Flocculation may play an important role in developing such a low-cost and high — throughput harvesting process (Brentner et al. 2011). During harvesting or dewa­tering of microalgae, a microalgal paste is produced with a dry matter content of 20 %. This implies a 400-40 times concentration from 0.5 or 5 up to 200 g dry matter L-1. Theoretically, this can be achieved in a single step using mechanical methods such as high-speed centrifuges or ultra — or micro-filtration membranes. Due to the large volumes of culture broth that need to be processed, however, the cost and energy inputs are extremely high. The energy inputs for harvesting by means of centrifugation are 14 MJ kg 1 of dry biomass, which is about 55 % of the energy content of the biomass (Norsker et al. 2011).

Several authors have suggested that low-cost harvesting of microalgae can be achieved by means of a two-stage harvesting process in which the biomass is pre­concentrated by means of flocculation prior to final dewatering (Pahl et al. 2013; Kim et al. 2013; Weschler et al. 2014). In flocculation, individual microalgal cells form larger aggregates or flocs. These flocs have much higher settling rates than individual cells and can easily be separated from the medium by means of gravity sedimentation to yield a microalgal slurry (Fig. 12.1). This slurry can be completely dewatered using a mechanical method such as centrifugation or filtration. Due to the large size of flocs compared to individual microalgal cells, the energy demand for mechanical dewatering will be much lower than for culture broth with freely sus­pended cells. Flocs have higher sedimentation rates than individual cells and can be separated from the medium with a low centrifugal force (Xu et al. 2012). When filtration is used, flocculation prior to membrane filtration results in higher mem­brane fluxes and thus a lower energy demand (Lee et al. 2012a, b).