Reduction of biogenic CO ₂ emissions, COD and nutrients in municipal wastewater via mixotrophic co-cultivation of <i>Chlorella vulgaris</i> – aerobic-activated sludge consortium

In this study, biogenic CO₂ emissions, COD and other nutrients (i.e. TP, TN and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:msubsup><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup><mml:mrow></mml:mrow><mml:mstyle><mml:mtext>-</mml:mtext></mml:mstyle><mml:mrow><mml:mi>N</mml:mi></mml:mrow></mml:math>) from aerobic treatment in municipal Wastewater Treatment Plants (WWTP) were quantified and reduced by phycoremediation using a mixotrophic co-cultivation of <i>Chlorella vulgaris</i> and activated sludge. It has been shown that the microalgae sludge consortium (A-ASR, R1) outperformed the normal-activated sludge system (ASR, R2). In fact, estimated biogenic CO₂ emissions with algae mark 1.20-fold higher removal, COD marks 1.40-fold higher removal, TP marks 1.70-fold higher removal, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:msubsup><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup><mml:mrow></mml:mrow><mml:mstyle><mml:mtext>-</mml:mtext></mml:mstyle><mml:mrow><mml:mi>N</mml:mi></mml:mrow></mml:math> marks 1.40-fold higher removal, compared to normal activated sludge (ASR, R2). Meanwhile, due to aeration, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:msubsup><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mn>3</mml:mn><mml:mtext>-</mml:mtext></mml:msubsup><mml:mrow></mml:mrow><mml:mstyle><mml:mtext>-</mml:mtext></mml:mstyle><mml:mrow><mml:mi>N</mml:mi></mml:mrow></mml:math> concentration increased in both reactors because some Ns were oxidized through nitrification. Furthermore, COD increased again during <i>C. vulgaris</i> stationary growth; thus, activated sludge addition every 4 days (optimal time) was implemented to maintain algae-bacteria balance. The results suggest that integrating the treatment of GHG emissions and water pollutants in a single, concurrent process can significantly enhance the sustainability and efficiency of wastewater treatment plants, which has not been explored comprehensively. Finally, by leveraging <i>C. vulgaris</i> capabilities for carbon and nutrients sequestration, this study can provide practical guidance for achieving carbon neutrality in a WWTP.