Chlorination of irrigation water impacts on chlorate and perchlorate residue in edible produce at harvest

Chlorination of irrigation water impacts on chlorate and perchlorate residue in edible produce at harvest

Purpose / primary objective

AHDB wishes to gather data on how growers are disinfecting their irrigation water and the impact this has on chlorate and perchlorate levels in edible produce at the point of harvest.


Chlorination - referred to in this Call for work as the use of chlorine compounds and systems for disinfection - is the cheapest and most effective way of disinfecting large volumes of water. In horticultural plant production, chlorine compounds such as chlorine dioxide and calcium or sodium hypochlorite are used to disinfect irrigation water. This is resulting in chlorate and perchlorate bioaccumulation in the crops, as plants take up the chlorinated water from soil. Chlorate and perchlorate contaminants in foods are of human health concern, because at high doses, both inhibit the uptake of iodine in the thyroid gland, disrupting thyroid function and the production of hormones involved in metabolism.

Before 2009, chlorate and perchlorate residue in food and feed was not an issue, as the Maximum Residue Levels (MRLs) that were permitted then took into account the use of sodium chlorate as a herbicide. Sodium chlorate uses were revoked in 2009, at which point the Commission defaulted to the default MRL of 0.01mg/kg, the Limit of Detection (LOD) for chlorate. No MRL was set for perchlorate.

Food Standards Authorities in the different countries were asked to monitor for residue - the reported detections across Europe were found to be over the LOD for chlorate. Reports were published on the occurrence of chlorate in fruits and vegetables. The European Commission proposed chlorate MRLs in November 2015 which were based on monitoring data collected across the EU; these MRLs were not accepted by different Member States for various reasons. Since then the Commission has been deliberating internally on the issue, and they have confirmed that they will be taking into account further monitoring data before any future proposals on chlorate MRLs are published.

On perchlorate, a provisional harmonised enforcement approach was agreed in July 2013; levels were further revised in March 2015. The Commission has very recently (2018) suggested the below for perchlorate in fruits and vegetables.



Suggested levels – 2018 (mg/kg) for discussion at 29/05/18

Fruits and vegetables


with the exception of


- Cucurbitaceae, kale, leafy vegetables  except


- - spinach, rucola, and herbs



In light of these contaminant issues, AHDB Horticulture pooled residues data in UK fresh produce in 2015 and 2016 (AHDB Horticulture Project CP 154). The data collected was submitted to the UK Food Standards Agency (FSA) and the Chemicals Regulation Directorate (CRD) of the UK Health and Safety Executive (HSE). The data also fed into the AHDB Horticulture Chlorate and Perchlorate Review, which was published in 2016. Although the data was limited, broad findings were:

  •  for chlorate - that leafy salads (eg lettuce) and baby-leaf crops (eg spinach), herbs; protected edibles crops such as tomatoes that are grown hydroponically and protected soft fruit irrigated with chlorinated water had levels above the chlorate LOD.

  •  Very limited data was available on perchlorate, so no conclusions could be drawn.

This project aims to collect further residues data on chlorate and perchlorate. Data collection will focus on chlorination practices that are employed by UK growers to disinfect irrigation waters only, with a view to gain insight on the impact that this has on the chlorate residue in the crop at the point of harvest.

All data gathered will be anonymised and processed before submission to the FSA and the CRD. It will provide UK evidence to inform the European Food Safety Authority (EFSA) MRL setting. The data will also be used to inform further work, if required.


This project aims to:

  •  collect and review UK growers’ chlorination disinfection practices and analyse the resulting chlorate and perchlorate data for the different crops.

           *AHDB will run a campaign to encourage growers to gather their own data and to send samples for chlorate and perchlorate testing at their own cost; the contractor will collect and analyse the data.

  • Further substantial information/data pertaining to the chlorine dioxide dosing of irrigation water that is used in outdoor spinach production will need to be collected and analysed. AHDB will identify five UK outdoor spinach growers who will be engaged in this activity.                                                 

Specific Objectives

The contracted work will produce the following outputs:

 1. The development of a detailed and statistically robust grower-friendly data collection protocol to advise growers on how to capture information that includes but is not limited to:

  • irrigation water source and quality

  • chlorate and perchlorate contamination in soil

  • how to collect and process irrigation water samples for microbial analysis before and after water disinfection

  • chlorine disinfection practice – dosing time, concentration used (when and how often; manual/automated; systems etc)

  • the number of irrigation events on the harvested / study crop from which data is pooled

  • how to sample fruit and vegetables ready for harvest and process the produce samples for chlorate testing

  • methodology for data collection

  • where to send soil and water samples for microbial testing and produce samples for chlorate testing (costs of analysis to be covered by participating businesses). Samples will need to be analysed at accredited laboratories and the methodology for chlorate and perchlorate testing employed by the labs will need to be accredited.

2.  A grower- friendly data collection template to support (1).

The protocol and template(s) will go out to all horticultural growers of edible crops. Interested parties are encouraged to include exemplar/draft protocols and templates in their submissions.

3.  The successful contractor will be expected to work with five UK growers of outdoor spinach crops; following a crop from drilling to harvest; documenting and capturing all necessary data as referred in (1).   Sufficient detailed information pertaining to the chlorine dioxide dosing of irrigation water that is used in outdoor spinach production and the impact this has on chlorate and perchlorate loading in spinach at the point of harvest to support MRL setting will need to be collected and analysed.

Only submissions from accredited contractors will be accepted, for there is need to ensure that the protocols developed will generate data that is deemed to be sufficiently robust for consideration in MRL setting.


The outputs listed below are considered a minimum; potential contractors are encouraged to outline other relevant knowledge exchange activities for the edibles growing industry. 

  1.  Grower friendly protocol and template for data collection

  2.  Data to be in a format that is easy to lift; for inputting into the EFSA monitoring data Excel templates.

  3.  A publication/review reporting on UK chlorination systems/practices for disinfection of irrigation water; with recommendations/guidance to promote best practice chlorination.

  4.  Final report to AHDB with recommendations for further work

  5.  Article for AHDB Grower magazine

  6.  Presentation at up to two relevant grower events

Project Duration

4 months

Examples of relevant publications from AHDB

AHDB Horticulture Keeping it Clean Page

AHDB Horticulture Project CP 154a Call

AHDB Horticulture Project CP 154a Report

AHDB Horticulture Contaminants Update


Prospective contractors are encouraged to submit a joint proposal if complementary expertise has been identified. AHDB may, if it is deemed desirable, request applicants to form a project consortium.


The indicative budget is in the range £55 – 57k excluding VAT

Completion and submission of the application form

Please refer to the guidance notes for completion of application forms. Applicants should complete the AHDB Research and KE Application Form - Full Proposal Large, completed forms must be emailed to no later than 12.00 noon on 3 August 2018.


Proposed timings for application and project delivery Stage of process


Call published

Friday 13 July 2018

Full proposal submission deadline

Friday 3 August 2018

Applicants informed of the outcome

Friday 10 August 2018

Anticipated start date

Monday 13 August 2018

Project duration

4 months (ending 13 December 2018)



If you have a specific question related to this call, please email As part of the open tender process, AHDB cannot discuss specific project details with you before submitting your proposal. View full Questions & Answers for this Research Call.

Assessment criteria

Please note that the assessment criteria have been slightly altered for this tender. To access the revised assessment criteria, please click here.