Desalination 285 (212) 226 231 Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal The possibility and applicability of coagulation-mbr hybrid system in reclamation of dairy wastewater Weiwei Chen, Jinrong Liu School of Chemical Engineering, Inner Mongolia University of Technology, Hohhot 151, PR China article info abstract Article history: Received 5 July 211 Received in revised form 1 October 211 Accepted 8 October 211 Available online 29 October 211 Keywords: Dairy wastewater Coagulation MBR The significant improvements of membrane technology in reliability and cost effectiveness have increased the reuse probability and recycling extent of dairy wastewater. However, membrane fouling still remains a major bottleneck in wide application. In order to solve the problem, this paper investigated the possibility and applicability of coagulation-membrane bioreactor (MBR) hybrid system in reclaiming dairy wastewater. A comparative experiment based on the removal efficiencies and the membrane performance was designed to achieve the purpose. The results showed that polyaluminium chloride as the appropriate coagulant in coagulation process was effective for turbidity removal. Coagulation process played a very important role in stabilizing the effluent of MBR and the level of transmembrane pressure. MBR was a crucial process in turbidity and aluminum removal. MBR had the capability to resist shock loading and to maintain the high COD removal. Biological flocs in MBR could improve the fouling level of membrane. The hybrid system reduced 98% COD from the original and COD value of the wastewater came down to 8 mg/l. The combination of coagulation with MBR presents the possibility and applicability to reclaim effluent in dairy industries. 211 Elsevier B.V. All rights reserved. 1. Introduction In most countries including China, the production of dairy wastewater has increased due to the steady rise in demand of milk and dairy products. Dairy wastewater essentially originates from the process wastewater due to the non-accidental losses of milk or dairy products, which is mixed with waters produced in various processing units as well as with water generated from living area [1 3]. The wastewater is characterized by the high biological oxygen demand (BOD) and chemical oxygen demand (COD) due to the presence of dissolved or suspended organic matter [4 6]. If without reasonable treatment, it would cause discomfort to the surrounding population because of the decomposition of high effluent. Furthermore, dairy wastewaters generally do not contain conventional toxic chemicals [7]. Thus, it is necessary to internally recycle process waters in order to reduce fresh water consumption and minimize effluent production [1]. Several biological treatment systems including aerobic and anaerobic processes have been used for dairy wastewater treatment. However, each of these systems has its own disadvantages caused by either high energy requirement or strong operational difficulty [3,5,6]. With significant improvements of membrane technology in reliability and cost effectiveness, the application of membrane filtration has tremendously increased in the field of wastewater reclamation. In dairy industries, some studies have reported the treatment Corresponding author at: No. 49 Aimin Street, Xincheng District, Hohhot, PR China. Tel.: +86 4716576661; fax: +86 471653298. E-mail address: jinrong_liu@126.com (J. Liu). and reuse possibility of dairy wastewater by means of membrane filtration. Balannec et al. [8 1] focused on the selection of membranes and the performance of dairy wastewater treatments for water reuse. They reported that one single membrane operation allowed the milk constituents to be concentrated in the retentate but was insufficient for producing water of composition complying with the requirements for drinking water [2]. Akoum et al. [1] investigated treatment of dairy process waters using a vibratory shear-enhanced filtration system (VSEP) and nanofiltration (NF) and reverse osmosis (RO) membranes. They found that the VSEP outperforms conventional crossflow filtration in NF in terms of permeate flux and permeate COD reduction. However, the performance yielded by VSEP is lower than the rotating disk due to its lower membrane shear rate [11]. Turan [12] investigated the effect of hydrodynamic conditions and fouling on the filtration performance of the NF and RO membranes in dairy wastewater treatment. The results demonstrated that the RO and NF membranes showed excellent performance in the removal of COD. The permeate flux was higher at higher transmembrane pressures and higher feed flowrates. Castillo et al. [13] and Bae et al. [14] proposed a new bioreactor structure used for the treatment of dairy industry wastewater. They reported that membrane participated in COD removal and the process allowed the treated water to be accepted for discharge into the environment. BOD removal was high and stable regardless of operation modes. Luo et al. [15] demonstrated that the two-stage ultrafiltration/nanofiltration (UF/NF) treatment of dairy wastewater was a viable and promising method to recycle water and nutrients for production of bioenergy. However, membrane fouling that arises from the 11-9164/$ see front matter 211 Elsevier B.V. All rights reserved. doi:1.116/j.desal.211.1.7
W. Chen, J. Liu / Desalination 285 (212) 226 231 227 accumulation and deposition of retentate, such as proteinous materials [16], hampers the wide application of the processes in dairy industries. Thus, these processes need to be further analyzed to define appropriate ways for controlling membrane fouling phenomena and minimizing operating cost according to the characteristics of dairy wastewater. In order to solve this problem, a new hybrid system of coagulation-mbr, which combines the promising technology of MBR with the mature, effective and economical pretreatment of coagulation [17,18], was investigated. Other similar studies have not been found on the possibility and applicability of the hybrid system in dairy wastewater treatment. A comparative experiment based on the removal efficiencies and the membrane performance was designed to achieve the purpose. 2. Materials and methods 2.1. Description of the setup The schematic of the hybrid system is shown in Fig. 1. The hybrid system includes two parts: a coagulation process and a MBR process. Coagulation was performed in coagulation tank including the rapid mix, slow mix and settling regime. The MBR was divided into a riser zone and two down-comer zones by one membrane module. The module is built with multiple flat sheet membranes arranged in parallel with defined spaces between every single plate. The characteristics of the membrane used in this study are shown in Table 1. The influent and the coagulant were pumped simultaneously by the dosing pumps from the feed tank to the coagulation tank in which mixing, coagulation and sedimentation were completed. After coagulation, the mixed liquor flowed into the MBR by gravity. Two water level sensors were respectively placed in the coagulation tank and the MBR to maintain the constant water levels. Feed pumps and solenoid valve were interconnected with the water level sensors, respectively. The effluent was intermittently drawn from the outside to the inside of the membrane by a peristaltic pump automatically controlled with a programmable logic controller (CPM1A-2CDR-A- V1, OMRON Co., Ltd, Shanghai, China). The filtration time and the relaxation time were 8 min and 2 min, respectively. The transmembrane pressure (TMP) was monitored by a pressure gauge set between the membrane module and the suction pump. In order to induce mixing in bioreactor, remove the particles deposited on the membrane surface and to provide oxygen for biomass growth, aeration with the intensity of.7 m 3 /h was imposed through a perforated pipe with a pore size of 3 mm beneath the membrane module. The capacity of MBR was 7 L/h and the hydraulic retention time was controlled at 1 h. Influent Rapid/Slow Coagulant Settling Coagulation tank Solenoid valve Module P Aeration pipe Liquid flow meter Suction pump Effluent Membrane bioreactor Fig. 1. Schematic diagram of the coagulation-mbr hybrid system. Air compressor Table 1 Characteristics of the flat sheet membrane used in this work. Manufacturer Shanghai SINAP Membrane Science & Technology Co., Ltd., China Configuration Flat sheet Width height thickness 22 mm 32 mm 6 mm Weight.4 kg Effective surface area.1 m 2 Pore size.1 μm Material Polyvinylidene fluoride (PVDF) ph resistance range 3 12 2.2. Wastewater and coagulants The dairy wastewater used in this work was collected from the wastewater treatment plant of a dairy company in Hohhot, and then was stored at 4 C to prevent degradation. Liquid milk and ice cream are the main products of the company. The dairy wastewater treated using UASB/CAS (i.e. up-flow anaerobic sludge blanket+conventional activated sludge) process in this plant, is produced by the liquid milk production line. The capacity of the wastewater treatment plant is 5 6 m 3 /day. The characteristics of raw dairy wastewater are shown in Table 2. Before feeding to the hybrid system, the wastewater was stabilized at room temperature for one day. As the chemical pretreatment of MBR, different types of coagulants such as inorganic (alum, aluminum sulfate, and ferric chloride), polymeric (polyaluminium chloride, PACl) and organic (polyacrylamide) were tested. PACl was purchased from local market and the others were analytical reagents. Stock solutions of alum, aluminum sulfate and ferric chloride were prepared by dissolving analytical reagents in deionised water at a concentration of 1 g/l, respectively. PACl stock solution was prepared by dissolving 1 g of a commercial grade solid PACl (Al 2 O 3 29%, Basicity 68%) in deionised water at a concentration of 1 g/l. 2.3. Operating conditions The experiment was divided into four stages. The first stage of investigation was the selection of optimal technological parameters of the coagulation process. The influence of the kind and amount of coagulant and the condition of operation on the efficiency of coagulation were determined. Jar-test used in this work performed 3 min rapid mixing at 12 rpm (perikinetic phase), 15 min slow mixing at 3 rpm (orthokinetic phase), and a settling phase. The polyacrylamide used as a flocculant or coagulant aid was added to the orthokinetic phase. It could enhance the formation of larger flocs and improve the rate of sedimentation [19]. The ph of the influent was adjusted with.1 N HCl or.1 N NaOH, before addition of the coagulant. The second stage was the inoculation and acclimatization of activated sludge in MBR. The MBR was initially inoculated with returned Table 2 Characteristics of the dairy wastewater. Parameters Unit Dairy wastewater Drinking water standards COD mg/l 142.8 2133.8 Turbidity NTU 16 33 Iron mg/l.91.3 Aluminum mg/l.52.2 Manganese mg/l.22.1 Lead mg/l.1.1 Arsenic mg/l.17.1 Chromium (6+) mg/l.15.5 Conductivity μs/cm 146 153 ph 6.7 7.1 Temperature C 21. 25.5
228 W. Chen, J. Liu / Desalination 285 (212) 226 231 activated sludge collected from a municipal wastewater treatment plant. After four weeks of operation, the biomass was allowed to acclimatize to the substrate and MBR reached a steady state. During the whole experiment, a certain amount of excess sludge was intermittently discharged from the bioreactor to maintain a relatively constant sludge concentration of 3 g/l. The third stage was carried out to assess the properties of the hybrid system. The effluent quality was evaluated in terms of turbidity, residual aluminum and COD. The membrane performance was evaluated from the increase rate of TMP value. The optimal conditions previously obtained in the first stage were applied in coagulation process. To further examine the possibility and applicability, the fouling level of the system was compared with the coagulation-mf system in the fourth stage. 2.4. Analytical methods Turbidity was measured by a turbidity meter (Model WGZ-2, Shanghai Precision & Scientific Instrument Co., Ltd, Shanghai, China). COD was determined by spectrometric method with 5B-3B instrument (Lan Zhou Lian-Hua Environmental Technical Co., Ltd, Lan Zhou, China). The ph was measured with a ph meter (Model PHSJ- 3F, Shanghai Precision & Scientific Instrument Co., Ltd, Shanghai, China). All analyses were performed in triplicate for each sample. The deviation of each measured parameter for each sample was less than 1%. 3. Results and discussion 3.1. Preliminary results of the coagulation process Coagulation is an important step to reduce the suspended and colloidal materials responsible for turbidity of the wastewater and also for the reduction of organic matters which contributes to the BOD and COD content of the wastewater [7]. Preliminary jar-tests were conducted to select the optimal technological parameters of the coagulation process. The efficiency is based on the turbidity reduction after coagulation. 3.1.1. The effects of coagulant types on coagulation efficiency Since natural organic coagulants are costly compared to other common coagulants, coagulation is mainly induced by metal salts, e.g. aluminum and ferric sulphates and chlorides. In this test, alum, aluminum sulfate, ferric chloride and PACl were tried to identify the appropriate kind of coagulant. After coagulation, the suspension was poured into a graduated cylinder for sedimentation. Supernatant samples were taken under the liquid surface for turbidity measurements. Each test gave good results in terms of removal efficiency. Fig. 2 shows the variation of turbidity reductions as a function of coagulant dosages at different coagulants. The high removal of turbidity was obtained at the dosage of 8 mg/l for each coagulant, and the value was higher than 9%. However, the minimum amount of coagulant was PACl, followed by ferric chloride at the same removal efficiency conditions. This result showed that the effectiveness of PACl and ferric chloride for turbidity removal, and the other two coagulants were abandoned simultaneously due to this fact. In further study, PACl was selected as the appropriate coagulant based on the following considerations: (1) The use of ferric chloride complicates the process of transport and storage, and increases the consumption of chemical due to high acid strength [2 22]. (2) At ph 7 8, it precipitates massively in the form of ferric hydroxide, and therefore dramatically increases the volume of sludge generated [23]. Turbidity removal ( ) 1 9 8 7 6 5 4 3 2 1 Alum Aluminum sulfate Ferric chloride Polyaluminium chloride 3 45 6 75 9 15 12 135 15 Coagulant dose (mg/l) Fig. 2. Variations of turbidity removal with dosage for different coagulants. (3) Although iron salt is a nutrient, effects on the microbial community are inevitable. Dosing of ferric iron salts to activated sludge produces inhibition of the normal metabolic activity of most microorganisms [24,25]. (4) In China, PACl has been widely adopted as the most common coagulant in drinking water plants due to the chemical instability of ferric chloride [26]. 3.1.2. The effects of coagulant dosages and ph on coagulation efficiency According to the result obtained above, we pursued the Jar-Test with PACl to determine the optimal coagulation conditions, i.e., coagulant dosage and ph of feed water. In coagulation processes, coagulant dosage and ph play an important role in determining the coagulation efficiency. Insufficient dosing of coagulant will result in undesirable treated water quality. On the other hand, higher dosages can result in high cost and health problems related to high level of residual aluminum. An optimum range of ph should be determined in coagulation process. The effects of coagulant dosage and ph adjustment by PACl on turbidity removal are illustrated in Fig. 3. It clearly shows that the removal efficiency of turbidity increases with the increase in coagulant dosage and ph adjustment till it reaches its highest value, after which the removal efficiency start to decrease. The optimum 8. 7.5 ph 7. 6.5 6. 1 9 8 7 6 5 4 3 4 5 6 7 8 9 11112 Coagulant dose (mg/l) Fig. 3. Effects of coagulant dosage and ph adjustment on turbidity reduction. Turbidity removal (%)
W. Chen, J. Liu / Desalination 285 (212) 226 231 229 dosage, which simultaneously ensures the efficiency and the economy of coagulation, was approximately 9 mg/l in this work. In general, increasing ph from the acidic to alkaline levels had a strong positive influence on reduction of turbidity. It could be seen that an optimum range of ph exists at 7.5 beyond which effluent quality deteriorated. At the PACl optimum dosage of 9 mg/l and ph of 7.5, turbidity removal could reach 98.95%. The higher efficiency at ph of around 7.5 should be due to the charge neutralization or electrostatic repulsion by charged complexes and the sweep flocculation by the hydroxides [5,19]. In more detail, the increase in turbidity removal efficiency is due to increase in concentration of various hydrolysis species which destabilize the colloidal particles. At the higher dosages, formation of large amount of hydroxide flocs was responsible for turbidity removal by sweep coagulation. The decrease in the turbidity removal efficiency at highest dosage was mainly due to charge reversal of colloids. 3.1.3. The effect of settling time on coagulation efficiency Based on the results obtained above, the settling time for coagulation was investigated. It can be seen from Fig. 4 that no more further reduction in turbidity was observed with settling time after 3 min. Approximately 97% turbidity reduction was obtained. Therefore, the settling time after 3 min could be considered as effective value for the coagulation process. 3.2. Overall performance of the hybrid system Since the coagulation process can not ensure 1% removal of organic compounds, and the metal ions remaining in the water exceed the concentration standards, in the following part of the studies, the hybrid process using the coagulation results was applied to meet the preset objective. Table 3 shows the overall performance of the hybrid system over UASB/CAS process. 3.2.1. Removal of turbidity and concentrations of residual aluminum During the whole period of operation, the effluent turbidity (Table 3) was maintained lower than a turbidity of.3 NTU regardless of turbidity concentrations in raw wastewater. It showed the system has excellent performance in particle removal due to the rejection of the membrane. This signifies that the permeate water has the possibility for reuse. It is well known that aluminum retained in the treated water is associated with Alzheimer's diseases and blood cancer. Although Table 3 The overall performance of the hybrid system over UASB/CAS process. Parameters Effluent of UASB/CAS process Effluent of the hybrid system Turbidity, NTU.21 1.75.1.26 COD, mg/l 17.6 54.61 5.7 8.8 Aluminum, mg/l.3.7 Chloride, mg/l 46.94 58 16 22.96 PACl as a polymeric coagulant is superior to those inorganic coagulants of aluminum salts in this aspect [7], the wide application of the system in dairy wastewater treatment is still dependent on the residual concentrations in effluent. It can be seen from Table 3 that the concentrations of residual aluminum in effluent were below.1 mg/l, which meet the requirements of the drinking water standard (far less than.2 mg/l). In conclusion, aluminum ions were removed to the permissible level during the hybrid process. MBR played a crucial part in aluminum removal and greatly decreased its potential threat. 3.2.2. Effluent quality Because of the high COD level of the dairy wastewater, COD concentration after coagulation still could not meet the disposal standard. In order to reuse the treated water in the dairy plant, a finishing step must be added. In the hybrid system, a MBR was applied as the final supplementary step to ensure the effluent quality. Comparing the removal efficiencies of MBR without/with coagulation pretreatment, it can be seen apparently from Fig. 5 that MBR had the capability to resist shock loading and could maintain the high COD removal regardless of the efficiency of coagulation. It implies that the use of MBR would allow the coagulation time to be much shorter than that required in coagulation tanks, and therefore the hybrid systems would save the space in coagulation process. In addition, the investment for this part would be reduced. MBR was an effective method for decreasing the COD caused by the presence of organic matter such as proteins and carbohydrates in dairy wastewater. The effluent of UASB/CAS process collected in local dairy industries was also analyzed and compared with the effluent of MBR without/with coagulation pretreatment. The results are shown in Fig. 6. As was observed, a great variation can be seen in the effluent COD of UASB/CAS process. The values of effluent in MBR fluctuated with the influent quality, but they are still lower than that in the effluent of Turbidity removal ( ) 1 95 9 85 8 75 7 65 1 2 3 4 5 Time (min) COD (mg/l) COD (mg/l) 25 2 15 1 5 45 375 3 225 15 75 a Influent without coagulation Effluent without coagulation 5 1 15 2 25 3 b Influent with coagulation Effluent with coagulation 5 1 15 2 25 3 Time (d) Fig. 4. Removal efficiency of turbidity with settling time. Fig. 5. The effluent of MBR (a) without and (b) with coagulation pretreatment.
23 W. Chen, J. Liu / Desalination 285 (212) 226 231 7 6 5 Effluent from dairy plant Effluent without coagulation Effluent with coagulation longer period of operation can be maintained before membrane regeneration. 3.3. Comparison of MF and MBR COD (mg/l) 4 3 2 1 5 1 15 2 25 3 Time (d) Fig. 6. Comparison of the MBR effluent with the UASB/CAS effluent. UASB/CAS process. It showed that MBR ensured the quality of effluent better over other present treatments, and coagulation process played a very important role in the stability of effluent. The system reduced 98% COD from the original and COD value of the wastewater came down to 8 mg/l. These values remained below the level acceptable for discharge into the natural environment. 3.2.3. Membrane performance of the hybrid system As an indicator for evaluating the membrane performance, the TMP required should be slightly lower because it fixes the acceptable duration of operation before membrane regeneration. It is no doubt that the differences in the characteristics of the filtered objects influence the TMP evolution. When wastewater was directly fed into MBR without coagulation pretreatment, it was observed that TMP value increased slowly from an initial value of 1.9 kpa to 4.6 kpa at day 23 and then increased rapidly to 11.3 kpa at the end of the experiment (Fig. 7). These values were obviously higher than those values obtained with coagulation pretreatment, implying that the hybrid system was feasible and membrane performance was improved due to the coagulation pretreatment. Because coagulation pretreatment effectively removed organics of raw water in the coagulation tank, the organic loading of MBR process was decreased [17]. In this hybrid system, the TMP was essentially at the close level of preliminary stage. It means a The value of critical flux (CF) is an important factor to evaluate the system performance because it is directly related to the rate of membrane fouling and the capacity of system. This method was used in our work to evaluate the two processes with the effluent of coagulation tank. According to the definition of CF, the preliminary and the last CF values of MBR were obtained by increasing the permeate flux with step increments, until a rapid rise in TMP was observed, indicating a severe fouling. The same experiment described above was repeated in the MF process. The development of CF at two different processes was shown in Fig. 8. It could be seen that the CF value decreases with the elapse of running time, and the value at the last stage decreased by 44.26% and 3.39% of the preliminary value, respectively. A reduction of critical flux indicated the occurrence of the fouling. However, the fouling rate of MBR remained very low compared with that in the MF process. Significant reduction of the CF value in the MF process could be attributed to the serious deposition inside the membrane pores or on the surface by residual colloidal particles. Although the percentage of colloidal particles ranging from 1 nm to 1. μm [27] was very low after coagulation process, the particle size was very close to the pore size of the membrane and made an important contribution to membrane fouling. In addition, the polymeric coagulant also had the potential to aggravate membrane fouling [28]. Based on the above discussion, we did not distinguish which effect was predominant but could conclude that large biological flocs could improve the fouling level of membrane. Formation of the porous cake and adsorption of the biological flocs had a strong positive effect on reduction of fouling rate. 4. Conclusions In this study, we presented the possibility and applicability of coagulation-mbr to reclaim effluent in dairy industry. The hybrid system was evaluated in terms of water quality and membrane performance to replace the existing treatment processes. We observed that PACl as the appropriate coagulant was effective for turbidity removal. Turbidity removal could reach 98.95% at the PACl optimum dosage of 9 mg/l and ph of 7.5. The settling time after 3 min could be considered as effective value for the coagulation process. 12 1 Without coagulation With coagulation 6 5 Preliminary stage Last stage TMP (kpa) 8 6 4 Critical flux (L/h/m 2 ) 4 3 2 2 1 5 1 15 2 25 3 Time (d) Coagulation-MF Coagulation-MBR The hybrid system Fig. 7. Variation of TMP without/with coagulation pretreatment. Fig. 8. The development of CF at two different processes.
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