Phosphorus removal from sidestreams by crystallisation of magnesium-ammonium-phosphate using seawater

In recent years, a number of biological-treatment processes have been developed for phosphorus removal, alone or in combination with nitrogen. In anaerobic/oxic and anaerobic/anoxic/oxic processes, it is necessary to lower the concentration of phosphorus in the sidestream from the sludge-disposal processes. To do this, the magnesium-ammonium-phosphate system has been developed in which phosphorus is removed in a side-stream process. Alternative systems require chemicals such as magnesium chloride as the source of magnesium and sodium hydroxide for pH control, and are economically less viable than that described in this paper in which seawater is substituted for the chemicals. Dissolved phosphorus removal of over 70% was achieved without pH control by feeding the sidestream which contained 50-111 mg/l dissolved phosphorus and had a pH greater than 7.77, together with a 9-10% flow of seawater into the reactor containing 4-13% of magnesium ammonium phosphate and which provided a retention of 29 mins.