- Email: [email protected]
Conference Report
CONFERENCE REPORT SLUDGE 8
prevent harm, but raises the question of how extensive the risk assessment needs to be. Should the risk assessment extend to risks which are possible but uncertain, as well as risks which are well understood? In one recent case, where Legionella bacteria had escaped into the street, with possible risk to passers-by, the institution concerned needed to guard against the risk of harm: it was not necessary to show that harm had actually occurred. In another case, a large chemical company was prosecuted by the Environment Agency about the leak of a chlorohydrocarbon,where the judge emphasized the failure of the company to assess the risk arising from a new form of piping on its premises. Rosalind then looked at some cases involving the Best Practicable Environmental Option (BPEO), the regulations through which risk is applied in environmental law, and the cost of ignoring risk assessment. The latter dealt with the prosecution and enforcement policy of the Environment Agency and the defence of `Due Diligence’. Keith Harris (Thames Water plc) gave an update on the new Thames Water sludge incineration plant at Beckton in east London, where a visit was arranged for the participants on the following day. The East London Sludge Incinerator is one of the most technologically advanced plants of its type, dealing with 180,000 tons of dry sludge solids per year and generating 11.4 MW of electricity which will reduce Thames Water’ s dependency on the National Grid. Keith reviewed operation of the installation, as well as the background and history of the incineration project, and mentioned that a challenging aspect of incinerator operation was the delivery of a sludge cake with a low enough water content, and ef® cient sludge de-watering was a critical prerequisite for the ¯ uidized-bed furnace. Some initial problems had also been experienced with the control software. The incineration process was designed to integrate with the day-to-day operation of the Sewage Treatment Works (STW) it served, to minimize problems arising from high sludge water content and variations in sludge water content. The installation meets stringent environmental standards, including noise attenuation and visual impact. Geoffrey Welburn (Meadow Vale Ventures, Swindon) gave an account of the production of lightweight building aggregates from sewage sludge and other wastes. Meadow Vale Ventures supply a plant that disposes of sludge economically. The sludges are mixed with ® llers, pelletized and ® red to give a lightweight aggregate, which has a good market and is used to produce concrete blocks and beams with good insulation properties. The plant is ¯ exible and can use a variety of sludges, ® llers and other waste materials. The process is well-proven, environmentally-friendly and pro® table, as it eliminates the need for land® ll, sludge de-watering, drying, digestion, composting, gasi® cation, incineration or spreading on land. A small-scale plant at Tilbury deals with 1250 tons of sewage sludge dry solids together with power station ash and clay, and a full-scale plant dealing with 10,000 tons/year of dry solids has been ordered, to give 14,000 tons of product/year in the year 2000. Geoffrey gave a list of possible waste materials, ® llers and waste fuel sources,
A one-day meeting on developments in sludge treatment and disposal, held at the University of Surrey on 22 September 1998, sponsored by the Royal Society of Chemistry (Industrial Affairs Division) Process Technology Group in association with the Institution of Chemical Engineers Environmental Protection Subject Group, and organized by William Hui (Thames Waste Management Ltd) and Mike Winkler (Chairman, IChemE EPSG) Report by Mike Winkler, who also chaired the meeting. Rosalind Malcolm (Barrister and Senior Lecturer in Law, University of Surrey) opened the proceedings with the now traditional discourse on developments in environmental law, and this year dealt with `Preventing environmental risk’ . Four principles underpin the implementation of European Environmental Law, which will play an increasing role in the application of European law in the UK. According to the Treaty of Amsterdam, 1998, EC `policy on the environment shall aim at a high level of protection taking into account the diversity of situations in the various regions of the Community. It shall be based on: · the Precautionary Principle and on the principles that · preventative action should be taken, that · environmental damage should as a priority be recti® ed at source and that · the polluter should pay.’ This policy links the precautionary and the preventative principles, and Rosalind discussed the difference between them. The preventative principle was outlined in the EC’ s ® rst action programme on the environment as `the best environmental policy consists in preventing the creation of pollution and nuisances at source, rather than subsequently trying to counteract their effects’ . While the preventative principle is thus about preventing a predetermined risk, as in, for example, Environmental Impact Assessment, the precautionary principle is about preventing a risk which might happen, although there may be no scienti® c agreement about the certainty of risk. Implementation of the precautionary and preventative principles errs on the side of caution, shifting the burden of proof on to industry to prove a generality that a product is safe, rather than to defend a claim that the product was harmful in a particular case. Examples of this are pathogens in the food chain and spreading sludge on agricultural land. Rosalind reviewed some recent relevant legal cases, notably one in which the court held that the precautionary principle was not directly applicable in the interpretation and application of English Law, but it was to be applied at the point when European policy was being developed. There are obvious links with risk assessment, with the contemporary approach emphasizing the importance of assessing risk to Trans IChemE, Vol 77, Part B, March 1999
99
100
CONFERENCE REPORT
including sewage sludge, power station ash and mine tailings. Other waste fuels that can be used include lignite or coal dust and crumbed tyres. The lightness of the aggregate produced is such that 50-storey buildings can be built on foundations designed for 35-storey buildings using conventional concrete. Sean Lynch (Northumbrian Water, Stockton) gave a presentation on the bene® ts of drying and gasi® cation of sewage sludge, dealing with the NWL plant on Teesside for drying and gasifying sludge brought in by sea. For regular delegates to SLUDGE meetings, this was an interesting update of the presentation given by Dave Pollington at SLUDGE 5 on the initial planning of the sludge drying project. A gas-turbine CHP plant generates the heat and electrical power required for sludge drying and gasi® cation, which in turn provides part of the gas for the gas-turbine, the remainder being made up with natural gas. Sludge dewatered in belt presses is dried thermally and pelletized, then pyrolized in a cracking process at 800 to 900°C to give a fuel gas consisting mostly of hydrogen, carbon monoxide and methane, leaving a 20% ash residue. The fuel gas has a calori® c value of 22 to 25 MJ/m3 , compared with the 38 MJ/m3 provided by the natural gas. Gerald Tetchner (Enertec Engineering Consultants) discussed centralized Anaerobic Digestion (AD) with particular reference to farm wastes. After outlining the scienti® c principles of AD, Gerald listed the types of agricultural, food industry and other wastes amenable to AD, with the proviso that straw length should not exceed 50 mm in any circumstances. Gerald mentioned the problems resulting from the hydrogen sulphide content of biogas generated by AD when used in gas engines, and listed the ® ve main components of a centralized AD system: · Material input, storage and handling; · Digesters; · Liquid storage and handling; · Fibre separation and composting; and · Combined heat and power (CHP) generation. Gerald reviewed digester construction, digester heating systems, digester retention times and described a typical centralized AD process for dealing with pig, cattle or poultry wastes. David Slater CB (OXERA Environmental Ltd, Oxford), also well known as formerly HM Chief Inspector of Pollution, discussed `Regulations, risks and options for sludge disposal’ . The trend is for regulations to become increasingly severe and increasingly severely enforced, while disposal options are decreasing. · Land® ll is no longer a long-term option, due to decreasing availability and the restrictions on co-disposal of organic material imposed by the forthcoming EC Land® ll Directive. This will nullify land® ll as a disposal route for sewage sludge. · Land spreading is rated as the BPEO by the Royal Commission on Environmental Pollution, and is becoming increasingly utilized by the water and agricultural industries, but is incurring increasing public concern about odour, heavy metals and pathogens entering the food chain. · Incineration, as we have seen, is being adopted by some water companies, but sludge is likely to be classi® ed as `hazardous waste’ , which will result in expensive emissions control, and will be subject to many new requirements under the forthcoming IPPC Directive. To be environmentally acceptable, incineration must also involve power generation or CHP.
Overall, the disposal option is seen as a choice between land use and incineration. In considering risks, Dr Slater highlighted in: · Land® ll: Gas generation. Pathogen and heavy metal contamination of groundwater are also risks in: · Agricultural land use: This is in any case a better longterm option than land® ll, but Persistent Organic Preparations (POPs) become a problem here. The dif® culties in eliminating heavy metals, POPs and endocrine disrupters from the input to STWs mean that these Persistent Toxic Elements (PTEs) will tend to accumulate in the soil to unacceptable levels. Pathogen content is seen as a more serious risk. · Incineration risks are well covered by industrial emissions regulations. The use of heat generated for de-watering and sterilizing sludge could also improve the risks involved in using sludge in agriculture. · Gasi® cation requires highly competent engineering, and in combination with power generation is likely to become the most attractive option. Dr Slater’ s ® nal recommendations were: · Separation of domestic and industrial ef¯ uents would give better control of Persistent Toxic Elements; · Treatment of residues from primary, secondary and tertiary treatment before disposal; · Sterilized sludge for use in agriculture could be the BPEO; · Complete energy conversion, via gasi® cation and CHP, is the most Politically Correct option. Keith Panter (Simon-Hartley Cambi UK) gave a presentation on thermal hydrolysis and anaerobic digestion for pasteurization, volume reduction and energy production. This followed on from the paper on sludge hydrolysis presented by Cambi A/S at SLUDGE 7 last year. While the use of sludge biosolids in agriculture is the least-cost BPEO, there is concern about its pathogen content. Pasteurization will therefore be needed, to assure agriculture and non-agriculture uses, with the requisite energy deriving from the sludge itself. The resultant process needs to be optimized for capital and operating costs incurred by de-watering, drying and incineration. Thermal hydrolysis of de-watered sludge combined with anaerobic digestion and thermal drying in a STW in Norway was found to give 30 to 50% more gas with about half the digestion and drying capacity of conventional systems. As it happens, the pasteurized digested sludge cake is preferred by local farmers and drying has proved unnecessary. One project is under construction in the UK by SimonHartley Cambi and others are under consideration. Fiona Dennison (Thames Water Utilities) concluded the proceedings with a presentation assessing management options for sludge disposal by applying the environmental tool Life Cycle Assessment (LCA). This study compared 15 different Thames Water wastewater treatment works, from which ® ve management regimes for centralizing sludge treatment and disposal were subjected to LCA in choosing the BPEO. Centralization of sludge treatment and disposal showed an environmental improvement, but composting showed minimal bene® t over digestion, depending on the packaging and disposal of the ® nal product. ... and something to look forward to: SLUDGE 9 will be held at the University of Surrey on 7 September 1999. For further details contact Val Hors® eld, (Tel: +44 1483 259239; Fax: +44 1483 259510, E-mail: v.hors® [email protected]). Trans IChemE, Vol 77, Part B, March 1999
prevent harm, but raises the question of how extensive the risk assessment needs to be. Should the risk assessment extend to risks which are possible but uncertain, as well as risks which are well understood? In one recent case, where Legionella bacteria had escaped into the street, with possible risk to passers-by, the institution concerned needed to guard against the risk of harm: it was not necessary to show that harm had actually occurred. In another case, a large chemical company was prosecuted by the Environment Agency about the leak of a chlorohydrocarbon,where the judge emphasized the failure of the company to assess the risk arising from a new form of piping on its premises. Rosalind then looked at some cases involving the Best Practicable Environmental Option (BPEO), the regulations through which risk is applied in environmental law, and the cost of ignoring risk assessment. The latter dealt with the prosecution and enforcement policy of the Environment Agency and the defence of `Due Diligence’. Keith Harris (Thames Water plc) gave an update on the new Thames Water sludge incineration plant at Beckton in east London, where a visit was arranged for the participants on the following day. The East London Sludge Incinerator is one of the most technologically advanced plants of its type, dealing with 180,000 tons of dry sludge solids per year and generating 11.4 MW of electricity which will reduce Thames Water’ s dependency on the National Grid. Keith reviewed operation of the installation, as well as the background and history of the incineration project, and mentioned that a challenging aspect of incinerator operation was the delivery of a sludge cake with a low enough water content, and ef® cient sludge de-watering was a critical prerequisite for the ¯ uidized-bed furnace. Some initial problems had also been experienced with the control software. The incineration process was designed to integrate with the day-to-day operation of the Sewage Treatment Works (STW) it served, to minimize problems arising from high sludge water content and variations in sludge water content. The installation meets stringent environmental standards, including noise attenuation and visual impact. Geoffrey Welburn (Meadow Vale Ventures, Swindon) gave an account of the production of lightweight building aggregates from sewage sludge and other wastes. Meadow Vale Ventures supply a plant that disposes of sludge economically. The sludges are mixed with ® llers, pelletized and ® red to give a lightweight aggregate, which has a good market and is used to produce concrete blocks and beams with good insulation properties. The plant is ¯ exible and can use a variety of sludges, ® llers and other waste materials. The process is well-proven, environmentally-friendly and pro® table, as it eliminates the need for land® ll, sludge de-watering, drying, digestion, composting, gasi® cation, incineration or spreading on land. A small-scale plant at Tilbury deals with 1250 tons of sewage sludge dry solids together with power station ash and clay, and a full-scale plant dealing with 10,000 tons/year of dry solids has been ordered, to give 14,000 tons of product/year in the year 2000. Geoffrey gave a list of possible waste materials, ® llers and waste fuel sources,
A one-day meeting on developments in sludge treatment and disposal, held at the University of Surrey on 22 September 1998, sponsored by the Royal Society of Chemistry (Industrial Affairs Division) Process Technology Group in association with the Institution of Chemical Engineers Environmental Protection Subject Group, and organized by William Hui (Thames Waste Management Ltd) and Mike Winkler (Chairman, IChemE EPSG) Report by Mike Winkler, who also chaired the meeting. Rosalind Malcolm (Barrister and Senior Lecturer in Law, University of Surrey) opened the proceedings with the now traditional discourse on developments in environmental law, and this year dealt with `Preventing environmental risk’ . Four principles underpin the implementation of European Environmental Law, which will play an increasing role in the application of European law in the UK. According to the Treaty of Amsterdam, 1998, EC `policy on the environment shall aim at a high level of protection taking into account the diversity of situations in the various regions of the Community. It shall be based on: · the Precautionary Principle and on the principles that · preventative action should be taken, that · environmental damage should as a priority be recti® ed at source and that · the polluter should pay.’ This policy links the precautionary and the preventative principles, and Rosalind discussed the difference between them. The preventative principle was outlined in the EC’ s ® rst action programme on the environment as `the best environmental policy consists in preventing the creation of pollution and nuisances at source, rather than subsequently trying to counteract their effects’ . While the preventative principle is thus about preventing a predetermined risk, as in, for example, Environmental Impact Assessment, the precautionary principle is about preventing a risk which might happen, although there may be no scienti® c agreement about the certainty of risk. Implementation of the precautionary and preventative principles errs on the side of caution, shifting the burden of proof on to industry to prove a generality that a product is safe, rather than to defend a claim that the product was harmful in a particular case. Examples of this are pathogens in the food chain and spreading sludge on agricultural land. Rosalind reviewed some recent relevant legal cases, notably one in which the court held that the precautionary principle was not directly applicable in the interpretation and application of English Law, but it was to be applied at the point when European policy was being developed. There are obvious links with risk assessment, with the contemporary approach emphasizing the importance of assessing risk to Trans IChemE, Vol 77, Part B, March 1999
99
100
CONFERENCE REPORT
including sewage sludge, power station ash and mine tailings. Other waste fuels that can be used include lignite or coal dust and crumbed tyres. The lightness of the aggregate produced is such that 50-storey buildings can be built on foundations designed for 35-storey buildings using conventional concrete. Sean Lynch (Northumbrian Water, Stockton) gave a presentation on the bene® ts of drying and gasi® cation of sewage sludge, dealing with the NWL plant on Teesside for drying and gasifying sludge brought in by sea. For regular delegates to SLUDGE meetings, this was an interesting update of the presentation given by Dave Pollington at SLUDGE 5 on the initial planning of the sludge drying project. A gas-turbine CHP plant generates the heat and electrical power required for sludge drying and gasi® cation, which in turn provides part of the gas for the gas-turbine, the remainder being made up with natural gas. Sludge dewatered in belt presses is dried thermally and pelletized, then pyrolized in a cracking process at 800 to 900°C to give a fuel gas consisting mostly of hydrogen, carbon monoxide and methane, leaving a 20% ash residue. The fuel gas has a calori® c value of 22 to 25 MJ/m3 , compared with the 38 MJ/m3 provided by the natural gas. Gerald Tetchner (Enertec Engineering Consultants) discussed centralized Anaerobic Digestion (AD) with particular reference to farm wastes. After outlining the scienti® c principles of AD, Gerald listed the types of agricultural, food industry and other wastes amenable to AD, with the proviso that straw length should not exceed 50 mm in any circumstances. Gerald mentioned the problems resulting from the hydrogen sulphide content of biogas generated by AD when used in gas engines, and listed the ® ve main components of a centralized AD system: · Material input, storage and handling; · Digesters; · Liquid storage and handling; · Fibre separation and composting; and · Combined heat and power (CHP) generation. Gerald reviewed digester construction, digester heating systems, digester retention times and described a typical centralized AD process for dealing with pig, cattle or poultry wastes. David Slater CB (OXERA Environmental Ltd, Oxford), also well known as formerly HM Chief Inspector of Pollution, discussed `Regulations, risks and options for sludge disposal’ . The trend is for regulations to become increasingly severe and increasingly severely enforced, while disposal options are decreasing. · Land® ll is no longer a long-term option, due to decreasing availability and the restrictions on co-disposal of organic material imposed by the forthcoming EC Land® ll Directive. This will nullify land® ll as a disposal route for sewage sludge. · Land spreading is rated as the BPEO by the Royal Commission on Environmental Pollution, and is becoming increasingly utilized by the water and agricultural industries, but is incurring increasing public concern about odour, heavy metals and pathogens entering the food chain. · Incineration, as we have seen, is being adopted by some water companies, but sludge is likely to be classi® ed as `hazardous waste’ , which will result in expensive emissions control, and will be subject to many new requirements under the forthcoming IPPC Directive. To be environmentally acceptable, incineration must also involve power generation or CHP.
Overall, the disposal option is seen as a choice between land use and incineration. In considering risks, Dr Slater highlighted in: · Land® ll: Gas generation. Pathogen and heavy metal contamination of groundwater are also risks in: · Agricultural land use: This is in any case a better longterm option than land® ll, but Persistent Organic Preparations (POPs) become a problem here. The dif® culties in eliminating heavy metals, POPs and endocrine disrupters from the input to STWs mean that these Persistent Toxic Elements (PTEs) will tend to accumulate in the soil to unacceptable levels. Pathogen content is seen as a more serious risk. · Incineration risks are well covered by industrial emissions regulations. The use of heat generated for de-watering and sterilizing sludge could also improve the risks involved in using sludge in agriculture. · Gasi® cation requires highly competent engineering, and in combination with power generation is likely to become the most attractive option. Dr Slater’ s ® nal recommendations were: · Separation of domestic and industrial ef¯ uents would give better control of Persistent Toxic Elements; · Treatment of residues from primary, secondary and tertiary treatment before disposal; · Sterilized sludge for use in agriculture could be the BPEO; · Complete energy conversion, via gasi® cation and CHP, is the most Politically Correct option. Keith Panter (Simon-Hartley Cambi UK) gave a presentation on thermal hydrolysis and anaerobic digestion for pasteurization, volume reduction and energy production. This followed on from the paper on sludge hydrolysis presented by Cambi A/S at SLUDGE 7 last year. While the use of sludge biosolids in agriculture is the least-cost BPEO, there is concern about its pathogen content. Pasteurization will therefore be needed, to assure agriculture and non-agriculture uses, with the requisite energy deriving from the sludge itself. The resultant process needs to be optimized for capital and operating costs incurred by de-watering, drying and incineration. Thermal hydrolysis of de-watered sludge combined with anaerobic digestion and thermal drying in a STW in Norway was found to give 30 to 50% more gas with about half the digestion and drying capacity of conventional systems. As it happens, the pasteurized digested sludge cake is preferred by local farmers and drying has proved unnecessary. One project is under construction in the UK by SimonHartley Cambi and others are under consideration. Fiona Dennison (Thames Water Utilities) concluded the proceedings with a presentation assessing management options for sludge disposal by applying the environmental tool Life Cycle Assessment (LCA). This study compared 15 different Thames Water wastewater treatment works, from which ® ve management regimes for centralizing sludge treatment and disposal were subjected to LCA in choosing the BPEO. Centralization of sludge treatment and disposal showed an environmental improvement, but composting showed minimal bene® t over digestion, depending on the packaging and disposal of the ® nal product. ... and something to look forward to: SLUDGE 9 will be held at the University of Surrey on 7 September 1999. For further details contact Val Hors® eld, (Tel: +44 1483 259239; Fax: +44 1483 259510, E-mail: v.hors® [email protected]). Trans IChemE, Vol 77, Part B, March 1999