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1、Department of Civil and Environmental Engineering Sungllyunllwan University, 300, Chunchun-dong, Jangan-gu, Suwon-Si 440-746, Repuhlic of KoreaKeywords:A2/O reactor MBR Nutrient removal TMPABSTRACTIn the present study, an advanced sewage treatment process has been developed by incorporating excess s

2、ludge reduction and phosphorous recovery in an A2O-MBR process. The A2O-MBR reactor was operated at a flux of 77 LMH over a period of 270 days. The designed flux was increased stepwise over a period of two weeks. The reactor was operated at two different MLSS range. Thermo chemical digestion of slud

3、ge was carried out at a fixed pH （11）and temperature （75） for 25% COD solubilisation. The released pbospborous was recovered by precipitation process and the organics was sent back to anoxic tank. The sludge digestion did not have any impact on COD and TP removal efficiency of the reactor. During th

4、e 270 days of reactor operation, the MBR maintained relatively constant transmembrane pressure. The results based on the study indicated that the proposed process configuration has potential to reduce the excess sludge production as well as it didnt detonated the treated water quality.2008 Elsevier

5、Ltd. 1. Introduction Excess sludge reduction and nutrients removal are the two important problems associated with wastewater treatment plant. MBR process has been known as a process with relatively high decay rate and less sludge production due to much longer sludge age in the reactor （Wen et al., 2

6、004）. Sludge production in MBR is reduced by 28-68%, depending on the sludge age used （Xia et al.,2008）. However, minimizing the sludge production by increasing sludge age is limited due to the potential adverse effect of high MLSS concentrations on membrane （Yoon et al., 2004）. This prob-lem can be

7、 solved by introducing sludge disintegration technique in MBR （Young et al., 2007）. Sludge disintegration techniques have been reported to enhance the biodegradability of excess sludge （Vlyssides and Karlis, 2004）. In overall, the basis for sludge reduction processes is effective combination of the

8、methods for sludge disintegration and biodegradation of treated sludge. Advances in sludge disintegration techniques offer a few promising options including ultrasound （Guo et al., 2008）, pulse power （Choi et al.,2006）, ozone （Weemaes et al., 2000）, thermal （Kim et al., 2003）, alkaline （Li et al., 2

9、008） acid （Kim et al., 2003） and thermo chemical（Vlyssides and Karlis, 2004）. Among the various disintegration techniques, thermo chemical was reported to be simple and cost effective （Weemaes and Verstraete, 1998）. In thermal-chemical hy-drolysis, alkali sodium hydroxide was found to be the most ef

10、fective agent in inducing cell lysis （Rocker et al., 1999）. Conventionally, the nutrient removal was carried out in an A2O process. It has advantage of achieving, nutrient removal along with organic compound oxidation in a single sludge configuration using linked reactors in series （Tchobanoglous et

11、 al., 2003）. The phosphoroes removal happens by subjecting phosphorous accumulating organisms （PAO） bacteria under aerobic and anaerobic conditions （Akin and Ugurlu, 2004）. These operating procedures enhance predominance PAO, which are able to uptake phosphorous in excess. During the sludge pretreat

12、ment processes the bound phosphorous was solubilised and it increases the phosphorous concentration in the effluent stream （Nishimura, 2001）.So, it is necessary to remove the solubilised phosphorus before it enters into main stream. Besides, there is a growing demand for the sustainable phosphorous

13、resources in the industrialized world. In many developed countries, researches are currently underway to recover the phosphoroes bound in the sludges of enhanced biological phosphorus removal system （EBPR）. The released phosphorous can be recovered in usable products using calcium salts precipitatio

14、n method. Keeping this fact in mind, in the present study, a new advanced wastewater treatment process is developed by integrating three processes, which are: （a） thermo chemical pretreatment in MBR for excess sludge reduction （b） A2O process for biological nutrient removal （c） P recovery through ca

15、lcium salt precipitation. The experimental data obtained were then used to evaluate the performance of this integrated system.2. Methods2.1. Wastewater The synthetic domestic wastewater was used as the experimental influent. It was basically composed of a mixed carbon source, macro nutrients （N and

16、P）, an alkalinity control （NaHCO3） and a microelement solution. The composition contained （L-） 210 mg glucose, 200 mg NH4C1, 220 mg NaHCO3, 22一34 mg KH2PO4, microelement solution （0.19 mg MnCl2 4H20, 0.0018 mg ZnCl22H2O, 0.022 mg CuCl2 2H2O, 5.6 mg MgSO4 7H2O, 0.88 mg FeCl36H2O, 1.3 mg CaCl2 2H2O）.

17、The synthetic wastewater was prepared three times a week with concentrations of 210 1.5 mg/L chemical oxygen demand （COD）, 401 mg/L total nitrogen （TN） and 5.5 mg/L total phosphorus （TP）.2.2. A2/O-MBR The working volume of the A2/O-MBR was 83.4 L. A baffle was placed inside the reactor to divide it

18、into anaerobic （8.4 L） anoxic （25 L） and aerobic basin （50 L）. The synthetic wastewater was feed into the reactor at a flow rate of 8.4 L/h （Q） using a feed pump. A liquid level sensor, planted in aerobic basin of A2O-MBR controlled the flow of influent. The HRT of anaerobic, anoxic and aerobic basi

19、ns were 1, 3 and 6 h, respectively. In order to facilitate nutrient removal, the reactor was provided with two internal recycle （1R）. IRl （Q= 1）connects anoxic and anaerobic and IR 2 （Q=3） was between aerobic and anoxic. Anaerobic and anoxic basins were provided with low speed mixer to keep the mixe

20、d liquid suspended solids （MLSS） in suspension. In the aerobic zone, diffusers were used to generate air bubbles for oxidation of organics and ammonia. Dissolved oxygen （DO） concentration in the aerobic basin was maintained at 3.5 mg/1 and was monitored continuously through online DO meter. The soli

21、d liquid separation happens inaerobic basin with the help of five flat sheet membranes having a pore size of 0.23 pm. The area of each membrane was 0.1 m2. They were connected together by a common tube. A peristaltic pumpwas connected in the common tube to generate suction pressure. In the common tu

22、be provision was made to accommodate pressure gauge to measure transmembrane pressure （TMP） during suction. The suction pump was operated in sequence of timing, which consists of 10 min switch on, and 2 min switch off.2.3. Thermo chemical digestion of sludge Mixed liquor from aerobic basin of MBR wa

23、s withdrawn at the ratio of 1.5% of Q/day and subjected to thermo chemical digestion. Thermo chemical digestion was carried out at a fixed pH of 11（NaOH） and temperature of 75 for 3 h. After thermo chemical digestion the supernatant and sludge were separated. The thermo-chemically digested sludge wa

24、s amenable to further anaerobic bio-degradation （Vlyssides and Karlis, 2004）, so it was sent to theanaerobic basin of the MBR2.4. Phosphorus recovery Lime was used as a precipitant to recover the phosphorous in the supernatant. After the recovery of precipitant the content was sent back to anoxic ta

25、nk as a carbon source and alkalinity supelement for denitrification.2.5. Chemical analysis COD, MLSS, TP, TN of the raw and treated wastewater were analyzed following methods detailed in （APHA, 2003）. The influent and effluent ammonia concentration was measured using an ion-selective electrode （Ther

26、eto Orion, Model: 95一12）. Nitrate in the sample was analyzed using cadmium reduction method （APHA, 2003）.3. Results and discussion Fig. 1 presents data of MLSS and yield observed during the operational period of the reactor. One of the advantages of MBR reactor was it can be operated in high MLSS co

27、ncentration. The reactor was seeded with EBPR sludge from the Kiheung, sewage treatment plant, Korea. The reactor was startup with the MLSS concentration of 5700 mg/L. It starts to increase steadily with increase in period of reactor operation and reached a value of 8100 mg/L on day 38. From then on

28、wards, MLSS concentration was maintained in the range of 7500 mg/L by withdrawing excess sludge produced and called run I. The observed yields （Yobs） for experiments without sludge digestion （run I） and with sludge digestion were calculated and given in Fig. 1. The Yobs for run I was found to be 0.1

29、2 gMLSS/gCOD. It was comparatively lower than a value of 0.4 gMLSS/gCOD reported for the conventional activated sludge processes （Tchoba-noglous et al., 2003）. The difference in observed yield of these two systems is attributed to their working MLSS concentration. At high MLSS concentration the yiel

30、d observed was found to be low （Visva-nathan et al., 2000）. As a result of that MBR generated less sludge.The presently used MLSS ranges （7.5一10.5 g/L） are selected on the basis of the recommendation by Rosenberger et al. （2002）. In their study, they reported that the general trend of MLSS increase

31、on fouling in municipal applications seems to result in no impact at medium MLSS concentrations （7一12 g/L）. The thermo chemical sludge digestion was started on day 70 by withdrawing sludge at the ratio of 1.5 Q/day. The sludge digestion period was divided into two phases namely, run II （day 70-139）

32、and run III （day 140-210）. During run II, the MLSS concentration in MBR was maintained around 7500 mg/L and for run III it was maintained around 10500 mg/L. Both of these two runs （II and III） demonstrate the role of sludge disintegration in controlling the excess sludge production. The Yobs for run II and III were found to be 0.03gMLSS/gCOD, respectively. It accounts for 58% and 75% of sludge reduction when compared to run I. The observed yield for run III was found to be lower than run II. This is due t