Electricity consumption and raw materials usage were the major running costs resulting from biomass production. Electricity consumption was of particular importance in this analysis as this contributes directly to carbon emissions. All car­bon dioxide consumed by the system was assumed to be supplied free of charge through flue gas from the nearby power station. The analysis of electricity con­sumption centred on the pumping and mixing of fluids in each of the production stages, and the electricity consumed by the centrifuge or filtration equipment in dewatering the culture.

In the cultivation stage, the electricity consumed in pumping carbon dioxide and water throughout the system was estimated using electricity consumption data from

Sazdanoff [30]. Sazdanoff’s consumption data were scaled-up volumetrically to meet the requirements outlined in Table 1. Also considered in the cultivation section was the electricity consumed in mixing the algal culture: by airlift pump in the reactor-style systems or paddle wheel in the raceway ponds. Electricity consumed by the airlift pumps and the paddle wheels was estimated using data from previous studies by Acien Fernandez [1] and Sazdanoff [30], respectively. The only major raw material considered in the cultivation section was the cost of culture medium, where unit costs were based on Molina Grima et al. [21] and the quantity required was developed using information provided in Danquah et al. [8].

All pumping to the dewatering unit operation were assumed to be associated with the cultivation section, thus the only running costs involved in a single-stage dewatering process was the operation of the different dewatering systems. The energy consumption of the single-stage dewatering options was estimated primarily using data provided in Molina Grima et al. [21] . For the dual-stage process, other running costs such as the flocculant and the mixing of algal broth in the flocculation tanks were also considered. Chitosan was the preferred flocculant, with costs esti­mated at US $11/kg [9]. The electricity consumption in the mixing during flocculation was determined using the data provided in Sinnott [32] .

A number of materials and solvents were required to extract saponifiable lipids from the dewatered biomass including ethanol, hexane and water. The quantities of the raw materials required were based on work by Ramirez Fajardo et al. ] 27], whereas costs were based on Molina Grima et al. [21].