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Operating a semi-continuous raceway pond allows to link pH and oxygen dynamics to the interaction between microalgae and bacteria

The impact of nutrient load and hydraulic residence time (HRT) on the dynamics of a microalgae/ bacteria consortia treating different kinds of urban influents (primary settled wastewater, supernatant of digestate dewatering) was investigated. A 80 L raceway pond was operated for over 8 months. Biomass productivity was 4.1 ± 0.2 g TSS/m2/d. While treating primary settled wastewater at 4 d HRT, nitrogen removal efficiencies were 80.1% ± 16.7% and 47.2% ± 24.8% for total Kjeldahl (TKN) and total nitrogen (TN), respectively. The relatively high TN removal suggests ammonia stripping since the pH value was close to 9.0. When increasing nutrient load with dewatering influent, the performances decreased. The HRT had to be increased to 8 d to recover a similar TKN removal efficiency (84.2% ± 9.8%) while TN removal remained negligible (1.0% ± 2.9%) due to a pH drop below 6.0. The time series decomposition technique was applied on dissolved oxygen (DO) and pH monitoring data to evaluate the dynamics of algal and bacterial processes: nitrification became the dominant process impacting pH and DO when the nutrient load increased. Algal photosynthesis decreased but slightly recovered when applying a higher HRT (8 d). Results from this study show potential for a fast and simple assessment of algal bacterial dynamics under different influencing factors.