Biological treatment of whitewater in a laboratory process in order to determine kinetic parameters for model development

Implementation of an in-mill biological treatment plant is one solution to the problems associated with closure of whitewater systems. It is, however, important to operate the treatment with low concentration of nutrients in the effluent. The effect on the COD reduction from decreased additions of NH₄-N and PO₄-P were investigated in three parallel aerobic suspended carrier reactors during treatment at 46 to 48°C of whitewater from a recycled paper mill producing liner and fluting. In the reference reactor, a COD reduction of 89% was achieved and 45.6 mg NH₄-N/(g COD reduced) and 11.6 mg PO₄-P/(g COD reduced) was consumed at an organic load around 20 kg COD/(m³·d). Reduced additions of NH₄-N decreased the COD reduction. Addition of 56% of the consumption of NH₄-N in the reference reactor resulted in a COD reduction of 80%. The response from decreased addition of PO₄-P was different compared to NH₄-N but it could not be determined if this is due to unsuitable experimental design or a different reaction mechanism. Reducing the addition of PO₄-P to 26% of the consumption of PO₄-P in the reference reactor, decreased the COD reduction to 83%. The main conclusion from the experiment is: biological treatment has the potential of treating whitewater from recycled paper mills with low effluent nutrient concentrations.