Synergistic cooperation between wastewater-born algae and activated sludge for wastewater treatment: Influence of algae and sludge inoculation ratios

► Cultivate algal–bacterial system with microorganisms from wastewater plant. ► Enhance biomass settleability with activated sludge. ► Enhance wastewater treatment efficiency at proper algae/sludge inoculum ratios. ► Identify the microbial consortium in cultures with different inoculum ratios. 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.