The process for conversion of the algal feedstock into biofuel dictates the type or types of harvesting methods that can be deployed. For instance, if the value proposition requires the use of residual biomass for animal feeds or valuable co­products, many of the simple settling, flocculation, and flotation methods may not be applicable since they require long time periods and addition of chemicals that could be detrimental to the biofuel and/or co-products.

The different techniques have different costs that must be absorbed by the final product portfolio (biofuel plus associated co-products). The overall cost hinges on the application (i. e., the process deployed by the company producing the biofuel). An in-depth study on harvesting technologies was conducted by the National Alliance for Advanced Biofuels and Bioproducts (NAABB) in 2011. Estimates for capital and operating costs associated with different harvesting techniques are provided in Table 14.2. These numbers were derived either from the NAABB report, literature, or by an engineering analysis and estimate by the authors.

Adjustments were made from the NAABB data to reflect energy and chemical costs on a per volume basis, assuming 0.5 g L 1 dry biomass concentration, which is a reasonable average for phototrophic cultures. In practice, costs for some methods, such as centrifugation, are impacted primarily by volume. Others, such as the chemicals required for chemical harvesting, are dependent on the mass of solids

in the stream. In this way, performance of the growth systems influences the types of harvesting systems that could be desired for the application.

Capital costs were scaled from the NAABB report to 100 m3 h 1 throughput using the 6/10ths rule of thumb for scaling capital costs and are presented as a ±40 % range. A caveat should be noted, many of these technologies are not yet at industrial scale, meaning that the actual industrialized costs could be significantly different than these projections. Particularly, the assumptions for chitosan floccu­lation and electrolytic harvesting stand out as requiring additional data to determine a more accurate scaled-up cost estimate.

Some OpEx components, such as labor and maintenance, were not included in the calculation. Electricity was assumed to cost $0.08/kWh. Hollow fiber members were estimated at a cost of $250/m2 with a 3-year life span, and the nickel alloy membranes were taken at $50/m2 with a 2-year life span. The details of the assumptions for the chemical dosing, cost of chemicals, and supporting data can be found in the NAABB final report (NAABB 2014).

Algal biofuel and bioproduct companies are currently pre-commercial or just recently in commercial-scale production of products based on algae and relatively closed with production data. While it is difficult to ascertain the exact process many of these companies are using for their harvesting technology, one can speculate from their Web sites, patent applications, and discussions with colleagues the probable harvesting methods being applied. Aurora Algae, Inc. (West Perth, Aus­tralia) is growing algae in open ponds photoautotrophically and harvesting using a DAF system supplied with flocculant (http://www. aurorainc. com/technology/ facilities/karratha-facility/). Looking at the Sapphire Energy Green Crude Farm information, it appears they use a DAF system in their process (http://www. sapphireenergy. com/GCFvideo). Heliae (Arizona, USA) uses a spiral plate centri­fuge system from Evodos for their harvesting for the production of astaxanthin (http://www. heliae. com/technology/?page=harvesting). Cellana (Hawaii, USA) uses selection of algae that settle rapidly combined with continuous centrifugation for their harvesting, and they have shifted away from biofuels to higher value products (http://cellana. com/technology/core-technology/). While Cyanotech (Hawaii, USA) is not in biofuels, they have developed a very efficient system for continuous centrifugation and washing over a moving screen in production of Arthrospira. Bioprocessalgae Inc. (Iowa, USA) grows their algae in enclosed systems and biofilms in order to harvest algae by washing down the biofilms and settling (http://www. bioprocessalgae. com/). Algenol (Florida, USA) has developed one of the systems that mostly bypasses the harvest of biomass and, instead, focuses on recovery of product (ethanol) through condensation on the bioreactor cover and processing of the water/ethanol product in their “direct-to-ethanol” process (http:// www. algenolbiofuels. com/direct-to-ethanol/direct-to-ethanol). Joule Unlimited (Massachusetts, USA) is a bit hard to decipher but appears to produce biofuels (e. g., ethanol) and bioproducts in enclosed PBRs and to collect the products directly rather than extraction from biomass (http://www. jouleunlimited. com/).