Environmental and economic assessment of sewage sludge handling options.

The environmental and economic consequences of four recycling and disposal options for municipal sewage sludge have been assessed. The four options were: agricultural application, co-incineration with waste, incineration combined with phosphorus recovery (Bio-Con) and fractionation including phosphorus recovery (Cambi-KREPRO). Life cycle assessment (LCA) was used to assess the environmental consequences, while effects in the district heating system were analysed using the energy system model MARTES. Co-incineration had the best energy balance, but without recovery of phosphorus. In the Bio-Con and Cambi-KREPRO processes both phosphorus and energy could be recovered. Compared to Cambi-KREPRO, Bio-Con was more effective in most respects, but suffered from higher emissions to air. Spreading sludge on agricultural land was the least preferable option from an environmental point of view. Energy was required for transportation, spreading and pasteurisation of the sludge, whereas the other three options enabled energy recovery. Spreading also caused release of nutrients and acidifying substances and transferred the content of heavy metals in the sludge to agricultural soil. The economic assessment showed that agricultural application had the lowest cost of the options, whereas co-incineration had the highest cost. The difference in cost between Bio-Con and Cambi-KREPRO was small, but since the technologies are new and untried in a commercial context, these results are uncertain. This study has shown that two sludge handling options, incineration and direct application to agricultural soil, have respectively economic and environmental restrictions. The development of relatively low cost phosphorus recovery technologies has the potential to reconcile the environmental and economic aspects of sustainability.