Recovery of nitrogen & phosphorus from microalgae by subcritical water

Abstract

Sustainable energy supply and its security are the prime concern for the establishment of the low carbon society. Microalgal biomass produced by mass cultivation has a high potential as a renewable and sustainable energy resource that is a tangible alternative to fossil energy resource. Since a microalga fixes atmospheric carbon dioxide as organic matters such as lipids and carbohydrates as a result of photosynthesis, microalgal lipids are considered to be a carbon-neutral feedstock for fuel production. Number of previous studies have shown that microalgae can be utilized as the source of energy as well as of biologically active substances, supplements, food and feed. However, only a limited number of studies have been conducted on utilization of microalgal biomass itself, after extraction of useful substances, as the nutrients source for its mass cultivation, although it is necessary to establish an economically feasible process to recycle nutrients to the mass cultivation system of the microalga. Cost of nitrogen and phosphorus would be a substantial part of the running cost of such mass cultivation systems without a nutrients recycling process. The results from this experiments of the sub-critical water (SCW) process, which has been carried out at relatively moderate temperature conditions of up to 250 °C and under the saturation steam pressure, is found as an unique process that can possibly recover both lipids as the feedstock and water soluble nutrients, N and P, necessary for the mass-cultivation of microalga simultaneously in a single process. Further, the experiments were carried out to optimize the SCW operational conditions to maximize lipids and nutrients recovery from microalgal biomass. The preliminary experimental data imply that effective and energy-saving extraction of the both classes of products is possible by applying the SCW technology to the microalgal biomass.