Removal of Trihalomethane Precursors Using the MIEXÂ® Dissolved

Chapter 16

Removal of Trihalomethane Precursors Using the MIEX Dissolved Organic Carbon Process in Combination with Granular Activated Carbon

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Mary Drikas, Mike Dixon, and Jim Y. Morran CRC for Water Quality and Treatment, Australian Water Quality Centre, SA Water Corporation, Private Mail Bag 3, Salisbury, South Australia 5108

A pilot plant study comparing the removal of trihalomethane (THM) precursors by granular activated carbon (GAC) following conventional treatment with and without MIEX pre-treatment was completed. The quality of the treated water following GAC filtration was found to be substantially better when incorporating MIEX. This difference was reduced when the empty bed contact time (EBCT) of the GAC filters was decreased from 20 mins to 5 mins and the GAC filter aged, however the removal of dissolved organic carbon (DOC) after GAC filtration remained marginally more effective with MIEX treated water. The additional DOC removal with MIEX pre-treatment also resulted in reduced chlorine demand and trihalomethane (THM) formation. THM formation following MIEX pre-treatment was considerably lower than could be achieved with conventional coagulation alone, becoming almost negligible following GAC filtration when the EBCT was 20 minutes. MIEX pre-treatment was shown to preferentially remove THM precursors, both by direct removal and by improving the effectiveness of GAC filtration.

Published 2008 American Chemical Society. In Disinfection By-Products in Drinking Water; Karanfil, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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228 The important role of natural organic matter (NOM) in the disinfection process has been well established (1-3). Studies have shown that removal of NOM reduces disinfectant demand and results in decreased formation of disinfection by-products (DBP) confirming that NOM provides the major source of precursors in formation of DBPs (7-7). Consequently removal of NOM has been adopted as one of the main mechanisms to reduce DBP formation. This has led to the optimisation of existing treatment processes and the development of new processes which focus on NOM removal. The MIEX DOC Process was developed specifically to remove NOM from raw water sources utilising a contact process rather than column filtration (6,8). The application of this process in operating treatment plants is increasing (9-13). As the MIEX DOC process only removes dissolved organics it is necessary to link the process with another technique to remove suspended matter. The Mt Pleasant Water Treatment Plant (WTP) in South Australia encompasses the MIEX DOC process and also enables comparison of two subsequent turbidity removal processes - conventional treatment (comprising coagulation, flocculation, sedimentation, and filtration) and submerged microfiltration (MF) (13). This has proven the effectiveness of operating MIEX in two possible scenarios - retrofitting into a conventional treatment plant or in a greenfield operation utilising MF to remove particulates. The successful operation of this plant has shown that a high level of removal of dissolved organic carbon (DOC) can be achieved using MIEX combined with a second step for removal of turbidity. This results in high quality water requiring low doses of chlorine to achieve stable disinfectant residuals with associated low levels of trihalomethanes. Whilst the use of MIEX significantly improves NOM removal, there are other water quality contaminants such as pesticides or algal metabolites that require additional processes for effective removal. Granular activated carbon (GAC) filters may be included in the treatment train for such a purpose. Moreover as GAC is a broad adsorbent it will also result in further NOM removal (14-16). The effect of combining these two N O M removal processes and the impact on disinfectant demand and the formation of disinfection by-products (DBP) was evaluated in a pilot plant study. The direct comparison of the efficiency of NOM removal by GAC following coagulation with and without MIEX pretreatment is discussed.

Methodology The Mt Pleasant WTP consists of two separate treatment trains; both utilise the MIEX DOC Process as the first treatment process but are followed by two different particulate removal streams - either conventional treatment (comprising coagulation, flocculation, sedimentation, rapid filtration) or microfiltration. Stream 1 at the Mt Pleasant WTP incorporates MIEX followed by conventional

In Disinfection By-Products in Drinking Water; Karanfil, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

229 treatment. During the period July 2005 to January 2007, MIEX was applied at an average resin dose of 8mL/L for 10 minute contact followed by sedimentation and removal of the resin before entering the conventional treatment stage, Coagulation during this period utilised an average alum dose of 6.5 mg/L (as A1 (S04)3.18H 0) and 0.2 mg/L poly DADMAC. A conventional pilot plant without MIEX pre-treatment was established on site at the Mt Pleasant WTP as a control stream to enable direct comparison of the water quality produced with the full scale operation of the MIEX pre-treated water. The conventional pilot plant consists of coagulation, flocculation, sedimentation and rapid filtration. The alum dose was chosen both using a model (17) and jar tests to achieve the optimum DOC removal (defined as the point of diminishing return, where an additional 10 mg/L alum produces 50,000Da. Thisfractionhas been shown to exert a significant chlorine demand, produce THMs and to be effectively removed by coagulation processes (and was also removed by the conventional treatment in this study) (22- 23). The high THMFP after MIEX treatment alone and the improvement noted after coagulation of the MIEX treated water is attributed to the presence and subsequent removal of this fraction. Molecular weight distribution after GAC filtration is shown in Figure 6 for the samples taken for Day 198 and includes the raw and MIEX treated waters for comparison. Similar to MIEX treatment, GAC filtration also removed organics



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Figure 5. Molecular weight distributions before GACfiltration(Day 198)

across the whole molecular weight range. However, as previously illustrated (refer Figure 1), the organics removal remained constant with the MIEX process whereas the efficiency of DOC removal with the GAC filtration decreased with time to the extent that after 300 days minimal reduction in organics was detectable. The decrease in adsorption capacity of GAC is well documented (16,24) and, whilst biodégradation may continue to occur after adsorption is exhausted, the extent of DOC removal is substantially lower at around 10-20% DOC removal. GAC filtration after conventional treatment (post GAC 1) reduced the organics across the complete molecular weight range and particularly the lower molecular weight range. This reduced the THMFP to 45 ug/L, similar to that achieved by MIEX followed by coagulation. Comparing the molecular weight scans in Figure 6, this suggests that removal of molecular weight below 600 Da and above 1000 Da did not significantly impact T H M formation. The key difference in molecular weight distribution between the conventional and MIEX coagulated processes after GAC filtration is the removal of the range between 600-1000 Da. This suggests that in addition to higher molecular weight compounds previously identified as responsible for THM formation (5-5), and easily removed by coagulation processes, compounds in this molecular weight range also produce notable contributions to THM formation (25). The low level of DOC remaining after GAC filtration following MIEX coagulation treatment and the absence of organics in the molecular weight scans



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Figure 6. Molecular weight distributions after GAC filtration (Day 198)

support the conclusion that MIEX pretreatment improves GAC effectiveness. This can be attributed to the removal of a broad range of organics both by reduction of the quantity reaching the carbon adsorption pores and by removal of larger organics that may cause pore blockage. This improvement is greater than can be achieved by conventional treatment alone but similar improvement could be achieved by other processes which removed a broad range of organics such as nanofiltration (26). Whether there is any impact on biogical growth on the carbon and resulting biodégradation is not clear at this time. Rapid fractionation was also used to determine whether the various processes were removing organics of different character. The fractionation data summarised in Figure 7 was obtained for the same samples (Day 198) as the molecular weight determination illustrated in Figures 5 and 6. This indicates that whilst conventional treatment decreased all fractions to some extent, MIEX (either alone or in conjunction with coagulation) removed all the charged fraction and more of the hydrophobic (VHA and SHA) fractions than coagulation. GAC filtration after conventional treatment only removed small additional quantities of all fractions confirming the residual presence of organic compounds of varying character. GAC filtration after MIEX and coagulation pre-treatment (post GAC 2), removed the remaining hydrophobic fractions, leaving only a proportion of the neutral fraction. As the neutral fraction has been identified as consisting generally of non-UV absorbing organics this confirms the lack of any observable peaks in the molecular weight scans which were detected using by UV absorbance.


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Figure 7 Rapidfractionation of the coagulation processes before and after GAC filtration (Day 198)

Conclusions This study compared the effectiveness of GAC filtration for removal of DOC and the subsequent formation of THMs following coagulation with and without MIEX pre-treatment over a 505 day period. The EBCT of the GAC filters was initially 20 mins but this was reduced to 5 mins after 312 days. The study has shown that the removal of DOC following GAC filtration was significantly better when the treatment train incorporated MIEX. After 200 days operation DOC after MIEX treatment alone was lower than conventional treatement after GAC filtration. This difference was reduced when the EBCT of the GAC fitlers was decreased from 20 mins to 5 mins and the GAC filter aged, however the removal of DOC after GAC filtration remained marginally more effective with MIEX treated water for the entire study period. The additional DOC removal with MIEX pre-treatment also resulted in reduced chlorine demand and THM formation. THM formation incorporating MIEX pretreatment was considerably lower than could be achieved with conventional coagulation alone, becoming almost negligible following GAC filtration when the EBCT was 20 minutes. MIEX pre-treatment was shown to preferentially remove THM precursors, both by direct removal and by improving the effectiveness of GAC filtration. This was attributed to removal of a higher proportion and a broader range of organics (as defined by molecular weight and fractionation) by MIEX than possible with conventional treatment. This enhanced the ability of GAC to remove further organics resulting in lower THM formation potential. The lower THM formation was maintained for the length of the study even after the EBCT of the GAC filter was reduced.


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References 1.


2.

3.

4.

5.

6.

7.

8.

9.

Edzwald, J.K.; Becker, W.C.; Wattier, K.L. Surrogate parameters for monitoring organic matter and THM precursors. Journal ofAmerican Water Works Association 1985, 77(4), 122-132. Reckhow, D. Α.; Singer, P.C. Chlorination by-products in drinking waters: from formation potentials to finished water concentrations. Journal of American Water Works Association 1990, 82(4), 173-180. Disinfection By-products in Water Treatment: The Chemistry of Their Formation and Control Minear, R.A.; Amy, G.L., Eds; Lewis Publishers: 1996. Martin-Mousset, B.; Croue, J.P.; Lefebvre, E.; Legube, B. Distribution and characterization of dissolved organic matter of surface waters. Water Research 1997, 31(3), 541-553. Kitis, M . ; Karanfil, T.; Kilduff, J.E.; Wigton, A. The reactivity of natural organic matter to disinfection byproducts formation and its relation to specific ultraviolet absorbance. Water Science and Technology 2001, 43(2), 9-16. Bursill, D.; van Leeuwen , J.; Drikas, M . ; Problems related to particulate and dissolved components in water: the importance of organic matter. Water Science & Technology: Water Supply 2002, 2(1) 155-162. Drikas, M . ; Chow, C.W.K.; Cook, D.; The impact of recalcitrant organic character on disifection stability, trihalomethane formation and bacterial regrowth: An evaluation of magnetic ion exchange resin (MIEX) and alum coagulation. Journal of Water Supply: Research and Technology - AQUA 2003, 57(2), 475-487. Morran, J.Y.; Bursill, D.B.; Drikas, M . ; Nguyen, H.; A new technique for the removal of natural organic matter, Proceedings of the AWWA WaterTECH Conference, Sydney, Australia, May 1996. Smith, P.; Botica, C,; Long, B,; Allender, B.; Design, construction, commissioning, and operation of the world's first large scale M I E X water treatment plant, Proceedings of the AWWA Annual Conference, New Orleans, LA, June 2002. Hammann, D.; Bourke, M.; Topham, Evaluation of a magnetic ion exchange resin and ozone to achieve EPA DBP standards at the village of Palm Springs. Proceedings of the AWWA Conference, Orlando, Florida 2003. Nestlerode, F.; Bourke, M . ; Teply, M . ; Installation of a magnetic ion exchange resin treatment process to improve TOC removal and meet EPA Stage 1 DBP standards at the Green Valley WTP., Proceedings of the CA/NV AWWA Conference, Reno, NV, 2005. Bourke, M.F.; Full-scale impact of a magnetic ion exchange process on downstream coagulation performance and distribution system DBP levels. Proceedings of the Water Quality Technology Conference, Denver, CO, November 2006. ®

10.

11.

12.



240 13. Drikas, M . ; Morran, J.Y.; Cook, D.; Bursill, D.B.; Operating the MIEX process with microfiltration or coagulation. Proceedings of the AWWA Water Quality Technology Conference, Philadelphia, USA, November 2003. 14. Summers, R.S.; Hopper, S.M.; Solarik, G.; Owen, D.M.; Hong, S.; Bench scale evaluation of GAC for THM Control. Journal of American Water Works Association 1995, 87(8), 69-80. 15. Newcombe, G.; Drikas, M . ; Assemi, S.; Beckett, R.; Influence of characterised natural organic material on activated carbon adsorption: I. Characterisation of concentrated reservoir water. Water Research 1997a, 31(5), 965-972. 16. Metz, D.H,.; DeMarco, J.; Pohlman, R.; Canon, F.S.; Moore, B.C.; Effect of multiple GAC reactivations on disinfection byproduct precursor removal. Water Science and Technology: Water Supply 2004, 4(4), 71-78. 17. van Leeuwen, J.; Daly, R.; Holmes, M.; Modeling the treatment of drinking water to maximize dissolved organic matter removal and minimize disinfection by-product formation, Desalination 2005, 177, 81-89. 18. Standard Methods for the Examination of Water and Waste Water; Method 4500-Cl (F); APHA, AWWA and WEF: Washington, DC, 1998; 20 Edition. 19. Chow, C.W.K.; Fabris, R.; Drikas, M . ; A rapid fractionation technique to characterise natural organic matter for the optimisation of water treatment processes. Journal of Water Supply: Research and Technology - AQUA 2004, 53(2), 85-92. 20. Fitzgerald, F.; Chow, C.W.K.; Holmes, M . ; Disinfectant demand prediction using surrogate parameters - a tool to improve disinfection control. Journal of Water Supply: Research and Technology - AQUA 2004, 55(6), 391-400. 21. Morran, J.Y.; Drikas, M . ; Cook, D.; Bursill, D.B.; Comparison of MIEX treatment and coagulation on NOM character, Water Science and Technology: Water Supply 2004, 4 (4), 129-137. 22. Lin, C F . ; Huang, Y.J.; Hao, O.J.; Ultrafiltration processes for removing humic substances: effect of molecular weight fractions and PAC treatment Water Research 1999, 33(5), 1252-1264. 23. Allpike, B.P.; Heitz, Α.; Joli, C.; Kagi, R. Abbt-Braun, G.; Frimmel, F.:Brinkmann, T.; Her, Ν.: Amy, G.; Size Exclusion Chromatography To Characterize DOC Removal in Drinking Water Treatment Environmental Science and Technology 2005, 39, 2334-2342 24. Newcombe, G.; Collett, Α.; Drikas, M . ; Roberts, B.;Granular Activated Carbon Pilot Plant Studies Water 1996, 23(3), 29-31. th


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25. Fabris, R.; Chow, C.W.K.; Drikas, M ; Practical application of a combined treatment process for removal of recalcitrant NOM - alum and PAC, Water Science and Technology: Water Supply 2004, 4(4), 89-94 26. Her, N.; Amy, G.; Park, H. R; Song, M ; Characterising algogenic organic matter (AOM) and evaluating associated NF membrane fouling, Water Research 2004, 38(6), 1427-1438


Removal of Trihalomethane Precursors Using the MIEXÂ® Dissolved

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