Water & Wastewater Treatment Magazine
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Pesticide Protection Metaldehyde cannot be removed effectively with standard drinking water treatment processes, but there are technologies that have the ability to treat the problematic pesticide By Robin Hackett www.wwtonline.co.uk | WWT | DECEMBER 2018 | 29 Innovation Zone Drinking water treatment W ith the European Drinking Water Di- rective (DWD) limit- ing individual pesticide levels to 0.1μg/l and total pesticides to 0.5µg/L, water companies face a difficult task in control- ling hard-to-treat pesticides such as metaldehyde. Many water companies have been working to reduce the prevalence of the chemi- cal, which is used for slug control, through catchment management approaches, and they may also use controlled abstraction processes whereby an affected raw water source is taken out of use when pesti- cides are detected. Even so, treatment can at times be the only option, but metaldehyde is not removed effectively by standard drinking water treatment processes such as chlorination and ozonation. Granular activated carbon (GAC) can be used, but the GAC beds require regular regenera- tion to do so. Here, we detail some of the systems that may be used to address the issue. TransPAC (Transvac Systems) Powdered activated carbon (PAC) can be used to adsorb most pesticides, including metaldehyde, without the operational issues associated with GAC. Transvac's TransPAC system is a fully containerised PAC dosing system that can be taken from site to site to treat issues as they arise, such as metaldehyde, algae bloom or taste and odour problems, with no requirement to involve M&E contractors. The system, which is custom- designed and comes with bulk-bag and silo feed options as standard, also features an HMI control panel so that the PAC dosing system can be adjusted to match the works flow. It has been created with the ambition to be hassle-free and low-maintenance, and its once- through inline dosing means there are no batch mixing tanks, while it flushes the lines clean on shut-down. The technology inside the secure container includes an intermediate hopper, 800-litre header tank, HMI control panel and Ejector dosing system. Actiflo Carb (Veolia) While the technology has been in use for around a decade, Veolia recently announced that – in a global first – it is to use its Actiflo Carb technology for the specific purpose of metaldehyde removal at an Affinity Water treatment plant a er successful trials with the utility. Actiflo Carb combines the fast flocculation and sedimentation performance of Veolia's Actiflo high-rate clarifier with the adsorption capacity of powdered activated carbon. The process sees the raw water initially entering the PAC contact tank to adsorb pollutants resistant to chemical clarification before going through coagulation, flocculation and sedimentation basins. A recirculation circuit with a specific hydrocyclone recovers clean microsand, returns the PAC to the contact tank and purges excess sludge from the process. Where used for drinking water, it treats non- organic matter resistant to clarification, pesticides, emerging micropollutants and micro-algae, as well as flavours and odours. There is also the option – depending on raw water composition and treatment objectives – to use the Actiflo Twin Carb: a dual-stage treatment involving an Actiflo clarification stage followed by an Actiflo Carb refinement stage. The Twin Carb version can cut total organic carbon (TOC) from 15 mg/l in the raw water to less than 2 mg/l in the treated water. The Affinity treatment works will use prefabricated Actiflo AS5 Carb and Multiflo MS1 FE lamella thickener technologies together with Actisand and PAC and dosing equipment to treat the metaldehyde at one of the boreholes from which the plant abstracts water. GOAL 3 DRINKING WATER QUALITY