Harvesting of Hartbeespoort Dam micro-algal biomass through sand filtration and solar drying

Abstract

Renewable energy sources such as biomass are becoming more and more important as alternative to fossil fuels. One of the most exciting new sources of biomass is microalgae. One of the major obstacles in the commercial production of microalgae as feedstock for biomass-to-liquid fuels, is the development of energy efficient and cost effective harvesting methods for the separation of micro-algal biomass from its growth medium. In this study, a promising method of harvesting micro-algal biomass from the Hartbeespoort Dam through a combination of sand filtration and solar drying was investigated, which could be used to increase the energy efficiency and cost effectiveness of an integrated biomass-to-liquids process. Micro-algal biomass was collected from the Hartbeespoort Dam and the wet biomass was allowed to separate from the aqueous phase for 24h through its natural buoyancy. The bottom aqueous layer was drained and the top green layer of wet biomass was poured onto metal palettes containing buildings sand and left in the sun to dry for 24h. An average dry weight of 7.6g of dried micro-algal biomass from the Hartbeespoort Dam was harvested after one day of sun-drying on a patch of 0.0484m2 or 497.7g of building sand. An average, annualized, volumetric harvesting yield of 4.6kgL−1a−1 of dry weight micro-algal biomass was achieved per liter of Hartbeespoort Dam pulp and an average, annualized, aerial harvesting yield of 47.3kgm−2a−1 of dry weight micro-algal biomass was achieved per square meter of drying area. Micro-algal biomass from the Hartbeespoort Dam was successfully harvested by sun-drying on building sand. The building sand substrate improves the separation of water from the wet micro-algal biomass. As water is absorbed into the sand, it increases the drying area and thus increases the drying rate of the micro-algal biomass. Solar radiation provides the energy to evaporate the moisture. Thermo-chemical liquefaction is one of the preferred methods to extract bio-oils from microalgae, but is very energy-intensive. After extraction of bio-oils, micro-algal biomass rests could be sand-filtered, sun-dried and combusted to provide heating for the liquefaction section. Sand filtration and solar drying has the potential to produce 9938GJha−1a−1 of renewable energy which could be used to offset the energy requirements of an integrated biomass-to-liquids process. Harvesting costs could also be reduced from 20% to 30% of the total cost of biomass-to-liquids production to 18–19% by utilizing sand filtration and solar drying.