Microfiltration employs the use of filtration media with <1 pm pores and a pressure differential created by flow to dewater biomass slurry. Various conformations of filtration media that are made from various materials are available at industrial scale, including hollow fibers, plates, and spiral wound. The membranes can be made of polymers (e. g., PVDF, PES, PS), ceramics, or metals. There is an established history of using microfiltration technology for the purpose of cell harvesting, as in the fermentation industry and wastewater treatment. Advantages of microfiltration are that the algal cells retain their structure, properties, and motility (Chen et al. 2011).

Microfiltration methods have been advancing rapidly since the early studies in algal harvesting. Stacked filters that are subject to blocking have been replaced by hollow fiber filters and tangential flow filters that are now being applied to dewa­tering of extremely dirty solutions, such as those in wastewater treatment plants (e. g., Koch Membrane Systems PURONPlus MBR (http://www. kochmembrane. com/Engineered-Systems/Standard/PURON-MBR. aspx). Blocking of the filter pores (membrane fouling) by algal and bacterially derived materials, due to their small cell sizes, is a chief problem of any of these microfiltration methods and is a concern for microfiltration (Bosma et al. 2003). The use of hollow fiber filters and tangential flow filters provides some scouring of the membrane surface to help reduce blockage and provide high flux rates.

Not much data are publically available on the performance of membrane fil­tration at an industrial scale for algal biofuel systems. Important performance values include flux rate (LMH or L m-2 h-1), recirculation rate (L min-1), and filter pressures (kPa). However, engineers at Phycal, Inc. performed studies at a subpilot scale (hundreds of liters) using polymeric, hollow fiber filtration membranes with

0. 2-pm pore sizes. A typical performance over an 8-h trial for the alga Chlorella vulgaris (Fig. 14.1.) demonstrated steady state flux rates around 150 LMH, and a final biomass concentration of 22 g/L. Other trials with similar performance metrics were able to reach 80 g/L, showing this membrane filtration system could provide primary and secondary harvesting for an algal biofuel production system that could tolerate significant water content, such as aqueous extraction and hydrothermal liquefaction. However, because flux rates are lower when biomass concentrations

are higher, scaled systems would have two or more filtration stages with different operating parameters, membrane pore sizes, and operating concentrations at each stage in order to optimize throughput and lower OpEx (e. g., pumping and mem­brane replacement).

Work on improved microfiltration in conjunction with algal biofuels continues with novel metallic membranes being developed, which are proposed to lower the membrane replacement costs (lower OpEx) and lower overall fouling to increase throughput (NAABB 2014).