Evaluation of Laboratory and Plant Scale Membrane Bioreactors in Treating Textile Wastewater

Transcription

1 ( Evaluation of Laboratory and Plant Scale Membrane Bioreactors in Treating Textile Wastewater T. A. Ferdian*, A. Reza* and T. Setiadi** * Department of Chemical Engineering, Faculty of Industrial Technology, Institut Teknologi Bandung, Jl. Ganesha No. 10 Bandung 40132, Indonesia **Centre for Environmental Studies-PSLH, Institut Teknologi Bandung, Jl. Sangkuriang 42A, Bandung, Indonesia ( tjandra@che.itb.ac.id) Abstract The Membrane bioreactor (MBR) technology has being developed in many countries in the world. Due to its ability to produce a high quality effluent, the MBR technology can be applied to improve the environment and it can also help reducing the water resources needed through water recycling. Apparently, this technology has not yet been widely applied to the textile wastewater treatment in Indonesia since there are a number of factors, such as economy and technical reasons. From the technical point of view, there is still a lack of information available on factors influenced the MBR performance in treating textile wastewater. Thus, this study was aimed to evaluate both the labscale and plant scale submerged hollow fibre MBR performance on textile wastewater treatment. The lab-scale MBR, which had an operating volume of 5 L and an aeration system at the bottom, was operated in the fed-batch mode and fed with 2 L/day of textile wastewater from a textile mill outside Bandung, West Java, Indonesia. It was operated in 25 days where permeate was extracted at a constant flux between 5-6 LMH (L/m 2 /h) and HRT (hydraulic retention time) of days. The MBR lab-scale result was compared to the plant-scale performance which operated in a constant flux of 5.6 LMH and HRT of 4.2 days. The result shows that the operation at the laboratory and plant scale were relatively at a stable flux (5-6 LMH) and TMP (trans membrane pressure) was never exceeded 0.7 bar (52.5 cmhg) during the 25 days experiment, so it did not indicate the occurrence of membrane fouling. Extracellular polymeric substances in the laboratory scale sludge was dominated by polysaccharides, while it was dominated by proteins in the plant scale. The filament index of laboratory sludge was 5; while on plant-scale was 4 according to the Eikelboom category, and sludge in both plants were dominated by the same filamentous microorganisms, i.e. Sphaerotilus natans. Moreover, there was no significant difference between the laboratory and plant scale MBR effluent quality, i.e. the COD removal of 85-91%, and % on the lab-scale, and plant-scale, respectively. Keywords Laboratory scale, plant scale, membrane bioreactor, textile wastewater INTRODUCTION The clean water needs in the developing countries are more difficult to achieve because of an increase trend on population, urbanization, water usage percapita and water pollution. This encourages many governments on establishing stricter regulations on water resources and water pollution (Copeland and Taylor, 2004). For example, excessive water use could be reduced by the water-saving campaign and groundwater use restriction. Wastewater efluent standards are also tightened up to protect water resources, such as river, lake and sea from pollution. These water management regulations made industries do their best efforts on finding a better wastewater treatment technology. The most common wastewater treatment technology applied in industries in the last half of 20 th century was activated sludge processes - ASP (Davis, 2010). With the limitation of water resource availability in many urban areas of developing countries, industries make best efforts to use their wastewater as a no-other-choice resource. Therefore, the conventional ASP technology will not be enough to deal with this kind of situation, a better and realible treatment technology should be applied. The membrane bioreactor (MBR) is one of the advanced technology in wastewater treatment nowadays. MBR that combined the activated sludge process and membrane separation process was first developed by Dorr-Oliver (Judd, 2006).

2 Currently, the MBR technology has been widespread applications but it is still developed in many countries around the world. Its applications can be beneficial to environment preservation, because it can produce a high quality effluent which is safe to discharge to water bodies or recycling purposes. However, it has been known that the main problem in membrane bioreactor operation is a membrane fouling. Fouling is a process leading to deterioration of flux because of surface or internal blocking of the membrane. Variety of substances might exist or appear in water, thus it could increase the membrane resistance by adsorption on the membrane surface, adsorption on the surface of the membrane pores, or by closing the pore membrane entirely. The LCOCD (Liquid Chromatography Organic Carbon Detection) analysis showed that the retention of substances > 5 kda was greater than 90% (Fane, et al. 2000). Among the factors affecting membrane fouling, extracellular polymeric substances (EPS), which consists of polysaccharides and proteins as major components, is considered as the main contributor. EPS is produced by microorganisms in MBR, mainly by bacteria (Bella et al., 2000; Mehrez et al., 2007) and/or filamentous bacteria (Wang et al., 2010). In Indonesia particularly, the MBR technology has been studied to treat industrial wastewater such as hazardous waste landfill leachate (Setiadi and Fairus, 2003) and textile mill wastewater (Setiadi, et al., 2005). However, this technology has not yet been widely applied to the textile wastewater treatment in Indonesia since there are a number of factors, such as economy and technical reasons. From the technical point of view, there is still a lack of information available on factors influenced the MBR performance in treating textile wastewater. Thus, this study was aimed to evaluate both the lab-scale and plant scale submerged hollow fibre MBR performance on textile wastewater treatment in Indonesia. MATERIALS AND METHODS Feed wastewater Wastewater fed into the membrane bioreactors at both laboratory scale and plant scale obtained from a textile mill in West Bandung, Indonesia. The average characteristics of raw wastewater are shown in Table 1. Before feeding to the MBR, the raw wastewater both in laboratory and plant scale was neutralized into ph of 7 by adding a required amount of sulfuric acid solution. In the plant scale, there was a mean to reduce oil and grease content and then the wastewater was equalized in an equalization basin. The wastewater used in the laboratory scale was collected from the equalization basin. Table 1 Wastewater characteristics Parameter Unit Value mg/l 200 COD mg/l 610,9 TSS mg/l 61 Phenol (total) mg/l 0,01 Chromium (total) mg/l < 0,06 Ammonia (as N) mg/l 1,09 Sulfides mg/l 0,11 Oil and grease mg/l 13 ph 9,04 BOD 5 20

3 MBR setup The MBR experimental set up in the laboratory scale is shown in Figure 1. Hollow fiber UF membrane modules made of PES (polyethersulfone) with 0.1 µm pore size and surface area 0.1 m 2 installed in a laboratory scale bioreactor with submerged configuration. The reactor has a maximum working volume of 10 L. Textile wastewater was stored in the 5 L container and neutralized to ph of 7 before feeding to the bioreactor using a peristaltic pump. The sludge was not acclimatized, due to it was collected from the plant-scale system. Permeate was extracted by using a diaphragm pump with a constant flux between 5-6 LMH (L/m 2 /h). Backwash was conducted periodically every 10 minutes by pumping back a portion of the permeate through the membrane for 1 minute. Aeration and mixing was carried out by passing air through air diffusers located at the bottom of bioreactor. An analog manometer installed at effluent stream to monitor TMP (trans-membrane pressure). Pressure data were recorded manually at the end of each filtration (right before backwash). Figure 1 Flow diagram of lab-scale textile wastewater treatment The schematic diagram of plant scale is shown in Figure 2. The configuration in the plant scale was more complicated than that of laboratory one. In the plant scale, wastewater having undergone a series of pretreatment (oil and grease reduction, equalization and neutralization) fed into two aerobic tanks (Aerobic Tank 1 and Aerobic Tank 2). In both tanks, the wastewater was mixed with activated sludge and aerated using surface aerators. Mixed liquor from the Aerobic Tank 1 was pumped into the Aerobic Tank 2 before feeding into the membrane bioreactor (UF Tank). Permeate was extracted by using a centrifugal pump through the 0.1 µm pore size PVDF (polyvinylidene fluoride) ultrafiltration membrane with a total surface area of 7200 m 2, while the returned activated sludge (RAS) was pumped back into the Aerobic Tank 1 and 2. Permeate was extracted by using a centrifugal pump at a constant flux of 5.6 LMH. Backwash was conducted periodically every 10 minutes by pumping back a portion of the permeate through the membrane for 30 seconds. Aeration and agitation were conducted with air flowing through the disc-diffusers located at the bottom of the membrane aeration basin. Comparison between the operating conditions in the laboratory experiments and plant scale are summarized in Table 2. Analytical method Characteristics of wastewater and permeate were analyzed in accord with the Standard Methods (1989). EPS (extracellular polymeric subtances) analysis was conducted with Dubois test for polysaccharides (Dubois et al., 1956), as well as Bradford (Bradford et al., 1974) and Lowry (Lowry, 1951) test for proteins.

4 transfer pump surface aerator uf pump dosing UF transfer pump scrapper Aeration tank2 dimension: #21x11 m disc diffuser uf backwash pump uf backwash tank sludge recycle mixer transfer pump surface aerator diffuser blower pit tank equalization tank dimension: 20x5x5 m neutralization tank dimension: 5x3x3 m uf blower RECYCLE/ EFFLUENT storage tank sludge pump Aeration tank1 dimension: #16x15 m sludge pump Figure 2 Flow diagram of plant-scale textile wastewater treatment Table 2 Laboratory and plant scale MBR experimental operating conditions Values Operating Conditions Unit Lab-scale Plant-scale Flux L. m -2. h ,6 Filtration/backwash duration filtration/backwash (minute/minute) 10/1 10/0,5 Hydraulic retention time 4,2 Days 2,5-3 (HRT) Sludge retention time (SRT) Days 25 - F:M ratio kg COD. kg MLSS -1. day -1 0,05 0,15 0,05 0,14 Aeration vvm 0,2 0,4 Maks. 0,125 Dissolved oxygen (DO) mg. L Temperature C 25 27,5 31 ph RESULTS AND DISCUSSION Flux and trans-membrane pressure (TMP) Flux value between laboratory scale and plant scale was nearly equal and relatively stable at the whole period of experimentation. This occurred because the flux at both scale was kept constant, so that the membrane cleaning caused by fouling was determined based on TMP parameters only. Flux that were in the range of 5-6 LMH (liter/m 2 /hour) can be classified as a low flux, compared with flux at a textile wastewater treatment reported by Brik et al. (2006) of 18 LMH or Bella et al. (2007) of 19 LMH. This low flux prompted the TMP never exceeded 45.9 cmhg at the laboratory scale. However the membrane operation time, between two consecutive chemical cleaning, at the plant scale was fairly short, i.e. about every two weeks in the plant membranes and no chemical cleaning for the laboratory membranes during 25 days of operation. This might be caused of different membrane type and age of membrane between the laboratory and plant scale. PVDF membranes used at the plant scale and the PES membranes used in the laboratory scale. In addition, the plant-scale membranes having experienced several fouling and chemical cleaning many times,

5 had been approximately operated for 3 years while the laboratory-scale membranes merely 6 months old. Mixed Liquor Suspended Solid MLSS comparison between laboratory and plant scale is shown in Fig 3. The MLSS measurement was carried out in the day 9 and 25 of operation. It can be seen that there was no significant differences between laboratory and plant scale MLSS. MLSS values had been stable at the end of the laboratory-scale experiments with a value of between mg / L, while plant scale values was kept constant to reduce membrane fouling intensity. Figure 3 The MLSS in the laboratory and plant-scale at different days of operation. Extracelluler Polymeric Substances (EPS) Figure 4 shows that the sludge in laboratory-scale experiments contained more polysaccharide (as glucose) than that of plant-scale sludge. On the other hand, the plant-scale sludge has had more protein (either with Lowry or Bradford method) than that of lab-scale one. Further discussion would be given in later to clarify the comparison of laboratory and plant floc forms. mg/l Glucose Protein - Lowry Plant Lab Protein - Bradford Figure 4. The comparison of EPS in the plant and laboratory sludge EPS presence in the activated sludge may reduce permeate flux due to the membrane fouling. This could be seen with the increase in resistance, being represented by the trans-membrane pressure. However, because of the low EPS generated in the experimentation, therefore the fouling occurred relatively insignificant in this experiment. It is in accordance with the finding of Ramesh et al. (2006) that polysaccharides were a major cause of fouling at a low F/M ratio indicated by a high

6 polysaccharide concentration in Dubois test. The low concentration of polysaccharide might be due to they were consumed again by microorganisms as substrates. Flocs comparison The comparison of microbial flocs and filamentous microorganisms in the sludge between laboratory and plant scale at the end of the experiment is shown in Table 3. It can be seen that both lab and plant-scale sludge having small flocs with bridging interflocs type. Chang et al. (2002) reported that the particle and floc size of sludge greatly affected the membrane fouling in MBR process. If the size is too small, then the floc and particles could pass through the pores of the membrane. However, most bacteria and floc size are much larger than membrane (MF or UF) pore size so it is more likely that the floc made a cake deposition on membrane surface, which then led to a membrane fouling. Table 3 Microphotograph of activated sludge floc Magn. Lab-scale Plant-scale 40x 100x The figure shows also that the number of filamentous microorganisms in the lab-scale sludge were more abundant than that of the plant-scale one. The filament index of laboratory sludge was 5; while on plant-scale was 4 according to the Eikelboom category (Eikelboom, 2000). This occurance could be several reasons, either a shorter SRT or less nutrient in the lab-scale operation. Moreover, in the both laboratory and plant scale operation, it is found that the same filamentous microorganism, i.e. Sphaerotilus natans, dominated in both sludge. Richard (2003) reported that Sphaerotilus natans might dominate at a low dissolved oxygen concentration. Effluent Quality The laboratory and plant scale MBR effluent quality in the 25 th day of operation were summarized in Table 4. Table 4 MBR performances during the 25 th days of operation Values Parameter Unit Plant Lab Feed effluent effluent 20 BOD 5 mg/l COD mg/l 610,9 155,8 94,3 TSS mg/l Phenol (Total) mg/l 0, Chromium (Total) mg/l <0,06 < 0,06 < 0,06 Ammonia mg/l 1,09 0,55 0,62 Sulfides mg/l 0,11 0,05 0,03 Oil and fat mg/l ph 9,04 8,25 8,21

7 Based on Table 5, the laboratory-scale MBR effluent had met all of the quality standards according to KEP-51/MENLH/10/1995 Appendix B.IX for the Indonesian textile mill effluent standard. On the other hand, the plant scale MBR effluent met most the quality standards except TSS (total suspended solids / total suspended solids) and COD (chemical oxygen demand) parameters. This was caused by there was a damaged in several membrane fibers at the plant during the 25 th day of operation, thus the sludge were carried out to the effluent. This problem could not be directly repaired because it was hard to find the leak point in big numbers of membranes. BOD 5 20 (biological oxygen demand over 5 days at 20 o C) of plant-scale effluent was not detected due to inaccurate analysis. The COD test was also conducted on day 21 th and 24 th samples, as shown in Table 5. Based on the table, both lab and plant scale COD removal ranged from 88.1% to 93.1%. The removal percentages are within COD removal in wastewater treatment reported by Brik et.al. (2006), i.e between 60% and 95%. Table 5. The COD removal throughout MBR operation Sample Average COD Day Wastewater (ppm) % Removal Raw Lab effluent 90 90,9 Plant effluent ,1 24 Raw Lab effluent 54 91,0 Plant effluent 41 93,1 Moreover, the color reduction did not have a significant difference between laboratory scale and plant scale. According to Brik et al. (2006), textile wastewater color reduction was not influenced by feed concentration and microbial biodegradation but the color adsorption phenomena into the biomass. It was proven by a similar color reduction percentage between the laboratory scale and plant scale might be due to the comparable value of MLSS in both operations. The similar MLSS has caused the quantity of dye adsorbed by the biomass in activated sludge did not differ significantly. Other evidence was the reddish color floc visible under the microscope resembled the color of the wastewater fed in this experiment. Conclusions From the experiment, the conclusions are described as follows: 1) Textile wastewater treatment at the laboratory and plant scale were operated at a relatively stable flux (5-6 LMH) and TMP (trans membrane pressure) was never exceeded 0.7 bar (52.5 cmhg) during the 25 days experiment, so it did not indicate the occurrence of membrane fouling. 2) Extracellular polymeric substances in the laboratory scale sludge was dominated by polysaccharides, while it was dominated by proteins in the plant scale. 3) The filament index of laboratory sludge was 5; while on plant-scale was 4 according to the Eikelboom category. 4) Both the laboratory and plant scale activated sludge were dominated by the same filamentous microorganisms, i.e. Sphaerotilus natans. 5) There was no significant difference between the laboratory and plant scale MBR effluent quality, i.e. the COD removal of 85-91%, and % on the lab-scale, and plant-scale, respectively.

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