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

Abstract

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 salts·L−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 chemical oxygen demand (COD) < 100 mgO2·L−1 and removal efficiencies of 95%. After 140 days, the sludge volume index decreased below 100 mL·g−1 reaching the threshold of good settling sludge. Sludge morphology clearly improved, with dense sludge flocs and less filaments being present. A maximum anaerobic dissolved oxygen carbon (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%. HIGHLIGHTS Dense and compact sludge flocs were achieved under high-saline conditions; Filamentous organisms were suppressed in the system, favoring the growth of glycogen-accumulating organisms; Settling characteristics of the sludge were improved to the point of the sludge being characterized as well-settling;

Keywords

• anaerobic selector
• glycogen-accumulating organisms
• high salinity
• petrochemical wastewater
• sludge bulking
• sludge densification