Sodium azide inhibition of microbial activities and impact on sludge floc destabilization

Absence of sludge deflocculation under prolonged (24 h or longer) conditions with dissolved oxygen (DO) less than 0.5 mg L⁻¹ was recently reported. The reduced aerobic microbial activity, was speculated, had been compensated by the activity of other bacterial (i.e. facultative) communities. To assess such a compensation mechanism and to better evaluate impact of overall microbial activity on the flocculation process, SBR sludge samples were inhibited by using sodium azide under various DO conditions. Sludge deflocculated only in the presence of sodium azide, regardless of DO conditions. This was linked to sodium azide's inhibitory effects on the microbes as indicated by the reduced ammonium and DOC removals. Extracellular potassium level in the mixed liquor of azide spiked samples also indicated simultaneous cell lysis. Fluorescence excitation emission matrix (FEEM) analysis of the extracted bound EPS and fluorescence quenching based interaction studies indicated sodium azide had interacted with the EPS components, and especially with the bound EPS proteins. The impact of such interactions on reduced floc stability needs consideration. This study confirmed the importance of overall microbial activity in the biological flocculation process and the role of bacterial communities, other than the aerobes, in mitigating deflocculation under low DO conditions.