Phosphorus Recovery from Sewage Sludge by Thermal Treatment and Use of Acids and Bases

摘要

In Sweden there are significant differences in opinions and policies between organisations and municipalities regarding the strategy for sludge management in the future. The national proposal of 75% phosphorus recovery from sewage sludge would make high demands on wastewater treatment plants around Sweden if realised. The proposal is at present under investigation by the Swedish Environmental Protection Agency (SEPA) as a commission from the government. The demand of phosphorus recovery has led to sludge applications besides agricultural use. Sludge fractionation is considered to be a sustainable method for sludge handling where the sludge is seen as the raw material and products are recovered. A sludge handling system with product recovery may involve the following different steps (Hultman and Levlin, 1997). First, sludge is dissolved by different methods such as physical (heat or pressure), mechanical (use of ultra sonics), biological or chemical methods (use of acids and/or bases). The treated sludge may be used directly or the liquid phase is separated from the solid phase, where product recovery will continue. In the Scandinavian countries phosphorus is mainly removed from wastewater by chemical precipitation. This causes the phosphorus to be strongly bound to metal ions and for phosphorus release, it may be necessary to use acids combined with high temperature and pressure. Technologies that reduce both sludge mass and volume, whilst producing a reusable sludge product, are becoming increasingly favoured. This chapter describes some of these methods.