There are 32 Scottish Water and 21 public-private partnership (PPP) sewage sludge treatment sites in Scotland. The majority of sludge destined for agricultural use are produced by the PPP sites (~110,000 tonnes dry solids per year).
Regulatory controls for treatment are split between waste water (which must be sufficiently treated to reach standards that allow it to be discharged into surface waters without undue harm), and sewage sludge treatment (which must be sufficiently treated to reach standards that allow them to be used in thermal energy recovery (incineration) or land-based markets – principally agriculture). There is a conflict whereby (for example) approaches that encourage hazards to partition to sludges (thus improving the quality of the treated waste water) can impact negatively on the quality and usability of the sludges.
There are several best practices guidance for the production, transportation, handling, storage and application of sewage sludge. If adopted augment the regulatory controls, especially with regard to the control of odours and other nuisances.
Most sludges supplied for agricultural purposes have undergone advanced anaerobic digestion, have been thermally dried, or lime pasteurised. These processes are undertaken in order to meet the requirements of the Safe Sludge Matrix (ADAS, 2001). While the focus of these treatments is on pathogen reduction (advanced treatment looks to achieve a 6 log reduction in E. coli and zero Salmonella), these treatment processes can also affect concentrations of other potential hazardous agents including those highlighted by the quantitative risk assessment.
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