The choice of a good harvesting method for algae is crucial to the efficiency of the entire process in terms of capital investments as well as operation costs. The key factors for comparison include high cake dryness in the separated algae and low specific energy demand during the process, that is, energy demand per unit mass of algae harvested.

In a single-stage harvesting process using disk stack centrifuges, the algae — water suspension is directly fed into the centrifuge. Inside the centrifuge the suspension is separated into a mostly clear water phase and an algae concentrate. The algae concentrate is drawn out periodically and has a fluid/ creamy consistency. The whole suspension has to be put into rotation to cre­ate a centrifugal force up to 10,000 g’s, thus the specific energy demand is relatively high. Therefore, this single-stage harvesting process is especially suitable for small and middle-sized facilities.

Disk stack centrifuges have been successfully operated for separation of two different liquid phases and solids from each other in a continuous process [28]. The operational principle of a disk stack centrifuge is described below. A good example of separation of two different liquid phases is separation of biodiesel methyl ester and by-product glycerin, whereas a good example of solid-liquid separation is dewatering algae from an algae-water suspen­sion. Alfa Laval offers a variety of sizes and types of disk stack centrifuges for such industrial applications [29]. In a disk stack centrifuge used for liq­uid-solid separation, the denser solids are pushed outward by centrifugal forces against the rotating bowl wall, and the less dense liquid phases form inner concentric layers. By inserting specially designed disk stacks where the liquid phases meet, a very high separation efficiency is achieved. The solids such as algae cakes can be removed manually, intermittently, or fully continuously, depending upon the specific process design and application. The separated liquid phase overflows in the outlet area on top of the bowl into recovery vessels, which are sealed off from each other to prevent poten­tial cross-contamination [29].

The Flottweg enalgy process [30] is a two-stage algae harvesting process consisting of (1) preconcentration via static settling, filtration, flocculation, or dissolved air flotation (DAF), and (2) bulk harvesting using the Flottweg Sedicanter® in order to dewater the algae suspension to concentrate. In con­trast to the aforementioned single-stage process, only a small, predewatered, part of the algae suspension is separated by centrifugation in this two-stage process, thus reducing the energy demand drastically. Whereas the pre­concentrator provides a clear water phase as an initial step, the Flottweg Sedicanter dewaters the algae concentrate to obtain a solids cake with 22-25% dry substance [30]. A schematic of the Flottweg two-stage harvesting process is shown in Figure 2.3.