraction of ammonia (about 1.2mg/L)still could be found in the effluent,indicating that incomplete nitrification was achieved in the single-point feed An-M(A/O)-MBR system.Studies also showed that dissolved oxygen(DO)concentration in the MBR zone was one of the important parameters affecting the performance of the system.DO value at the range of 2.0～3.0mg/L was optimal in this research.2.Studies showed that many parameters,such as DO concentration in the aerobic compartments of the multiple A/O reactor,the sludge retention time(SRT),the hydraulic retention time(HRT),the influent COD concentration and the influent distribution ratio,could affect the performance of the step-feed An-M(A/O)-MBR system.The results of this study were as following;(1)The step-feed An-M(A/O)-MBR system exhibited good performance on organic substances removal. The COD removal efficiency was high and stable(more than 94%)throughout the operation although influent COD concentration fluctuated in the range of 120-1200 mg/L;(2)DO concentration in the aerobic compartment in multiple A/O reactor had great effect on the performance of nitrogen and phosphorus removal.Under the conditions imposed,high nutrient removal could be obtained when DO concentration was at 0.8～1.2mg/L.The average removal efficiency of TN and TP was 74.81% and 71.41%;(3)Giving attention to nitrogen removal and phosphorus removal simultaneously,under the conditions imposed,good performance could be obtained when SRT was at 26～27d;(4)When the HRT was at 8.7h、6.96h and 4.97h,the average removal efficiencies of TN and TP were 73.15%、79.76%、81.98% and 67.79%、80.99%、92.16%,respectively.Under the conditions imposed,Long HRT introduced low OLR in the system,resulting in the decrease of nitrogen and phosphorus removal due to the decrease of bioactivity.That’s why removal efficiencies of TN and TP increased with the decrease of HRT;(5)Low COD level(120～200mg/L)could cause the deficiency of organic carbon source for denitrification and for phosphorus release. And high COD level(1110～1200mg/L)could change the ecospecies in the system because of the excessive high OLR;(6)More influent quantity flowing into the multiple A/O reactor was favorable for nutrients removal.But too much influent distributed to the multiple A/O reactor(distribution rate was 4;6)could shorten the retention time of a fraction of influent,and could lead to the deficiency of organic substances for phosphorus release in the anaerobic reactor as well,resulting in the deterioration of process performance 3.Studies of activated sludge characteristics showed;(1)MLVSS/MLSS values in all reactors were in the range of 0.76～0.85,indicating that good bioactivity was kept throughout the operation.The MLVSS/MLSS value demonstrated a decrease trend along the system flow,implying that stable phosphorus release and uptake existed in the step-feed An-M(A/O)-MBR system.In addition,the MLVSS/MLSS margin between in the membrane tank and in the anaerobic tank could reflect the phosphorus removal performance of the system.The more the margin value,the better the phosphorus removal efficiency could be achieved in the system;(2)There was no obvious sludge volume index(SVI)difference in the anaerobic reactor,the multiple A/O reactor and the membrane reactor,and the SVI variation showed the same trend in these reactors.Under normal condition,the SVI value in the membrane reactor was at 65～90mL/g,so the sludge had good sedimentation property;(3)The special configuration and operation mode ensured the diversity of microorganism population in the step-feed An-M(A/O)-MBR system.So stable and complex microbial ecosystem was built up and the ability of microorganism self-adjustment and self- improvement could be enhanced when the influent quality changed a lot.Meanwhile,the interaction among different microorganism populations could also improve the performance of the system;(4)Activated sludge in the step-feed An-M(A/O)-MBR system had strong ability for COD adsorption.OLR exhibited great influence on the adsorption character. The COD adsorption quantity increased with the increase of OLR.4.Characteristics of membrane fouling were as following;(1)In the A-(A/O)~n-MBR process,a characteristic three-stage TMP profile was observed with an initially extended period of slow and a fast TMP rise followed by a sudden transition to a rapid rise;(2)During long-term operation,characteristics of membrane fouling varied with the mix liquor characteristic and the membrane flux adopted.High-quality sludge characteristics and low membrane flux was helpful to alleviated membrane fouling;(3) Physical cleaning methods,such as air flushing and water flushing,were only effective to alleviate membrane fouling in short time,but they could be used as supplementary methods to remove fouling caused by cake layer and part of incompact clogging and gel layer;On-line oxidant cleaning method was effective to remove fouling caused by pore clogging and gel layer; On-line acid cleaning and on-line alkaline cleaning could be used as supplementary methods for oxidant cleaning; Off-line chemical cleaning was the most effective method for fouling control,which could ensure perfect regeneration of the membrane;(4)The rapid TMP rise was mainly due to surface cake accumulation, which belonged to reversible resistance and could be removed by tap water washing. Fouling caused by gel layer,which belonged to irreversible fouling,could be removed efficiently by chemical cleaning,and the effect of NaClO+HCl cleaning procedure was superior to that of HCl+NaClO one because NaClO could oxidize gel layer formed mainly by macro-molecular organic substances,while HCl could remove inorganic particles formed by Ca、Mg,Al,etc.5.According to the mass-balance equations of each composition in the anaerobic reactor,each compartment of the multiple A/O reactor and the aerated membrane reactor were established respectively,a series of dynamic models of each composition along the An-M(A/O)-MBR system flow were built and computed.The results showed that good consistency between the measured and the simulated values of each composition was achieved,indicating these dynamic models were suitable for simulating the variation of each composition in the An-M(A/O)-MBR system. Therefore,the dynamic models were useful in system design,control and prediction.