Outdoor pilot trial integrating a sidestream microalgae process for the treatment of centrate under non optimal climate conditions

This study was carried out to assess the efficiency of a pilot-scale bubble-column reactor to remove nitrogen in centrate from the biosolid dewatering of a municipal wastewater treatment plant whilst producing biomass for agricultural purposes. The column was inoculated with a mixed community of Scenedesmus and Chlorella spp. and operated outdoor in batch for 55 days and in continuous for further 130 days. In continuous, the average daily biomass productivity was 40 +/- 62 mg TSS L-1 d(-1) and the average NH4+-N removal was 20 +/- 10 mg L-1 d(-1). Nitrification was fostered by photo-oxygenation leading to the oxidation of 34 +/- 27% of the incoming ammonia nitrogen. Microalgal and bacterial activity inside the column was analyzed by the Generalized Linear Models in order to understand the main factors affecting the process performances. Microalgal growth was affected positively by the NH4+-N content in the influent and negatively by the amount of TSS entering the system, probably due to the competition between microalgae and bacteria for phosphorus and other nutrients. The removal rate of NH4+-N was positively affected by NH4+-Nin (influent concentration) and by pH, whose increase fosters stripping, and decreased for increasing NH3-N concentrations, responsible for inhibiting nitrifying bacteria. NH4+-N oxidation was the result of complex interactions between algae and bacteria and was also affected by flow and solar radiation. No other specific limiting factors have been highlighted. The possibility of improving the process performance by controlling pH, by supplying off-gas as CO2 additional source, appears as an interesting option. In view of a scale-up, the most relevant expected result would be the energy saving due to the decrease in the oxygen demand for nitrification in the water line. The microalgal biomass grown on centrate was suitable for agricultural use due to its low contamination by heavy metals.