Heatphos process for recovering phosphorus from bio-P sludge before anaerobic digestion

Sewage sludge generated in Bio-P processes (also known as enhanced biological phosphorus removal processes) typically contains 3-5% of its dry weight as phosphorus (P). Approximately 60-80% of the P is stored as polyphosphate (polyP) which is a heat-labile polymer of inorganic phosphate (P_i). This chapter describes a simple technology, named "Heatphos," for leaching polyP from Bio-P sludge by heating at 70 °C for about 1 h and recovering P_i from the leachate by the addition of CaCl2 without needing to adjust pH to a high value. Heating sludge for polyP leaching can also improve the digestive efficiency and thus biogas productivity in the subsequent anaerobic sludge digestion at both mesophilic (typically 35-40 °C) and thermophilic (50-60 °C) temperatures. The heat energy required for polyP leaching can be supplied from biogas generated by anaerobic sludge digestion. The demonstration plant with a capacity of treating Bio-P sludge of 0.36 m3/ day has showed that P could be recovered from Bio-P sludge at a rate of approximately 10 kgP/day (c. 3.5 tP/a).