From sewage sludge to giant grass: Crispin Aubrey reports on a project in Cornwall that has found a solution to a waste problem and also provides a clean energy source
Monday 20 July 1992
The idea involves combining sewage sludge, which might otherwise be disposed of in the sea, with domestic rubbish, which is usually buried in landfill dumps. The resulting mixture would be spread on derelict ground, making it suitable for planting with the woody grass, which has an extremely high energy potential when burnt. The scheme would also address a fourth problem: what should we do with the large areas of countryside which could soon be abandoned by traditional agriculture?
The key element is the giant grass miscanthus. A native of the Far East, it grows quickly to around 3m. When harvested it has a moisture content of only 20 to 30 per cent compared with wood's 40-50 per cent. It is therefore ideal for burning in industrial boilers and small power stations, generating relatively clean energy, from a renewable resource.
The attractions of miscanthus stem from its status as a 'C4' plant, which means it has a highly efficient method of photosynthesis, converting more light and heat into energy than other plants. The only other C4 plant commercially grown here is forage maize. Miscanthus flourishes with a low fertiliser input, and is suited to northern Europe. It is also carbon neutral, extracting as much carbon dioxide during growth as is expelled when it is burnt.
Miscanthus is being cultivated in Denmark, Austria and Germany, with yields of over 30 tons per hectare (12 tons per acre) - much higher than that of other energy crops such as willow and poplar. A 12-megawatt biomass-fuelled combined heat and power plant will open in Amberg, Bavaria, this summer, fed by a mixture of miscanthus, willow, poplar and straw. About 1,000 hectares (2,500 acres) of miscanthus would be needed to supply the station's annual fuel supply.
In Britain, John Harvey, an agronomist with the Ministry of Agriculture, Fisheries and Food, has been growing miscanthus for eight years at his home near Exeter. 'A problem is the high cost of propagation from stem pieces,' he says, 'but micropropagation and production from seed will bring the price down.'
This spring the Maff started a trial with large plots of miscanthus in Cambridgeshire, Devon and Herefordshire. Mr Harvey is about to start work with a project in west Cornwall where offices and greenhouses will be heated by boilers burning crops including miscanthus, poplar and willow. He has just completed a review of the potential of miscanthus for the Government's Energy Technology Support Unit.
'If we're thinking of reducing carbon emissions, these are the sorts of thing we should be looking at,' says Faith Culshaw, of ETSU. 'If we could achieve the European yields here, it would be an extremely valuable crop.'
Developing interest in miscanthus coincides with a dilemma for water companies such as South West Water. Pressure to clean up their disposal routes means they have larger amounts of sewage sludge to deposit elsewhere. Devon County Council also has a problem. It has 300,000 tons of rubbish a year to dispose of and is running out of landfill sites.
The two have combined in a pounds 1m project, supported by the Department of the Environment, to compost together municipal rubbish (screened for pollutants) and the sludge. The first plant is due to open this month near Plymouth, with an initial throughput of 500 tons of the mixture per week. One use for this compost would be on derelict land: Cornwall has thousands of acres left from tin mining works. Miscanthus would be grown on the blanket.
Care would have to be taken to ensure that poisonous residues in the ground, such as copper and arsenic, were not taken up by the plants and dispersed into the atmosphere during burning. A European Community-funded study will begin next spring to look into this.
If miscanthus does flourish across the South-west, it will be good news for a farming area under pressure to 'set aside' land from food production. There are other uses for the crop apart from energy production. In Germany it has been used as a source of cellulose for paper manufacture, and by an oil company to produce hydrogen.
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