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.
Highlights
A previous study has shown that the incorporation of a microalgae oil production system into an existing wastewater treatment plant (WWTP) using flue gas and waste heat can reduce environmental burden [2]
A scenario is shown in which a microalgae oil production system is partially installed in the Awano WWTP (Figure 6)
Because photosynthesis is essential for microalgae production, the water depth needs to be reduced to allow for sunlight transmission
Summary
A previous study has shown that the incorporation of a microalgae oil production system into an existing wastewater treatment plant (WWTP) using flue gas and waste heat can reduce environmental burden [2]. A scenario is shown in which a microalgae oil production system is partially installed in the Awano WWTP (Figure 6). HRT and algal biomass productivity are closely related to each other and further optimization research is in progress. The water depth is reduced from 2.5 m to 0.8 m and the HRT is increased from 1.2 to 4.0 days, compared to the existing OD process. If microalgae cultivation is possible at a water depth of 5 m, which is currently under investigation at the University of Tsukuba, the capacity of the cultivation tank is approximately six times greater than the assumed water depth of this study (0.8 m) and so the two tanks will be able to treat 979 m3 /d (115%) of sewage and all the treated water can be treated using the algal system
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