Phosphorus recovery potential in Sofia WWTP in view of the national sludge management strategy

•Measured and estimated phosphorus content of different sludge streams in Sofia WWTP.•Total phosphorus balance in Sofia WWTP.•Estimated recovery potential of 170-250 t phosphorus/year from digested sludge or ash.

In view of the vital importance of phosphorus and its increasing scarcity as a natural resource, phosphorus recovery has recently gained significant scientific and technical interest. The aim of this study is to support the implementation of the Bulgarian national sludge management strategy, particularly its objective to achieve more efficient resource management of phosphorus in waste water treatment plants (WWTPs). The study estimates the potential for phosphorus recovery from the municipal WWTP at Sofia, the largest in Bulgaria, serving some 14% of the Bulgarian population. The phosphorus content of five process streams (thickener supernatant, dewatering centrate, sludge before digestion, sludge after digestion and ash) is analysed on the basis of one year of measurements. It is estimated that 170-250 t phosphorus could be recovered annually from the WWTP from digested sludge or ash, depending on the phosphorus recovery technology used.