Resilience of a Combined Chemical-Physical and Biological Wastewater Treatment Facility

This work is aimed at assessing the effects of pretreated aqueous wastes stream-fed to a biological nutrient removal (BNR)-activated sludge plant. Aqueous wastes (mainly landfill leachate and liquid residues from chemical and pharmaceutical factories) are pretreated by means of a traditional coagulation-flocculation stage and subsequently sent to the BNR stage together with municipal wastewater. Pretreated aqueous wastes contribute up to about 75% of the influent load, in terms of chemical oxygen demand (COD; average concentration 23,000 mg/L). The resilience of the system, that is, the capacity to maintain high and stable performances despite sudden perturbations of the input conditions, was evaluated by interrupting the aqueous waste dosage and then progressively restoring the initial condition. The consequent variation of the influent load and composition caused a reduction of the biomass activity, which led to a decrease of the nitrogen removal efficiency (from 89% down to 73%). More than 30 days were required for restoring typical working conditions (in terms of influent load, process performance, and biomass activity), the sludge retention time (SRT) being 23 days.