Is it time you built a biobed?

The news that lines biobeds can be registered with the Environment Agency as an exemption is good for pesticide users.

Biobeds are a very effective and relatively inexpensive way of treating pesticide washings and run-off from pesticide handling areas.

A biobed is basically an organic filter system. Because most pesticides cling to organic matter, when contaminated washings or run-off water from the filling area goes through a biobed the pesticides stick to the organic material and clean water is left behind. The pesticides stay in the biomix where they are then broken down by the micro-organisms in the mix.

In this way, biobeds will remove over 99 per cent of the pesticides put onto them.
Provided the biobed can be sited in a secure place, 10m away from a watercourse and 50m away from any spring, well or borehole, they should be suitable for most agricultural or commercial horticultural operations using pesticides.

Above or below ground

Typically a biobed will consist of a 1m-deep pit lined with a heavy-duty synthetic liner, filled with a biomix consisting of one part peat substitute, one part topsoil and two parts wheat or barley straw. The biomix should be pre-composted and turfed over before any pesticides are put onto it.

But it doesn’t have to be a pit. It could be an above-ground container, for example, an old skip would be ideal provided it is sound.

Research being conducted by ADAS and funded by the HDC is developing a biobed specifically for the protected crop and hardy nursery stock sectors, the findings of which are due to be reported in 2008.

There are two basic systems. The direct system is where the sprayer is filled over the biobed and any drips, spills and washings drop straight into the biobed. The indirect system is where the separate handling area must be impermeable, bunded and fitted with a drain so that liquid is collected and stored securely. This liquid is then pumped from the temporary store through a drip irrigation system so that it is spread evenly onto the adjacent biobed. Both systems require a pump in the bottom of the biobed to transfer treated water for final irrigation or re-use.

Management is pretty straightforward. The biomix will need topping up annually as it decomposes and compacts over time. The drains in the handling area will need to be kept clear of sediment and will eventually need cleaning from time to time, with soil residues placed directly onto the surface of the biobed. In the longer term the whole biomix will need replacing. Further research is ongoing but current advice is this must be done after five years.

Spent biomix will need to be stored for 12 months to be sure all the pesticides have been degraded before being spread onto the land. It is important to remember, however, that a biobed is no substitute for good practice in the mixing and filling area.

In 1998, the Cherwell Study showed that pesticide handling areas can be significant sources of water contamination and that simple changes to operator practice — collecting foil seals, not letting empty rinsed packs drain into the yard and so on — could reduce this contamination significantly.

So why build a biobed? The plain fact of the matter is that doing nothing is not an option. Environmentalists, water companies and the public simply don’t want to see pesticides in water.

Moreover, the Government is using the amount of pesticide finding its way into water as one of its measures to assess the performance of the Voluntary Initiative, and the Water Framework Directive, which is just around the corner, will increase the pressure to keep water clean, as it will require all surface and ground waters to have good chemical and ecological status.

The answer for sensitive crops

Even when following best practice some drips and spills in the pesticide handling area can occur. Unless these are collected they are likely to contaminate water. A bunded pesticide handling area draining to a biobed is an effective solution.

It is illegal to dispose of pesticide waste to areas of soil or grass unless the Environment Agency grants a Groundwater Authorisation and even then those areas can only be used once a year. Of course sprayers can, and indeed should, be washed out in the field and the tank washings sprayed out on an area left under-dosed for this purpose.

But what about when you are spraying sensitive crops and the equipment needs rinsing two or three times between treatments? A biobed provides the answer, allowing these extra washings to take place in the handling area.  

Dr Paul Fogg is a pesticide specialist at ADAS and is involved in the DEFRA-funded Environment Sensitive Farming initiative

How to build a biobed

From the start, the whole concept of a biobed has been that it should be something which can be easily built and managed on-farm. Detailed instructions are provided in the Environment Agency guidelines, but in summary this is the procedure:

•    Not counting the turf layer, a biobed must be at least 1m deep and no more than 1.5m deep in the centre.
•    The pit can be any shape to suit the tank or lining system. Soil side slopes of 30-35° work well. These should be blinded with 25mm sand over which a geotextile membrane should be laid, followed by a 1.5mm thick liner of a type suitable for a small reservoir. The liner must be fitted in one seamless piece to prevent any leakage.
•    A 100mm bonded outlet is required in the bottom of the liner to allow treated water to be pumped to the surface for re-use.
•    A coil of 100mm perforated drainage pipe should be laid in the base of the pit on top of the liner to prevent the biomix material clogging the pipe. The lined hole should then be filled with the biomix and turfed over.
•    The direct biobed will also need a steel grid capable of supporting the fully laden sprayer.
The cost of a DIY biobed will be in the range £1,000 to £3,000 depending on its size and materials required.
Direct systems are more expensive because of the steel grid.
If substantial improvements to the handling area are required to ensure containment, costs will increase.

Further information

Dr Fogg is the author of the Environment Agency’s new guidance booklet on biobeds. The booklet provides detailed advice on siting, design and construction along with operation and management.

The Biobed Guidance and Exemption Pack is available from the Environment Agency’s agricultural waste line on 0845 603 3113.There is also a website at

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