An algal–bacterial culture, composed of wastewater-born algae and activated sludge, was cultivated to
treat domestic wastewater and accumulate biomass simultaneously. The influence of algae and sludge
inoculation ratios on the treatment efficiency and the settleability of the accumulated biomass were
investigated. There was no significant effect of the inoculation ratios on the chemical oxygen demand
removal. Comparatively, the nutrients removal and related mechanism were varied with different
inoculation ratios. The highest nitrogen and phosphorus removal efficiencies were observed with 5:1
(algae/sludge) culture (91.0 ± 7.0% and 93.5 ± 2.5%, respectively) within 10 days, which was 5–40% higher
and 2–4 days faster than those with other inoculation ratios. The biomass settleability was improved
with the assistance of sludge, and the 1:5 (algae/sludge) culture showed the best settleability. Furthermore,
16S rDNA gene analysis showed that the bacterial communities were varying with different algae
and sludge inoculation ratios and some specific bacteria were enriched during operation.