Illuminated septic tank – A microalgae assisted onsite wastewater treatment system

To reduce the anthropogenic nutrient loading into natural waterways, onsite wastewater treatment is essential for households without access to municipal sewer systems. However, many conventional septic tanks significantly underperform the centralized wastewater treatment. In the present study, a novel illuminated septic tank inoculated with the consortium of microalga Chlorella vulgaris and aerobic bacteria was developed by modifying the conventional configuration. The inclusion of microalgae facilitated aeration inside the septic tank through photosynthetic oxygen evolution, and the microalgae-bacteria consortium enhanced the wastewater treatment performance. With the operation at 15 ℃ and a hydraulic retention time of 2.5 days, the lab-scale algae-assisted septic tank significantly decreased the concentrations of ammonium-nitrogen (NH₄ ⁺-N), soluble phosphorus (P), total P, and total chemical oxygen demand by 54%, 65.2%, 78.5%, and 90.7% in the discharged effluent, respectively (p < 0.05), whereas the conventional septic tank without microalgae did not exhibit any treatment of NH₄ ⁺-N or soluble P and the total P removal efficiency was lower by 50%. Meanwhile, the photosynthetic activity of microalgae altered the gas composition in the tank headspace by consuming carbon dioxide, emitting oxygen, and inhibiting the evolution of methane, which further benefited the aeration requirement and contributed to the mitigation of greenhouse gas emissions from septic systems.