Feasibility of phosphate precipitation from digested anaerobic sludge in a continuous aerated reactor

This study investigates the effectiveness of a complete stirred and aerated reactor for deliberate phosphate precipitation as struvite from anaerobic sludge in a domestic wastewater treatment plant and, to achieve the optimum operational conditions in order to reduce the potential formation of struvite downstream of the digester and decrease the phosphate (P) and nitrogen (N) load on the treatment plant. The reactor was fed with anaerobic sludge and operated at different hydraulic retention times (HRT) and aeration rates. Theoretical calculations indicated that a minimum pH of 7.5 would be necessary to attain struvite precipitation to its potential. To reach this pH value, a minimum HRT of 1.0 h with aeration flow rate of 46.7 m³ air/h/m³ reactor was necessary. Struvite precipitation kinetics were observed to be much faster than other minerals that can precipitate and a decrease in HRT promotes a precipitate richer in struvite. An increase in the HRT to 2.0 h, for the same aeration rate led to a higher phosphate precipitation with calcium. However, for all HRT and air flows studied, the precipitate obtained can be considered to be poor in struvite (maximum 0.72 mmol/l) and it was responsible for around only 4 and 0.5% decrease in the phosphate and ammonia load on the plant, respectively.