The recycling to agricultural land is an important outlet for sewage sludge and other sludges but it must be controlled in order as to obtain agricultural benefit from the sludge whilst protecting human and animal health and the environment at large. Current practices in Europe are based on the requirements of the 1986 Directive on the use of sewage sludge in agriculture (86/278/EEC). Since that time new technologies have become available for sludge treatment, more pathogens associated with the food chain have been identified and the concerns of the public relating to acceptable risk have changed.
This report identifies for a number of different types of sludges those pathogens that may present a risk to human, animal or plant health. It considers the factors in sludge treatment processes, particularly the newer processes, that influence lethal effects on pathogens, and makes recommendations for the parameters to be applied to these processes to produce treated sludges that present minimum risk.
The constraints on the use in agriculture of sludges that have not been subjected to advanced forms of treatment are discussed.
Proposals are made for the measures to be adopted for the quality assurance of treated sludge and for the validation of treatment processes and new treatment plants as follows:
It is not practicable to monitor treated sludge for the presence of pathogens and surrogates should be used for routine evaluation of treatment plant performance and sludge quality.
Surrogates should be organisms commonly found in sludges that have similar resistance to treatment as pathogens. E. coli (or enterococci) and Clostridium perfringens are suggested. Numbers of E. coli should not exceed 1000 per gram (dry weight) and it is tentatively recommended that spores of C. perfringens should not exceed 3000 per gram (dry weight).
Because of the diversity of microorganisms and hosts it is not possible to suggest suitable surrogates for systems treating vegetable waste. However, a treatment plant designed to hygienise sewage sludge or sludges of animal origin will also destroy plant pathogens.
Validation of new processes and new treatment plants using advanced processes
New treatment processes and plants will need to be validated.
A reduction of at least 4 log10 of added Salmonella and the destruction of viability of Ascaris ova is suggested for plants treating sewage sludge or animal waste by advanced processes.
Conventional treatment processes
These processes should achieve at least a 2 log10 reduction in pathogen numbers as evidenced by reduction in numbers of E. coli. Use of conventionally treated sludge on the land must be restricted according to the controls set out in the report to further reduce the risk of pathogen transmission.