Implementation of an anaerobic selector step for the densification of activated sludge treating high-salinity petrochemical wastewater

Sludge bulking is a common challenge in industrial biological wastewater treatment. Leading to difficulties such as bad sludge settling and washout, which is a problem also encountered in the petrochemical industry. Anaerobic feeding strategies can be used to induce the growth of storage-capable organisms, such as glycogen-accumulating organisms (GAO), leading to denser sludge flocs and better settling. In this study, the implementation of an anaerobic feeding strategy was investigated for high-salinity petrochemical wastewater (+35 g saltsmiddotL-1), using a sequencing batch reactor. Influent, effluent and sludge characteristics were analyzed throughout the operational period, which can be divided into three stages: I (normal operation), II (increased influent volume) and III (longer anaerobic mixing). Good effluent quality was observed during all stages with effluent COD , 100 mgO2middotL-1 and removal efficiencies of 95%. After 140 days, the sludge volume index decreased below 100 mLmiddotg-1 reaching the threshold of good settling sludge. Sludge morphology clearly improved, with dense sludge flocs and less filaments being present. A maximum anaerobic DOC uptake was achieved on day 80 with 74% during stage III. 16S rRNA amplicon sequencing showed the presence of GAOs, with increasing relative read abundance over time from 1 to 3.5%.