Milk Sludge Fertilising Willow

Dale Farm’s environmentally aware staff have come up with a novel solution for reducing the company’s carbon footprint – using sludge from the waste water treatment plant at Dunman to grow willows.

Traditionally the sludge, which is a normal by-product of dairy processing, has been spread on farmland – where it acts as a valuable fertiliser. However, the Nitrates Directive has effectively reduced the amount of land that can be treated this way and other outlets had to be examined. After reviewing a range of possible options, it was decided that applying the sludge to Short Rotation Coppice Willow for producing renewable energy biomass, was an environmentally friendly solution that was also cost effective.

The scheme, which is operated in partnership with Rural Generation Ltd, began on a trial basis in June. It uses bespoke machinery that injects the sludge below the surface of the ground in a willow plantation in the Cookstown area, relatively close to the Dunman factory.

A permit had to be attained from the Environment and Heritage Service to allow the sludge to be used this way. The effluent has to be treated using a belt press, which increases the solid content to approximately 12% – which results in fewer transport movements and thus reduces the fuel and labour required.

Willow production is a renewable and sustainable energy crop that is harvested and used to produce either heat or electricity. Harvesting is usually on a two yearly rotation when the willow is cut and chipped, and some local Councils are currently using willow as an alternative energy source for heating e.g. swimming pools.

Willow production does not contribute to global warming – the only carbon dioxide released upon burning is that which was fixed from the air whilst the willow was growing, making it “carbon neutral” unlike other fuels such as coal and oil.

The use of sludge as a soil conditioner and source of nitrogen on willow plantations is a good example of an environmentally sustainable, low carbon route for recycling an organic waste stream. It is well established in Europe and N America, but is still relatively new in N Ireland and Dale Farm is the first local company to investigate this opportunity.

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