Comparative performance of Scenedesmus Sp. cultivated in sewage wastewater in 1L bench top vessels and A 30L reactor for algal growth and biofuel production

Abstract

Scaling up algal cultivation from bench-scale to pilot reactors is necessary for advancing biofuel production. Various operational parameters such as nutrient availability, light illumination, pH, aeration and several others are interdependent and influence algal growth; as well as the hydrodynamics of the growth environment such as the height and diameter of the vessel. This study compares the batch cultivation performance of Scenedesmus sp. in sewage wastewater using 1L bench-top vessels and a 30 L pilot-scale bubble column reactor, focusing on biomass productivity, growth rates and energy efficiency. Cultivation was conducted at pH of 8, with an 18-hour light: 6-hour dark cycle and a 2:2 Bold’s Basal Medium (BBM) to sewage wastewater ratio, between 10 cm and 30 cm diameter reactor vessels. Ultraviolet (UV) sterilization of wastewater was performed to eliminate microbial competition within the wastewater source. The aim is to analyse reactor hydrodynamics, the scalability of biofuel production and operational challenges encountered during scale-up. Necessary algal concentrations and inoculation volumes were established during the upscaling process. The 1 L reactor achieved an average biomass productivity of 64 × 104 cells/mL/day, and a growth rate of 0.79 day⁻ ¹ under optimized conditions. In contrast, the 30 L reactor achieved a lower biomass productivity of 46.89 × 104 cells/mL/day and a growth rate of 0.577 day⁻ ¹, primarily due to inefficiencies in flow, culture depth, sheer stress, light intensities and other necessitating factors. However, the 30 L reactor demonstrated potential for higher biomass yield when adjustments to aeration and distance of light source were applied, with a projected improvement of 29 % in biomass recovery following system optimization. Additionally, CO2 injections (3 %) at regular intervals maintained optimal pH concentration. The study highlights that hydrodynamics is crucial, with the 1 L benefiting from uniform mixing, while the 30 L reactor experienced localised shear forces that negatively impacted algal growth. Nonetheless, the 30 L vessel demonstrated higher biomass yield potential due to its ability to accommodate larger volumes. This research provides insights for optimizing algal cultivation at larger scales, contributing to the development of scalable and energy-efficient biofuel production processes.