Performance of mixed LED light wavelengths on nutrient removal and biogas upgrading by different microalgal-based treatment technologies

Micro-algal treatment technologies are an efficient and effective method for simultaneous biogas slurry nutrient removal and biogas upgrade. Three treatment technologies were investigated, utilizing different selected strains (mono-cultivation of microalgae, co-cultivation of microalgae with fungi, and co-cultivation of microalgae with activated sludge). Strains were cultivated by using biogas slurry as a nutrient medium in a photo-bio-reactor at various mixed LED treatments. A red: blue LED ratio of 7:3 and 5:5 was considered optimum for co-cultivation of microalgae with either fungi or activated sludge, to support COD and TP removal, with 65.57%–74.29% and 70.83%–76.69% removed, respectively. The system with co-cultivated microalgae and activated sludge, demonstrated high N removal efficiency at a red: blue LED ratio of 5: 5. The most effective light mixture ratios for biogas upgrade were found to be red: blue = 7: 3 and 5: 5, with the highest performance in the co-cultivated microalgae with fungi system, while the optimum red: blue ratio for both biogas slurry nutrient removal and biogas upgrade was 5:5 for the co-cultivation of microalgae with fungi system. The optimal cultivation times for mono-cultivated microalgae, co-cultivated microalgae with fungi and co-cultivated microalgae with activated sludge, were 7 d, 9 d and 8 d, respectively. These results show the high potential for combined strain systems in nutrient removal and biogas upgrading, with algal-fungal symbiotic systems showing significant promise.