Evaluation of daily and seasonal variations in a semi-closed photobioreactor for microalgae-based bioremediation of agricultural runoff at full-scale

Abstract Microalgae-based treatment systems are a suitable solution for agricultural wastewater treatment in order to tackle eutrophication. Valuable biomass can be produced in photobioreactors while removing nutrients from agricultural runoff. However, little is known about experiences of microalgae systems for bioremediation of agricultural wastewater. This study evaluated for one year the performance of a full-scale semi-closed photobioreactor treating agricultural runoff and partially treated domestic wastewater, located in a coastal area in Barcelona (Spain, 41.288 N and 2.043 E, UTM), characterized by Mediterranean climate. The evaluation of the daily variations highlighted specific features of the photobioreactor, such as the cooling capacity of the lateral open tanks, which avoided the extremely high temperatures that would be reached in closed systems, eventually reporting daily variations of temperature similar to those in open systems. The open tanks also allowed for oxygen release, maintaining the dissolved oxygen concentration below inhibiting levels. In turn, the paddle wheels were able to aerate the mixed liquor during night when the dissolved oxygen concentration was lower than the saturation value. Regarding the dynamics of nitrogen removal, ammonium was rapidly removed after feeding. However, nitrate concentration was reduced after feeding but not completely removed in winter. Regarding orthophosphate, its concentration was practically negligible during the whole experimentation due to the removal in the photobioreactor and the low influent concentration. It can be concluded that during the warm seasons, the nutrients removal capability of the system was underused, and in spite of the satisfactory biomass concentration, biomass growth was likely limited by the low concentration of nutrients in the photobioreactor during most part of the day. On the contrary, biomass growth in winter was limited by phosphorus and the low water temperature. The insight gained in this study may aid to achieve more accurate operational designs of semi-closed photobioreactors for agricultural runoff bioremediation.