to read the full report - Ecolateral by Peter Jones

48
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy
AEA/ED45551/Issue 1
However disposal to land may reduce in coming years as legislative and practical constraints limit this
route. 67 The available land bank may reduce due to a reluctance to have food products grown on land
treated with this waste, although the growth of energy crops would provide a (small) mitigating land
availability. 67 The impact of the Nitrates Directive (Nitrate Vulnerable Zones Regulations) may be a
reduction in the volume of sludge that can be applied in certain areas.
Landfill
A small amount of sludge is disposed of to landfill, which is largely used only as a fall back option as it
can be used at very short notice. 56 This route is not sustainable in the longer term and the costs
associated with it are increasing as landfill tax levels go up and void space is reduced. In addition, due to
the high water content of bio-solids only a limited number of sites are willing to accept it due to the
potential impact on the landfill sites’ leachate management programme.
Incineration
Incineration is the second largest disposal route, generally used by the larger treatment works. 56 The
heat generated is often used on site to meet the heating needs of the process, i.e. to dry the bio-solids,
and generate electricity. Several stages of cleaning of the flue gases are incorporated within the process
to ensure they meet EU emission limits set by the Waste Incineration, the Integrated Pollution Prevention
and Control (IPPC) and the Landfill Directives. 68 The calorific value of bio-solids is dependent on the type
of material, its water content (the degree of dewatering) and the concentration of organic matter present,
solids content, calorific values and biomass contents vary according to the type of treatment.
Incineration is a relatively expensive option, so it is only of interest in areas where alternative disposal
methods are not available or the cost of transportation is prohibitive.
Incineration of bio-solids involves burning the sludge at 600-900°C to destroy the organic content, leaving
a residue of mineral ash for final disposal (usually to landfill or controlled waste re-use such as land
reclamation). The net calorific value of dried bio-solids, i.e. its energy content after deducting the heat
required to evaporate the remaining water is around 23MJ/kg of volatile solids. 69
Co-incineration
The bio-solids, either as dewatered cake, but generally as dried pellets, can be burnt in adapted (WID
compliant) plants, often co-fired with coal. It is possible to burn bio-solids with municipal waste, however,
the burner design system must be capable of handling both fuels.
67 Thames Water 25 year Sludge Strategy, 2008, http://www.thameswater.co.uk/cps/rde/xbcr/SID-54091C65-CAD380F4/corp/sludge-strategy.pdf
68 Waste Incineration Directive http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2000:332:0091:0111:EN:PDF, Integrated Pollution
Prevention and Control, original legislation, http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31996L0061:EN:HTML, Landfill Directive
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:1999:182:0001:0019:EN:PDF
69 Sewage Sludge Disposal: operational and environmental issues, Bruce A.M. and Evans, T.D., Foundation of Water Research, 2002.