Harvesting of the algal biomass is one of the greatest energy consumers in the process chain for algal biofuel production. Many options exist for extracting the algae with each having their own advantages and disadvan­tages. Low energy harvesting is favoured but options are limited by cell sizes. If the algae being harvested are of a large 70 pm [61] the algae can be filtered. Ideally, if the conditions allow, gravity filtration is possible and this requires very little energy input. This would be the optimal solu­tion economically and environmentally due to low energy requirements. Alternatively, should cell size allow, the biomass can be pressure filtered requiring slightly more energy but providing a higher removal efficiency. In most cases it is likely that flocculation would be required to allow the biomass to settle or float more readily. Conventional flocculants appear favourable in terms of harvesting yield yet may cause issues downstream due to contamination. Ideally if flocculation is necessary, bio-flocculation could be carried out using bacteria or other algae strains to obtain flocs, but further research is required to fully understand in which conditions this is possible. Alternatively organic flocculants such as chitosan could provide a more sustainable option but, again, efficiency of biomass re­moval using chitosan requires further study. It is likely that if flocculation is necessary, a combination of flocculants would be required for the most sustainable solution. Following flocculation, sedimentation or flotation of the biomass should be a successful method of harvesting without signifi­cant energy input.

Centrifugation of algal biomass could be necessary if biomass of high solids content is required. Due to the energy requirement, centrifugation should, where possible, be avoided but due to the high and rapid recovery it could provide a necessary step. The least energy-intense processes for harvesting algae are sedimentation/flotation and gravity filtration as there is little energetic input. Ideally biomass would be filtered or settled using sedimentation due to low inputs however with the majority of strains this may not be practical without prior treatment. Flocculation with the least intensive and damaging flocculants should be used if necessary and cen­trifugation a last option for dewatering.