Environmental evaluation: Final disposal alternatives of sludge from municipal wastewater treatment plants

Environmental issues concerning wastewater treatment are considerable. Legislation is continuously refining the requirements for the level of removal of various pollutants in treated water. This often leads to a higher production of sludge, although ways to eliminate this sludge are becoming increasingly restricted. Most sludge has traditionally been incinerated (with energy recovery), disposed of in landfill sites or dumped into the sea. Today, however, ocean dumping has been banned in many areas. In Europe, landfill regulations for organic waste will become more and more stringent until 2015, when landfilling will be banned completely. Incineration is heavily regulated and facing strong social opposition, as the incineration of sludge leads to small atmospheric emissions of substances such as mercury and dioxins and generates ashes that need to be landfilled or handled as hazardous waste. Recycling sludge as fertilizer on agricultural soils is held by many governments to be the best environmental option, adding nutrients and organic content to the soil, although is banned in some areas. However, the content of heavy metals, pathogens and persistent and toxic organic compounds in sewage sludge make stringent regulations necessary; this option is being increasingly questioned by farmers. The aim of this study is to assess alternatives for the final disposal of sewage sludge from wastewater treatment from a life cycle assessment (LCA) point of view in order to identify the best environmental alternatives. This study proposes five alternatives for final sludge applications: agricultural uses, compost plant, cement plant, incineration and landfill. LCA methodology is used to evaluate the environmental profile of the different studied alternatives in order to establish in a global, rigorous and objective way the most positive environmental alternative. Among the alternative final destinations of sludge studied here, the best environmental option is to combine agriculture use (fertilizer replacement) by using the sludge in a cement plant (as fuel replacement and raw material reduction) because this emphasizes their respective advantages and diminishes their respective disadvantages.