BACKGROUND Microalgae-bacteria-based processes are among the most promising low-cost technologies to treat livestock wastewaters. The current literature reports the need for pretreatment or dilution of piggery wastewater for adequate microalgal growth. The aim of this study is to optimize the potential of microalgal-bacterial communities to treat undiluted and untreated piggery wastewater by investigating the influence of some operational parameters such as phosphorus and CO2 availability and hydraulic retention time on the nitrogen removal efficiency and biomass productivity. RESULTS The microalgal community (dominated by Chlorella spp.) developed quickly and remained quite stable. The rates of biomass production and NH4-N removal were 55 +/- 30 mg TSS L-1 day(-1) and 13 +/- 3 mg NH4-N L-1 day(-1) respectively. CO2 adjustment had a positive effect on microalgal growth and NH4-N removal. CONCLUSION Data confirm the ability of the microalgal-bacterial consortium to grow on undiluted and untreated piggery wastewater under semi-continuous conditions. Synergy between algae and bacteria seems positive since photosynthesis produces the oxygen needed for ammonia oxidation. (c) 2019 Society of Chemical Industry