Though N and/or P are most frequently the limiting nutrients in industrial applications of the activated sludge process, attention has to be given also to other potentially limiting nutrients, especially Fe, Mg and Mo. Two different mechanisms exist in nutrient-limited biomass; multiple limitation of the growth rate and single nutrient limitation of the biomass crop.

It is suggested that effluent concentrations of both NH3-N and NO−3-N are used as control parameters for nitrogen dosing adjustment. It is also suggested that dosing of both ammonia and nitrate to supplement nitrogen is considered for situations sensitive to total nitrogen discharge. The rationale for this is that excessive concentration of nitrate can be easily denitrified while excessive concentration of ammonia is not easy to remove in pulp and paper wastewaster treatment plant conditions.

Soluble orthophosphate is not an adequate control parameter for phosphate dosing since some of the phosphate precipitates can be utilized by microorganisms. Appropriate methods for assessing their availability to microorganisms in real situations are still missing.

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