Enhanced Microalgal Growth and Effluent Quality in Tertiary Treatment of Livestock Wastewater Using a Sequencing Batch Reactor

Effect of harvesting and hydraulic retention time (HRT) on the performance of a sequencing batch reactor (SBR), growing a green alga Botoryococcus braunii, was investigated. In this continuous tertiary treatment, relieving limitations of light, inorganic carbon, nitrogen, and phosphorous can make photoautotrophy active through the rapid fixation of the building blocks into microalgal biomass together with heterotrophy promoted by organics and self-shading effect. Analysis of the results reveals that the control over CO₂ supply and the extension of solid retention time (SRT) are the keys to maintaining higher biomass productivity and better treatability in the mixotrophic SBR. Among HRTs tested, the shortest HRT of 2 days could demonstrate the best removal efficiencies of ammonia (98.8%) and total phosphorus (96.2%) while keeping the highest specific growth rate of 0.23 day⁻¹. Those results provide understanding on the impact of settling sequence, which extends SRT to 5~7 days and prevents significant limitations of light and essential building blocks. With the interplay between photoautotrophic and heterotrophic metabolisms of microalgae, this study identifies how the mixotrophic SBR perform resource recovery during tertiary treatment of livestock wastewater, and how limitation is associated with the effluent quality in the SBR.