Success claimed for co-composting of waste with sewage sludge

A commercial plant to compost municipal waste with sewage sludge may soon become a reality following the success of a demonstration project in south-west England. But the lack of any quality standards for composts may restrict the product to the lowest value end-uses and could deter investment in a large-scale facility.

The COWS co-composting project began at Chelson Meadow in Plymouth last August. Its aim was to determine the most cost-effective large-scale method of producing "safe" compost. The quality of compost produced from organic household waste which has been separated at source has already been proven (ENDS Report 218, pp 15-18 ).

The participants' interest in the project stemmed from the growing squeeze on existing disposal outlets for sewage sludge and municipal waste. South West Water (SWW) currently dumps about half the sewage sludge produced in the region at sea, but this practice will be banned at the end of 1998. Meanwhile, annual sludge generation in the area is projected to increase from about 30,000 dry tonnes at present to 52,000 tonnes by 2011, mainly under the impact of the 1991 EC Directive on urban wastewater treatment.

For Devon Waste Management, the disposal company of Devon County Council, the problem is that existing landfill capacity in the region will be exhausted ten years hence, and new sites are proving increasingly difficult to find. Composting could reduce the amount of municipal waste going to landfill by up to 70%.

With the backing of a £409,000 grant under the Department of Trade and Industry's DEMOS scheme, SWW set up a windrow facility. In-vessel composting and aerated stack piles were ruled out on cost grounds, but the facility is semi-enclosed to exclude rain and minimise leachate, and to reduce odours and other airborne pollutants.

Incoming municipal waste is not screened because this was shown to remove too much organic matter. Large items, however, are removed by hand before it is mixed in various ratios with sewage sludge, shredded and seeded with previously composted waste. It is then left for 15 days at 55° C to ensure sufficient pathogen kill, while being regularly turned. Next, the material is screened to remove plastic, metal and other large contaminants before being left to stabilise for 2-3 months in the open. The process reduces 100 tonnes of input material to 50 tonnes of compost and 22 tonnes of waste.

Composting can cause adverse environmental impacts if mismanaged, with leachate, litter, vermin and odours all potential problems. But the most serious risk of co-composting, Dr Ian White of Devon County Council's Waste Control Unit told a DEMOS seminar in June, is that of airborne pathogens released during turning. These have to be controlled or the composting operation conducted away from public areas.

Tests by SWW have shown that no pathogens, as represented by Salmonella as an indicator organism, occur in the final product. Only Rhodotorula, a common yeast found in most organic wastes, was found in any significant quantities in airborne emissions from the plant. Faecal coliforms were detected on one occasion. Independent tests on off-site migration of airborne pathogens are now under way.

The COWS trial, due to end in September, has gone well enough to enable South West Water and Devon Waste Management to consider scaling it up to take 20,000 tonnes of both sewage sludge and municipal waste each year. "We now have sufficient knowledge to consider moving forward to a commercial basis," says SWW's Tom Young. A decision is expected in July.

Estimated capital costs of £6-700,000 and operating costs of £4-5 per tonne would be covered by a gate fee of £15 per tonne. Gate fees in the region are typically of this order so only a small shift in prices, perhaps brought about by a landfill tax, would make composting the cheapest route.

The calculations exclude revenues from compost sales and recycling credits. But markets are unlikely to be high value ones. The current product is "not suitable for sale in garden centres because there is not a 100% removal of glass and plastic," says Tom Young, although glass pieces could be reduced further by crushing.

One of SWW's concerns is that the compost market could be damaged if poor-quality products are pushed into end-uses for which they are unsuited. Like most other composters, Mr Young argues that compost standards are needed. "People are not willing to invest without knowing what the future standard will be. We can't take this much further without one."

SWW claims that the COWS product meets the existing EC standards for the disposal of sewage sludge to agricultural land. Copper, zinc, cadmium and nickel levels are generally at half the lower EC limits. But on one occasion lead levels exceeded the lower EC limit of 750 mg/kg of dry matter, although the average has been 321 mg/kg. Analyses are currently being carried out for mercury.

The compost is also being tested in growth trials with maize. If it proves to be "beneficial" then disposal to land will be exempted from waste management licensing, eliminating one market barrier. Other potential outlets include daily cover at landfills, land reclamation and engineering schemes such as road embankments.

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