Control of membrane fouling by Hydrogen Sulfide (H 2 S) diffusion

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1 Control of membrane fouling by Hydrogen Sulfide (H 2 S) diffusion Hasan Mahmud Khan Water and Environmental Engineering, Department of Chemical Engineering, Lund University P.O. Box 124, SE-221, Lund, Sweden Abstract The major problem of widespread applications of membrane bioreactors (MBRs) in water and wastewater treatment is quick membrane fouling and consequently it s operation cost in term of fouling mitigation although this technology ensures better treatment efficiency and high effluent quality. This work presents an experimental study on membrane fouling in MBR operation at different concentrations of hydrogen sulphide (H 2 S). Nine samples have been collected from MBRs to investigate the sludge and membrane properties before and after the application of, 25 ppm, 5 ppm, and 1 ppm concentrations of H 2 S. It was found that protein, carbohydrate, total organic carbon (TOC) of eeps and TOC of SMP decreased and ph increased with the increase of H 2 S concentration in feed whereas zeta potential showed no trends. Key works : MBR, membrane fouling, eeps, SMP, TOC, ph, H 2 S diffusion Introduction MBR, the technology of membrane separation of activated sludge, is the activated sludge treatment together with the biological sludge separation by micro- or ultra-filtration membranes with pore size of normally 1 nm to.5µm to generate particle-free effluent (Lesjean et al. 27). MBR technologies have been potentially improved after an intensive research in last decade. It is gaining popularity in water and wastewater treatment sector due to its better quality of effluent compare to conventional treatment process, high treatment efficiency, less foot print and small reactor requirements, good disinfection capability and higher volumetric loading (Liang et al., 27 & Judd, S., 26). It offers a complete separation of solid-liquid in the biological degradation process by activated sludge, truly physical retention of bacterial floc and other suspended solid within bioreactor (Le- Clech et al., 26). However, its still challenging in widespread applications of MBR for rapid fouling of membrane resulting quick decline of permeate flux and high maintenance cost in rigorous physical and chemical cleaning (Meng et al. 26). Foul elements deposit and form a cake layer on the membrane surface in consequence of blocking and narrowing the membrane pores. After a certain time trans-membrane pressure increases sharply or permeates flux drops below the significant level under constant pressure. This fouling propensity is the major obstacle of MBR applications in commercial sector over conventional systems. The main objective of this work is to control the membrane fouling through H 2 S diffusion. Extracellular polymeric substances (EPS) EPS are the composition of polymeric materials consist of carbohydrates, proteins, nucleic acids and have been found at or out side the cell surface and in the intercellular opening of microbial aggregates. It is the construction material of biofilm, flocs. Carbohydrates and proteins are the foremost components in extracted EPS and are responsible for membrane fouling. EPS provide a highly hydrated gel where microorganisms are embedded and they hamper flux to permeate in the MBRs. Specific EPS resistance could be estimated from the filtration resistance (m -1 ) divided by EPS density on the membrane surfaces (kgtoc/m 2 ) and it is between 1 16 and 1 17 m/kg. Recent studies showed that proteins have strong positive correlation to membrane resistance whereas carbohydrates 1

2 have moderate correlation due to low amount. According to the current researches, proteins are the most important factor of membrane fouling (Meng et al., 26). Soluble microbial product (SMP) SMP are the large amount of soluble organic substances that are produced from bacterial populations in bioreators. SMP level is extremely important for biological treatment process as treatment efficiency and effluent quality highly depends on SMP amount in treatment reactors. It is well established that majority of soluble organic substances come from SMP in effluent in biological treatment systems and its concentration set up the discharge level of chemical oxygen demand (COD) and total organic carbon (TOC) in effluent. Furthermore, some SMP have certain characteristic like toxicity and metal chelating properties which affect the metabolic activities of microorganisms both in receiving water and in treatment process. In some case, it lessens the specific respiration rate of microorganisms. Therefore, for better efficiency of treatment systems, less concentration of SMP is desirable (Liang et al., 27). SMP could be again categorized into two as utilization associated products and biomass associated products. Although humic substances (humic and fulvic acids), carbohydrates and proteins have been identified successfully as major components of SMP, it is still unclear of its precise composition (Liang et al., 27). However, most of previous studies concentrated on SMP in conventional biological treatment plants but fewer have been focused to grasp the impact of SMP in MBR. It is found from study that depending of the operation condition SMP are liable from 26 to 52% of membrane fouling in microfiltration and ultrafiltration membranes (Liang et al., 27). Materials and Method Hydrogen Sulphide (H 2 S) diffusion Air and mixture of air & H 2 S was sprayed into sludge for getting different concentration of H 2 S in sludge. Pressure and flow rate were 2 bars and 4 ml per min respectively and was aerated for 15 min and after then kept it for 1 min for homogeneous distribution of H 2 S in sludge. High concentration of H 2 S (2 mg/l) has been diluted by mixing of compressed air to get a certain concentration of H 2 S. Two gas flow meters were used to control the air and H 2 S gas flow and connected to the gas cylinder by plastic tube line. This mixture of gases has been applied to feed by diffuser which was connected at the end of source tube. Concentration of 25 ppm (.75 mg H 2 S/g biomass), 5 ppm (.15 mg H 2 S/g biomass) and 1 ppm (.3 mg H 2 S/g biomass) of H 2 S were sprayed in three samples with flow rate of 4 ml/min and each sample is amount of 2 liters. Only air was pumped into the sludge to get zero concentration H 2 S sample for same duration with flow rate 4 ml/l. These samples were analyzed for eeps, SMP, ZP and ph. H2S Tank Air Tank Regulator Regulator H2S Flowmeter Air Flowmeter 2 m tubeline 2 L Glass Cylinder Figure 1: Schematic setup for H 2 S diffusion Dead End Filtration The whole experiment was consisted of dead end filtration cell, reservoir, compressed air and computer program for measuring weight of collected permeate. Around one liter of aerated sludge is poured in about 2 l volume closed feed reservoir and pump to dead end filtration cell (11 ml volume capacity, 1.59 X 1-3 m 2 membrane 2

3 area). PVDF membrane (.189 cm 2 ) with pore size of.22 µm is placed at on porous media at the end of cell and rubber circular ring is placed on the membrane to prevent leakage. Dead end cell is completely air tight and connected to feed tank by rubber tube. Compressed air cylinder is connected to the tank through pressure gauge and pressure stabilizer for pumping sludge from tank to cell. Applied air pressure is controlled by EL PRESS P-72CM pressure transducer installed in the line tubing, which is then connected to an EL PRESS electronic regulator for a more accurate control of pressure. Filtrated supernatant weight is measured every 6s by computer linked balance and computer program calculate the flux rate. Air tank Automatic pressure regulator Pressure Controller Reservoir Dead-end filtration cell Mass balance Fig 2: Dead-end filtration set-up Permeat e Computer Source of biomass Biomass has been collected from North Head MBR and Malabar MBR which was originally from North Head s MBR. Both sewages go to MBR after primary treatment and are a mixture of municipal and industrial waste. Sludge age of Malabar is 2 days and about 13L of sludge is wasted each day to keep the sludge age at 2 days and hydraulic retention time (HRT) is about 1 hours. Volume of MBR is 25 l and the total membrane area is 2 m 2 with constant flux at 12 l/h. Nine samples have been used for the experiment marked from A to I. s have been randomly selected for different tests and observed the effect of H 2 S on this particular sludge characteristic. 3 SMP and EPS extraction Heating method has been followed to extract SMP and EPS. is centrifuged first at 2 rpm and supernatant is filtrated through 1.2 µm pore size filter paper for SMP extraction. The suspension was diluted with.98% NaCl solution, mixed for 5 min and kept it 8 C temperature for 1 min. This temperature has been controlled by high pressure stream sterilizer (ES-315). After then it was centrifuged at 25 rpm and supernatant was again filtrated using 1.2 µm pore size filter and collected for EPS extraction. Carbohydrate analysis Phono-Sulfuric acid method proposed by Hanson and Philips, 1981 was used to determine the carbohydrate concentration of SMP and extracted EPS. In presence of sulfuric acid, phenol and carbohydrate form colored aromatic compounds and measured it as an absorbance at 48 nm wave length by spectrophotometer. Concentration of carbohydrate then was determined from the value of absorbance using carbohydrate calibration curve which was developed from standard solution. Concentration, mg/l Carbohydrate Calibration y = x R 2 = Absorbance at 48 nm Fig 3: Carbohydrate calibr. curve at 48 nm Protein analysis Modified Lowry method has been used to measure the concentration of protein of SMP and extracted EPS from filtrate which was modified by Peterson (Peterson, 1977). In this method, bonded proteins react to lowry reagent which contains copper ions and Folic-Ciocalteu reagent composed of phosphomolybdate and phosphotungstate. When lowey reagent is added to the filtrate, copper ions together with protein form a

4 complex alkaline solution, Buiret Chromophore and further addition of Folin- Ciocalteu reagent reacted protein develops blue color that is detectable by spectrophotometer under the wavelength from 5 to 75 nm. Protein concentration is then calculated from calibration curve. mg/l Proteins Protein calibration y = x R 2 = Absorbance 65 Fig 4: Protein calibration curve at 65 nm Zeta Potential (ZP) ZP is the electrical double layer that is surroundings the particles in a colloidal solution or suspension (Lindquist, 23). Ionic characteristics and dipolar features of the colloidal particles dispersed in the solution are the causes of their electrically charged. Particles are surrounded by the opposite charges which is called fixed layer and outside of fixed layer there is the cloud area composed of positive and negative ions. This cloud area is called diffuse double layer. ZP is considered to be the electric potential of this inner area including this conceptual "sliding surface". As this electric potential approaches zero, particles tend to aggregate. It has been measured of new and foul membrane by PAAR EKA-Electro Kinetic Analyzer RV 4.31C at different ph value. Total Organic Carbon (TOC) Sludge is centrifuged and filtrated through membrane with nominal pore size.45 µm in order to get particle free clear sample. Few drops of 98% H 2 SO 4 are added to remove any inorganic carbon from supernatant and TOC is then measured by total organic carbon analyzer (TOC-5 A). 4 Results and Discussion Effect of H 2 S on Chemical Characteristics: ph ph Effect of H 2 S on ph A C E F G Fig 5: Variation of ph No H2S 5 ppm H2S 1 ppm H2S Wastewater treatment process largely depends on ph of biomass. It is observed from obtained result that ph value moderately increased with increase of H 2 S concentration. This elevation of ph value might be able to contribute positively on membrane fouling propensity when original ph of biomass is low. It is well known that drop of ph under certain level makes the wastewater treatment process difficult. Therefore, removal of H 2 S from activated sludge could decrease the ph of sludge and might lead higher fouling in MBR because the living species could not work properly at low ph. Zeta Potential (ZP) ZP of membrane represents the surface charge of membrane. It is found from study, it has strong negative correlation with fouling resistance. Resistance increase when ZP drop and it plays a significant role in membrane fouling (Meng et al., 26). ZP of virgin and fouled membrane has been determined and results show that ZP of fouled membrane is higher than virgin membrane but it was not found any impact of H 2 S that influence the ZP of membrane surface. From test result, it could be said that fouling definitely affect the ZP of membrane surface because clear difference has been

5 found between the virgin and all fouled membranes but the effect of H 2 S in different concentration is ambiguous. ZP unevenly varies with H 2 S concentration. Effect of H 2 S on Biopolymeric Composition of Biomass: Extracellular Polymeric Substances (EPS) Extracellular polymeric substances are consisting of protein, carbohydrate and carbon compound that are mainly liable for membrane fouling. Four samples have been tested to determine the concentration of protein, carbohydrate and total organic carbon in extracted EPS. The results which were found from lab test are given below: Protein Protein Concentration, mg/l Effect of H 2 S on eepsp C D F G No H2S 5 ppm H2S 1 ppm H2S Fig 6: Protein conc in eeps at different H 2 S From the bar chart it is found that protein concentration of one set of sample (D) does not give any correlation with different concentration of H 2 S diffusion but in other three set of samples protein concentration decreases with increase of H 2 S diffusion in sludge. According to Meng et al. (26) fouling resistance increase with increase of protein concentration, of H 2 S in sludge could lead to decrease the fouling resistance and consequently increase the filterability of membrane. As there is no standard method for extraction of EPS, the obtained result might not be quite accurate but in general it could be said that diffusion of H 2 S in activated sludge could enhance the permeability of membrane. Dissolved matter is mainly responsible for irreversible fouling (Yamato et al 26) so that application of H 2 S might lead to reduce the irreversible fouling. 5 Carbohydrate Carbohydrate Concentration, mg/l Effect of H 2 S on eepsc C D F G No H2S 5 ppm H2S 1 ppm H2S Fig 7: Carbohydrate of eeps at different H 2 S conc. Concentration of carbohydrate in three samples out of four samples dropped when H 2 S was sparged in higher concentration. Carbohydrate has poor correlation with fouling resistance (Meng et al., 26) so that H 2 S diffusion is less effective technique to reduce the membrane resistance in term of carbohydrate. However, results give the positive sign of filterability improvement when H 2 S was sparged in sludge. Total Organic Carbon TOC, mg/l Effect of H 2 S on TOC D F G No H2S 5 ppm H2S 1 ppm H2S Fig 8: TOC of eeps at different H 2 S conc Constituents of TOC like polysaccharides, humics are adsorbed on the membrane surface and increase the fouling resistance by reducing the pore size. Adsorption tendency is 46% for polysaccharides and 16% for humics in the feed (Huber, 1998). It was found from lab results that concentration of TOC slightly drops with increase of H 2 S diffusion in feed water. According to Huber s (1998) study, it could be concluded that of H 2 S improve the permeability of membrane as it reduces TOC in feed.

6 Soluble Microbial Products (SMP) SMP is soluble EPS in feed water. It is mainly composed of protein, carbohydrate and total organic carbon. In our experiment, four samples were used to determine the concentration protein and carbohydrate and five samples for total organic carbon. Obtained results are given below: Protein Table 1: Protein conc. (mg/l) in SMP Run C D F G No H 2 S ppm H 2 S ppm H 2 S ppm H 2 S Concentration of protein in SMP has been determined after sparing of H 2 S in four different concentrations. Most results show that protein concentration is very low and it does not vary with H 2 S concentration. Carbohydrate Table 2: Carbohydrate conc. (mg/l) in SMP Run C D F G No H 2 S ppm H 2 S ppm H 2 S ppm H 2 S After diffusion of H 2 S it was not found a significant variation of carbohydrate concentration in SMP sample and according to obtained result, effect of H 2 S on concentration of carbohydrate in SMP is not countable. Total Organic Carbon TOC, mg/l Effect of H 2 S on TOC D F G H I No H2S 5 ppm H2S 1 ppm H2S TOC has been measured for five set of samples and in two test results, TOC was not vary significantly when applied H 2 S in sludge but other three set of results show that TOC decreases with the increase of H 2 S diffusion. As total organic carbon play an important role of in fouling resistance, application of H 2 S in feed water deduce the TOC and consequently help to increase the filterability of membrane in MBRs. Conclusion This research works have focused on the impact of H 2 S diffusion on membrane fouling in MBR operation. ZP, ph, SMP, eeps have been used to investigate the effect of H 2 S application on membrane fouling behavior. From the obtained result, following specific conclusions could be drawn out: 1. Application of H 2 S in feed water assisted significantly to decrease the concentration of eeps and SMP TOC in sludge. It was found other studies that eeps and SMP have a vital role to enlarge the fouling resistance (Liang et al., 27). Therefore, diffusion of H 2 S enhanced permeability of membrane in MBR operation by limiting fouling resistance as it reduced the concentration of eeps and SMP compounds. 2. It is quite clear from obtained results that ph of biomass increased with the increase of H 2 S concentration. This elevated ph value facilitates the activities of the living microorganisms which could improve the whole treatment performance. 3. ZP of virgin membrane is lower than the fouling membrane but the effect of H 2 S application on zeta potential is dubious because it unevenly varied with different concentration of H 2 S. Fig 9: TOC of SMP at different H 2 S conc 6

7 List of Abbreviations COD - Chemical oxygen demand eeps - Extracted extracellular polymeric substances HRT - Hydraulic retention time MBR - Membrane bioreactor PVDF - Polyvinylidene fluoride SMP - Soluble microbial products TOC - Total organic carbon ZP - Zeta Potential fouling in MBRs (MBRs) caused by membrane polymer materials, Journal of Membrane Science, 28, (27, June 1) Acknowledgements I would like to thank to University of New South Wales, Australia for assistance and materials support, and to Lund University, Sweden for partial financial and sincere academic support to carry out this research work. References Huber S. A. (1998). Evidence for membrane fouling by specific TOC constituents, Desalination 119, Judd, S. (26). The MBR Book: Principles and Applications of Membrane Bioreactors in Water and Wastewater Treatment, Elsevier, Oxford. Le-Clech, P., Chen, V. and Fane, T.A.G. (26). Fouling in MBRs used in wastewater treatment, Journal of Membrane Science 284, Lesjean, B. and Judd, S. (27). MBR Network, Facts on MBR tech, (online) Available: (27, February 4). Liang, S., Liu, C. and Song, L (27). Soluble microbial products in MBR operation: Behaviors, characteristics, and fouling potential, Water Research 41, Lindquist, A. (23). About Water Treatment, Helsingborg, Sweden : Kemira Kemwater. Meng, F., Zhang, H., Yang, F., Zhang, S., Li, Y. and Zhang, X. (26). Identification of activated sludge properties affecting membrane fouling in submerged MBRs, Separation and Purification Technology 51, Yamato, N., Kimura, K., Miyoshi, T. and Watanabe, Y. (26). Difference in membrane 7