REEDBED WASHINGS TREATMENTS A SAVER
Setting up on-farm treatment for parlour washings could save on slurry storage and
spreading costs without risking polluting water courses. Shelley Wright reports
A REEDBED system being developed by SAC could offer producers a cost-effective alternative to irrigating dairy parlour washings onto land or storing it with slurry.
Until recently, reedbeds have been best known for treating domestic sewage after it leaves septic tanks in rural areas where connection to the mains sewer is not available.
But since 1999, with funding from the Scottish Executive, SAC has been investigating and developing a reedbed system to treat farm effluent which is farmer-friendly in construction and operation.
It has installed a system to treat all dairy washings from the 200-cow unit at its Auchincruive Farm, near Ayr. And, according to Stewart Moir, SAC water and waste management consultant, initial results are encouraging.
The pollution strength of any effluent is measured by its biochemical oxygen demand (BOD), which shows how much oxygen the effluent strips from water. On farms, dairy washings are second only to silage effluent in BOD.
A typical reedbed system consists of a shallow excavation, often lined with a PVC liner, which is filled with 60-80cm (24-31in) depth of gravel. The gravel is then planted with the common reed, Phragmites australis.
"Its easy to construct, any producer or drainage contractor could install one," adds Mr Moir.
Effluent flows through the gravel and is cleaned by bacteria that grow on the stones. Although some bacteria survive without oxygen, others need it and are supplied with their oxygen requirement from the roots of the reeds. This results in a mixture of bacteria which allow effective treatment of effluent, reducing BOD, suspended solids and nutrient levels.
At Auchincruive, the system under development has two elements. First, dairy washings are collected and pre-treated in an aerobic sequence batch reactor. This is a tank which sits on a 3m x 6m (10ft x 20ft) concrete apron adjacent to the parlour wall of the dairy unit.
All the wastewater from the dairy unit, some 3000 litres/day, is collected and held in the tank, he says. Air is bubbled through to provide oxygen to bacteria which begin the process of digesting the effluent and reducing its BOD. The washings are then discharged to the Reedbed Treatment System for final treatment.
Results from Jan 2000 to May 2001 show that the dual process is consistently effective in reducing the BOD of dairy washings to an acceptable standard for discharge into a watercourse or recycling for yard washing.
However, one problem was quickly identified, he explains. The lack of nutrients in dairy washings was leading to stress in the reeds, with leaves turning yellow and being more prone to fungal infection.
That could be solved by adding fertiliser to the reedbed, but at Auchincruive it has been rectified by the start of the second phase of the study – treatment of silage effluent. Again, the effluent is held in a tank first to allow initial treatment, then it is passed through the same reedbeds as the dairy washings.
Nutrients from the silage effluent have been sufficient to meet the reeds requirements and they are now growing healthily.
Although preliminary results with silage effluent are encouraging, says Mr Moir, the BOD is not yet sufficiently reduced to allow it to be discharged into water courses or recycled for yard washing after treatment.
The reedbed system of treating dairy washings offers many advantages over storing them with slurry. Mr Moir lists the key ones as extended slurry storage; increased concentration of N, P and K for each cu m of slurry; fewer loads of slurry to spread and fewer days required for spreading. There is also reduced potential of odour nuisance and less potential run-off from land.
Mr Moir justifies the costs based on a typical 90-cow dairy unit with modern buildings in an area with an annual rainfall of 1100mm (44in).
"The best case scenario is that your storage requirement for slurry alone is 1400 cu m, costing £30,000. In the worst case scenario, where storage is needed for slurry, washings and run-off it would require 3400 cu m of storage costing £50,000."
"That gives a £20,000 difference for a treatment system." Although SACs own system at Auchincruive cost exactly that amount, Mr Moir stresses that additional expense was incurred for scientific monitoring and recording equipment.
"A typical cost would be about £15,000, maybe even as low as £10,000 when farm labour is used," he says.
He also calculates annual savings of £1500 for a 90-cow herd. This includes £500 worth of water charges by recycling dairy washings for yard cleaning and £1000 less for spreading dairy washings onto land with slurry.
Although the research is still in its infancy, Mr Moir believes a commercial system should be available in the next three years.
The next step in its development will include further work on automating the system and developing a feedback system that reduces energy input.
A scaled-up system is also planned to cope with the effluent from SACs 400-plus herd at Crighton, Dumfries. *
• Working in practice.
• Proving cost-effective.
• On farms in three years.