Recovery potential of aerobic sludge biomass from Co (II) stress in sequencing batch reactors

Heavy metals in higher concentrations are often encountered in domestic sewage of developing and under-developed countries. High metallic concentrations can stress reactor sludge biomass morphology impeding its performance in organics reduction. However, the extent of damage and ability of sludge biomass to recover from the metallic stress is not fully understood. Also, there is no protocol to identify and prevent the sludge biomass from metallic stress in fully functional sewage treatment plants (STPs). This study investigates performance, metabolic activity, morphology, and settling characteristics of the sludge biomass under different Co(II) stress conditions. The extent of recovery in biomass, when the supply of Co(II) metal ion was discontinued in the inlet stream, was explored. The study also proposed a protocol based on simple settling characteristics of sludge biomass to get an early indication of metal infiltration to prevent potential damage to the biomass morphology. Four sequencing batch reactors (SBRs) with Co(II) ion concentrations of 0 (designated as RCo0), 5 (RCo5), 25 (RCo25), and 75 mg/L (RCo75) in the feed were operated with a cycle time of 12 h. Reactors were operated for 35 days with Co(II) in the feed (termed as stressed phase operation) followed by 24 days of operation without Co(II) in the feed (termed as recovery phase operation). Results show that COD removal in reactor RCo75 reduced to 48% on the 10th day of stressed phase operation, showing a lag in COD removal due to metallic stress. The activity of biomass in reactors RCo5, RCo25, and RCo75 was reduced by 39%, 45%, and 49%, respectively, in the stressed phase compared to the biomass in control reactor. Recovery in COD removal efficiency and specific biomass activity were observed in all the reactors after the removal of metallic stress. The settleability of sludge biomass in reactors RCo25 and RCo75 was significantly affected. Transformation in the shape of flocs in reactor RCo25 and RCo75 biomasses revealed the prolonged effect of metallic stress, which was observed to be irreversible even during the recovery phase operation.