Diurnal and nocturnal pH control in microalgae raceway reactors by combining classical and event-based control approaches

Exploring microbial growth dynamics in a pilot-scale microalgae raceway fed with urban wastewater: Insights into the effect of operational variables

Microalgae-based wastewater treatment is a promising technology efficient for nutrient recycling and biomass production. Studies continuously optimize processes to reduce costs and increase productivity. However, changes in the operational conditions affect not only biomass productivity but the dynamics of the overall microbial community. This study characterizes a microalgae culture from an 80 m2 pilot-scale raceway reactor fed with untreated urban wastewater. Operational conditions such as pH, dissolved oxygen control strategies (On-off, PI, Event-based, no control), and culture height were varied to assess microbial population changes. Results demonstrate that increased culture height significantly promotes higher microalgal and bacterial diversity. pH, dissolved oxygen and culture height highly affects nitrifying bacteria activity and nitrogen accumulation. Furthermore, the system exhibited high disinfection capability with average Logarithmic Reduction Values (LRV) of 3.36 for E. coli and 2.57 for Clostridium perfringens. Finally, the fungi species detected included Chytridiomycota and Ascomycota, while purple photosynthetic bacteria were also found in significant abundance within the medium.

Influence of pH and dissolved oxygen control strategies on the performance of pilot-scale microalgae raceways using fertilizer or wastewater as the nutrient source

Dissolved oxygen concentration and pH are controllable and cost-effective variables that determine the success of microalgae-related processes. The present study compares different control strategies for pH and dissolved oxygen in pilot-scale microalgae production systems. Two 80 m2 raceway reactors were used, one operated with freshwater plus fertilizer and the other with wastewater as the nutrient source. Both were in semi-continuous mode at a fixed dilution rate of 0.2 day-1. A comparison between the classical On-Off and more advanced pH control strategies, such as PI and Event-based control, was performed, focusing on biomass productivity and the influence of all the process parameters on microalgae growth; "No control" of pH was also assayed. The results show that Event-based control was the best algorithm when using freshwater plus fertilizer. In contrast, no significant differences were observed using the different control strategies when wastewater was the nutrient source. These experiments were performed through selective control strategy, prioritizing pH over dissolved oxygen; however, it was demonstrated that they did not allow to achieve satisfactory dissolved oxygen removal results, especially for the fertilizer system. After modifying the gas diffuser configuration and improving the mass transfer, independent on-off strategies have been developed, permitting effective control of both variables and increasing productivity by up to 20% in both systems. Concluding, a detailed analysis of the energy demand for each strategy implemented in terms of gas consumption and gas flow to biomass ratio is provided.