Chapter 15
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Combined Treatments for Enhanced Reduction of Trihalomethane Precursors Rolando Fabris, Christopher W.K. Chow and Mary Drikas CRC for Water Quality and Treatment, Australian Water Quality Centre, SA Water Corp. Private Mail Bag 3, Salisbury, South Australia 5108
Research has determined that there is a portion of natural organic matter (NOM) that cannot be removed by coagulation processes. As such, alternative treatment technologies must be applied to achieve improvements in drinking water quality. Three different powdered activated carbons (PACs) were applied in combination with coagulation to treat a high dissolved organic carbon (DOC) source water with a focus on reducing trihalomethane formation potential (THMFP). A steam-activated, coal-based carbon was shown to most effectively reduce DOC and THMFP. In addition, a combined treatment protocol utilising adsorbent technologies (MIEX and PAC) with coagulation was effective in reducing DOC by up to 96% and THMFP by up to 97%. Due to differing mechanisms of NOM removal, the technologies applied were complimentary in removing DOC of various character, including material that is typically recalcitrant to coagulation. ®
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© 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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Introduction Many published studies have determined that there is a portion of the natural organic matter (NOM) that cannot be removed by coagulation processes (1-4). This material can be termed recalcitrant NOM and is primarily composed of low molecular weight hydrophilic neutral organics such as polysaccharides, proteins and amino sugars (5,6). Previous investigations have also established that coagulants alone were incapable of producing required reductions in chlorine reactivity and disinfection by-product (DBP) formation in some water sources (7). As such, alternative treatment technologies must be considered if further improvements in drinking water quality are to be achieved. Alternative treatment technologies for NOM removal fall into four main categories: biological degradation; oxidative processes such as UV irradiation and ozonation; adsorbent technologies such as ion-exchange resins, metal oxides and activated carbon; and membrane filtration, specifically nanofiltration and reverse osmosis. In terms of practical application, perhaps the simplest of the alternative technologies to implement into existing plant infrastructure is activated carbon and more specifically, powdered activated carbon (PAC). The ability to dose periodically into the treatment plant with minimal changes to operation makes this a popular choice for removal of algal metabolites or micro-pollutants, however the use of PAC specifically for NOM removal is less prevalent. This concept is explored further through the work presented in the following section. Of greater complexity to implement, but developed specifically for NOM removal, MIEX®, a magnetic ion-exchange resin, has shown effectiveness in enhancing DOC removal, while reducing the need for chemical based treatments, such as coagulation (8,9). The enhanced aggregation and settling of the resin due to magnetic properties allows application in stirred contacters that can cause operational issues for many other ion-exchange resins. It has shown effectiveness for removal of a wide range of organic compounds of various molecular weights and can also reduce the dosing requirement for disinfectants and hence reduces DBP formation (10-12). In practical applications, adsorbents will also typically require a clarification step to remove or reduce turbidity, such as traditional coagulation/media-filtration or microfiltration (MF). As most of the described techniques are not practical to apply in isolation, it is common to combine several technologies which are complimentary to each other in removing selected contaminant materials efficiently and economically. In such a way, both particulates and dissolved species, such as NOM, algal metabolites and micro-pollutants can be targeted within the treatment strategy. This is conducive to the 'multi-barrier' approach to drinking water treatment that has gained considerable interest and approval in recent years (13).
In Disinfection By-Products in Drinking Water; Karanfil, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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Evaluation of Activated Carbon Activated carbon has long been used as a treatment technology for the removal of algal toxins, tastes and odours and micropollutants, such as pesticides and pharmaceuticals. It is also one of the most effective methods for removal of recalcitrant NOM and this can be optimised through careful choice of carbon material based on selection of the pore volume distribution and adsorption kinetics (14-17). While granular activated carbon (GAC) is preferred for long term application of activated carbon, PAC, through virtue of greater surface area, requires lower doses and less contact time to produce equivalent removals of target compounds (18) and is more easily applied in a laboratory based investigation. Three different powdered activated carbons were applied in combination with alum to treat a high DOC source water with a focus on improving NOM removal, and specifically recalcitrant THM precursors.
PAC evaluation protocol The source water used for this investigation was Myponga Reservoir, located about 50km south of Adelaide, Australia. The water is sourced via surrounding catchment and is generally considered a high colour and DOC source (115 Hazen units and 14.5mg/L), respectively, at the time of the investigation. Aluminium sulphate stock solution (20,000mg/L as A1 (S0 )3.18H 0) was prepared by dilution of liquid aluminium sulphate solution (7.5% as A1 0 ) with ultrapure water. Preliminary jar testing indicated that a dose of 100mg/L of aluminium sulphate (alum) was optimal for DOC removal. The jar testing procedure employed has been previously described (3) All jar tests were pH controlled to between 6.2 and 6.4. The three PACs used were chosen to represent a variety of source materials and activation methods. Carbon A is a steam activated, coconut based carbon, which is primarily microporous (
