Risks from BSE via Environmental Pathways
Burning Rendered Products in Power Stations
Disposing of BSE Infected Cattle in Incinerators
Disposing of BSE Carcases in Landfills
Thruxted Mill Rendering Plant: Risk Assessment of Waste Water Disposal
for the Environment Agency June 1997MANAGEMENT SUMMARY
This report presents an overview of the risks associated with BSE (Bovine Spongiform Encephalopathy) via environmental pathways. The study was initiated so that the main issues of concern to the Environment Agency could be identified and to help determine what options were available for dealing with any issues raised. The report has been prepared by DNV Technica on behalf of the Environment Agency, and concentrates on the risks to human health in England and Wales. Risks via the food chain and occupational exposures are not addressed, as these are not within the remit of the Agency.
The risk model uses the following main elements, which are described in detail in the report:
Identification of pathways. The study has identified the various pathways for possible transmission of the BSE agent to humans through the environment. It includes identification of hazards which might result in unexpected causes of transmission.
Infectivity of cattle waste products. The study has combined available information to obtain best-estimates and confidence ranges of the human infectivity of cattle waste products, including carcases of cattle confirmed to have BSE, as well as tissues from cattle which are infected but not diagnosed. These are the source terms for the analysis.
Transport and fate of cattle waste products in the environment. The study has used event tree analysis to estimate the quantity of cattle waste products which might travel through the environment and might ultimately be ingested by humans.
Risk results have been presented in the form of the consumption of infectivity, measured in terms of the annual fraction of the dose needed to cause infection, for average individuals, and for the entire population of England and Wales.
The available information indicates that the degree of infectivity (i.e. the potential for human infection) in BSE-infected brain is very high. A cautious best-estimate of the infective oral dose as assumed for this assessment is 1g for humans, and 0.1g for cattle. This suggests that the brains of the estimated 20,000 infected cattle still alive at the beginning of 1996 have a major potential for causing human infections.
Despite this, the risks to human health through environmental pathways are estimated to be very low. The societal risk results show that the total ingestion of infectivity by all the population of England and Wales through environmental pathways in 1996 was approximately 3 ID50 units and that the maximum individual exposure was less than one millionth of an infective dose per year. These risks are considered to be negligible. A best-estimate of risk of death would be virtually zero.
The largest potential contributor appears to be the ingestion of infectivity through drinking water abstracted from the ground. The analysis carried out indicates that the infectivity in the ground water comes primarily from excreta from live infected cattle, and the blood from the slaughter of OTMS cattle spread on land. No BSE infectivity has ever been detected in either blood or excreta, but for the purposes of the assessment it has been assumed that some infectivity is present at levels well below the limit of detection. It should also be noted that blood from OTMS cattle is no longer spread on land, but from January 1997 is collected for treatment and destruction.
The explanation for the low risks is the range of measures which have already been taken to minimise the risks to people from BSE in cattle. These measures include the incineration of all confirmed BSE cases, enforcing high standards of separation of potentially infected material in abattoirs, and to render for waste the resulting carcases. The main residual potential for infection would be the accidental failure of the safeguards which have been established.
for the Environment Agency June 1997MANAGEMENT SUMMARY
It has been proposed that Meat and Bone Meal (MBM) and tallow produced from rendering the carcases of cattle slaughtered under the over thirty month scheme be disposed of by burning in coal fired power stations. This material may contain small quantities of BSE (Bovine Spongiform Encephalopathy) infectivity. The Environment Agency has commissioned DNV Technica to assess the risks to human health that may result from this activity.
The report has assessed risks of infection for humans from all emissions and waste products from power stations burning MBM and tallow. The approach has been to assess the amount of infectivity in the raw material, to assess the proportion of infectivity destroyed by incineration and then to analyse the transport and fate of any remaining infectivity in all waste streams through all possible environmental pathways.
The risk results have been presented using two risk measures, societal risk and individual risk. The societal risk is expressed as the total number of infective units (human oral ID50 units) ingested by all people in England and Wales (assuming all the MBM is burned there). It is therefore a measure of the total number of infections that could occur. The individual risk is expressed as the infective units per year ingested by the most exposed individual. It is therefore a measure of the maximum risk to any one person. Both these measures assume that there is no safe threshold and that the risk is directly proportional to dose. These are very pessimistic assumptions and will overestimate the risk from small doses.
The societal risk, i.e. the total human ingestion of infectivity from burning all the 1996 MBM in power stations, is estimated to be approximately 2 x 10-4 human ID50 units. Thus, the risk of any human infections is extremely small, and the most likely result would be no infections at all. Compared to the background societal risk of approximately 50 sporadic CJD cases in England & Wales per year this is clearly a negligible risk.
The maximum individual risk, i.e. the ingestion of infectivity by the most exposed person from burning all the 1996 MBM in power stations, is estimated to be approximately 3 x 10-11 human ID50 units. Thus, even on the pessimistic assumption that there is no safe threshold and hence that risk is directly proportional to dose, there would be less than a 1 in 1000 million chance of infection for the most exposed individual. Compared to the Health & Safety Executive (HSE) guideline that a 1 in 1 million risk per year would be broadly acceptable and would not alter ordinary behaviour in any way, this is clearly a negligible risk.
In relative terms, the main pathway for societal risk is predicted to be via water supplies derived from ground water polluted by effluent from the power station, contaminated by MBM spilled from the raw material handling equipment. The most exposed individual is predicted to be a person drinking untreated surface water contaminated by liquid effluent from a power station. Although other environmental exposure pathways have been considered, their risks are estimated to be even lower than the main ones discussed above. It is important to note that the risks, which are very low, are dominated by pathways affected by spillages of MBM, and not by any of the products of combustion. This emphasises the importance of good housekeeping and material handling practices for this type of operation.
It is concluded that burning MBM from cull scheme cattle in power stations would produce negligible risks of infection of people via environmental pathways. Both the individual and societal risks are very small, and would not normally be a cause for concern. An uncertainty analysis has shown that the risks may vary by several orders of magnitude above or below those estimated here, but even a more pessimistic analysis would not give risks which were other than negligible compared to other risks in daily life.
for the Environment Agency June 1997MANAGEMENT SUMMARY
The Environment Agency has commissioned DNV Technica to assess the risks to public health associated with incinerating cattle infected with BSE (Bovine Spongiform Encephalopathy) in animal carcase incinerators. The study is based on an on-going overview study of risks to human health arising from BSE infected material passing through environmental pathways.
The report has assessed risks of infection for humans arising from the incineration of cattle infected with BSE in animal carcase incinerators. The approach has been to assess the amount of infectivity in the raw material, to assess the proportion of infectivity destroyed by incineration and then to analyse the transport and fate of any remaining infectivity in all waste streams through all possible environmental pathways.
The risk results have been presented using two risk measures, societal risk and individual risk. The societal risk is expressed as the total number of infective units (human oral ID50 units) ingested by all people in England and Wales (assuming all infected carcases are burned there). It is therefore a measure of the total number of infections that could occur. The individual risk is expressed as the infective units per year ingested by the most exposed individual. It is therefore a measure of the maximum risk to any one person. Both these measures assume that there is no safe threshold and that the risk is directly proportional to dose. These are very pessimistic assumptions and will overestimate the risk from small doses.
The societal risk, i.e. the total human ingestion of infectivity from incinerating all 1997 BSE carcases in incinerators, is estimated to be approximately 5 x 10-3 human ID50 units. Thus, the risk of any human infections is extremely small, and the most likely result would be no infections at all. Compared to the background societal risk of approximately 50 sporadic CJD cases in England & Wales per year this is clearly a negligible risk.
The maximum individual risk, i.e. the ingestion of infectivity by the most exposed person from incinerating all the 1997 BSE carcases in incinerators, is estimated to be approximately 10-9 human ID50 units. Thus, even on the pessimistic assumption that there is no safe threshold and hence that risk is directly proportional to dose, there would be only a 1 in 1,000 million chance of infection for the most exposed individual. Compared to the Health & Safety Executive (HSE) guideline that a 1 in 1 million risk per year would be broadly acceptable and would not alter ordinary behaviour in any way, this is clearly a negligible risk.
It is concluded that incinerating BSE animal carcasses in incinerators would produce negligible risks of infection of people via environmental pathways. Both the individual and the societal risks are very small, and would not normally be a cause for concern. An uncertainty analysis shows that the risks vary by several orders of magnitude above or below those estimated here, but even a more pessimistic analysis would not give risks which were other than negligible compared to other risks in daily life.
This report presents an assessment of the risk to human health from the disposal of BSE infected carcases to landfill sites. It has been estimated that some 6000 carcases were disposed of in 59 different landfill sites around the UK in the early stages of the epidemic, (from 1988 to 1991). The study brings together the expertise on BSE from the members of the Spongiform Encephalopathy Advisory Committee (SEAC), with knowledge about behaviour in the environment and environmental pathways in a risk assessment framework. It draws on other work carried out by DNV for The Environment Agency on the risks from BSE infectivity through environmental pathways.
Any risk that there may be from the burial of these carcases is most likely to be due to possible contamination of leachate which could then contaminate water supplies. It is considered that there is no risk from the BSE infective agent associated with landfill gas. SEAC considered the issue of BSE carcases in landfills in May 1996, and concluded ì..it was unlikely that leachate from landfill sites would present a significant risk..î
There are uncertainties in many aspects that affect this assessment; the number of carcases, the potential infectivity to people, the behaviour of the infective agent in the landfill and in the environment, and the amount of any infectivity in the leachate from a landfill. This means that the results of the assessment will have a significant level of uncertainty, and should be used with caution. However, assumptions made throughout the study have erred on the side of caution, and thus the results should represent an upper estimate and may be used to indicate whether or not there is a potential problem that requires further investigation.
Preliminary data on all 59 sites were obtained by the Environment Agency from its local offices and the data compared with that held by MAFF. A ranking system, based on a source-pathway-target model, was designed to take account of the number of carcases, the proportion of infectivity in the site, degree of containment and proximity to water sources. This was used to identify six sites for a more detailed assessment, which is presented in this report.
The study results have been presented in terms of the consumption of infectivity by someone continuously exposed to the water supply considered to be most at risk from each site. This is presented as the fraction of the dose needed to cause infection that is consumed in one year. The individual risk results range from 2 x 10 -8 to 7 x 10 -10 infective units per year. These are very small fractions of a potentially infective dose. If there is no threshold dose for infection this would imply a risk of death of 2 x 10 -8 or less; in reality the risk would be closer to zero. None of these results take account of the effects of attenuation of infectivity in the ground, which would reduce the risk significantly, depending on the length of the flow path. The risk of infections are all well below an individual risk of 1 in a million years, which is generally regarded as an acceptable level of risk (e.g. by the Health and Safety Executive in assessing risks to the public from major hazard sites).
It is concluded that the risk estimates for contamination of water supply with BSE infectivity are all well below any level that would be considered to be of significance.
Whilst it is not readily possible to test the groundwater from boreholes in the vicinity of these landfill sites for the presence of BSE infectivity, groundwater is routinely tested at most sites for other contaminants that would indicate contamination by leachate, and public water supply sources are tested regularly for a range of possible contaminants. For the sites considered in this report there are no reports of leachate contamination in a water supply. As long as this remains valid then it is possible to state confidently that there is no risk to public health due to contamination of water supplies by BSE infectivity due to the disposal of carcases in landfill sites.
Final Report January 1997 EXECUtive summaryA Risk Assessment of waste water disposal options for the rendering plant at Thruxted Mill, near Canterbury in Kent, has been carried out for the Environment Agency. The plant was in the process of installing new effluent treatment facilities in order to meet consent conditions set by the Agency, when it became one of the plants in England and Wales authorised to render Specified Bovine Material (SBM) and cattle from the government cull scheme. This raised concern as to the potential for the agent suspected of causing Bovine Spongiform Encephalopathy (BSE), to contaminate groundwater which is abstracted 2km away for public water supply.
The objective of this risk assessment is to assess the risks of exposure to BSE infectivity from the currently proposed option for treatment and disposal of waste water at Thruxted Mill together with a number of alternative options, and to make an assessment of their acceptability in order to support the Agencyís decision making as to the preferred option. The options considered in this study are:
Option 1 Current Proposal - primary and secondary treatment on site followed by disposal underground through a sub surface soakaway system adjacent to the plant.
Option 2 Mains Drainage - discharge to sewer following treatment on site as for Option 1. Primary, secondary and tertiary treatment at Chartham sewage treatment works, followed by discharge to River Stour.
Option 3 Tankering Offsite - treatment on site as for Option 1 followed by removal by road tanker for treatment at Weatherlees treatment works before discharge to Stour Estuary.
Option 4 Historical disposal method - partial treatment on site and disposal to land surface. This represents the situation at the site prior to commissioning any new treatment facilities.
The study has considered both normal and abnormal releases of material and operating conditions at the Mill, and the transport of infective material in the environment under different conditions. The concentration of infective material to which humans may potentially be exposed, and the probability of exposure, have been calculated for each different condition.
The main conclusions of the study are as follows:
The level of infectivity in the treated effluent from Thruxted Mill is extremely low under normal operating conditions (2 x 10-8 human oral ID50 units per litre).
The level of infectivity in the drinking water supply at the nearest boreholes to Thruxted Mill (Godmersham and Chilham) under normal plant operating and environmental conditions is estimated to have a median value of 1 x 10-12 human oral ID50 units per litre; an individual would need to consume 2 litres per day for over 1000 million years to reach an ID50 dose. This is not considered to present a significant risk, even taking into account the high level of uncertainty in predicting BSE risks to humans.
The highest level of contamination of the drinking water supply predicted for Option 1 is 1 x 10-6 human oral ID50 units per litre. This would require a significant release of infectivity directly into the effluent collection tank at the Mill, for the treatment not to be working and for less than expected dilution to occur in the groundwater. The probability of this combination of conditions would be very small (less than one in a million years).
The Average Ingestion Rate of infectious units through possible exposure of the public water supply calculated for Option 1 is 1.2 x 10-11 infectious units per day. Thus, even if doses could accumulate in the body, it would take some 200 million years of exposure to build up to one human oral ID50 unit. This represents a negligible risk of infection.
The results show that all Options present extremely low risks of exposure to BSE infectivity and would be considered negligible in comparison to established risk criteria accepted for use in the United Kingdom. Option 3 presents the lowest risk of exposure; however, this lower risk of exposure would be more than negated by the risk of fatal road accidents due to the tanker movements. Option 3 would also result in other environmental impacts from the use of tankers.
The recent improvements to the plant (installation of the DAF unit and improved raw material handling area) have resulted in a significant reduction in the risk of exposure to BSE infectivity
Substantial uncertainties exist in the assessment of BSE risks to humans. The main uncertainties affecting the prediction of risks from the Thruxted Mill effluent relate to the species barrier, the number of infected cattle supplied to the plant, the environmental behaviour of the infective material (i.e. dilution and adsorption of infectivity in the ground), the probability of accidental releases from the plant and the effectiveness of the effluent treatment plant.
Throughout this study assumptions have been made which err on the side of caution. This will mean that the results presented will tend to overestimate the risks. This is particularly true for the proposed scheme (Option 1) as factors such as:
may have all been underestimated by many orders of magnitude. These will mean that Option 1 is even safer than predicted, and will clearly be the preferred Option.