Optimization and operation of a demonstrative full scale microalgae harvesting unit based on coagulation, flocculation and sedimentation

Abstract

Biomass harvesting is a bottleneck in microalgae culture and microalgae-based wastewater treatment systems. Sedimentation coupled to coagulation and flocculation has been experimentally studied for a long time, however, full scale sedimentation experiences have not been addressed so far in literature, and scaling up from experimental results may not be straightforward. The aim of this study was to evaluate the performance of a harvesting unit in a demonstration scale microalgae biorefinery for wastewater treatment and resource recovery. The unit consisted of coagulation, flocculation and sedimentation within a lamella settler with a volume of 0.7 m(3). A high correlation between the influent turbidity and the coagulant dose could be established, so that the coagulant dose could be determined daily just measuring the influent turbidity without performing any jar-test. Suspended solids concentration in the clarified water was always <26 mg.L-1 and the turbidity <8 NTU. The volume of harvested biomass ranged between 0.8 and 2.5% of the influent mixed liquor volume. This harvested volume was also well correlated with results of standard V-30 tests of biomass from the flocculation zone, which was used as a test for harvesting control. Suspended solids concentration in the harvested biomass ranged from 5 to 20 g.L-1. Additionally, a pulse harvesting strategy (with sudden discharges at high flow rate) was used to avoid biomass accumulation inside the settling tank. The present study demonstrates how a full scale coagulation, flocculation and sedimentation process for microalgae separation and concentration can be operated by means of very few simple tools.