Recirculation of phosphorus : an optimization analysis of processes to recycle phosphorus from sewage sludge

Sammanfattning: Phosphorus is a limited resource and for countries like Sweden without domestic mines, it becomes increasingly important to recycle this resource. Sewage sludge is rich in phosphorus and has traditionally been used as a fertiliser via direct application to farmland. There are many technologies that have been developed to recover P at the wastewater treatment plant or from sludge ashes, but the adaption of these technologies is slow. In Sweden the most common method of utilisation is still recycling through direct application to farm land, but this still only encompasses 25% of the produced sewage sludge. The Swedish EPA has suggested the objective to recycle 40% of the phosphorus in sewage sludge should be recycled to farmland by 2018, without risk of harming people or environment. Along with this target, new regulations have been suggested, which means that stricter limit values for residue in sludge is to be expected. Thus it will become more difficult to apply sludge to farmland. This makes it important to consider the effects of sludge quality when making strategic decisions. It is thus relevant to investigate which is the least costly processes with potential for recycling phosphorus from sewage sludge, under the suggested new regulations. This study compares the effect of four processes using linear programming to estimate the lowest cost operation, with Kungsängsverket as a case plant. The processes include two wastewater treatments, EBPR and conventional treatment with AS and chemical P precipitation, plus two additional processes; upstream management (Revaq) and recovery from ashes (CleanMAP). EBPR in combination with Revaq was found to be the optimum process combination. It was however rather difficult to make a reliable estimation of construction costs and therefore the results ought to be viewed with caution.