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 degrees from horizontal. If reliable, shallow-angle flow of sedimented algae is achieved, increased clarification rates may be realized (e. g., 63% enhancement at 20 degrees versus 55 degrees 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 inverted perpendicular 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 degrees. (C) 2013 Elsevier B.V. All rights reserved.