Photoacclimation caused by high frequency flashing light assists Chlorella sp. M-12 wastewater treatment and biomass accumulation in dark color biogas slurry

Using microalgae to treat biogas slurry can realize wastewater treatment and nutrients recovery. However, the dark color of biogas slurry causes serious light attenuation. Flashing light was used to solve light attenuation, while improving biomass production and pollutant removal. Higher frequency (f) and lower duty cycle (φ) flashing light conditions increased biomass concentration and pollutant removal. With 100 Hz-0.3 φ-60 μmol photons m⁻² s⁻¹, the biomass concentration, biomass productivity and NH₄⁺-N removal reached the highest values of 255 mg L⁻¹, 8.45 mg L⁻¹ day⁻¹ and 55.2%, which were 35.8%, 33.6% and 44.2% higher than those in continuous light (100 Hz-1.0 φ-60 μmol photons m⁻² s⁻¹), respectively. With 100 Hz-0.3 φ-60 μmol photons m⁻² s⁻¹, the total chlorophyll content increased by 25.4% and the Chl a/b ratio decreased by 35.1% compared with that in continuous light at the fourth day, which showed stronger photoacclimation. The photoacclimation and the higher luminous intensity caused the enhancement of biomass concentration in dark color biogas slurry, and f had more effect than φ. In addition, the same quantity of biomass produced under the condition of 100 Hz-0.3 φ-60 μmol photons m⁻² s⁻¹ decreased 25.1% light energy consumption compared with continuous light. Flashing light could be used to solve the light attenuation effect caused by dark color biogas slurry. It provided a special method for light provision when using microalgae to treat dark color biogas slurry.