Quantifying Nutrient Assimilation in Activated Sludge

Nutrient assimilation was revisited using a steady state model developed from the activated sludge modeling concepts. While nitrogen assimilation is only related to organic particulate fractions, the role of inorganic complexes and precipitates was accounted for in phosphorus assimilation. It is shown that the influent aluminum background concentration may contribute significantly to phosphorus assimilation, and that in 3.6% of cases, calcium phosphate may precipitate in activated sludge.A global sensitivity analysis was conducted with the Monte Carlo method to identify the most sensitive model parameters and input variables. The nutrient content of the unbiodegradable mixed liquor fractions emerged as the most sensitive parameters and were calibrated using databases and some assumptions. Most of the remaining variability of nutrient assimilation then comes down to four variables (three influent concentration ratios and SRT), which were used to prepare charts to predict assimilation in design and operation. For an SRT of 20 days, it is proposed to update the typical empirical BOD5:N:P ratio from 100:5:1 to 100:5.7:1.4.