An experimental investigation of microalgal dewatering efficiency of a belt filter system

Abstract

Profitable large-scale production of biofuel from microalgae has not yet been demonstrated. A major bottleneck is high operational cost of microalgal harvesting. This is due to small cell size and dilute microalgal suspension. A belt filter system is preferred over other dewatering technologies as it has lower energy consumption. However, a microalgal feed concentration of 10 - 40 g dry wt. /L is required prior to dewatering on a belt filter system. The objective of this study was to investigate the microalgal dewatering efficiency of a belt filter system. A prototype belt filtration system designed for feed concentration of 50 g dry wt. /L was used for this investigation. A mixed laboratory culture of freshwater species dominated by three eukaryotic green microalgae (Chlorella vulgaris, Scenedesmus sp., and Kirchneriella sp.) was cultivated in wastewater effluent. Bench-scale gravity filtration tests were conducted to determine the filtration belt mesh needed for the prototype system. Based on the test results a 70 micron mesh size resulted in the highest microalgal recovery rate and was subsequently used for all dewatering tests conducted in this study. Belt dewatering tests conducted on untreated microalgal suspensions - pond water at the KU Field Station and stationary growth phase samples from the microalgal lab culture - resulted in negligible recovery. The highest concentration of microalgal suspension available for testing on the prototype belt filtration system was 6 g dry wt. /L obtained from biomass settling tanks at the Lawrence, Kansas domestic wastewater treatment plant that resulted in 84% biomass recovery. To further investigate this, 54 Liters of 4 g dry wt. /L were produced from bench-scale flocculation using an alum dosage of 200 mg/L at pre-test pH value of 6.5. Results of belt dewatering tests indicated that the percent of microalgae recovered for 4 g dry wt. /L suspension, 46%, was significantly lower than 6 g dry wt. /L suspension. Sealed filter section would likely improve the microalgal recovery (subsequently reducing the number of filtration passes required for maximum microalgal recovery).