Water companies have a regulatory requirement to carry out risk assessments of water supply, to identify the potential for contamination within the catchment, and the performance of treatment for removing contaminants to protect the health of the consumer.
These risk assessments are carried out using a Drinking Water Safety Plan (DWSP) framework for individual systems, and are identifying chemical or microbial contaminants arising from industrial or agricultural activity, or from sewage effluent discharges. Many of these are organic micropollutants which may have health implications, or exceed drinking water quality standards, at the very low concentrations (µg/l or ng/l levels) found. The DWSP approach needs to identify the potential for contamination of water sources with these chemicals, and ensure that control measures are in place to mitigate the risks, either by preventing contamination or by removing the contaminants in water treatment. The control measure can also include monitoring of raw and treated waters for the contaminants.
Pesticides have been identified as potential contaminants in water for many years. In the EU, the regulations are laid out in the Council Directive 98/83/EC “on the quality of water intended for human consumption”. This specifies that the concentration of individual pesticides should not exceed 0.1 µg/l, the total concentration of pesticides should not exceed 0.5 µg/l and the water should be “wholesome”. These standards are incorporated in regulations of Member States, including the UK. Many pesticides such as the urons, acid herbicides and triazine compounds were identified in the 1990s, and treatment implemented to deal with these. The recent implementation of DWSPs and the development of new products have raised the awareness of other pesticides of emerging concern, some of which are not as readily removed by water treatment. Metaldehyde, used in slug pellets, is of particular concern in this respect at the present time, but other biocides such as clopyralid, ethofumesat, metazachlor, propyzamide, quinmerac and triclopyr are also being identified as potential problems in meeting the pesticide standard, although not necessarily in relation to toxicity or wholesomeness at the very low concentrations occurring. Because removal of some of these compounds by water treatment is ineffective or uncertain, control at source through guidance on application may be the more appropriate control measure.
Other organic micropollutants of emerging concern include perfluoro-octane sulphonate (PFOS), used historically in fire-fighting foams, and endocrine disrupting compounds (EDCs) such as steroid oestrogens from pharmaceuticals and bisphenol A, used in the production of some plastics and resins. Some naturally occurring organic micropollutants, particularly the cyanotoxins and taste and odour compounds from algal growth, can also be a concern in water supply, and need to be included in the DWSP approach. There is a need to monitor the water quality for compounds of concern, both in the untreated raw water and in the finished product, and remove compounds that will exceed the regulated limits. The DWSP should identify which of the very wide range of organic micropollutants need to be monitored for a particular site. At the present time, on-line systems for monitoring of organic micropollutants are not widely available, and are expensive and complex. Such systems are therefore used only for highest risk sites, and for other treatment works, monitoring by manual sampling and laboratory analysis would be used, supported by liaison with environmental and agricultural agencies, and with users of the chemicals, principally farmers.
Generally, water treatment can be highly effective at removing most organic micropollutants, provided design and operating conditions are appropriate. Activated carbon is used in the granulated (GAC) or powdered (PAC) form. PAC is dosed in to the water and removed by subsequent treatment. It can be implemented as needed, and may be more cost effective compared with GAC for intermittent contamination. However, GAC is generally the preferred option for most treatment works, as it is easier to handle and provides greater security through continuous use. It is used in purpose built adsorbers, and is regenerated when the adsorption capacity has been reached and breakthrough of the contaminants from the GAC bed occurs. The key factors which determine the effectiveness of this treatment process are competitive adsorption from background natural organic matter, GAC type, adsorber design (particularly empty bed contact time, EBCT) and the GAC regeneration regime. The increasing costs of GAC treatment and the wider range of GAC types available have made it important to procure the most appropriate GAC for the application. Furthermore, optimised regeneration has become a key factor for cost savings on already tight operational budgets.
As an alternative or complement to adsorption by GAC, oxidation using ozone, UV, chlorine, or advanced oxidations processes (AOPs) can be used to reduce the concentration of contaminants. Chlorine, widely used as a disinfectant in water treatment, can be effective for some pesticides. However, it is generally not used for this application because of its very limited capability for degradation of organic micropollutants. Ozone is commonly used for degradation of contaminants, reduction in natural colour and as a disinfectant. Some organic micropollutants are more amenable to ozonation than activated carbon adsorption. Ozone is therefore often used in combination with GAC to deal with a wide range of organic micropollutants. The increase in the GAC bed life and decreased operational costs can offset the cost for the upstream ozonation, to provide overall a more secure and cost effective treatment.
The use of UV has become increasingly popular for disinfection purpose, particularly for the inactivation of Cryptosporidium, but UV also has the potential for degradation of pesticides and other organic compound. The UV dose needed for degradation is however significantly higher than for disinfection only. Advanced oxidation processes utilise combinations of hydrogen peroxide and ozone or UV to produce the highly reactive hydroxyl radical (HO·) which can rapidly degrade many organic micropollutants. The use of AOPs is not widespread for water treatment at the present time, but is developing rapidly.
WRc is currently carrying out work for UK Water Industry Research Ltd, on behalf of the UK water companies, to evaluate the removal of metaldehyde and other emerging pesticides by water treatment. The work is aimed at addressing many of the issues highlighted above and to help provide a balanced approach to dealing with problem pesticides, taking into account control through application as well as removal by treatment.
For further information please contact Jörgen Jönsson at Jorgen.Jonsson@wrcplc.co.uk