Effects of CO2 concentration on nutrient uptake and starch accumulation by duckweed used for wastewater treatment and bioethanol production

Abstract

Abstract The aquatic macrophytes commonly known as duckweed has been successfully used in wastewater treatment plants during decades. Besides the efficiency of these plants to remove nutrient from wastewater, duckweed has drawn increasing attention for bioethanol production due to its high biomass and starch production. Recently several studies have been evaluating techniques to promote starch accumulation in duckweed biomass and thus improve ethanol yield. Therefore, the present study aimed to evaluate the effect of CO2 concentration ([CO2]) and availability in nutrient removal and starch accumulation by duckweed grown in photobioreactors (PBRs). Thus, duckweed was grown in hermetic PBRs (24 L) exposed to three different CO2 concentrations (C1-1,500; C2-6,000 and C3-100,000 ppm), as well as a control group (CC-380 ppm), without CO2 replacement for a seven-day test period. The decay of NO3 - and PO4 - was monitored along the test, as well the [CO2] and biomass growth rates. The results showed that in C1 and C2, duckweed quickly consumed the CO2 in the gas phase, causing a reduction of nutrient removal efficiency and the consumption of storage starch. By contrast, the higher [CO2] improved the starch content by approximately 150%, from 9.6 to 24.7%, and presented the best results for nitrate and phosphate removal (82 and 79% from 308 mgNO3 L-1 and 28 mgPO4 L-1, respectively).The findings pointed that [CO2] is an important parameter to be monitored in closed duckweed systems, and CO2 supply could improve the starch content and nutrient removal rates.