Operating costs related to instability in a pulp and paper activated sludge treatment system

Wastewater treatment by the activated sludge treatment process has been widely implemented in the pulp and paper industry; however, this process can be difficult to operate, particularly during sludge bulking events. Preventing noncompliance events can normally be achieved effectively in wastewater treatment plants (WWTP) by manual control, but this can be costly. Indeed, the use of various relief chemical products constitutes a significant part of a WWTP’s operating costs. Moreover, the use of these reactive rather than proactive methods does not target the specific causes of problems and only temporarily removes their effects. This paper analyzes the operating costs of an activated sludge process treating the effluent from a thermomechanical pulp mill in eastern Canada, in particular the costs associated with abnormal or transient system behavior. A specific treatment cost of $725/t BOD was calculated, highlighting the importance of chemical products, particularly for sludge thickening and settling, and of energy. Costs associated with instability were also reported, and it was found that 11.0% of the total operating costs were directed at relieving the treatment system from an unstable state.