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BACKGROUND: The use of microalgae-bacteria systems is particularly attractive for wastewater treatment, and the generated biomass can be further used for methane production. The aim of this study was to evaluate the influence of the hydraulic retention time (HRT) on the organic matter, nutrient removal, settling properties and the biochemical methane potential using a granular microalgae-bacteria system in a high rate algal pond.

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This research investigates the effect of mixing wavelength light photoperiods (12 h blue, 8 h blue: 4 h green, 4 h blue: 8 h green, and 12 h green) and N/P ratios (1.3 to 8.3) on the growth microalgae-bacteria systems, organic matter, and nutrients removals. The highest microalgae-bacteria growth performance (μ = 0.2 d−1, 481.1 ± 15.3 mg DW L−1) was observed when 8 h blue: 4 h green mixed wavelength and the low N/P ratio were used. For both N/P ratios, the biomass productivity was favored when using the blue light dominated in time. Mechanisms of nitrogen removal by assimilation depend on the N/P ratio, achieving assimilation between 49 and 65% at a low N/P ratio. High nitrogen removals (>50%) showed a strong relation with alkalinity culture conditions (pH > 8.5). The mixing of wavelength photoperiods seems to be a promissory strategy to achieved high biomass productivity and nutrients removal. However, for the optimal conditions, the N/P ratios in the wastewater should be considered.

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