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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

Metaldehyde pellets are used to control slugs on cropsMetaldehyde pellets are used to control slugs on crops

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.

THE INNOVATIONS

The 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 below).

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 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.

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.


FACTFILE: CarboPlus - Micropollutant removal solution

What is it? CarboPlus is a proven treatment for micropollutants such as pesticides (including metaldehyde), THM precursors and pharmaceuticals. It utilises the adsorption capacity of activated carbon to bring a compact solution to the growing problem of tackling hard to remove micro substances. Easy to operate, efficient, and simple to incorporate into an existing process stream, CarboPlus is being used within industrial wastewater as well as drinking water treatment systems.

What is innovative about it? The most innovative aspect of CarboPlus is that it uses a fluidised bed of activated carbon within a combined contact and separation reactor together with a continual means of carbon replacement to remove micropollutants. The fluidised bed and carbon replacement approach allows CarboPlus to provide a simple and reliable solution because it maintains a controlled carbon adsorption and removal capacity within a reactor using familiar media. This approach also improves on contact time for a given amount of carbon and gives the ability to adjust the carbon contact time to suit seasonal variations in raw water quality. The simple hydraulics of the process allows water to pass through the fluidised bed of activated carbon separating naturally by gravity with little or no need for chemical usage, and with control and mechanical aspects constrained to carbon injection and removal.

How it was developed? CarboPlus is a patented solution developed by the French utility group SAUR; as an operator itself, SAUR identified the growing knowledge and tightening standards on micropollutants in water and wastewater as both a business and consumer risk. To address this risk they started work to create a solution that met its needs of being reliable, cost efficient, and easy to operate with very low operational cost. Development of the process moved from laboratory to full-scale pilot over a 10-year period resulting in full site operation in 2010.

What benefits does it bring? The system has delivered and exceeded in operation the anticipated removal rates across a range of pesticides and pharmaceuticals on clean water, industrial water and wastewater sites including the difficult-to-remove metaldehyde. The compact system size has reduced construction costs and enabled its installation alongside existing processes. It also uses little energy and boasts low or no chemical consumption - therefore making little impact on operational cost - and makes highly efficient use of media in terms of contact time and dosage flexibility.

Who is using it at the moment? The CarboPlus process is used in more than 20 different sites in France with capacities from 0.3 to 72 MLD. Most recently it has been installed in a new plant near Nantes where it was inserted between existing processes with no interruption to water supply.
SAUR is working with Capita to bring the process to the UK and interest is already being shown by a number of clients, including water companies concerned about metaldehyde.

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