Risk assessment of the use of source-segregated anaerobic digestates in GB agriculture

Chambers B1, Gale P2, Litterick A3, Longhurst P4*, Taylor M1, Tompkins D5, Tyrrel S4, 1ADAS, 2AVHLA, 3Earthcare Technical, 4Cranfield University, 5WRAP


Abstract This paper reports on the completion of a quantitative assessment of residual risks for PAS110 biofertilisers. The work focuses on the practical use of source-segregated biofertilisers as agricultural soil amendments to ensure the protection of crops, humans, animals and the wider environment. Risks from human and animal pathogens, organic compound contaminants and plant pests and diseases are considered. The study details the basis for calculating the potential for exposure to hazards using a source-pathway-receptor method to determine the extent to which controls are needed to ensure protection from harm. Scenarios that consider the highest plausible combination of hazards arising from AD feedstocks, process parameters, land application rates and differing crop categories are used which indicate the extent to which risks could occur. In conclusion, the work provides evidence for process and agricultural management practices to enable the safe use of these high value and low cost soil amendments.

Keywords: Anaerobic digestion (AD), agriculture, biofertilisers, digestates, PAS110, risk assessment

Introduction Anaerobic digestion offers an opportunity for the generation of renewable energy, and also for the recovery of the nutrients in food waste via the production of quality digestates. To achieve this, long term, sustainable markets for AD digestates are required. Whilst biofertilisers offer numerous benefits to agriculture including replacements for energy-intensive inorganic nitrogenous fertilisers their use must be complemented by guidance based on a robust understanding of safety in use. This is particularly the case on land where crops are grown for human consumption. Therefore a robust approach to risk assessment is necessary to inform the use of these materials and provide evidence-based guidance to ensure good agricultural practice in the use of biofertiliser.

The project built upon previous qualitative risk assessment work using whole wet digestate as the basis for risk assessments, recognising that the publically available specification (BSI:PAS110) includes ‘whole digestate, separated liquor and separated fibre derived from the anaerobic digestion of source-segregated biodegradable materials’. Each stage of the assessment builds upon published research as well as datasets and previous projects available from WRAP.

In determining the scope of the assessment, the controls addressed within the PAS110 define the extent of risks to be considered. These include: defined inputs to AD Plant, e.g. source-segregated feedstocks; the influence of supply agreements with for example local authorities and commercial firm, e.g. on the QA of feedstock supply; plant process control including corrective actions in event of failures; pasteurisation. These include the ABPR specification and requirements where digestates are moved between farms; as well as sampling and analysis including pathogens, potentially toxic elements (PTEs), physical contaminants, biochemical stability, and quality controls at the input stage.

Two toxicological principles, of exposure and potency, underpin the research supporting the development of this risk assessment. Firstly, for there to be a risk of harm there must be exposure to a hazard or hazardous agent. Without exposure there can be no risk. Secondly, the dose at the point of exposure must be sufficient enough to cause harm. Living organisms are routinely exposed to hazards which they tolerate and are resistant to. Here, the method and summary findings are set out from determining the highest plausible exposure that a sensitive receptor can be exposed to from the transfer of a hazard from its original source, see Figure 1.


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