Microalgae Oil Production Using Wastewater in Japan—Introducing Operational Cost Function for Sustainable Management of WWTP

Abstract

A comparative evaluation of economic efficiency was performed for native polyculture microalgae oil production in an oxidation ditch (OD) process wastewater treatment plant (WWTP). A cost function was developed for the process. The operational cost per 1 m3 of wastewater (w.w.) was 1.34 $/m3-w.w. in the existing scenario, 1.29 $/m3-w.w. in algal scenario A (no cost for CO2 and waste heat) and 1.36 $/m3-w.w. in algal scenario B (no cost for CO2). The conditions were set as follows: hydraulic retention time (HRT): 4 days, microalgal productivity: 0.148 g/L and daily treatment volume: 81.6 m3-w.w./d. The cost differences were related to the increase in polymer flocculants for algae separation (+0.23 $/m3-w.w), carbon credits from CO2 absorption (−0.01 $/m3-w.w), the sales of biocrude (−0.04 $/m3-w.w) and sludge disposal (−0.18 $/m3-w.w). Hence, the introduction of the algae scenario was the same cost-effective as the existing scenario. Microalgae oil production in an OD process WWTP can serve as a new energy system and reduce the environmental load in a society with a declining population.

Biocrude Oil Production by Integrating Microalgae Polyculture and Wastewater Treatment: Novel Proposal on the Use of Deep Water-Depth Polyculture of Mixotrophic Microalgae

“…A WTW-LCA that analyzed the impact of algal biodiversity on biofuel production showed that certain polycultures improved GHG and EROI by 16% and 20%, respectively, compared to the best monoculture [75]. It has also been suggested that in a 0.8 m depth polyculture of microalgae, based on 86% heat recovery in the HTL process previously reported, WTG-LCA of combining algae biomass/biocrude-oil production and wastewater treatment reduced the CO 2 emissions of the conventional wastewater treatment system by 33% [8]. Thus, the benefits for reduced CO 2 emission and increased net energy-gain are clearly demonstrated for the proposed biocrude oil production by HTL employing the polyculture system for wastewater treatment.…”

“…This model suggests that algae fuel production is economically feasible combined with carbon credits that promote the realization of a low-carbon society. Ishizaki et al [8] suggested that a 0.8-m depth polyculture-based wastewater treatment had the same cost-effectiveness as the conventional wastewater treatment, even though the resulting biocrude oil was sold at the same price as fossil crude oil. Thus, further reduction of MFST can be expected by introducing a combination of algae and sewage treatment to the NERL model.…”

“…In contrast, the total area of sewage treatment plants in Japan is 1.37 million ha, and it is necessary to increase the algae biomass productivity per land area to achieve the Japanese demand for crude oil by using the existing footprint of sewage treatment plants. Additionally, microalgae have some issues for the future commercial application in reference to requirements (8), (9), (11), and (12), such as a lower energy return on investment (EROI) and higher biomass production cost than land plants, the necessity for a large land area, and vulnerability to environmental and biological stresses, which should be solved for future use as an alternative to fossil fuel. In particular, the traditional lipid extraction-transesterification method for producing biofuel from lipid rich microalgae is energy intensive because it requires the drying of cultured microalgae.…”

“…However, the differences of biochemical composition combined with various optimal conditions make general prediction and operation of HTL difficult [4]. Additionally, developments on conventional HTL processes to increase EROI, lower capital and process costs, and accelerate CO 2 reduction through technology innovation are needed [4][5][6][7][8]. The development of innovative HTL technology to solve these issues is steadily progressing [4,9,10].…”