Is the reuse of sewage sludge (SS) for agricultural purposes a safe procedure? This is a key issue that must be addressed, especially when this practice has been fostered over the last 30 years as the most suitable for sludge disposal. In the last years, the number of wastewater treatment plants (WWTPs) has increased and consequently, sludge production. SS/compost contains a high organic matter content and nutrients that are extremely valuables for soils. These characteristics make them prone to be used as fertilisers. However, the potential presence of significant amounts of organic contaminants has raised concern. In fact, the land application of SS/compost may be considered as a risk to humans and the environment due to their persistence, bioaccumulation and toxicity.
This project aims to contribute to increase the knowledge on the safe use of SS/compost in cropland and to promote improvements in these practices, trying to increase its use in Portugal (up to 10%), promoting a circular economy. It will be focused on the analysis of 4 classes of organic pollutants - polycyclic aromatic hydrocarbons, polychlorinated biphenyls, synthetic musks and volatile methylsiloxanes. To the authors? best knowledge, this work will fill an important gap, being the first study of its kind to be performed in Portugal.
First, a national monitoring plan will be established, collecting SS/compost in urban WWTPs. This will allow to determine the pollutants? levels, physicochemical/agronomic characteristics of these residues and the variation of the quality of SS/compost according to the origin of waste, sociodemograhic aspects and seasonality. This is crucial to define new procedures and crop management conditions, trying to minimise pollutants uptake. Then, application of SS/compost to agricultural fields will be studied and plant uptake assessed using 3 crops - Solanum lycopersicum (tomato), Zea mays (maize) and Phaseolus vulgaris (bean) ? those with a higher production/consumption in Portugal. Studies will be performed in growth chambers under controlled conditions and in farmlands, reproducing real production conditions. This part will be performed in the Agrarian Campus of Vairão. Furthermore, tests in amended-soils should be performed (sorption, dissipation, leaching) to know if this practice induces negative effects and how can these be avoid. Environmental and human risk asssessment studies will be carried out in order to understand the impacts of this practice.
At the end, the following questions should be answered: Are those pollutants present in SS/compost? At what levels? Is there any plant uptake when SS/compost is applied in crops? If so, is there a risk to human health? Is there any accumulation and negative impact on soil?
This project faces one of the major societal challenges, the conversion of a residue in a high-value product. So, raising awareness and encouraging the safe reuse of SS/compost will have a great impact in economic activities.