Reed-beds are a unique and simple way to treat dirty water, but are they a panacea for all farmers?
Peter Marks, managing director of Duntech Irrigation Systems, says they can be used to clean up dirty water from livestock farms, treat industrial effluent, or capture and clean run-off from saturated fields.
But a cure-all they are not.
“Many producers want to treat raw slurry and reed-beds just won’t do that.
They can break down almost anything, but there is a limit to the amount of nitrogen they can handle.”
Reeds pump oxygen into their root zone and sustain bacteria which clean the water.
The maximum ammonia the bacteria can remove is about 2000mg/litre, and this rate is reduced by about 15% in winter due to colder temperatures.
To create a reed-bed, farmers must first calculate the amount of dirty water they have, and how polluted it is, says Mr Marks.
Dirty water from a parlour or concrete yard will be fine – effluent from a weeping wall system will not.
A reed-bed will cope with 20mm of dirty water across its surface each day – so it is the surface area of the bed which must increase with water volume and pollutant strength, not depth.
“You really should get some professional advice if you’re thinking of installing one, as they are site specific.”
The area must be flat and will require a liner when the subsoil is permeable.
The lagoon should be about 1m deep, and 700mm of soil and gravel is laid before the reeds are planted at a rate of four plants/sq m.
“Norfolk reed is the most efficient aerator, which quite often farmers have on their land already.
Otherwise you could get some from the Environment Agency when it clears water courses.”
An entry pipe is placed at the top of the pond, with a drainage pipe at the other end, linked to a flexible hose.
This can be raised or lowered to adjust the water level in the pond.
“It is important to keep the reed-bed flooded.”
The cost of creating a reed-bed depends on the site, but varies from about £20/sq m to £60/sq m, says Mr Marks.
Planning permission is not required, but when the bed discharges into a watercourse the Environment Agency must be informed.
One pond is usually sufficient, although some farmers prefer to alternate usage between two, he adds.
Reed-beds take three years to reach full capacity, working at 50% in the first year and 75% in the second.
However, they require no maintenance after that, apart from flushing out the drainpipes, and will double their cleaning ability after 10 years.
“It’s an everlasting asset once you’ve got it.”