Guest post by Dr Richard Cooper, University of East Anglia
A two year research program (2013–2015) by UEA academics, in partnership with the Environment Agency, farm managers and precision farming experts Farm Systems & Environment, found that a lined compost-straw-topsoil biobed reduced total pesticide concentrations in waste machinery washings by over 90%, thus minimising both surface water and groundwater pollution risk.
The biobed facility
In 2013, a biobed facility was installed at Manor Farm, Salle Park Estate, Norfolk, UK to treat pesticide contaminated machinery washings. The facility consists of three main components:
- Wash-down unit: a 20 m x 9 m enclosed wash-down unit is used to both remove pesticides residues from farm machinery and to contain any pesticides spilt during the filling of the pesticide sprayer. A drain running down the centre of the unit channels contaminated washings into a concrete storage tank (the input sump);
- Biobed: the biobed itself is an uncovered, indirect, lined (impermeable geomembrane) design covering an area of 49 m2 (7 m x 7 m) to a depth of 1.2 m, thus providing a large surface area for the sorption and subsequent biological and photo-degradation of pesticide residues. The organic bio-mix matrix material is composed of a 1:2:1 mix of peat-free compost, chopped wheat/barley straw and local topsoil. The surface is seeded with grass. Contaminated water from the input sump is pumped onto the biobed surface via a trickle irrigation system, with the leachate collected at the base of the biobed in a concrete output sump;
- Drainage field: the leachate from the output sump is pumped onto a 200 m2 (20 m x 10 m) grass drainage field via a second trickle irrigation system buried just below the surface to promote further removal of residual pesticide residues. A network of 20 porous pots were installed across the drainage area at 45 cm and 90 cm depth to monitor soil water pesticide concentrations at depth for signs of further removal or accumulation.
Pesticide removal efficiency
Pesticide concentrations were analysed in water samples collected fortnightly between November 2013 and November 2015 from the biobed input and output sumps and from the 20 porous pots buried within the drainage field. In total, 86 different pesticides were detected in the system and the biobed was effective at reducing the concentrations of them all. Results revealed that the biobed removed 68–98% of individual pesticides within the contaminated washings, with mean total pesticide concentrations reducing by 91.6% between the biobed input and output sumps. Drainage field irrigation removed a further 68–99% of individual pesticides, with total mean pesticide concentrations reducing by 98.4% and 97.2% in the 45 cm and 90 cm depth porous pots, respectively. The average total pesticide concentration at 45 cm depth in the drainage field (57 µg L-1) was 760 times lower than the mean concentration recorded in the input sump (43,334 µg L-1).
Table 1: Biobed pesticide removal efficiencies for the 15 most abundant pesticides detected.
The biobed facility required limited maintenance following its construction in 2013. The biobed matrix was topped up with fresh material in July 2015 as decomposition of organic material had reduced the depth of the bio-mix. There was no evidence of a reduction in the pesticide removal performance of the biobed over its two years of operation, nor was there any evidence of seasonality in degradation efficiency indicating the design was able to withstand cool, wet UK winters.
Costs to set up
The cost of the biobed including the pipework, pumps, liner, matrix material and labour, was around £4300 and the cost of the drainage field infrastructure was around £1700. Replenishment of the matrix material two years after construction cost £8 per square metre. The most expensive part of the system was £90,000 for the construction of the large wash-down unit and equipping it with mains electricity and steam cleaning equipment, however, simpler facilities would be recommended for wider roll-out.
Agricultural pesticide pollution threatens both riverine and groundwater systems and therefore on-farm mitigation measures are required to reduce the amount of pesticides entering these freshwater environments. This study shows that a three-stage biobed can successfully reduce pesticide pollution risk from contaminated machinery washings on a large commercial arable farm.
This research by Richard Cooper, Peter Fitt, Kevin Hiscock, Andrew Lovett, Lee Gumm, Steve Dugdale, Justin Rambohul, Antony Williamson, Lister Noble, James Beamish and Poul Hovesen was published in the Journal of Environmental Management in June 2016 – “Assessing the effectiveness of a three-stage on-farm biobed in treating pesticide contaminated wastewater” doi:10.1016/j.jenvman.2016.06.047.