Dairy Event 2010: Recycling run-off lowers costs

Filtering dirty water through a system of integrated constructed wetlands could save livestock farmers money on storing and disposing of run-off.

An experimental integrated wetland was established in 2007 at the ADAS Pwllpeiran Research and Development farm in Ceredigion to test this type of concept ahead of its possible use on commercial livestock units.

A series of five shallow ponds covering 0.33ha was constructed at this 1300ha upland farm, which supports a flock of 1300 sheep and 60 Welsh Black beef cattle. Each pond is about 25.5cm (10in) deep and the ponds have been planted with a range of native aquatic plants including Bullrush, Common Sedge, Canary Reed, Marsh Marigold and Common Rush.

A main concern regulators have with using integrated wetlands for dealing with agri-effluent is infiltration into groundwater. The concept relies on a combination of chemical and biological processes to remove organic matter and nutrients from dirty water and the soil is an integral part of this process. This makes artificial lining of the ponds not only expensive, but also less appropriate. Instead the pond construction relies on compacted clay to form a bund. When identifying a site, the more clay the better. There should be a minimum of 20% otherwise clay would need to be brought on site adding to costs, says Aldwyn Clarke, the facility’s senior scientific officer.

An obstacle at Pwllpeiran was a network of old field drains. When encountered during construction, these were removed or re-routed. To ensure the ponds were not leaking, each pond was connected via a chamber fitted with flow meters. This enabled volumes flowing in and out of each pond to be monitored which helped to identify any leakages. The value of this was highlighted when a major leak occurred in the biggest pond in the second year.

Dense vegetation had built up in the pond which made it difficult to locate the source of the leak, but it was eventually traced to a field drainage pipe that had not been seen during construction. “It was 7in below the base and would have proven impossible to detect without monitoring,” explains Mr Clarke.

Because the system at Pwllpeiran is experimental, the staff have full control over what goes into it. In addition to flow meters between ponds, each also has lysimeters to monitor infiltration. The final pond, the sampling pond, is butyl-lined to enable the system to be contained if any problems occurred before the water is discharged into the local watercourse.

The final pond is in reality a source of clean water which could be of value in times of drought. A further benefit of the ponds is that each has been constructed with a 1m free board. If a major spillage occurred, such as a slurry tank rupturing, slurry could be contained in the ponds to avoid a major pollution incident.

For a system like this to work it must be gravity fed which means the ponds are at graduated heights. An alternative would be a pumping system, but this would add greatly to the construction and running costs.

Wetlands will vary in size because they are governed by the total yard area of the agricultural holding. Work in Ireland suggests they should be two to three times the size of the yard surface area, depending on rainfall.

At Pwllpeiran, the wetland has been effective in reducing nutrients and organic matter in dirty water, says Mr Clarke. “They have also created a really good habitat for wildlife including invertebrates, amphibians and even a family of mallards.”

Although the Environment Agency is following the study at Pwllpeiran closely it does believe farmers should consider reducing effluent volumes in other ways.

Bob Merriman, agriculture specialist at Environment Agency, Wales, reckons large-scale systems like the one at Pwllpeiran may not be suitable for treating raw slurry, silage effluent or large amounts of dirty water, but he sees them as providing a good barrier for reducing diffuse pollution and some aspects of farm drainage.

“An important factor for farmers is to reduce the amount of dirty water on their farms in the first place. There are a number of simple things that can be done to separate clean and dirty water,” says Mr Merriman. “The clean water can then be used for stock watering or washing down collecting yards, as well as offering a cost effective solution for reducing water bills. It also means less slurry to manage or spread and a better quality slurry with a higher fertiliser value to help crop yield.”