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.
Highlights
The group of aquatic plants commonly named duckweed has been successfully used in effluent treatment systems, mainly for agricultural and municipal wastewater (Skillicorn et al, 1993; Körner and Vermaat,1998, Mohedano et al, 2012a, Zhao et al, 2015)
Of full scale treatment ponds where nitrogen could be removed by many ways, in PBR the nitrate uptake by duckweeds is the only way for nitrogen removal, the growth rate is directly related to nutrient removal
It seems evident that a lack of CO2 causes growth inhibition in lower [CO2] treatments and the high [CO2] in C3 does not cause inhibitory effects. This means that carbon dioxide is consumed until the compensation point, as described by Muller et al (1977), which states that carbon fixation in the duckweed Lemna minor ceases under [CO2] values lower than 40 ppm
Summary
The group of aquatic plants commonly named duckweed has been successfully used in effluent treatment systems, mainly for agricultural and municipal wastewater (Skillicorn et al, 1993; Körner and Vermaat,1998, Mohedano et al, 2012a, Zhao et al, 2015). The photosynthetic activity in plants with C3 metabolism (such as duckweeds) may be improved by increasing CO2 and light, which not happen in C4 metabolism plants In this sense, Andersen et al (1984) demonstrated a growth rate improvement of 46% for Lemna giba exposed to 6,000 ppm of CO2 compared with exposure to normal atmospheric concentration (350 ppm of CO2). Andersen et al (1984) demonstrated a growth rate improvement of 46% for Lemna giba exposed to 6,000 ppm of CO2 compared with exposure to normal atmospheric concentration (350 ppm of CO2) Taking into account their application in full-scale wastewater treatment plants, the biogas produced in anaerobic digesters such as UASBs should be a potential source of CO2 (before or after burning) to be used in duckweed pond enrichment (Byrns, et al 2012). The CO2 enrichment technique is commonly used for biomass gain in greenhouses for vegetable growth; their effect on duckweed intended for biofuel production and nutrient recovery has not been assessed or reported in the scientific literature
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