How to eliminate the toxic impact of extreme salinity on the ecology of activated sludge? Experimental verification with pickle plant effluents

BACKGROUND: The study defined a sustainable management scheme to eliminate the toxic impact of extreme salinity on the ecology and performance of an activated sludge process. The scheme was implemented on a plant treating pickle plant effluents involving significant flow and salinity transients. It was conducted in two phases. Firstly, the plant was operated without attenuating extreme salinity transients, enabling observation of all adverse impacts on the microbial ecology. The second phase was implemented with a new management scheme tempering all salinity gradients revealing the recovery of the microbial ecology into a stable and sustainable state. RESULTS: Initially, the microbial community could not cope with rapid salinity increase and exhibited significant changes resulting in the predominance of filamentous microorganisms, disruption of the floc structure and almost total loss of eukaryotic microorganisms. Settling problems and deterioration of effluent quality were observed, followed by plasmolysis and repeated total loss of the biomass. The new waste management strategy allowed only transients limited to ±500 μS cm⁻¹ in the influent; in this way, the microbial ecology steadily improved. The effluent chemical oxygen demand could be maintained below 80–90 mg L⁻¹ with no appreciable particulate matter escape. CONCLUSIONS: Results identified sharp salinity transients as the key factor triggering total destruction of activated sludge. The novel scheme provided conclusive evidence that a stable microbial community could be maintained even when continuously exposed to a salinity level of around 10 000 μS cm⁻¹ in the reactor, provided that variations remained limited to ±500 μS cm⁻¹, ensuring satisfactory effluent quality.