The role of potassium (K) and magnesium (Mg) in enhanced biological phosphorus removal (EBPR) by activated sludge was studied using a bench-scale continuous-flow A/O system. A synthetic wastewater containing all the nutrients required for EBPR was used as the influent feed for the control phase of the experiment. The influent feed to the test phase of the experiment was changed to totally limit specific cations. The results clearly indicated that both K and Mg were absolutely required for successful EBPR. Failure of EBPR occurred when either K or Mg were eliminated from the influent. The molar ratio of K:P during anaerobic release and aerobic uptake was observed to be 0.22 mol/mol, while Mg:P was 0.30 mol/mol. Calcium was not required for successful EBPR. Neither calcium, iron, nor sodium were co-transported with phosphorus during release and uptake.

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