Biofouling Control by Biological Activated Carbon Filtration: a Promising Method for WWTP Effluent Reuse

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1 ID : 95 Session: domestic wastewater reuse Biofouling Control by Biological Activated Carbon Filtration: a Promising Method for WWTP Effluent Reuse Peter van der Maas 1,2, Elbert Majoor 3, Jan C. Schippers 4 1. Waterleidingmaatschappij Drenthe (WMD), P.O. Box 18, 9400 AA Assen, The Netherlands 2. WLN, P.O. Box 26, 9470 AA Zuidlaren, The Netherlands ( p.vd.maas@wln.nl) 3. Waterschap Velt en Vecht, P.O. Box 330, 7740 AH Coevorden, The Netherlands ( e.majoor@veltenvecht.nl) 4. Global Membrains, P.O. Box 2147, 5202 CC 's-hertogenbosch, The Netherlands ( j.schippers@globalmembrains.nl) Keywords: biofouling control; biological activated carbon; effluent reuse; nutrient limitation ABSTRACT This paper describes the novel application of biological activated carbon filtration (BACF) with oxygen supply to control biofouling on reverse osmosis (RO) membranes without using biocides. Pilot research was performed in the scope of large scale (10,000 m 3.d -1 ) ultra pure water production in Emmen (The Netherlands) using wwtp effluent as water source. The results obtained show that BACF with oxygen supply is a promising technology for the prevention of biofouling on reverse osmosis membranes. In case the wwtp effluent was only pretreated by ultrafiltration (UF), an exponential increase of the RO trans membrane pressure was observed after only 3 days of operation, indicating the occurrence of biofouling on the RO module. In case the wwtp effluent was pretreated by UF and additional BACF with oxygen supply, biofouling was not observed during the entire runtime of the experiment (120 days). It is presumed that BAC filters remove easily degradable carbon, which is present in the wwtp effluent and not removed ultrafiltration. The consequence is a lower concentration of growth limiting substrate in the RO feed water. It is concluded that, especially from environmental point of view, BACF with oxygen supply is an attractive alternative to conventional biofouling abatement with biocides. INTRODUCTION By 2010, a major part of the effluent of wastewater treatment plant (WWTP) Emmen (Nieuw Amsterdam, The Netherlands) will be used for the production of boiler feed water. NieuWater BV, a joint venture of Waterschap Velt & Vecht and Waterleidingmaatschappij Drenthe, will supply this ultra pure water (UPW) to NAM BV. The latter company will use this water (e.g. as steam) for oil recovery from the Schoonebeek oil field in the North East of the Netherlands. The ultra pure water (conductivity < 0.2 µs/cm) will be produced from wwtp effluent by the application of integrated membrane technology, i.e. ultrafiltration (UF) and double pass 1

2 revered osmosis (RO), followed by electro deionization (EDI). The capacity of the UPW facility will be 10,000 m 3.d -1. Since wwtp effluents contain a relatively high concentration of nutrients, they have a high potential for creating severe biofouling on RO membranes (Fleming et al., 1997). Biofouling increases the feed channel pressure loss and flux reduction. This seriously hampers the application of spiralwound RO membranes in case the feed water is wwtp effeluent. Biofouling on RO membranes can be controlled by the application of biocides (Kim et al., 2009). However, since these chemicals may affect the environment, the use of biocides should be minimized from environmental and ecological point of view. To avoid the use of biocides, the challenge of this project was to control biofouling by means of nutrient limitation. This paper describes the novel application of biological activated carbon filtration (BACF) with oxygen supply to control biofouling on RO membranes without using biocides. METHODS Pilot research was conducted using a small scale (2 m 3 /h) ultrafiltration installation followed by two step BACF with a residence time of, respectively, 10 and 30 minutes (Fig 1). Biofouling was monitored by means of the increase of the trans membrane pressure (TMP) as well as the decrease of the membrane transfer coefficient (MTC), using a RO installation with one 4 inch module. A second RO installation was used to monitor biofouling with and without the dosing of a biocide (Fig.1). The UF installation (Zeenon Zeeweed 500) was operated at a net flux 28 l.m -2.h -1. The membranes were backflushed every 6 minutes and the concentrate was drained twice a day. Chemical cleaning was carried out twice a week using caustic soda (30 min., ph 10.5) and Divos2 (JohnsonDiversey, The Netherlands) at ph 3 for 20 minutes. The activated carbon in the BAC filters (Norit 830P bedheight 1.5 and 1.9 m, respectively) was already completely loaded with NOM by means of filtration wwtp effluent pretreated by UF prior to this research. Pure oxygen (Linde, Germany) was supplied to the BAC filters to avoid anaerobic conditions using a dissolved oxygen (DO) setpoint of 5 mg.l -1 for the BAC filtrate. The oxygen supply was controlled on setpoint automatically by means of mass flow controllers (Brooks, The Netherlands). The BAC filters were flushed back once or twice a week, i.e. depending on the resistance of the filterbed, with a bed expansion of 45%. Both RO installations (4 inch modules, Hydranautics ESPA2) were operated with a flow rate of 1 m 3.h -1 and a recovery of 10%, corresponding to a flux of 13 l.m -2.h -1. Prior to the membrane module, the feed water passed a cartridge filter (Logisticon, The Netherlands) with a pore size of 1 µm. No antiscalant was dosed. During the research period the RO module fed with BAC filtrate was cleaned two times (days 63 and 78, i.e. when the normalized pressure drop exceeded 15 kpa) according to the following protocol: flushing with permeate, followed by NaHSO 3 (30%), ph adjustment to 10.5 using caustic soda at 35 C. Soaking (15 min.) and circulation (15 min.) 6 times. Then the membrane was flushed with RO permeate until chemicals are removed (indicated by ph). Then an acid cleaning was performed using Divos2 at ph 2.6. Soaking (30 min.) and circulation (30 min.) 2 times. Finally, the membrane was flushed with RO permeate. To investigate the effect of BACF on biofouling, the reference RO module was fed directly by UF permeate with optional dosing of the biocide 2,2-dibromo-2-cyano-acetamide (DBNPA, DOW, The Nehterlands). In case DBNPA was dosed, this was done to the RO feedwater (just before the cartridge filter) once a day during 30 minutes at a concentration of 20 ppm. 2

3 Figure 1 Simplified flow scheme of the pilot facility. The biofilm formation rate (BFR) was monitored to quantify the biofouling potential of the RO feed water (Vrouwenvelder et al., 2003). The Mass Transfer Coefficient (MTC) of the RO membranes was calculated according to Van de Lisdonk et al. (2001), but without correction for conductivity (i.e. osmotic pressure) of the feedwater. The waterquality was monitored via weekly sampling and analyses according to WLN methods conform or based on international standards. RESULTS AND DISCUSSION Effect of BACF on biofouling In the absence of BACF, an exponential increase of the TMP was observed after only 3 days of operation (Fig. 2), indicating the occurrence of biofouling on the RO module. This observation confirms the high potential for biofouling on RO when wwtp effluent is used as a source. The biocide DBNPA showed to be able to stop biofouling (period day 31-47), but the pressure drop increased again when the DBNPA dosage was stopped (day 48). In case the feed water was pretreated by BACF and oxygen supply, a slight increase of the pressure drop started after app. 40 days of operation. However, an exponential increase, typically for biofouling, was not observed during the entire runtime of the experiment (120 days). The MTC for both membranes stayed rather constant at a value of approximately m/(s.kpa), indicating that fouling occurred predominantly inside the spacers of the membrane and not, or much less, on the membrane surface. 3

4 Figure 2 Normalized pressure drop (dpn) over the RO module with (blue line) and without (red line) BACF and oxygen supply as pretreatment. Autopsy of the RO module that was fed with wwtp effluent only pretreated with UF confirmed substantial biofouling (Fig. 3). Visual inspection of the cartridge filters showed severe biofouling after 7 days of operation in case BACF was absent, while the cartridge was relatively clean in case BACF was applied as additional pretreatment (Fig. 3). These results show that biological activated carbon filtration as pretreatment lowers the bioufouling potential of RO feed waters. This was confirmed by monitoring the accumulation of adenosine triphosphate (ATP) on glass pellets in the biofilm monitor (Fig. 4). In case BACF is absent, strong ATP accumulation, i.e. biological growth, was observed from day 21. The ATP accumulation was much lower in case the water had passed BACF. From the steepness of the curves, the biofilm formation rate can be estimated: roughly 40 and 400 pg ATP.cm -2.d -1 for the BAC filtrate and UF permeate, respectively. In case DBNPA was dosed, the BFR amounted < 1 pg ATP.cm -2.d -1 (data not shown). Figure 3 Biofouling observed on the RO membrane (left) and cartridge filters (right). The upper cartridge filter was fed with wwtp effluent pretreated with UF, the lower cartridge was fed with BAC filtrate. 4

5 ATP (pg/cm2) days Figure 4 Accumulation of ATP on glass pellets in the biofilm monitor fed with UF permeate (red line) and BAC filtrate (blue line). Effect of BACF on water quality Biological activated carbon filtration with oxygen supply lowers the load of organic substances (measured as DOC, COD, UV-absorption), ammonium, iron and manganese to the RO membranes (Table 1). Although determination of the growth limiting compound was beyond the scope of this study, it is plausible that the lower biofouling (potential) on RO membranes is caused by limitation of readily degradable organic carbon, e.g. quantified as Assimilable Organic Carbon (AOC) (Van der Kooij, 1992) which most likely is just a very small fraction of the total amount of DOC. According to Van der Kooij et al. (2007) low AOC concentrations are essential for biofouling prevention. Table 1 Effect of BACF on water quality (mean values, n = 20) parameter Unit Permeate UF Filtrate BACF1 Filtrate BACF2 Aluminium ug/l ATP ng/l BOD-5 mg/l O DOC mg/l COD mg/l O Turbidity FTU Membrane fouling index s/l Iron mg/l Manganese mg/l O 2 mg/l NH 4 -N mg/l NO2-N NO3-N PO 4 -P mg/l mg/l mg/l UV-absorption 254 nm Abs/m

6 It is presumed that easily degradable carbon is oxidized in the BAC filters, either directly from the waterphase and/or indirectly via sorption to and desorption from the activated carbon (Fig. 5). Biological activated carbon filtration is widely applied in drinking water production for reducing the regrowth potential of drinking water during distribution. In most cases, strong oxidation is applied prior to BACF using ozone for DOC removal, (Van der Hoek et al., 2000), UV for disinfection (Van der Veer, 2002) or UV/H 2 O 2 for advanced oxidation (Kruithof et al., 2005). Although (easily degradable) organic compounds can be adsorbed directly by the granular activated carbon (Bonné et al., 2002) it is known that biological processes enhance AOC removal in activated carbon filters (Polanska et al., 2005). D S AC B Figure 5 Presumed role of activated carbon: biofilm carrier and sorbent for substrate. AC = active carbon, B = biofilm, S = sorption, D = desorption and conversion of substrate. The oxygen consumption by the BAC filters varied between 5 and 40 mg O 2.l -1, depending on the quality of the wwtp effluent as well as temperature. The highest biological activity was observed during summer, i.e. at temperatures above 15 C. In that situation, the filters were flushed once per one or two days, to prevent clogging by biomass accumulation. During the first minutes after flushing, the BAC filtrate contained high concentrations of biomass (10-15 µg/l ATP). To prevent a high biomass load to the RO membranes, the BAC filtrate was discharged during the first hour after flushing. During that time, the ATP concentration of the filtrate dropped to approximately 30 ng/l, i.e. comparable with UF permeate (Table 1). Most oxygen consumed by the BAC filters is used for nitrification: oxidation of 4.9 mg/l NH 4 -N, i.e. the average effluent concentration during the research, to NO 3 -N consumes 22 mg/l O 2, theoretically. Note that also denitrification occurred in the BAC filters. The nitrogen balance with data from Table 1 indicates that approximately 35% of the oxidized nitrogen is denitrified, most likely in the inner spheres of the biofilm, i.e. where oxygen is absent. Sison et al. (1995) investigated denitrification in biological activated carbon filters and anthracite using sucrose as external carbon source. Contrary to anthracite, activated carbon showed a stable and constant nitrate removal while the carbon sources was injected only once a day, indicating that organic carbon is adsorbed from the water phase and subsequently desorbed to serve as carbon source in biological oxidation processes like denitrification. These observations indicate that the adsorption and desorption properties of activated carbon enhance the stability and capacity of biological filtration processes and they are in line with the presumed role of activated carbon in the present research (Fig. 5). 6

7 The combination of oxidation and (biological) activated carbon filtration showed to remove iron and manganese from the UF permeate by 60 and 80 %, respectively (Table 1). Besides nutrient limitation, the lower metal content of the RO feed water may have contributed to the stay away of biofouling on the RO module, since iron and manganese oxides may stimulate the induction of biofilm formation. CONCLUSION This paper demonstrates that biological activated carbon filtration with oxygen supply is a promising technology for the prevention of biofouling on reverse osmosis membranes. Most likely, biofouling is limited due to nutrient limitation. It is presumed that BAC filters remove easily degradable carbon, present in the wwtp effluent and not removed by UF. The consequence is a lower concentration of growth limiting substrate in the RO feed water. Although biofouling can also be prevented by dosing the biocide DBNPA, it can be concluded that this dosing is very critical, i.e. biofouling continues immediately in case the dosing is stopped, e.g. due to a operation failure. Oxygen supply followed by BACF showed to be a robust technology for biofouling prevention on RO membranes. Especially in case of wwtp effluent reuse it is, due to the high biofouling potential of wwtp effluents, an attractive alternative to conventional biofouling abatement with biocides. Therefore, this technology may be an important stimulus for effluent reuse. Acknowledgements The authors thank Reinder de Valk (WLN) and Simon Dost (WMD) for their contribution to the pilot research and Rob Bos (WMD) for carefully reading the manuscript. References Bonné, P.A.C., Hofman J.A.M.H. and Van der Hoek J.P. (2002). Long term capacity of biological activated carbon filtration for organics removal. Water Supply, 2, Flemming H.C., Schaule G., Griebe T., Schnitt J. and Tamachkiarowa A. (1997). Biofouling the Achilles heel of membrane processes. Desalination, 113, Kim D., Jung S, Sohn J, Kim H. and Lee S. (2009). Biocide application for controlling biofouling of SWRO membranes an overview. Desalination, 238, Kruithof J.C., Kamp P.C., Martijn B.J., Belosevic M. and Williams G. (2005). UV/H2O2 Treatment for Primary Disinfection and Organic Contaminant Control at PWN s Water treatment Plant Andijk, Proceedings of IUVA Worldcongress, Whistler, Canada. Polanska M., Huysman K and Van Keer C. (2005). Investigation of assimilable organic carbon (AOC) in flemish drinking water. Wat. Res., 39, Sison N.F., Hanaki K. and Matsuo T. (1996). Denitrification with external carbon source utilizing adsorption and desorption capability of activated carbon. Wat. Res., 30, Van der Kooij, D Assimilable organic carbon as an indicator of bacterial regrowth. J. AWWA 84(2): Van der Kooij, D., Heijnen W. Cornelissen E., Van Agtmaal J., Baas K. and Galjaard G. (2007). Elucidation of membrane biofouling processes using bioassays assessing the microbial growth potential of feed water. Proceedings AWWA Membrane Technology Conference, Tampa Bay US. 7

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