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Reduction of faecal coliform bacteria in sewage effluents using a microporous polymeric membrane
PII: S0043-1354(97)00344-8
Wat. Res. Vol. 32, No. 5, pp. 1417±1422, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
REDUCTION OF FAECAL COLIFORM BACTERIA IN SEWAGE EFFLUENTS USING A MICROPOROUS POLYMERIC MEMBRANE STEVEN W. TILL1, SIMON J. JUDD1* and BOB MCLOUGHLIN2 School Of Water Sciences, Cran®eld University, Cran®eld, Bedfordshire, MK43 0AL U.K. and 2Scimat Ltd., Dorcan 200, Murdock Road, Swindon, SN3 5HY U.K.
1
(First received November 1996; accepted in revised form August 1997) AbstractÐA study has been undertaken to assess the performance of an extruded, polymeric, tubular, micro®ltration membrane with respect to its ability to disinfect and clarify sewage euents. Membranes of two dierent pore sizes were investigated and the membrane module operated cyclically with 10 min on and 0.5 min o. Both membranes were seen to be eective at improving the physiochemical properties of the sewage euents, with signi®cant reductions in suspended solids (SS), chemical oxygen demand (COD) and turbidity. The smaller pore size membrane, however, was more ecient at removing faecal coliform bacteria (FCB) from the euents with a rejection that was comparable with existing membrane systems. It was seen that breakthrough of FCB occurs at the beginning of a cycle and that removal eciency quickly rises to a higher and more stable level of removal during the cycle. The performance, with reference to FCB rejection, against the primary euent was signi®cantly better than for the secondary euent for the larger pore size membrane. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐcross¯ow micro®ltration, faecal coliform bacteria, disinfection, sewage euent
INTRODUCTION
Municipal wastewater can contain the causative organisms of many infectious diseases such as typhoid, hepatitis and bacterial or amoebic dysentery. If drinking or recreational waters are subjected to faecal pollution then the risk of infection amongst the users of such waters is possible. Disinfection of sewage euents reduces this risk and helps to attain water quality standards such as the European Community bacteriological criteria for bathing waters (The Council of European Communities, 1976), and methods currently used for this duty include chlorination, ozonation and ultra-violet irradiation. However, all of these methodsÐand in particular the chemical methodsÐare capable of producing undesirable disinfection by-products (DBPs) and their ecacy depends much upon the quality of the feed water. Membranes oer an alternative for removal of faecal coliform bacteria (FCB) that requires no pretreatment of the euent prior to disinfection, as normally required by alternative unit process options, as well as producing a high-quality clari®ed euent. In addition, membrane systems impart no residual to the euent. Chlorine is known to react with organic compounds in the euent to form harmful chlorinated by-products (Chow and *Author to whom all correspondence should be addressed.
Roberts, 1979; Cooper et al., 1986). Although generating far lower levels of chlorinated DBPs, ozone is also claimed to react with many organic compounds to generate toxic products (Falk and Moyer, 1978). Lastly, UV irradiation is ostensibly limited to solutions having a transmissivity great enough to permit a sucient dose to reach all regions of the irradiated solution. Moreover, it has also been shown that UV can react with aromatic compounds and nitrate, both present in sewage euents, to produce compounds that exhibit mutagenic activity (Suzuki et al., 1982). In the past, membranes have been considered unsuitable for wastewater treatment due to high operating costs resulting from available membranes having low resilience, resulting in generally unsatisfactory reliability, and a short operation period between cleaning cycles. However, increasingly stringent standards (The Council of European Communities, 1991) are making the use of membrane systems more economically viable. This investigation was undertaken to assess the performance of the membrane treatment of primary and secondary sewage euents using a novel and ostensibly inexpensive polymeric membrane. Membranes of two dierent pore sizes were examined (0.45 mm and 1.2 mm) with particular reference to the ecacy of removal of FCB as well as the ability to improve the chemical and physical quality of the euent.
1417
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Steven W. Till et al. MATERIALS AND METHODS
Pilot plant The pilot-scale plant (Fig. 1) consisted of a 2.4 m length of the tubular membrane having an internal diameter of around 15 mm when in operation and a wall thickness of approximately 80 mm. The module was operated in the cross¯ow mode. The membrane, provided by Scimat (Swindon, U.K.), was fabricated by extrusion and ®brulation of polypropyleneÐa hydrophobic materialÐand its surface chemistry was not modi®ed. The method of manufacture ensured a narrow eective pore size distribution. The intrinsic mechanical weakness of the membrane demanded support in the form of a woven nylon sleeve, through which the membrane was threaded (Fig. 2). All aqueous feed waters were taken from the Cran®eld University Sewage Treatment Works. Primary settled euent, taken from the primary sedimentation tank at the works, or the secondary euent, from the biological trickling ®lters, was delivered to a continuously stirred feed tank and subsequently pumped through the membrane tube. Retentate was delivered back to the feed tank and permeate to drain. The pressure drop across the membrane was monitored by pressure gauges at the inlet and outlet manifolds. Operating retentate ¯ow rates of 25 l/ min (equivalent to a cross¯ow velocity of 2.4 m/s) were employed throughout with a corresponding transmembrane pressure of 0.6±0.8 bar. Partial cleaning of the membrane was achieved through its periodic collapsing under reduced feed pressure. The elastic properties of the membrane polymer are such that on removing the feed pressure the membrane reverts from its tubular con®guration to the ¯at con®guration of manufacture, thus dislodging accumulated solids. This cleaning procedure was automated by electronic timers actuating the feed pump. The system was operated in a cyclic
fashion, being ON for 10 min (membrane in operation) and then OFF for 0.5 min (cleaning period), this constituting one cycle (10.5 min). Experimental runs lasted for approximately 10 h (57 cycles). All of the permeate from each 10.5 min cycle was collected and the average ¯ux calculated accordingly. Physicochemical testing was carried out on the combined sample, such that data represented mean values for the entire cycle. Physicochemical testing Samples for suspended solids (SS) and total chemical oxygen demand (COD) analysis were taken at regular intervals during an experimental run and processed within 3 h of collection. Between 4 and 6 pairs of feed and permeate samples were analyzed for each run. SS determinations were carried out according to the standard methods (Department of the Environment/National Water Council, 1980) using a 0.2 mm pore size ®lter paper for the ®ltration step in place of the conventional 1.2 mm ®lter paper. Total COD determinations were carried out using a Hach COD Reactor (Model 45600) according to the procedure by Gibbs (1987). If dilution of the sample was necessary a 1:1 dilution with deionized water was carried out prior to heating. Absorbance was measured at a wavelength 420 nm with a Phillips PU8620 Spectrophotometer and COD values taken from comparison with a standard curve. Turbidity readings for the feed and permeate were taken using a Hach Ratio Turbidimeter. Bacteriological testing Due to the length of time involved in performing the bacterial tests only a limited number of samples could be analyzed. The system was operated until a steady ¯ux was
Fig. 1. Schematic representation of cross¯ow micro®ltration pilot scale plant.
Micro®ltration of sewage euents
1419
Fig. 2. Schematic representation of tubular membrane when in operation, showing the polypropylene membrane threaded through the woven nylon support and the direction of feed and permeate ¯ow. attained prior to taking regular samples across a single cycle. The pilot plant was operated for approximately 140 min (14 cycles) and following this 5 samples were collected, in sterile containers, over a period of one 10.5-min cycle. Samples were analyzed immediately following collection. FCB were enumerated using the standard membrane ®lter technique (Department of the Environment, 1982). Samples were asceptically ®ltered through 0.2 mm ®lter papers and then placed on ®lter pads impregnated with membrane lauryl sulphate broth (Oxoid MM615). They were then incubated at 308C for 4 h, to allow the bacteria to overcome shock and then further incubated in a circulating water bath at 44.58C for 14 h. Dilutions were made using quarter strength Ringers solution (Oxoid BR52).
RESULTS AND DISCUSSION
The physicochemical parameters of the primary and secondary treated and untreated euents are summarized in Table 1. The recycling of the retentate to the feed tank was seen to have a negligible eect in concentrating SS and COD in the feed. Moreover, the mean COD and turbidity rejection per cycle was found to be largely independent of time following the initial equilibration period of 140 min. The decrease in ¯ux over the course of the cycle was not quanti®ed, although it was noted that
the rapid decline to a more stable level generally occurred in the ®rst 2 min of the cycle. Reduction of faecal coliform bacteria Investigation of the permeate quality during one 10.5-min cycle, from start-up until collapse, revealed that breakthrough of FCB is seen for both membranes initially, giving reduced rejection (Fig. 3). Rejection of FCB rose rapidly during the ®rst 3 min of the cycle before reaching equilibrium. The performance of both membranes appeared to improve on challenging with primary rather than secondary euents. When the 1.2 mm pore size membrane was used to treat the secondary euent the initial breakthrough of FCB was such that a 0.4 log reduction was observed. The stabilization period for FCB rejection was observed to be longer under these conditions, and the average log reduction value across the entire cycle was only 1.3. When used to treat the primary euent the 1.2 mm pore size membrane was seen to perform signi®cantly better. Initial breakthrough of FCB was less (achieving a 2.3 log reduction) and a stable level of FCB rejection was achieved more quickly. The average log reduction of FCB throughout the cycle was 3.3.
Table 1. Mean values for feed and permeate water quality for primary and secondary euents: whole cycle average values at equilibrium Primary Euent Parameter Bacterial counts (CFU*/100 mL) Log reductions of FCB COD (mg/L) SS (mg/L) Turbidity (NTU) Temperature (8C) *CFU: colony forming unit.
Pore size 0.45 mm 1.2 mm 0.45 mm 1.2 mm 0.45 mm 1.2 mm 0.45 mm 1.2 mm 0.45 mm 1.2 mm
Feed
Secondary Euent
Permeate 7
2.7 10 1.0 107 4.8 3.3 194.0 242.2 110 98.8 80.8 97.9 14.5
2
9.1 10 1.1 104 54.6 83.8 1.8 2.1 2.3 2.4
Feed
Permeate 6
2.2 10 3.8 106 4.1 1.3 134.3 145.9 62 65.7 42.7 48.4 15.5
4.4 102 3.9 105 47.0 54.5 1.5 1.5 3.3 3.9
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Steven W. Till et al.
Fig. 3. Rejection of FCB during the period between startup and collapse of the membrane during a single 10.5 min cycle. ÐWÐ 0.45 mm (PE); - - r - - 0.45 mm (SE); ÐRÐ 1.2 mm (PE); - - r - - 1.2 mm (SE).
Although the 0.45 mm pore size membrane was seen to perform less eectively when used to treat the secondary euent the reduction of FCB was still greater than that obtained using the 1.2 mm pore size membrane with either euent. Initial log reduction of FCB was 3.1 with an average log reduction of 4.1 over the cycle. The best performance was seen when the 0.45 mm pore size membrane was used to treat the primary euent. Initial log reduction of FCB was 4.3 with mean log reduction of 4.8 per cycle. In both cases a relatively stable level of removal was reached within the ®rst 3 min of the cycle. Both membranes were seen to be competitive with existing membrane systems in terms of FCB removal when used to treat the primary euent (Table 2). The 1.2 mm pore size membrane was largely ineffective in removing FCB from the secondary euent, although the 0.45 mm membrane performance was still comparable with existing systems. Analysis of treatment with the 1.2 mm pore size membrane
showed that the concentration of FCB in the permeate was at least an order of magnitude greater when challenged with the secondary euent compared to the primary euent (Table 1). This may be explained by the dierent nature of the euents. The fact that the 1.2 mm pore size membrane was seen to reject more FCB when challenged with the primary euent, which had a higher concentration of FCB than the secondary euent, suggests that some constituent of the primary euent is capable of forming a dynamic layer on the membrane surface such that the removal of FCB is enhanced. Analysis of treatment using the 0.45 mm pore size membrane failed to show a similar relation between the two euent types, suggesting that the smaller pore size does not bene®t from any deposit onto the membrane surface that may form from the primary euent. The breakthrough of FCB is important from a consideration of the operating conditions of the system. The eect of the cycle times on the ¯ux was investigated by varying the length of time that the system was operated for and the length of time of the cleaning period. Four dierent cycle time combinations were investigated: 10 min ON/1 min OFF, 5 min ON/1.5 min OFF, 10 min ON/2 min OFF and 10 min ON/5 min OFF (Fig. 4) It was seen that the equilibrium ¯ux was almost independent of the cycle mode, i.e. having the system turned o for longer periods does not facilitate a more eective cleaning regime. Ecient FCB rejection occurs after approximately 3 min and so if shorter cycle times are employed the overall performance of the membrane, with respect to FCB removal, will be diminished. Cycles incorporating longer operation and shorter cleaning times thus improve the overall membrane performance. Physiochemical analysis The mean whole-cycle SS removal eciencies for the 0.45 mm and 1.2 mm pore size membranes were >99% and 98% respectively (Table 2). These values did not signi®cantly change with time once equilibrium had been reached, and compared favourably with those reported for other systems (Gosling and Realey, 1992; MacCormick, 1992; Job and Realey, 1993). Associated with this removal of SS was a reduction in the COD of the euent. For
Table 2. Comparison of membrane performances Average Log Reduction FCB
Reduction SS (%)
Reduction Turbidity (%)
Reduction COD (%)
Membrane
Pore size (mm)
PE
SE
PE
SE
PE
SE
PE
SE
Scimat Scimat Memcor Stork Renovexx
1.2 0.45 0.2 0.05±0.2 0.5±1.5
3.3 4.8 3.8* 2.5* 3.3*
1.3 4.1 >5$ NA NA
98 >99 92* 096* >91*}
98 >99 >99$ NA NA
98 97 94% NA 98}
92 94 99% NA NA
64.0 72.5 NA NA 60±68*}
62.5 72.0 NA NA NA
*Job and Realey (1993); $MacCormick (1992); %Kolega et al. (1991); }Gosling and Realey (1992). NA = Not Available; PE = Primary Euent; SE = Secondary Euent.
Micro®ltration of sewage euents
Fig. 4. Average ¯ux versus time representation for dierent cycle time combinations. - - Q - - 10 min ON/1 min OFF; ÐRÐ 5 min ON 1.5 min OFF; - - . - - 10 min ON/ 2 min OFF; ÐWÐ 10 min ON/5 min OFF.
each membrane the reduction in COD was about the same regardless of the euent type. The 0.45 mm pore size membrane had a higher reduction in COD (072%) than the 1.2 mm pore size membrane (063%). This can be attributed to the increased solids removal by the smaller pore size membrane. The reduction of turbidity was higher for the primary euent than for the secondary euent but approximately equal for both pore sizes. The removal of turbidity using the Memcor system showed a higher eciency with the secondary euent than with the primary euent (Kolega et al., 1991). The dierences are almost certainly due to diering euent compositions in each study. Cleaning requirement Memcor, Stork and Renovexx systems incorporate sophisticated regeneration/cleaning techniques which incur downtime. The Memcor system requires a 60±90 s backwash every 12±15 min. The Renovexx system relies on the periodic formation of a dynamic membrane of hydrolyzed aluminium coagulant on an inert non-selective polymeric substrate. The cleaning system used in this study for the Scimat membrane is thus comparatively simple and the downtime involved not unreasonably long. Having said this, the equilibration time of 140 min could not be countenanced in a real system, and the performance of the membrane over this initial period needs to be analyzed further. CONCLUSIONS
Reduction of FCB using the Scimat membrane at the two dierent pore sizes was comparable with existing commercial membrane systems when used to treat the primary euent. The 0.45 mm pore size membrane was particularly eective with the primary euent. The 1.2 mm membrane was not eective in reducing FCB when used to treat the secondary euent, whereas the smaller pore size membrane was still comparable with available membranes.
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Breakthrough of FCB was seen immediately upon start-up of the cycle, reducing the overall removal eciency. The removal of FCB was seen to increase within the ®rst 3 min to a more stable level. The 1.2 mm pore size membrane was seen to take 5 min to reach a similarly stable level when used to treat the secondary euent. The precise cycling mode for operation and cleaning does not appear to have a signifcant eect on the ultimate permeate ¯ux. The eectiveness of the system could therefore be increased by utilising longer operation times and shorter cleaning times. Reduction of SS was greater for the 0.45 mm pore size membrane than for the 1.2 mm pore size membrane. There was no dierence in the removal of SS between the primary and secondary euents. The Scimat membranes were at least as eective at removing SS as existing membrane systems and were seen to achieve greater removal of SS when treating the primary euent, implying the formation of a permselective dynamic layer. Reduction of turbidity did not appear to depend upon membrane pore size. However, greater removal was achieved for the primary euent than for the secondary euent. Greater COD removal was also observed for the smaller pore size membrane, an observation consistent with the corresponding greater solids removal recorded at lower pore sizes. REFERENCES
The Council of European Communities (1991). The urban wastewater treatment directive (91/271/EEC). Ocial Journal Of The European Communities, 34(L-135), 40± 45. The Council of European Communities (1976). The bathing water directive (76/160/EEC). Ocial Journal Of The European Communities, L-31, 1±7. Chow B. and Roberts P. (1979). Halogenated organics production in sewage disinfection with chlorine dioxide and chlorine. Presented at the National Conference On Environmental Engineering Processe, July 9±11th, San Francisco, California, USA. Published by ASCE. Cooper W. J., Villate J. T., Oh E. M., Slifker R. A., Parsons F. Z. and Graves G. A. (1986). Formation of organohalogen compounds in chlorinated secondary wastewater euent. In Proceedings Of The Fifth Conference On Water Chlorination: Chemistry, Environmental Impact And Health Eects, 1986. Department of the Environment (1982) Department of Health and Social Security and Public Health Laboratory Service. The bacteriological examination of drinking water supplies: reports on public health and medical subjects No. 71: methods for the examination of water and associated materials. Published by HMSO, London, UK. ISBN 0-11-75675-9. Department of the Environment/National Water Council (1980). Suspended, settleable and total dissolved solids in waters and euents. In Methods For The Examination Of Water And Associated Materials. Published by HMSO, London, UK. Falk H. L. and Moyer J. E. (1978). Ozone as a disinfectant of water. In Ozone/Chlorine Dioxide Oxidation Products Of Organic Materials, (Edited by Rice R. G. and Cotruvo J. A.). Published by Ozone Press
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International, Cleveland, Ohio, USA. ISBN 0-91865002-X. Gibbs C. R. (1987). Introduction to chemical oxygen demand. In Technical Information Series, Booklet Number 8. Published by Hach Company, Colorado, USA. Gosling P. and Realey G. (1992). Experience with the Renovexx micro®ltration system. Presented at the Disinfection Of Urban Waste Water conference. 16th September 1992. Job G. and Realey G. (1993). Water disinfection using micro®lters. Presented at the East Anglian branch meeting of IWEM, 18th November 1993, University of Essex, Colchester.
Kolega M., Grohmann G. S., Chiew R. F. and Day A. W. (1991) Disinfection and clari®cation of treated sewage by advanced micro®ltration. Wat. Sci. Tech. 23(79), 1609±1618. MacCormick A. B. (1992). Continuous micro®ltration in water and waste applications. Presented at the Innovative Wastewater Treatment Technologies Export Opportunities conference. Organised by AWWA. 2nd July, 1992, Technology Park, Perth, Australia. Suzuki J., Okazaki H., Nishi Y. and Suzuki S. (1982) Formation of mutagens by photolysis of aromatic compounds in aqueous nitrate solution. Bulletin of Environmental Contamination Toxicology 29, 511±516.
Wat. Res. Vol. 32, No. 5, pp. 1417±1422, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
REDUCTION OF FAECAL COLIFORM BACTERIA IN SEWAGE EFFLUENTS USING A MICROPOROUS POLYMERIC MEMBRANE STEVEN W. TILL1, SIMON J. JUDD1* and BOB MCLOUGHLIN2 School Of Water Sciences, Cran®eld University, Cran®eld, Bedfordshire, MK43 0AL U.K. and 2Scimat Ltd., Dorcan 200, Murdock Road, Swindon, SN3 5HY U.K.
1
(First received November 1996; accepted in revised form August 1997) AbstractÐA study has been undertaken to assess the performance of an extruded, polymeric, tubular, micro®ltration membrane with respect to its ability to disinfect and clarify sewage euents. Membranes of two dierent pore sizes were investigated and the membrane module operated cyclically with 10 min on and 0.5 min o. Both membranes were seen to be eective at improving the physiochemical properties of the sewage euents, with signi®cant reductions in suspended solids (SS), chemical oxygen demand (COD) and turbidity. The smaller pore size membrane, however, was more ecient at removing faecal coliform bacteria (FCB) from the euents with a rejection that was comparable with existing membrane systems. It was seen that breakthrough of FCB occurs at the beginning of a cycle and that removal eciency quickly rises to a higher and more stable level of removal during the cycle. The performance, with reference to FCB rejection, against the primary euent was signi®cantly better than for the secondary euent for the larger pore size membrane. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐcross¯ow micro®ltration, faecal coliform bacteria, disinfection, sewage euent
INTRODUCTION
Municipal wastewater can contain the causative organisms of many infectious diseases such as typhoid, hepatitis and bacterial or amoebic dysentery. If drinking or recreational waters are subjected to faecal pollution then the risk of infection amongst the users of such waters is possible. Disinfection of sewage euents reduces this risk and helps to attain water quality standards such as the European Community bacteriological criteria for bathing waters (The Council of European Communities, 1976), and methods currently used for this duty include chlorination, ozonation and ultra-violet irradiation. However, all of these methodsÐand in particular the chemical methodsÐare capable of producing undesirable disinfection by-products (DBPs) and their ecacy depends much upon the quality of the feed water. Membranes oer an alternative for removal of faecal coliform bacteria (FCB) that requires no pretreatment of the euent prior to disinfection, as normally required by alternative unit process options, as well as producing a high-quality clari®ed euent. In addition, membrane systems impart no residual to the euent. Chlorine is known to react with organic compounds in the euent to form harmful chlorinated by-products (Chow and *Author to whom all correspondence should be addressed.
Roberts, 1979; Cooper et al., 1986). Although generating far lower levels of chlorinated DBPs, ozone is also claimed to react with many organic compounds to generate toxic products (Falk and Moyer, 1978). Lastly, UV irradiation is ostensibly limited to solutions having a transmissivity great enough to permit a sucient dose to reach all regions of the irradiated solution. Moreover, it has also been shown that UV can react with aromatic compounds and nitrate, both present in sewage euents, to produce compounds that exhibit mutagenic activity (Suzuki et al., 1982). In the past, membranes have been considered unsuitable for wastewater treatment due to high operating costs resulting from available membranes having low resilience, resulting in generally unsatisfactory reliability, and a short operation period between cleaning cycles. However, increasingly stringent standards (The Council of European Communities, 1991) are making the use of membrane systems more economically viable. This investigation was undertaken to assess the performance of the membrane treatment of primary and secondary sewage euents using a novel and ostensibly inexpensive polymeric membrane. Membranes of two dierent pore sizes were examined (0.45 mm and 1.2 mm) with particular reference to the ecacy of removal of FCB as well as the ability to improve the chemical and physical quality of the euent.
1417
1418
Steven W. Till et al. MATERIALS AND METHODS
Pilot plant The pilot-scale plant (Fig. 1) consisted of a 2.4 m length of the tubular membrane having an internal diameter of around 15 mm when in operation and a wall thickness of approximately 80 mm. The module was operated in the cross¯ow mode. The membrane, provided by Scimat (Swindon, U.K.), was fabricated by extrusion and ®brulation of polypropyleneÐa hydrophobic materialÐand its surface chemistry was not modi®ed. The method of manufacture ensured a narrow eective pore size distribution. The intrinsic mechanical weakness of the membrane demanded support in the form of a woven nylon sleeve, through which the membrane was threaded (Fig. 2). All aqueous feed waters were taken from the Cran®eld University Sewage Treatment Works. Primary settled euent, taken from the primary sedimentation tank at the works, or the secondary euent, from the biological trickling ®lters, was delivered to a continuously stirred feed tank and subsequently pumped through the membrane tube. Retentate was delivered back to the feed tank and permeate to drain. The pressure drop across the membrane was monitored by pressure gauges at the inlet and outlet manifolds. Operating retentate ¯ow rates of 25 l/ min (equivalent to a cross¯ow velocity of 2.4 m/s) were employed throughout with a corresponding transmembrane pressure of 0.6±0.8 bar. Partial cleaning of the membrane was achieved through its periodic collapsing under reduced feed pressure. The elastic properties of the membrane polymer are such that on removing the feed pressure the membrane reverts from its tubular con®guration to the ¯at con®guration of manufacture, thus dislodging accumulated solids. This cleaning procedure was automated by electronic timers actuating the feed pump. The system was operated in a cyclic
fashion, being ON for 10 min (membrane in operation) and then OFF for 0.5 min (cleaning period), this constituting one cycle (10.5 min). Experimental runs lasted for approximately 10 h (57 cycles). All of the permeate from each 10.5 min cycle was collected and the average ¯ux calculated accordingly. Physicochemical testing was carried out on the combined sample, such that data represented mean values for the entire cycle. Physicochemical testing Samples for suspended solids (SS) and total chemical oxygen demand (COD) analysis were taken at regular intervals during an experimental run and processed within 3 h of collection. Between 4 and 6 pairs of feed and permeate samples were analyzed for each run. SS determinations were carried out according to the standard methods (Department of the Environment/National Water Council, 1980) using a 0.2 mm pore size ®lter paper for the ®ltration step in place of the conventional 1.2 mm ®lter paper. Total COD determinations were carried out using a Hach COD Reactor (Model 45600) according to the procedure by Gibbs (1987). If dilution of the sample was necessary a 1:1 dilution with deionized water was carried out prior to heating. Absorbance was measured at a wavelength 420 nm with a Phillips PU8620 Spectrophotometer and COD values taken from comparison with a standard curve. Turbidity readings for the feed and permeate were taken using a Hach Ratio Turbidimeter. Bacteriological testing Due to the length of time involved in performing the bacterial tests only a limited number of samples could be analyzed. The system was operated until a steady ¯ux was
Fig. 1. Schematic representation of cross¯ow micro®ltration pilot scale plant.
Micro®ltration of sewage euents
1419
Fig. 2. Schematic representation of tubular membrane when in operation, showing the polypropylene membrane threaded through the woven nylon support and the direction of feed and permeate ¯ow. attained prior to taking regular samples across a single cycle. The pilot plant was operated for approximately 140 min (14 cycles) and following this 5 samples were collected, in sterile containers, over a period of one 10.5-min cycle. Samples were analyzed immediately following collection. FCB were enumerated using the standard membrane ®lter technique (Department of the Environment, 1982). Samples were asceptically ®ltered through 0.2 mm ®lter papers and then placed on ®lter pads impregnated with membrane lauryl sulphate broth (Oxoid MM615). They were then incubated at 308C for 4 h, to allow the bacteria to overcome shock and then further incubated in a circulating water bath at 44.58C for 14 h. Dilutions were made using quarter strength Ringers solution (Oxoid BR52).
RESULTS AND DISCUSSION
The physicochemical parameters of the primary and secondary treated and untreated euents are summarized in Table 1. The recycling of the retentate to the feed tank was seen to have a negligible eect in concentrating SS and COD in the feed. Moreover, the mean COD and turbidity rejection per cycle was found to be largely independent of time following the initial equilibration period of 140 min. The decrease in ¯ux over the course of the cycle was not quanti®ed, although it was noted that
the rapid decline to a more stable level generally occurred in the ®rst 2 min of the cycle. Reduction of faecal coliform bacteria Investigation of the permeate quality during one 10.5-min cycle, from start-up until collapse, revealed that breakthrough of FCB is seen for both membranes initially, giving reduced rejection (Fig. 3). Rejection of FCB rose rapidly during the ®rst 3 min of the cycle before reaching equilibrium. The performance of both membranes appeared to improve on challenging with primary rather than secondary euents. When the 1.2 mm pore size membrane was used to treat the secondary euent the initial breakthrough of FCB was such that a 0.4 log reduction was observed. The stabilization period for FCB rejection was observed to be longer under these conditions, and the average log reduction value across the entire cycle was only 1.3. When used to treat the primary euent the 1.2 mm pore size membrane was seen to perform signi®cantly better. Initial breakthrough of FCB was less (achieving a 2.3 log reduction) and a stable level of FCB rejection was achieved more quickly. The average log reduction of FCB throughout the cycle was 3.3.
Table 1. Mean values for feed and permeate water quality for primary and secondary euents: whole cycle average values at equilibrium Primary Euent Parameter Bacterial counts (CFU*/100 mL) Log reductions of FCB COD (mg/L) SS (mg/L) Turbidity (NTU) Temperature (8C) *CFU: colony forming unit.
Pore size 0.45 mm 1.2 mm 0.45 mm 1.2 mm 0.45 mm 1.2 mm 0.45 mm 1.2 mm 0.45 mm 1.2 mm
Feed
Secondary Euent
Permeate 7
2.7 10 1.0 107 4.8 3.3 194.0 242.2 110 98.8 80.8 97.9 14.5
2
9.1 10 1.1 104 54.6 83.8 1.8 2.1 2.3 2.4
Feed
Permeate 6
2.2 10 3.8 106 4.1 1.3 134.3 145.9 62 65.7 42.7 48.4 15.5
4.4 102 3.9 105 47.0 54.5 1.5 1.5 3.3 3.9
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Steven W. Till et al.
Fig. 3. Rejection of FCB during the period between startup and collapse of the membrane during a single 10.5 min cycle. ÐWÐ 0.45 mm (PE); - - r - - 0.45 mm (SE); ÐRÐ 1.2 mm (PE); - - r - - 1.2 mm (SE).
Although the 0.45 mm pore size membrane was seen to perform less eectively when used to treat the secondary euent the reduction of FCB was still greater than that obtained using the 1.2 mm pore size membrane with either euent. Initial log reduction of FCB was 3.1 with an average log reduction of 4.1 over the cycle. The best performance was seen when the 0.45 mm pore size membrane was used to treat the primary euent. Initial log reduction of FCB was 4.3 with mean log reduction of 4.8 per cycle. In both cases a relatively stable level of removal was reached within the ®rst 3 min of the cycle. Both membranes were seen to be competitive with existing membrane systems in terms of FCB removal when used to treat the primary euent (Table 2). The 1.2 mm pore size membrane was largely ineffective in removing FCB from the secondary euent, although the 0.45 mm membrane performance was still comparable with existing systems. Analysis of treatment with the 1.2 mm pore size membrane
showed that the concentration of FCB in the permeate was at least an order of magnitude greater when challenged with the secondary euent compared to the primary euent (Table 1). This may be explained by the dierent nature of the euents. The fact that the 1.2 mm pore size membrane was seen to reject more FCB when challenged with the primary euent, which had a higher concentration of FCB than the secondary euent, suggests that some constituent of the primary euent is capable of forming a dynamic layer on the membrane surface such that the removal of FCB is enhanced. Analysis of treatment using the 0.45 mm pore size membrane failed to show a similar relation between the two euent types, suggesting that the smaller pore size does not bene®t from any deposit onto the membrane surface that may form from the primary euent. The breakthrough of FCB is important from a consideration of the operating conditions of the system. The eect of the cycle times on the ¯ux was investigated by varying the length of time that the system was operated for and the length of time of the cleaning period. Four dierent cycle time combinations were investigated: 10 min ON/1 min OFF, 5 min ON/1.5 min OFF, 10 min ON/2 min OFF and 10 min ON/5 min OFF (Fig. 4) It was seen that the equilibrium ¯ux was almost independent of the cycle mode, i.e. having the system turned o for longer periods does not facilitate a more eective cleaning regime. Ecient FCB rejection occurs after approximately 3 min and so if shorter cycle times are employed the overall performance of the membrane, with respect to FCB removal, will be diminished. Cycles incorporating longer operation and shorter cleaning times thus improve the overall membrane performance. Physiochemical analysis The mean whole-cycle SS removal eciencies for the 0.45 mm and 1.2 mm pore size membranes were >99% and 98% respectively (Table 2). These values did not signi®cantly change with time once equilibrium had been reached, and compared favourably with those reported for other systems (Gosling and Realey, 1992; MacCormick, 1992; Job and Realey, 1993). Associated with this removal of SS was a reduction in the COD of the euent. For
Table 2. Comparison of membrane performances Average Log Reduction FCB
Reduction SS (%)
Reduction Turbidity (%)
Reduction COD (%)
Membrane
Pore size (mm)
PE
SE
PE
SE
PE
SE
PE
SE
Scimat Scimat Memcor Stork Renovexx
1.2 0.45 0.2 0.05±0.2 0.5±1.5
3.3 4.8 3.8* 2.5* 3.3*
1.3 4.1 >5$ NA NA
98 >99 92* 096* >91*}
98 >99 >99$ NA NA
98 97 94% NA 98}
92 94 99% NA NA
64.0 72.5 NA NA 60±68*}
62.5 72.0 NA NA NA
*Job and Realey (1993); $MacCormick (1992); %Kolega et al. (1991); }Gosling and Realey (1992). NA = Not Available; PE = Primary Euent; SE = Secondary Euent.
Micro®ltration of sewage euents
Fig. 4. Average ¯ux versus time representation for dierent cycle time combinations. - - Q - - 10 min ON/1 min OFF; ÐRÐ 5 min ON 1.5 min OFF; - - . - - 10 min ON/ 2 min OFF; ÐWÐ 10 min ON/5 min OFF.
each membrane the reduction in COD was about the same regardless of the euent type. The 0.45 mm pore size membrane had a higher reduction in COD (072%) than the 1.2 mm pore size membrane (063%). This can be attributed to the increased solids removal by the smaller pore size membrane. The reduction of turbidity was higher for the primary euent than for the secondary euent but approximately equal for both pore sizes. The removal of turbidity using the Memcor system showed a higher eciency with the secondary euent than with the primary euent (Kolega et al., 1991). The dierences are almost certainly due to diering euent compositions in each study. Cleaning requirement Memcor, Stork and Renovexx systems incorporate sophisticated regeneration/cleaning techniques which incur downtime. The Memcor system requires a 60±90 s backwash every 12±15 min. The Renovexx system relies on the periodic formation of a dynamic membrane of hydrolyzed aluminium coagulant on an inert non-selective polymeric substrate. The cleaning system used in this study for the Scimat membrane is thus comparatively simple and the downtime involved not unreasonably long. Having said this, the equilibration time of 140 min could not be countenanced in a real system, and the performance of the membrane over this initial period needs to be analyzed further. CONCLUSIONS
Reduction of FCB using the Scimat membrane at the two dierent pore sizes was comparable with existing commercial membrane systems when used to treat the primary euent. The 0.45 mm pore size membrane was particularly eective with the primary euent. The 1.2 mm membrane was not eective in reducing FCB when used to treat the secondary euent, whereas the smaller pore size membrane was still comparable with available membranes.
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Breakthrough of FCB was seen immediately upon start-up of the cycle, reducing the overall removal eciency. The removal of FCB was seen to increase within the ®rst 3 min to a more stable level. The 1.2 mm pore size membrane was seen to take 5 min to reach a similarly stable level when used to treat the secondary euent. The precise cycling mode for operation and cleaning does not appear to have a signifcant eect on the ultimate permeate ¯ux. The eectiveness of the system could therefore be increased by utilising longer operation times and shorter cleaning times. Reduction of SS was greater for the 0.45 mm pore size membrane than for the 1.2 mm pore size membrane. There was no dierence in the removal of SS between the primary and secondary euents. The Scimat membranes were at least as eective at removing SS as existing membrane systems and were seen to achieve greater removal of SS when treating the primary euent, implying the formation of a permselective dynamic layer. Reduction of turbidity did not appear to depend upon membrane pore size. However, greater removal was achieved for the primary euent than for the secondary euent. Greater COD removal was also observed for the smaller pore size membrane, an observation consistent with the corresponding greater solids removal recorded at lower pore sizes. REFERENCES
The Council of European Communities (1991). The urban wastewater treatment directive (91/271/EEC). Ocial Journal Of The European Communities, 34(L-135), 40± 45. The Council of European Communities (1976). The bathing water directive (76/160/EEC). Ocial Journal Of The European Communities, L-31, 1±7. Chow B. and Roberts P. (1979). Halogenated organics production in sewage disinfection with chlorine dioxide and chlorine. Presented at the National Conference On Environmental Engineering Processe, July 9±11th, San Francisco, California, USA. Published by ASCE. Cooper W. J., Villate J. T., Oh E. M., Slifker R. A., Parsons F. Z. and Graves G. A. (1986). Formation of organohalogen compounds in chlorinated secondary wastewater euent. In Proceedings Of The Fifth Conference On Water Chlorination: Chemistry, Environmental Impact And Health Eects, 1986. Department of the Environment (1982) Department of Health and Social Security and Public Health Laboratory Service. The bacteriological examination of drinking water supplies: reports on public health and medical subjects No. 71: methods for the examination of water and associated materials. Published by HMSO, London, UK. ISBN 0-11-75675-9. Department of the Environment/National Water Council (1980). Suspended, settleable and total dissolved solids in waters and euents. In Methods For The Examination Of Water And Associated Materials. Published by HMSO, London, UK. Falk H. L. and Moyer J. E. (1978). Ozone as a disinfectant of water. In Ozone/Chlorine Dioxide Oxidation Products Of Organic Materials, (Edited by Rice R. G. and Cotruvo J. A.). Published by Ozone Press
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International, Cleveland, Ohio, USA. ISBN 0-91865002-X. Gibbs C. R. (1987). Introduction to chemical oxygen demand. In Technical Information Series, Booklet Number 8. Published by Hach Company, Colorado, USA. Gosling P. and Realey G. (1992). Experience with the Renovexx micro®ltration system. Presented at the Disinfection Of Urban Waste Water conference. 16th September 1992. Job G. and Realey G. (1993). Water disinfection using micro®lters. Presented at the East Anglian branch meeting of IWEM, 18th November 1993, University of Essex, Colchester.
Kolega M., Grohmann G. S., Chiew R. F. and Day A. W. (1991) Disinfection and clari®cation of treated sewage by advanced micro®ltration. Wat. Sci. Tech. 23(79), 1609±1618. MacCormick A. B. (1992). Continuous micro®ltration in water and waste applications. Presented at the Innovative Wastewater Treatment Technologies Export Opportunities conference. Organised by AWWA. 2nd July, 1992, Technology Park, Perth, Australia. Suzuki J., Okazaki H., Nishi Y. and Suzuki S. (1982) Formation of mutagens by photolysis of aromatic compounds in aqueous nitrate solution. Bulletin of Environmental Contamination Toxicology 29, 511±516.