The influence of the hydraulic retention time (HRT) (2 and 4 days) and the carbon/nitrogen ratio (C/N) (7, 8 and 9) of the wastewater on the treatment of synthetic domestic wastewater was evaluated in a new anoxic-aerobic algal-bacterial photobioreactor configuration operated at solids retention time of 10 d by biomass recycling and withdrawal. The removal of chemical oxygen demand remained between 84% and 89% regardless of the operational conditions. However, the decrease in the HRT from 4 to 2 d entailed reductions in the removal of total nitrogen (TN) and P-PO43− from 87 ± 2% to 62 ± 2% and from 22 ± 5% to 11 ± 1%, respectively. On the other hand, the decrease in the C/N ratio of the wastewater from 9 to 8 and 7 at a HRT of 2 d induced TN removals of 62 ± 4% and 48 ± 4%, respectively. In contrast, P-PO43- removals unexpectedly increased from 11 ± 1% at a C/N ratio of 9 to 53 ± 3% and 47 ± 5% at C/N ratios of 8 and 7, respectively. Finally, biomass settling and recycling supported the enrichment of an algal-bacterial population with good settleability characteristics (suspended solids removals in the settler ∼98%), being Chlorella vulgaris the dominant microalga specie at a C/N ratio of 9 which was gradually replaced by Phormidium sp., as a result of the reduction in the C/N ratio of the wastewater.
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