Magnesium requirement for biological removal of phosphate by activated sludge

The effects of magnesium on excess uptake of phosphate in an aerobic-anaerobic activated sludge process were examined by the fill and draw procedure. The alternation of anaerobic and aerobic conditions in one cycle of fill and draw process was varied many ways.

The presence of sufficient magnesium was necessary for uptake of excess phosphate. When sludge contained more phosphorus than the upper limit of phosphorus content in the usual aerobic activated sludge, 2.5% by weight, magnesium was also contained in more than an ordinary amount (0.5%). Their contents in the sludge at the end of each cycle of the process were correlated with each other by a linear equation with the correlation coefficient of 0.99. When magnesium concentration was insufficient for the uptake of excess phosphate, its concentration in the treated water was of the order of 0.1 mg/l.

In the first anaerobic period both phosphate and magnesium were released, and in a successive aerobic period they were taken up again. The weight ratio of differential amounts of phosphorus and magnesium released or taken up changed with time in one cycle.

Dynamic behaviors of phosphate and magnesium removal against the step change of feed magnesium concentration also showed a stoichiometric relationship supporting the correlation equation above mentioned.