PILOT EVALUATION OF ION EXCHANGE, COAGULATION AND MICROFILTRATION FOR TREATING SURFACE WATER AT SOUTH WEST WATER, UK. Abstract.

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1 PILOT EVALUATION OF ION EXCHANGE, COAGULATION AND MICROFILTRATION FOR TREATING SURFACE WATER AT SOUTH WEST WATER, UK Jumeng Zheng, PWN Technologies, Dijkweg 12, 1619 HA, Andijk, The Netherlands Phone: Gilbert Galjaard, PWN Technologies, Andijk, The Netherlands Holly Shorney-Darby, PWN Technologies Andijk, The Netherlands David Metcalfe, South West Water, Exeter, the United Kingdom Chris Rockey, South West Water, Exeter, the United Kingdom Abstract A process comprising ion exchange, in-line coagulation and ceramic microfiltration (MF) was tested for treating surface waters in southwest England. The membrane could operate at the nominal flux of 109 lmh, and at maximum 185 lmh. The process is robust, and can handle abrupt water quality changes by adjusting the coagulant dosage and with the normal course of backwashes (BW) and chemical enhanced backwashes (EBW). The process also produces excellent water quality, with, on average 83 percent removal of dissolved organic carbon (DOC) and nearly complete removal of suspended solids. It is concluded that the process comprising ion exchange, coagulation and ceramic MF is well-suited for treating the surface waters. Introduction South West Water (SWW) is a water utility in south-west England, providing water and sewerage services in Devon, Cornwall, and small section of Dorset and Somerset. SWW owns and operates 29 water treatment works (WTW) to serve approximately 1.6 million residents. SWW conducted a pilot-scale evaluation of suspended ion exchange (SIX ), in-line coagulation adsorption (ILCA ), and ceramic MF at the Crownhill Water Treatment Works (WTW) from March 2013 to May The aim of this pilot study was to observe the performance of this treatment train when treating the reservoir and river sources and to determine design parameters for a full-scale plant. Early testing focused on an evaluation of the SIX process to determine the optimum resin dose and contact time with respect to highest removal rates of DOC. This was determined to be 18 ml/l resin with 30 minutes of contact time. In May 2013, the ceramic membrane equipment was started, to treat the ion exchange treated water. Ion exchange alone could not control the fouling of the ceramic membrane because: 1) different water sources had varying water quality; 2) there was significant seasonal variation of the water quality; and, 3) there existed very special properties of the biopolymers. Generally, it was thought that the biopolymer fraction of DOC does not adsorb UV light. The biopolymer in the SWW source water had strong UV adsorbing properties [Zheng et al. (2014)]. In September 2013, clarified water from existing WTW was used as the feed, with and without pilot-scale ion exchange. During this phase of testing it became clear that coagulation was needed for sustainable membrane operation. The ILCA 1

2 reactor was designed and then installed in December 2013, and trials began in January 2014 with a polyaluminum chloride (PACl) coagulant. The pilot with SIX, ILCA and ceramic MF continued until May Throughout the pilot study, the results from first phase of testing, namely when treating the ion exchanged feed, clarified feed with or without ion exchange have been summarized and presented at 2014 AMTA conference [Shorney-Darby et al. (2014)]. This paper presents results from the second phase, after adding the ILCA to the process train. Objectives The goals for the pilot plant were focused on finding the sustainable operating parameters for the ion exchange, in-line coagulation and ceramic membrane systems so that design of a full-scale plant could be made. Experimental Testing was conducted at the Crownhill WTW, which treats three source waters Burrator Reservoir, Tavy River, and Tamar River the same as the future WTW. It was a containerized pilot plant comprising SIX, ILCA and ceramic MF. Figure 1 illustrates the process diagram and Figure 2 shows the ILCA reactor and membrane vessel. Other detailed information of the pilot plant can be found elsewhere [Shorney-Darby et al. (2014), Metcalfe et al. (2015)]. Figure 1 Flow Diagram of the Process (note ph control chemical dosing point was moved to downstream of the membrane feed tank in March 2015) 2

3 Figure 2 ILCA Contactor (left) and Membrane Vessel (right) The ion exchange resin used for testing was Lewatit S5128 resin (Lanxess Deutschland GmbH). The resin concentration was 18 ml/l in most of time and the contact time was constant of 30 minutes. The coagulant used is PACl (Brenntag WAC). The coagulant dosage was in the range of 0.5 to 4 ppm as Al 3+, e. g., 10 to 80 ppm as product. The coagulant dosage was adjusted by the feed water quality change, determined by jar testing. The coagulation ph was controlled to 6.4. The contact time in the coagulation contractor was from 2.4 to 4 minutes depending on the membrane feed flow rate. The ceramic membrane pilot unit consisted of one 25 m 2 ceramic membrane with nominal pore size of 0.1 micron (Metawater, Japan). A virgin membrane element was used when starting the pilot in May The element was replaced by another new membrane in March Another major change was that, the ph control and coagulant dosing control automated in the early of May The tests were conducted run by run. Most of the time, the membrane was operated at flux of 112 or 109 lmh, representing the nominal flux for the full scale plant (when treating the Burrator water in spring, autumn and winter); or at a flux of 185 lmh, representing the maximum flux (when treating the river water in summer). After treating a certain amount of water, there was regular BW or chemical EBW. Initially two types of EBWs were used: NaOCl EBW (100 ppm free chlorine) and HCl/peroxide EBW (ph 2.3, peroxide 100 ppm). Later on, the NaOCl EBW was replaced by NaOH EBW (ph 12.3). Table 1 the shows the major settings after starting the inline coagulation (i.e., test runs 22 to 43). Most of the time, each run treated 625 m 2 (runs 22 to 37, total filtration time 135 hours at 185 lmh, 223 hours at 112 lmh)). Since run 38, each run treated much more water. For instance, the run 40 was started on 10 October 2014 and it was finished on 23 December 2014 as planned for long-term stable operation testing. An ozone clean-in-place (CIP) (for replacing conventional chemical CIP) was evaluated, and an ozone mini-cip (for replacing EBW) was also tested in the autumn of These proved very successful at cleaning the membrane in a short time. Details of these ozone cleanings can be found in elsewhere [Zheng et al. (2015)]. 3

4 Table 1. Overview of Pilot Tests at SWW Test Run Source Water Pretreatment Flux (lmh) BW nterval (min) No. BW until EBW BW method 22 Burrator IX + coag xA 1xB 23 Burrator IX + coag xA 1xB 24 Burrator IX xA 1xB 25 Burrator IX + coag xA 4xB 26 Burrator IX + coag xA 4xB 27 Burrator IX + coag xA 3xB 28 Burrator IX + coag xA 4xB 29 Burrator IX + coag x[C+B] 30 Burrator IX + coag x [C+B] 31 Burrator IX + coag x [C+B] 32 Burrator IX + coag x [C+B] 33 Burrator IX + coag x [C+B] 34 Burrator IX + coag x[A+B] 35 Burrator IX + coag x[A+B] 36 river IX + coag x [C+B] 37 river IX + coag x [C+B] 38 river IX + coag x [C+B] 39 Burr. & river IX + coag x [C+B] 40 Burrator IX + coag xC 1x[C+B] 40a Burrator IX + coag xC 1x[C+B] 41 Burrator IX + coag xC 1x[C+B] 42 Burrator IX + coag xC 1x[C+B] 42a Burrator IX + coag xC 1x[C+B] 43 Burrator IX + coag xC 1x[C+B] 43a Burr. & Tavy IX + coag xC 1x[C+B] IX denoting ion exchange; and coag. meaning coagulation; A, B and C denoting NaOCl EBW, HCl/peroxide EBW and NaOH EBW, respectively; 1xA 1xB denoting applying type A EBW and type B EBW, alternatively; 1xA 4xB, 1xA 3xB denoting applying one time type A EBW and three/four times type B EBW; 1x[C+B] denoting combined B and C EBWs; meaning applying type C firstly, coming back to normal filtration for one term (meaning for instance filtrating minutes at 112 lmh), applying type B EBW. 3xC 1x[C+B] denoting three times single type C EB and one time the combined EBW. CIP procedures during pilot-testing A CIP was conducted after each run and it targeted for specific flux of 300 lmh/bar (temperature corrected to 10 C). For the chemical CIPs in the pilot, a variety of procedures were used to clean the old membrane (the one installed in May 2013) but in general, a chemical CIP comprised NaOH (ph >12.5) overnight, and if necessary, NaOCl soaking (> 500 mg/l) and/or low ph soaking (HCl, ph < 2.5 with or without peroxide). For the new membrane (the one installed in March 2015), only one CIP was performed. It started with one hour HCl/peroxide soaking (ph 2.7, peroxide 100 ppm); followed by one hour NaOH recirculating / soaking / recirculating (ph 12.2); then followed by high concentration NaOCl recirculating / soaking / recirculating over 4

5 weekend (free chlorine ca ppm at ph 11.9, noting much more higher free chlorine concentration). In general, the CIP procedure took two or three days. Water analysis Natural organic matter (NOM) was analyzed via size exclusion chromatography liquid chromatography organic carbon detection (SEC-LC-OCD) at Het Water Laboratorium (the Netherlands). This method determines the DOC concentration and classifies the chromatographable DOC (CDOC) into a series of NOM fractions, which are then classified as biopolymers, humic substances, building blocks, low molecular weight (LMW) neutrals and LMW acids [Huber et al. (2011)]. Results and Discussion The SIX -ILCA -ceramic MF pilot started on January 2014 and ended on May During this period, vast amount of data was generated, but only the significant data and milestones are reported herein. Water quality The water feed to the pilot was from Burrator reservoir in spring, autumn, and winter; while in summer (July, August and September) the water source was river sources (i.e., the river Tavy and river Tamar). Figure 3 shows the DOC change of raw water throughout the year of These data show that the river supplies had elevated DOC concentrations when compared to the Burrator supply. It is also evident that during autumn months, the DOC levels in the Burrator Reservoir were higher (i.e, approximately 3 to 6 mg/l) than in the winter/spring (i.e., approximately 1 to 3 mg/l). The DOC also abruptly changed, with up to 9.2 mg/l during Tavy spate and as high as 5.8 mg/l during Burrator spate. The average feed water DOC concentration for the year was 2.9 mg/l. Figure 3 also shows the DOC of the membrane filtrate. Most of the year, the treated water DOC was below 1 mg/l, and the average concentration was 0.5 mg/l. This equates to, on average, 83 percent DOC removal, which was excellent. It clearly shows the high level of water quality that can be achieved after the ion exchange, coagulation and ceramic MF treatment. This treatedwater DOC concentration is far lower than what was achieved by the SWW conventional treatment of surface water, and it was comparable to the DOC concentration of the groundwater sources of SWW. Figure 4 illustrates the OCD/UVD spectra for the Tamar water. It was sampled on the August The figure clearly shows the biopolymer fraction (i.e., the peak at 29 minutes retention time) had very strong UV adsorbing properties. The same was observed for the water from the Burrator reservoir [Zheng et al. (2014)]. Generally, it was thought that the biopolymer fraction does not adsorb UV light, so the high UV adsorbing properties of the biopolymer fraction was rather unique. Figure 5 shows NOM removal through the ion exchange, in-line coagulation and ceramic MF treatment process (sampling on 16 August 2014, Tamar spate). The CDOC for the raw water was 6666 µg/l. It was reduced to 2682 µg/l after the ion exchange, and further reduced to

6 signal (-) µg/l after the coagulation. The CDOC for the filtrated water was 1055 µg/l. The major DOC reduction was realized by the ion exchange and the reduction was also significant by the coagulation. However, the ion exchange and coagulation have different capabilities in NOM fraction removal. As clearly demonstrated in Figure 5, ion exchange removed a large portion of humics and LMW organics, but it had little impact on the biopolymer fraction; while coagulation removed the biopolymers and remaining humics. The combined ion exchange and coagulation pre-treatment proved essential to stabilize the downstream ceramic MF, and also to remove disinfection by-products precursors [Metcalfe et al. (2015)]. As expected, the ceramic MF rejected nearly all suspended solids (i.e., particles larger than 1 micron), as indicated by the reading from the particle counter. Figure 3 DOC of Raw and Treated Water for OCD UVD retention time (minute) Figure 4 LC-OCD Analysis of the Raw Water (Tamar, 16 August 2014) 6

7 OCD signal raw water SIX treated SIX and ILCA treated permeate retention time (minute) Figure 5 Illustration of NOM Removal upon the Process (Tamar supply, 16 August 2014) Membrane performance Membrane performance was dependent on pre-treatment. Figure 6 illustrates the transmembrane pressure (TMP) development with the SIX alone as pretreatment (run 24) and the combined SIX and coagulation pretreatment (run 25). It is clear that the membrane performance was much better with the combined SIX and coagulation pretreatment as compared to SIX alone. This suggested the coagulation step was the main fouling control method with removing the biopolymer fraction. Figure 6 Comparing Influence of Different Pretreatments, Treating Burrator Water at 112 lmh Figure 7 shows the impact of different EBW methods, e. g., the combined EBWs with NaOH EBW and HCl/peroxide EBW (run 34), and the combined EBWs with NaOCl EBW and HCl/peroxide EBW (run 35). It can be easily seen that NaOH EBW was much better for fouling control. The reason is still unknown and it requires further study. The combined NaOH EBW and HCl/peroxide itself worked very well, which could be ascribed to: 1) the NaOH EBW 7

8 effectively removed organics; 2) the HCl/peroxide EBW removed/prevented inorganic fouling; 3) the low ph EBW after the NaOH EBW helped to minimize the strong interaction between the organics and the membrane; 4) the membrane surface charge was being neutralized by the combined EBWs. Figure 7 Comparing Influence of Different EBW Method, Treating Burrator Water at 185 lmh After establishing the pretreatment method and the EBW method, efforts were made to optimize the process, e.g., the BW frequency and EBW frequency. It was found that stable operation was achieved with: 1) a filtration time of 110 minutes at 109 lmh (i.e., filtration load until BW= 200 l/m 2 ); 2) EBW occurring after 8 BWs (i.e., filtration load until EBW=1800 l/m 2 ); and, 3)three single NaOH EBWs followed by one combined NaOH EBW and HCl/peroxide EBW (run 42). The TMP development under such conditions is presented in Figure 8. Figure 8 Membrane Performance at Optimized Operational Conditions The last test with the old membrane was to further explore the membrane performance when the filtration time was increased to minutes at 109 lmh (filtration loading until BW 300 8

9 l/m 2 ), run 42a. As seen in Figure 9, the TMP after the EBW was about 70 kpa and the fouling rate was 2.75 kpa/day. On 19 March 2015, the old membrane was replaced by a new one and the operation continued under the same conditions (run 43). The TMP after BW/EBW was about 9 kpa and the fouling rate was even negative, showing much better performance as compared to the old membrane. Figure 9 Comparing the Old and New Membrane, 109 lmh with Burrator Supply Encouraged by this observation, we increased the flux to 185 lmh (run 43a). The TMP profile is shown in Figure 10. An extremely stable operation was again observed with the apparent fouling rate of 0.31 kpa/day (by linear fitting). On 13 April 2015, the coagulant ran out for about 3 hours. This caused a rapid TMP rise. After turning on the coagulant, the TMP was stabilized. But normal BW could not restore the TMP. However the NaOH EBW was effective and reduced the TMP to a similar level to that seen prior to the loss of coagulant. Figure 10 New Membrane Performance at 185 lmh, Burrator Supply 9

10 The tests continued with Tavy supply with the same operational settings (run 43 a). During this period, the ph control and coagulant dosing control was automated on 1 May and SIX was turned off on 11 May. Figure 14 shows the TMP development and online DOC change. It can be seen there were two periods when the river Tavy went into spate conditions and the raw water quality deteriorated significantly (maximum online DOC approximate 10 mg/l). There was also one occasion when the coagulant stopped for about 1 hour and then came back (as indicated in the red number 1). Despite these events, the operation was still very stable, with the TMP being restored after the BW and EBW. The only noticeable impact was slight increase in the baseline TMP (from about 21 to 25kPa after the EBW) following the period of operation without coagulant. Operation without SIX did not have a significant impact on the membrane performance but as previously discussed, the application of SIX removed additional DOC and therefore improved the finished water quality. Figure 11 New Membrane Performance at 185 lmh, Tavy Supply CIP efficiency The initial specific flux of the old membrane was 1210 lmh/bar (virgin membrane). The specific flux after the CIPs was in the range of 257 to 678 lmh/bar with an average of 384 lmh/bar. The specific flux of 678 lmh/bar was obtained after the ozone CIP. As compared to conventional chemicals, ozone was found to be the most efficient, with faster kinetics (2 to 4 hours with 3 ppm ozone) and higher specific flux recovery (about % recovery) [Zheng et al. (2015)]. The new membrane had the initial specific flux of 1290 lmh/bar, similar with the old one. However, the CIP was very efficiency and it almost completely recovered the specific flux (1250 lmh/bar). Obviously, the old and the new membranes showed different performance. It implies the better the membrane could be cleaned by CIP, the better could be the performance. 10

11 Conclusion Pilot testing to treat surface water at SWW has shown the following: 1. Abrupt raw water quality changes, reservoir and river spate often occurred; 2. Excellent water quality was achieved after the ion-exchange, coagulation, and ceramic MF treatment; 3. Sustainable operation was achieved at 109 lmh (for treating the reservoir water) as well at 185 lmh (for treating the river water); 4. The sustainable operation was achieved by combined ion exchange and coagulation pretreatment with the later one playing a key role; 5. A full size WTW has been designed according to the pilot data. References Huber, S. A., Balz, A., Abert, M., Pronk, W. (2011), Characterisation of Aquatic Humic and Non-Humic Matter with Size-Exclusion Chromatography Organic Carbon Detection Organic Nitrogen Detection (LC-OCD-OND), Water Research, 45, Metcalfe, D., Rockey, C., Jefferson, B., Judd S., Jarvis, P. (2015), Removal of Disinfection by- Product precursors by coagulation and an innovative suspended ion exchange Process, Water Research, 87, Shorney-Darby, H., Galjaard, G., Metcalfe, D., Rockey, C. (2014), Ceramic Membrane Filtration of a Surface Water treated with Ion Exchange, Proceedings AWWA AMTA Conference, Las Vegas. Zheng, J., Galjaard, G., Shorney-Darby, H. (2014), Ceramic Microfiltration Influence of Pretreatment on Operational Performance, Presented on Singapore International Water Week 2014; accepted publishing on Water Practice & Technology, Vol 10 No 4, Zheng, J., Galjaard, G., Shorney-Darby, H. (2015), Ozone for Cleaning the Ceramic Membrane, Poster, Water Quality Technology Conference, Salt Lake City. 11