Filter works wonders

27 February 1999




A bed of residues

Might a DIY on-farm compost pit – a biobed – swallow our pesticide residue problems?

Gilly Johnson takes a look at new research.

PESTICIDE residues in water are difficult and expensive for water companies to eliminate. Point source contamination – from accidental spillage, and washing and rinsing equipment such as sprayer tanks or containers – is considered a major culprit.

Enter the biobed. This is a new idea, poached from environmentally-conscious Sweden, but with a British twist. The principle involves letting naturally occurring soil organisms, contained in a bed of soil-based compost, clean up dilute pesticide waste on the farm. Pesticide residues in sprayer washings are degraded into harmless material before they have the chance of contaminating groundwater – thats the theory. Can it work in practice?

Research scientist Paul Fogg of the Soil Survey and Land Research Centre, Cranfield University, is putting the biobed on trial on three farms this season, with funding from a group of influential sponsors including the Department of Environment, Transport and the Regions, the Environment Agency, and MAFF. One biobed has been built on Crops diarist John Fentons farm at Blacktoft, near Yokefleet, North Humberside.

The first step was to dig a pit to hold the biobed mixture, on a site adjoining the farm yard, in a convenient position for washing off the sprayer. The sprayer is parked on a concrete pad, equipped with a central drain. A connecting pipe carries run-off water onto the neighbouring biobed.

The pit is lined with a butyl sheet; this is the main difference between the British system and the Swedish version. In the UK the authorities will not allow any escape of the materials within the bed into the groundwater, and so a sealed system is required.

Size is also different. In Sweden, the pit is just 50cm deep with clay at the bottom; in the UK the biobed is taking on a larger dimension with a capacity of 30cu metres, and 1.5m deep in the centre, with sloping sides – able to cope with perhaps 8,000 litres of spray washings a year.

To protect the 1mm plastic sheet from lumps and bumps, a layer of sand lines the pit. Another layer of sand sits on top of the liner to act as a water sump.

The recipe for the compost mix is half straw, and half equal parts of a soil and peat combination. Mr Fogg does not intend to seed the compost with additional bacteria; hes hoping there will be enough soil organisms within the top soil already to provide adequate inoculum.

As a safeguard, the fate of selected pesticide residues will be studied, in mini laboratory versions of the biobed this season. "We want to understand exactly what is happening inside – theres very little data available at this stage." Some agchem molecules could be more toxic than others to the organisms in the compost system; Mr Fogg reckons that fungicide and insecticide residues, applied later in the season, could have more impact than herbicides.

The idea is that the biobed should reach a steady state of degradation, so that some pesticide may be always detectable but levels dont build up. "The last thing we want to do is create a toxic pit in the farmyard…"

Costings, which do not include the concrete platform for the sprayer, work out at about £1,400. With labour, the cost rises to about £2,500. The biggest expense is the peat (£600), with the plastic liner coming second. Sixty small bales of straw were used for the Blacktoft pit. The farms own top soil was used.

Some teething problems have occurred. The biobeds have had to cope with exceptionally wet conditions this winter at all three sites. Waterlogging lowers the temperature, and could affect the degradation process. Initially, the compost generates heat as soon as the elements are mixed together. The biobed environment then stabilises, and for fastest degradation, it should operate at about 25íC. Tent roofs are being built over the biobeds to keep further rain away and encourage some drying out.

"Were trying to achieve an environment within the biobed thats neither too dry nor too wet," says Mr Fogg. "Aerobic conditions are needed." In theory, the biobed should never need emptying if the water balance is right. He expects to see the surface of the bed fall by about 10cm/year, and this will be topped up with more of the compost mix. In the summer, the surface of the biobed will be grassed over.

If the biobed system proves successful, then it might be a simple way of reducing pesticide residues in shallow groundwater, and one that growers could build themselves. "I would still expect the first sprayer washing to be done on the field. But the biobed would provide a way of disposing of the second and third washings back in the yard," says Mr Fogg. "It would be another tool, to be used in conjunction with other good practice techniques."

Already many growers have expressed a keen interest in the experimental biobeds, particularly in the one at Morley Research Centre, Norfolk. Farm manager Andrew Thurston is a convert. "If it works, it would seem to make sense as a practical, cost-effective answer. Our telephone hasnt stopped ringing with members making enquiries about the biobed since its been on view.."

Filter works wonders

FARM manager Ian Margetts wont be changing his spray washing-out system to meet the new groundwater regulations despite generating nearly 20,000 litres of dirty water each year.

He doesnt need to because the system installed nearly 10 years ago still produces crystal-clear water inside 24 hours. The key is a Carboflo charcoal filter treatment plant which cost just £2,500 in 1990 thanks to the 50% grant then available from MAFF.

All non-field spraying related activities on the 1,000ha Malshanger Estate, near Basingstoke, are carried out on a high-grade, silicone-sealed concrete pad beneath which are three 5,000-litre tanks. Waste water from the concrete surface, the chemical store, the emergency shower facilities and the preparation area drains into two of these tanks. From there the waste water is pumped in batches of 1,000 litres to the Carboflo.

Once the water has been through the mixture of activated charcoal and chemical agents which remove the toxic components, the remaining purified water is stored in the third tank for eventual application to any land without a licence.

"We tend to allow each batch 24 hours in the Carboflo, simply because this fits in with the daily farm routine, but the manufacturers claim you can do it in just a few hours," says Mr Margetts.

Filter works wonders

FARM manager Ian Margetts wont be changing his spray washing-out system to meet the new groundwater regulations despite generating nearly 20,000 litres of dirty water each year.

He doesnt need to because the system installed nearly 10 years ago still produces crystal-clear water inside 24 hours. The key is a Carboflo charcoal filter treatment plant which cost just £2,500 in 1990 thanks to the 50% grant then available from MAFF.

All non-field spraying related activities on the 1,000ha Malshanger Estate, near Basingstoke, are carried out on a high-grade, silicone-sealed concrete pad beneath which are three 5,000-litre tanks. Waste water from the concrete surface, the chemical store, the emergency shower facilities and the preparation area drains into two of these tanks. From there the waste water is pumped in batches of 1,000 litres to the Carboflo.

Once the water has been through the mixture of activated charcoal and chemical agents which remove the toxic components, the remaining purified water is stored in the third tank for eventual application to any land without a licence.

"We tend to allow each batch 24 hours in the Carboflo, simply because this fits in with the daily farm routine, but the manufacturers claim you can do it in just a few hours," says Mr Margetts.


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