The purpose of this study is to compare the spectral indices for a two-dimensional river algae map using an unmanned aerial vehicle (UAV) and an unmanned surface vehicle (USV) hybrid system. The UAV and USV hybrid systems can overcome the limitation of not being able to effectively compare images of the same region obtained at different times and under different seasonal conditions, when using a method of comparing and analyzing with absolute values in remote sensing. Radiometric correction was performed to minimize the interference that could distort the analysis results of the UAV imagery, and the images were taken under weather conditions that would minimally affect them. Three spectral indices, namely, normalized difference vegetation index (NDVI), normalized green–red difference index (NGRDI), green normalized difference vegetation index (GNDVI), and normalized difference red edge index (NDRE) were compared for the chlorophyll-a images. In field application and correlational analysis, the NDVI was strongly correlated with chlorophyll-a (R2 = 0.88, p < 0.001), and the GNDVI was moderately correlated with chlorophyll-a (R2 = 0.74, p < 0.001). As a result of comparing the chlorophyll-a concentration with the in-situ chlorophyll-a imagery by UAV, we obtained the RMSE of NDVI at 2.25, and the RMSE of GNDVI at 3.41.
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A B S T R A C TThis study investigated the feasibility of using pressure assisted forward osmosis (PAFO) for shale gas wastewater treatment. PAFO combines osmotic gradient across a membrane with external pressure together, which was expected to obtain higher flux than forward osmosis (FO). Experiments were performed in a laboratory-scale PAFO system, which allows the application of external pressure up to 10 bar on the feed solution side. Deionized water and three kind of synthetic shale gas wastewater, including low range, medium range, and high range wastewaters, were used as feed solutions and NaCl was used as a draw solution. The water flux was improved up to 10-15% by applying external pressure to FO when low range and medium range wastewaters were treated. However, the effect of the external pressure was significantly reduced when the high-range wastewater was treated. After FO treatment, air gap membrane distillation was successfully applied to re-concentrate the draw solutes.
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