Phosphorus recovery from wastewater – Risk assessment for recycling in agriculture
In recent years several ways of recovering phosphorous from municipal wastewater have been developed. Depending on the applied technology the recovered products vary significantly concerning the concentrations of heavy metals and organic residues. Within the boundaries of data quality and present uncertainties a comparative risk assessment of seven secondary phosphorus fertilizers, sewage sludge, raw ash and triple super phosphate has been conducted for PCDD/Fs, PCBs, PAHs, As, Cd, Cr, Cu, Hg, Ni, Pb and Zn. Local exposure assessment was done using the kinetic model of the European Union’s Technical Guidance Document for all substances accounting for both fertilization and average atmospheric deposition. For substances of concern (Cd and Zn) the exposure was additionally refined using a solute transport model (HYDRUS-1D) and a precipitation model (Visual MINTEQ-software). An annual fertilizer amount equivalent to 60 kg P2O5/ ha × year by these products is assumed. In order to account for potential accumulation a time span of 100 years is modelled. Results indicate that out of the selected 11 (groups of) chemicals only cadmium and zinc are of concern. Regarding soil organisms, zinc is of concern for sludge, raw ash and one of the seven secondary phosphate fertilizers in case of soil-pH above pH 6.0. Regarding groundwater, cadmium and zinc are of concern below pH 6.0 since mobilization at this pH level increase significantly. No risk is expected regarding the endpoint humans. Among the investigated products struvites have shown the lowest phosphorus-specific heavy metal contents. For ash related products more data from full scale operations are needed to reduce still existing uncertainties like the influence of raw wastewater quality and WWTP operation on the final product.