WATER REQUIREMENT AND ACIDIFICATION POTENTIAL OF ALGAL BIOFUEL PRODUCTION

Abstract

Microalgae are currently considered a potential feedstock for the production of biofuels. It emulates other bioenergy crops in terms of growth rate, biofuel yields, higher energy content, no competition with food supply and ability to grow in wastewater. However, water consumption in algal biofuel production is a concerned area. S Algae are generally cultivated in open ponds where a lot of water is lost due to evaporation. Beside this, algae organisms have about 80-85% water content, which is lost entirely or fractionally in subsequent steps of heat drying. Both these water demand (evaporation and water lost due to heat drying) constitute the direct water requirement for biofuel production. At the same time electricity is required at various stages of production like for circulating nutrients in the pond, harvesting, pumping water, drying etc. Water is consumed in the required electricity generation depending upon the source of electricity and the technology employed. This constitutes the indirect water demand. It was also observed that one could not recycle water endlessly from the cultivation ponds. Heavy metals build up in the cultivation pond and hinder the growth of algal biomass. Among the heavy metals Cu was the most toxic to the algal cells. In this study the direct water demand for 98.75% water recycling was estimated to be 3338 m3 per ton of dry algal biomass in a year when Cu was the limiting toxic metal. This value was reduced to 1485 m3 when Zn was the limiting metal. The variation of direct water requirement with various input parameters such as density of algae in pond (d), generation time (t) and number of cycles (C) was studied. Water requirement was found to be most sensitive for density and number of cycles, which in turn depends upon the toxicity limit of heavy metal and its concentration in cultivation pond. - For the indirect water requirement water consumption in five hydroelectric power plants and eight coal thermal power plants of India was estimated collecting relevant data. Average indirect water requirement for providing 1579 kWh (considered electricity requirement when diluted effluent from anaerobic digester of sludge) was computed to be 324 m3 per ton of dry algal biomass. Acidification potential for algal biofuel production was estimated by considering the acidifying gases released by the thermal power plants of India for providing 1105 kWh (considering 70% of required electricity to be provided by thermal power plants) and was estimated to be 9.24 kg SO2 eq.