Reduced sludge growth at high bulk liquor dissolved oxygen induced by increased secondary cell maintenance

Sludge reduction by physico-chemical methods results in the buildup of chemicals, which may require further treatment. Owing these reasons various biologically sustainable methods of sludge reduction including the application of high oxygenation have been successfully tested. Experiments on actual sewage in two lab-scale sequencing batch reactors (SBRs) were conducted under normal (1.5-2.5 mgDO/L) and high dissolved oxygen (DO) (HDO: 3-6.5 mgDO/L) regimes. It was observed that microorganism allocated substrate between maintenance and growth in the form of maintenance coefficient. Which could be induced by endogenous respiration owing to high solids retention time (SRT), predation on bacteria, chemical toxicity, adverse environment, and viral attack on bacteria. The wastewater treatment process may experience one or more maintenance inducing factors; nevertheless, high SRT and prevailing environmental conditions are imminent and thus considered as primary maintenance (m_p), while remaining are classified as secondary maintenance (m_s). Average yield coefficient reduction at HDO was 32.7% and 28.2% compared to stoichiometric and at normal DO, respectively. The observed primary and secondary maintenance was 0.11gCOD/gVSS.d (±0.01) at an SRT of 25.2 d (±2.0) and 0.096 g 0.1 gCOD/gVSS.d (±0.045) at an SRT of 24.2 d (±3.6d), respectively. The results obtained under the study are not as precise as on pure culture and defined substrate, nevertheless, it gives an idea that how stress factors inducing maintenance need to be addressed more seriously and objectively while managing our efforts on sludge reduction.