Evaluating spent mushroom waste for remediation of DDT-contaminated soils with focus on risks and costs

Research Project, 2024 – 2025

Former plant nursery sites contaminated with persistent organic pollutants, mainly insecticide DDT, occupy very large areas in Sweden. DDT that is usually detected in low to moderate concentrations in topsoil frequently poses the risks to the environment and sometimes to human health. From a geological point of view, these areas are often characterised by very similar conditions and consist of fine sandy soils that provide very good conditions for plant growth. Areas also have high natural values. These conditions are difficult, or impossible, to preserve and restore when traditional remediation methods that relies on excavation and disposal of contaminated masses are being used. Furthermore, these methods may lead to extensive waste generation and consumption of large amounts of energy for transport and natural materials for backfilling, as well as substantial emissions of greenhouse gases and accident risks from transport to and from the site. Alternative remediation solutions should therefore be sought to treat these enormous amounts of contaminated topsoil at former plant nurseries in Sweden. Gentle remediation options (GRO) are such alternative low-impact and low-cost nature-based solutions that rely on the intrinsic capability of plants, fungi and bacteria often in combination with soil amendments to detoxify the soil and improve its functioning. Still, GRO application in Sweden is constrained by a time pressure and the lack of knowledge. Long remediation time that is often required for GRO may not be an issue for former plant nurseries; however, the knowledge gaps remain unfilled. In the recent decade, waste from the industrial production of the second most popular edible oyster mushroom (P. ostreatus), also referred to as spent mushroom waste (SMW) of P. ostreatus, was identified as an effective soil amendment for biodegrading POPs and oil products in soils. It was, for example, demonstrated that the residual mycelium (a root-like system) of P. ostreatus in SMW excretes enzymes that can, in symbiosis with soil native bacteria, effectively biodegrade DDT, PCB and PAH in soils. Demonstration and validation studies at both lab- and pilot-scales, however, remain scarce. Furthermore, the optimal soil:SMW ratio has not been corroborated in scientific studies, although it has implications for both risk reduction over time and the associated remediation and societal costs. There are no studies on the effects of SMW-aided GRO on reduction of risks posed by DDT to the soil ecosystem, which is a risk object that governs a generic soil guideline value (often a measurable remediation target) for this and many other soil contaminants in the Swedish risk assessment model for contaminated soils. Neither the remediation nor societal costs of SMW-aided GRO are explored in scientific literature to support decision-making. To fill these gaps, this project aims to investigate the technical and economic possibilities of risk reduction at DDT[1]contaminated sites by GRO that involve SMW. The specific objectives are: (1) to set up a pot experiment where SMW from industrial production of oyster mushroom is mixed with soils collected from a site historically contaminated with DDT; (2) to evaluate risk reduction achieved over time in the pot experiment; and (3) to estimate the costs of this SMW-aided GRO per tonne contaminated soils at the site of their origin to both a problem-owner and the society. The project is carried out in collaboration between Chalmers University of Technology, Swedish University of Agricultural Sciences (SLU), Swedish Geotechnical Institute (SGI), Swedish Geological Survey (SGU) and the Swedish mushroom growing industry.