Treatability of domestic wastewater by anaerobic baffled reactors with different inoculums

In this study, the treatability of domestic wastewater results from two identical lab-scale continuous-flow anaerobic baffled reactors (ABRs), inoculated with different seed (flocculent and granular) sludges, were presented at varying hydraulic retention times (HRTs) ranging between 8.55-18 h corresponding to organic loading rates (OLRs) up to 0.49 kg COD/m ³/day. Seventeen baffles were placed with 0.05-m intervals along the reactors. Both reactors were operated at ambient (room) temperature observed in the range of 11-27°C depending on the climate changes (psychrophilic and/or sub-mesophilic). Raw sewage was taken fresh from the canteen and toilets of the University Campus, and presented mostly a weak character (COD_tot = ≤30-700 mg/L) although pre-sedimentation was not applied. ABRs were both seeded as 35% by flocculent and granular sludge taken from anaerobic reactors treating the waste-waters produced from different food companies. At first, the aim of designing two identical reactors was to operate them at the same HRT simultaneously in order to investigate and compare the performances with different inoculums. However, similar HRTs at both ABRs could not be provided and, thus, the anaerobic treatability of the two reactors was evaluated individually. ABR with flocculent seed indicated an average COD_tot removal of 84% at an average HRT of 17.8 h (T_avr = 13.5°C). Average COD_tot, removals were 67 and 72% at average HRTs of 12.8 h (T_avr=19°C) and 11.1 h (T_avr=24°C), respectively. Results of the ABR with granular seed indicated average COD_tot, removal rates of 63 and 75% at average HRTs of 9.5 (T_avr=18°C) and 8.55 h (T_avr=24°C), respectively. Though influent COD _tot concentrations fluctuated significantly, the effluent levels were consistently lower than 100 mg/L. However, COD_sol removals gave inconsistent results at all HRTs in both reactors due to very low influent concentrations (≤60 mg/L) not representing a typical sewage. Besides, more than 90% TSS were kept in the reactors and could be removed from the effluents. Alkalinity and pH values were monitored on a regular basis and almost no biogas production could be observed as expected from most anaerobic systems treating such diluted streams at relatively low temperatures.