Use of sewage sludge in Austria - I. A comprehensive case study

The possibilities and basic requirements for long-term agricultural utilization of sewage sludge were studied at the wastewater treatment plant (WWTP) at Mödling. The complete process was considered, from the origin of the wastewater to sludge production and utilization. The following chief components of this large-scale 3-year program, are discussed: systems analysis and modeling of the catchment area, sewage-sludge production and quality, experiments on sewage-sludge composting, and demonstration field experiments. The systems analysis provided the background information and demonstrated that the studied catchment area is predominantly residential with evenly distributed small and medium-sized businesses. Mathematical modeling resulted in a ranking list of suspects as polluters of wastewater and, hence, of sewage sludge. This list was qualitatively verified, although satisfactory correlation of measured and predicted loads is pending. Sewage-sludge analysis showed that its quality meets current, but not scheduled, legal requirements. Experiments were performed to improve the structure and the stabilization level of sewage-sludge composting. Additionally, demonstration field trials were nm for a period of 3 years, with comparisons of sewage sludge, compost, and mineral fertilizers. The results showed no distinct differences in terms heavy metals in the soil or taken up by plants. It was shown that sewage sludge and/or compost utilization can be integrated into agricultural practices. However, additional improvements in sewage-sludge quality will be necessary if adverse effects are to be avoided in the long term. The only practicable, feasible means of improving sewage sludge is to improve wastewater quality.