Sewage sludge a safe bet?
By John BurnsSouth-west correspondent
MISCANTHUS straw is proving particularly good for composting with undigested sewage sludge in trials designed to find a safe and publicly acceptable way to recycle sludge onto farmland and improve the profitability of industrial crops.
The trials could provide a blueprint for composting sewage sludge to meet the Safe Sludge Matrix standards agreed by the British Retail Consortium, representing multiple retailers, and UK Water, representing UK water companies.
The final blueprint will include costings as well as methods for controlling risks to human health where the composts are used for producing food and minimising pollution of land, water or air during the composting process and its later storage and application to land.
"After that, it will be up to water companies to negotiate with farmers about payments for composting the sludge on their farms," says ADAS environmental consultant Giles Perryman, who is running the trials in Devon and Cornwall.
Mr Perryman thinks it is likely the lowest costs overall would be achieved if the composting was done on farms that grew the straw and also used the compost. Miscanthus and oilseed rape straw have proved more suitable than cereal straw.
Current on-farm trials involve wind-rowing the sewage sludge, covering it with Miscanthus, and mixing it in. The windrow mixing and aerating machine, which is imported from Belgium, could be shared by a group of farms, says Mr Perryman.
Samples taken at the beginning and end of the composting process are analysed for harmful organisms and samples taken during the process are tested for E coli to check the temperature has been raised enough to destroy pathogens.
Probes in the wind-rows record temperature changes between mixings. It is hoped the final protocol will rely largely on temperature monitoring and control instead of expensive lab tests to control pathogen levels in the final product.
On delivery, the undigested sludge typically has 700,000-800,000 counts/gram. After three weeks of composting, levels are often below 10 counts – the minimum detectable.
After a further five weeks, all samples have undetectable levels of E coli and other pathogens.
Temperature in the wind-row is maintained at 65-75C by aerating whenever it falls below 60C. "Heavy winter rainfall slows down composting and can induce anaerobic activity," says Mr Perryman. "But mixing in more straw soon restores the process and the temperature rapidly rises again." Winter composting may take six weeks or more.
The project is also looking at composting sludge on the headlands of fields where it will be spread later.
The trials are financed by the MAFF/EU Objective 5b programme for parts of Devon and Cornwall, with match funding from ADAS, South West Water, the Environment Agency and local authorities. *
• Sewage sludge + straw.
• Aerated in windrows.
• Miscanthus/OSR best.
• Temp kills bugs.
• 16t/ha = 1t/ha cereal boost.
• Worth £3.83/t fresh.
Composting miscanthus straw and sewage sludge (right) appears to be promising in trials, says ADAS environmental consultant Giles Perryman (below).
The sludge used in the trials is partially processed raw sewage. It has been screened and settled and liquid drained off, leaving a "blancmange" of 20-25% dry matter, says Mr Perryman. It has not been digested or heat-treated. In its raw, uncomposted state the Safe Sewage Matrix would only allow its use on fields growing industrial crops.
March applications of 26t/ha (10t/acre) of the composted sludge to land before ploughing and spring barley drilling have so far shown an increase in yield even on plots that also received 250kg/ha of inorganic nitrogen. The biggest response to compost, at 1t/ha (0.4t/acre), was on plots given 0 or 50kg/ha of inorganic N. At the higher N rates, the average response was 0.3t/ha (2.4cwt/acre). The compost was estimated to supply 250kg/ha total N in a shallow-soiled field that had been in continuous cereals in an area of low sulphur deposition. "The yield responses could have been due to slow release of nitrogen late in the growing period," says Mr Perryman. The value of the compost based on its NPK content and crop yield response has initially been estimated by the ADAS team at £3.83/t fresh or £15.76/t dry matter.