How inoculation affects the development and the performances of microalgal-bacterial consortia treating real municipal wastewater

To date, little is known about the start-up of photobioreactors and the progressive development of stable microalgal-bacterial consortia with a view to the full-scale treatment of real wastewater. Two photo-sequencing bioreactors, one inoculated with Chlorella vulgaris (RC) and one with the absence of inoculum (RW), were fed with real municipal wastewater and run in parallel for 101 days. The influence of the inoculation was evaluated in terms of pollutant removal efficiency, excess sludge production, solids settleability and microbial community characteristics. No significant differences were observed in the removal of COD (89 ± 4%; 88 ± 3%) and ammonium (99 ± 1%; 99 ± 1%), mainly associated with bacteria activity. During the first weeks of acclimation, Chlorella vulgaris in RC promoted better P removal and very high variations of DO and pH. Conversely, under steady-state conditions, no significant differences were observed between the performances of RC and RW, showing good settleability and low effluent solids, 7 ± 8 and 13 ± 10 mg TSS/L respectively. Microbiome analysis via 16S rRNA gene sequencing showed that, despite a different evolution, the microbial community was quite similar in both reactors under steady state conditions. Overall, the results suggested that the inoculation of microalgae is not essential to engender a photobioreactor aimed at treating real municipal wastewater.