Water & Wastewater Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/741165
24 | NOVEMBER 2016 | WWT | www.wwtonline.co.uk can be treated but it is expensive and not necessarily the right option – the best option is usually to tackle it at source." Education initiatives with farmers – for example, passing on information about where and when to use pellets safely – have been showed to be effective, as has the promotion of alternative techniques for killing slugs, such as raking and dessication. However perhaps the most promising development in the catchment management arena has been the alternative slug pellets based on ferric phosphate. These kill slugs in a different way to metaldehyde, with the invertebrates typically dying underground rather than on the surface. Currently, they are slightly more expensive than metaldehyde pellets, and so questions remain about who should pay for farmers to use these alternative pesticides, or whether they should be forced to do so (see column). Some water companies have forged ahead by offering incentives for farmers to change their practices. One case study of this is Northumbrian Water's Pesti-wise initiative, under which agriculturalists can apply for grants to help purchase equipment such as precision slug pellet applicators and straw rakes, improve sprayer filling infrastructure as well as buy other precision equipment such as sprayer auto section shut-off. T he UK's first full-scale treatment works that can remove metaldehyde is Anglian Water's Hall Water Treatment Works, serving the city of Lincoln, which was opened in 2014. It uses membrane and GAC filtration, in combination with UV disinfection and chemical dosing. Heavy-duty treatment options were required for the new treatment works since it takes its water direct from the River Trent, a relatively low quality water source in which pesticides are frequently found. While GAC filtration is a proven way of removing metaldehyde, it comes at a price: Anglian has estimated that retrofitting these treatment processes at all of its works would cost £600M, an amount which would lead to a 21% increase in customers' bills on its own. The efficiency of activated carbon filtration in removing micropollutants such as metaldehyde can be boosted by using carbon replacement, as demonstrated by SAUR's patented CarboPlus technology (see factfile, opposite). Despite the availability of such treatment solutions, many in the industry believe the best approach to metaldehyde is tackle it at source through catchment management initiatives. This view was supported by Marcus Rink, Chief Inspector at the Drinking Water Inspectorate, who told WWT's Drinking Water Quality conference last month: "Metaldehyde Innovation Zone: metaldehyde A pest of a problem ● Amongst the agricultural pesticides that find their way into raw water sources, metaldehyde - which is used in slug control – is the most problematic for the water treatment process. In the latest WWT Innovation Zone special, we look at the innovative approaches water companies are taking to combat it THE CHALLENGE Metaldehyde, a chemical used in the majority of slug pellets, is very difficult and costly to remove from water and so presents a significant challenge for the water sector. Of the 69 compliance failures which were attributed to pesticides last year in England and Wales, 65 were the result of metaldehyde, according to the Drinking Water Inspectorate's annual report. The levels present are usually not high enough to represent a health risk, but any failure to reach the standards of the Drinking Water Directive is a concern for water companies and represents a reputational risk with customers. The issue is highly seasonal one, as pesticides are largely used to protect crops in the warmer months. Moreover, since pesticides typically enter watercourses through surface run-off, they can be entirely absent in raw water sources when it is dry before suddenly appearing in significant quantities when it rains. Water companies are able to stop these pesticides getting into supply by controlled abstraction – i.e. taking a raw water source out of use when pesticides are present and using an alternative, unpolluted source instead. However, this tactic is clearly only effective when water is plentiful, and when resources are stretched thin, the water company can have no choice but to use the affected source. End-of-pipe treatment solutions to remove metaldehyde tend to be expensive, and for the reasons above, can represent a poor capital investment as they are only occasionally used. Instead, water companies in recent years have sought to reduce the metaldehyde risk by working with farmers in catchment management initiatives. Some have also called for a ban on the use of metaldehyde – at least in high risk areas. However, this would have its own complications for the government, as agriculture is not the only user of the chemical, and the definition of a high risk area would be open to challenge. 65 The number of drinking water compliance failures in England and Wales in 2015 due to metaldehyde Le : Anglian Water's Hall WTW. Right: Northumbrian Water's Pesti-wise initiative Innovation Zone: metaldehyde