Digestate color and light intensity affect nutrient removal and competition phenomena in a microalgal-bacterial ecosystem

• O.D. of diluted (1/10) supernatant digestate ranged between 0.19 and 2.35 at 680 nm.
• Digestates absorb throughout the visible spectrum.
• Nitrogen removal up to 8.5 mgN-NH4+NH4+ L−1 d−1 in colored media.
• Growth of microalgae limits growth of ammonia-oxidizing bacteria.
• Chlorella is a better competitor for light than Scenedesmus.

During anaerobic digestion, nutrients are mineralized and may require post-treatment for optimum valorization. The cultivation of autotrophic microalgae using the digestate supernatant is a promising solution; however the dark color of the influent poses a serious problem. First, the color of the digestates was studied and the results obtained using three different digestates demonstrated a strong heterogeneity although their color remained rather constant over time. The digestates absorbed light over the whole visible spectrum and remained colored even after a ten-fold dilution. Secondly, the impact of light and of substrate color on the growth of Scenedesmus   sp. and on nitrogen removal were assessed. These experiments led to the construction of a model for predicting the impact of influent color and light intensity on N removal. Maximum N removal (8.5 mgN-NH4+NH4+ L−1 d−1) was observed with an initial optical density of 0.221 and 244 μmolE m−² s−1 light and the model allows to determine N removal between 15.9 and 22.7 mgN-NH4+NH4+ L−1 d−1 in real conditions according to the dilution level of the influent and related color. Changes in the microalgae community were monitored and revealed the advantage of Chlorella over Scenedesmus under light-limitation. Additionally microalgae outcompeted nitrifying bacteria and experiments showed how microalgae become better competitors for nutrients when phosphorus is limiting. Furthermore, nitrification was limited by microalgae growth, even when P was not limiting.