The benefits of utilizing waste streams such as food waste, human excreta or increasing the fertilization potential of animal manure in agricultural systems are clear; they are an abundant pool for nutrients and organic matter which can benefit amongst others soil texture and soil fertility and thereby crop yields. However, the recycling of waste streams can pose potential safety risks. Biological contaminants (e.g. concentrations of viruses/bacteria/zoonoses), chemical residues (e.g. from use of drugs/biocides/feed additives/medicines) or undesired physical particles (e.g. plastic nanoparticles) contained in waste streams can pose potential risks to human health, animals and the environment. For instance, the application of sewage sludge as agricultural fertilizer can pose risks with regards to heavy metals, hormones and antibiotics. In addition, the direct application of unprocessed livestock manure can pose risks due to the spreading of antibiotics, pathogens and hormones. Past research has focused on studying the risks for a limited part of the system and hazards have been identified for different waste streams. However, an integrated assessment on the stock and flows of waste streams and their potential safety risks has not yet been performed for circular agricultural systems.
Here we propose to develop a spatially explicit stock and flows model of waste streams in agriculture including their level of selected chemical, physical and biological substances. The distribution of waste streams and associated contaminants will be modelled based on balancing the nutrient budgets at farm and regional level using an existing nutrient-cycling model combined with new data on contaminant levels in waste streams. In The Netherlands, (too) high amounts of manure are applied in particular areas due to extensive livestock production, while areas with mainly crop production need to import nutrients. Closing the nutrient gap between livestock and arable farms means to better connect their inputs (i.e. harvested crop for livestock feed) and outputs (i.e. manure for crop needs) at the smallest scale possible. Identified locations with nutrient deficits and surpluses will provide opportunities for recycling waste streams, where the contaminant levels will be assessed in a semi-quantitative way (e.g. by classifying numerical ranges into risk assessment categories). The output of the model will be then coupled with a safety assessment procedure developed by another junior researcher focusing on risk assessments. The overall project outcome will be a semi-quantitative safety assessment tool to simulate the potential impact of novel waste stream management scenarios in agriculture to the environment and human and animal health.