The effect of varying food-to-microorganism ratios on phosphate uptake in an activated sludge environment

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Virginia Polytechnic Institute and State University


An economical process for the removal of nutrients to receiving waters has become a difficult problem of major proportions. It was the purpose of this study to clarify the importance of the food-to-microorganism ratio (F/M) and to evaluate the nature of phosphate uptake in the activated sludge environment. The investigation consisted of two batch studies which allowed the F/M to vary as follows: 0.20, 0.75, 1.25 and 2.00. The phosphate concentration (PO₄) was allowed to vary in the first batch study and was kept constant in the second batch study. Measurements of pH, soluble COD and soluble PO₄ were made on all samples taken. Certain samples were measured for sludge total phosphorus content.

Results obtained showed that both the biological and chemical processes have roles in phosphate uptake, but it was concluded that phosphate removal is predominately biochemical in nature. It was also found that physical adsorption was not the primary mechanism of phosphate uptake in an activated sludge environment, and the best overall soluble phosphate removal occurred when the food-to-microorganism ratio ranged between 0.75 and 1.25.