- 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