Sewage sludge: a burning issue
Health and environmental concerns are making the spreading of sludge on agricultural land less attractive. The alternatives, however, are far from established as Chris Webb reportsThe spreading of dewatered sewage sludge on agricultural land developed from methods of applying livestock slurry and farmyard manure. Since the 1980s there have been many major improvements in sludge application techniques, driven largely by statutory requirements and the need to spread the material cost-effectively and accurately, with minimal risk of pollution or public offence from odour.
The spreading of organic waste including sewage sludge, animal manure, blood and gut material from abattoirs and composted wastes has assumed an important agricultural role since the cessation of dumping sewage sludge at sea. Specialised self-propelled spreading vehicles, soil injection techniques using umbilical systems or tankers, and sophisticated application rate control technology have emerged, and water companies now keep detailed land management records to keep a check on heavy metal concentrations and sludge composition details at individual sites.
But concerns about pollution and its effects on human health abound. In Scotland, for example, worries over environmental issues arising from land spreading led to a wide-ranging re-evaluation of the practice in 1998, when the Scottish Environment Protection Agency (SEPA) launched a major review. This has resulted in Sam Galbraith, the Scottish Executive's minister for the environment, introducing tough new controls on the practice. The measures sought a requirement for the industry to demonstrate the agricultural benefit from spreading and the strict prohibition of untreated septic tank sludge being spread on land. Consideration of blood and gut contents from abattoirs has been deferred pending the report of the joint Executive/Food Standards Agency E.Coli Task Force, which is due this year.
SEPA chief executive, Tricia Henton, while welcoming the minister's actions, questioned whether they go far enough, stressing that sewage sludge spreading on agricultural land is a matter of increasing concern in terms of disease transmission, nuisance and pollution. In particular, she said SEPA would recommend comprehensive land management plans for any farm spreading organic waste on land. Without such an approach, there would remain a risk that successive applications of organic wastes and inorganic fertilisers would collectively result in detrimental environmental impacts.
SEPA warned the current Prevention of Environmental Pollution from Agricultural Activity code of good practice was ineffective, because much of its content was not a statutory requirement. There was also evidence the code is not widely used by farmers.
SEPA's strategic review highlighted a number of factors leading demonstrably to pollution, including the injection of liquid waste into land with field drains - which proved to be one of the most common causes of water pollution. The agency has called for the adoption of a fully integrated regulatory regime from producer to application site.
Events in Scotland have highlighted what the waste handling community is already addressing as a matter of some urgency: a need to find alternatives to spreading sludge on land. At Cranfield University, researchers are taking matters further, by working on means of reducing the amount of sludge initially produced. They are looking into the use of biochemical inhibition to reduce biomass production during activated sludge treatment, and manipulating the bacterial slime layer so sludges can be dewatered more easily.
In the meantime however, the cessation of dumping at sea, coupled with the increasing restrictions on the use of sewage sludge on land and the potential unsustainability of landfill, are increasing water companies' dependence on incineration to dispose of sewage sludge.
According to Gillian Hand-Smith of Babcock Water Engineering, alongside concerns arising from nitrate-sensitive zones and the new Ministry of Agriculture, Fisheries and Food (MAFF) soil, water and air codes, the influence of supermarkets on farming techniques is affecting levels of sludge use on agricultural land - particularly where crops could potentially be affected by beet rhizoma or brown potato rot. Hand-Smith also says the proposed new directive on waste incineration will regulate sewage sludge incineration and gasification technology.
Developments under the 1999 Safe Sludge Matrix initiative included the cessation of surface spreading of digested sludge onto grassland used for grazing. Only deep injection of digested sludge to grassland is now allowed. More regulation is to follow in the form of Sludge (Use in Agriculture) Regulations, the third draft of which state that sludge should not be used on soils with a pH of less than 5.0, or water-saturated, flooded, frozen or snow-covered ground. The regulations state sludge should be spread in such a way as to not cause sludge run-off and minimise soil compaction as well as the production of aerosols.
Water companies and their contractors insist the vast majority of these requirements are already observed, and exhaustive risk assessments are carried out at individual sites before spreading takes place. As a result, relatively little spreading took place during the period when the recent flooding was at its worst. The deluge highlighted a related problem associated with the storage of biosolid cake on farmland. At present there are few legal restraints relating to storage, and little by way of statutory guidance. According to Cranfield's lecturer in water sciences, Dr Elise Cartmell: "Storage of biosolid cake on land should be registered as an exempt activity under the Waste Management Licensing Regulations (1994). However, regulation of the activity is ambiguous, with the Environment Agency [in England and Wales] taking a risk-based approach centred around place and period of storage. This position is likely to change in the future through the revision of the exempt activities and possibly through the new Sludge (Use in Agriculture) Regulations." The third draft of the regulations state that measures must be taken and precautions put in place to prevent the leaching from sludge stored before use.
Putting the heat on
Dr Simon Judd, a Cranfield reader in water sciences, is interested in the development of thermal treatment for sewage disposal - the viability of which he says is influenced by cost, legislation and public perception. "Gasification is generally considered to be 'greener' and more economically viable at smaller scales, making it attractive to some local authorities, but it has yet to be proven in the UK for a 100% sewage sludge matrix. Incineration is a more established technology but tends to be viable only at larger scales [> 15,000 tonnes dry sludge per annum], and invokes strong public aversion."
While gasification technology is more established in countries such as Germany, where gate fees are substantially higher (>£100/tonne) than those in the UK, there remains a credibility gap with the process in the UK. This has been compounded by problems recently encountered with projects at two water utility sites. On the other hand, a recent pan-European study, in which Cranfield took part, established that co-gasification of palletised sewage sludge with crushed coal could achieve acceptable carbon conversion efficiencies (up to 82%) with none of the processing problems associated with the sewage sludge matrix alone.
One of the latest commercial co-gasification projects, a collaboration between TXU Europe and Anglian Water, is a venture to build a 10MW combined heat and power (CHP) plant based on the gasification of biosolids in Corby, Northamptonshire. An environmental impact assessment is currently being carried out for submission to the planning authority and Northamptonshire County Council later this year. Exhaust from a 5MW gas turbine will be used to dewater the biosolids, and this material will then be gasified through a set of low NOx burners. The heat will be used to raise steam, which will be used in the combined cycle plant. There is an increasing feeling such schemes are the way ahead at a time when sustainable development is being encouraged. This has prompted much debate in environmental circles as to whether it is right that co-combustion technology should face the same legislative constraints as waste incineration, and particularly whether small-scale co-generation projects like Corby should be subject to the same EU rules as a CHP plant that does not use its heat energy for such an environmentally significant end.
We are bound to see many such 'distributed generation' projects in the future. Forthcoming legislation is to compel the power utilities to provide 10% of their generated power from renewable sources by 2010, and changes to pricing policy place more emphasis on more reliable sources of renewable energy.
These factors tend to make the use of municipal biosolids more favourable. Gasification of sewage sludge is nonetheless hindered with technical difficulties which do not affect other biosolids such as wood-chip, powdered coal or grass cuttings, and which are proving difficult to overcome. It is a case of "watch this space".