Assuring the microbiological quality of water used to irrigate salad crops: an assessment of the options available

Headline. Although no evidence to link the irrigation of salads to disease outbreaks in the UK has been found, the industry should be seen to be taking the issue of irrigation water quality seriously. The strategy should be to take proactive measures to pre-empt the adoption of unnecessarily cautious standards within grower protocols. - Background and expected deliverables. Concerns have been expressed by some of the major supermarkets that salad vegetables may become contaminated with pathogens as a result of crop irrigation using poor quality water sources. As salads are likely to be eaten raw and will have received a minimal level of processing there are fears that consumers may be put at risk if irrigation water quality is not controlled. To assure product quality and to protect consumer confidence, some supermarkets may set stringent irrigation water quality standards in future grower protocols. Product quality is of paramount importance to growers, processors, retailers and consumers alike. However, the salad growing industry advocates a proportionate, science-based approach to the development of grower protocols rather than the adoption of an excessively precautionary principle. The aim of this project was to generate the baseline information needed by the industry to respond in a positive way to the concerns of retailers. The findings should help to inform the decisions and actions necessary to demonstrate and assure the quality of these products. The work should also support and contribute to the broader commercial objectives of the HDC regarding the efficient use of water. - Summary of the project and main conclusions. The following research tasks were undertaken in pursuit of the project’s aim. 1 A review of literature relating to the microbiological quality of irrigation water and of salad crops. 2 A survey of current UK irrigated salad production to assess current usage and underlying trends. 3 An evaluation of the technological and economic feasibility of on-farm water treatment options. 4 An analysis and discussion of the data collected and development of recommendations for the industry. The main conclusions of this work are: Irrigation water is one of many potential sources of contamination of salads. No published direct evidence has been found to link the irrigation of salads to disease outbreaks in the UK. However, there is a clear potential for this to occur. Published laboratory trials have shown that pathogens associated with poor quality irrigation water may survive on lettuce until harvest. Epidemiological investigations (not from UK) have indicated a link between disease and poor quality irrigation water. On occasions, some UK salad crops are probably irrigated with water of a lower microbiological standard than that recommended for comparable uses (e.g. reuse of wastewater for irrigation and bathing). The actual extent to which this occurs should be quantified and reviewed. The lack of guidance on irrigation water quality is a deterrent to proper water quality monitoring as most growers are unsure how they should respond to the data that is generated. This situation should be corrected as a matter of priority. It is reported that some of the multiple retailers in the UK favour a standard for irrigation water close to that which would meet the requirements for drinking water (i.e. absence or infrequent presence of E. coli in 100 ml water). Our review of standards suggests that this may be an unnecessarily cautious and expensive option. A grower faced with doubts about water quality appears to have four options: Demonstrate existing water is of adequate quality; Treat existing water; Change water source; Relocate crop. A site specific water resources study should be undertaken before assuming that treatment is necessary. Where water quality cannot be assured by management or sourcing strategies, treatment technologies may be considered. Of the many options, three technologies are likely to be suitable: ultra-violet (U/V) treatment, thermal treatment, and sand filters. U/V is considered to be attractive when taking all of the factors into account. Thermal treatment is the most rigorous and reliable. With heat recovery, such treatment could be viable in some cases. Sand filters offer the most farmer-friendly solution but these systems offer less assurance of water quality. - Financial benefits. There are no direct financial benefits to be gained by growers from this work. It may be prudent for growers to take proactive measures to improve monitoring procedures to pre-empt the adoption of unnecessarily cautious (and costly) standards in future grower protocols. - Action points for growers. There is likely to be increasing scrutiny of the microbiological quality of irrigation water. It is advised that growers review their monitoring strategy as a matter of priority. Regular sampling of water sources, at least monthly during the irrigation season, for faecal indicator bacteria would be a good start. The development of such a dataset would aid future decisions regarding the acceptability of particular sources.