Enhanced sediment flow in inclined settlers via surface modification or applied vibration for harvesting microalgae

Inclined settlers may be used to improve algal-harvesting economics by enhancing the clarification rate for a fixed sedimentation velocity and areal footprint. For a fixed plate area, smaller inclination angles from horizontal result in increased settling area and clarification rate. To ensure flow of the settled-cell sediment for continuous operation, standard designs utilize a minimum angle of 55° from horizontal. If reliable, shallow-angle flow of sedimented algae is achieved, increased clarification rates may be realized (e.g., 63% enhancement at 20° versus 55° from horizontal). To study the critical sediment-flow angle, we use an angle-of-repose model, which includes an additional normal-force term to account for adhesion. The inclusion of this term is supported by observations that the critical sliding angle decreases with the sediment mass/thickness. To reduce the critical sliding angle, we evaluated the sediment flow of settled, green alga Chlorella vulgaris on several low-adhesion surfaces described in the biofouling literature and demonstrated sediment flow at angles as low as 10° using a negatively-charged surface. This surface fouled after immersion for 24 h in spent culture medium, presumably due to the deposition of residual organic matter. Additionally, applied vibration is demonstrated to achieve sediment flow at angles as shallow as 20°.