Performance evaluation of a two-phase anaerobic reactor coupled with microalgae photobioreactors for slaughterhouse wastewater treatment in Ethiopia

Wastewater from the slaughterhouse industry is characterized by high levels of organic matter and nutrients. Due to this, it cannot be handled by a single or standalone treatment technique such as anaerobic digestion, as it does not provide effluent pollutant (organic matter and nutrients) concentrations that meet the desired discharge limits. The objective of this study was to develop an integrated biological wastewater treatment system that would take slaughterhouse wastewater, produce biogas in an anaerobic digestion unit by removing major organic matter, and remove most of the nutrients and residual organic matter in a microalgae photobioreactor. The integrated bench-scale treatment system indicated perceptible performance, with overall removal efficiencies of 99% of TCOD, SCOD, and BOD, 93–99% of TDS, 99% of TN, 98–99% of NH₄⁺-N and PO₄⁻³, 90–95% of NO₃⁻ and TP, and 97–98% of SO₄⁻². In addition, 189.5 mL/day of biogas and 128.4 mL/day of methane, with an average composition of 67.69%. Furthermore, from the Chlorella species, Scenedesmus species, and their co-cultures’ crude lipid, the biodiesel content of 72–79%, 67–78%, and 71–81%, respectively were obtained. The final effluent of the photobioreactor meets the minimum acceptable national environmental quality standards of Ethiopia. In general, from the lipid composition analysis of microalgae biomass in this study, all three (Chlorella sp., Scenedesmus sp., and co-cultures) strains cultivated in two-phase AD effluent can be noted as suitable feedstock for biodiesel.