10:45 AM - 12:15 PM
[AHW18-P19] Evaluation of the practicality of nano-bubble technology for decomposing dissolved hydrogen sulfide in Lake Ogawara
Keywords:Brackish Lake, Oxidative decomposition, Hydrogen Sulfide, Nanobubble, Oxygen, Ozone
The ultimate goals of this study are to improve the water quality of eutrophic brackish lakes, recover and resource phosphorus, and establish efficient hydrogen sulfide (H2S) removal technology. Lake Ogawara is a brackish lake in Aomori Prefecture located in the northernmost part of the main island of Japan. There, high concentrations of phosphorus accumulate in a deep high-salinity layer. Therefore, we consider that phosphorus can be efficiently recovered by focusing on this high-salinity layer. However, high concentration of H2S is also present in this layer, hampering phosphorous recovery. H2S is harmful to the human body and corrodes machinery. Accordingly, H2S must be removed before phosphoric acid (PO43-) in the water can be recovered. Therefore, our first goal was set to establish an efficient method for removing H2S from the lake water.
First, as basic information, it is necessary to clarify the amount of H2S in high-salinity water. Therefore, we surveyed a vertical profile of H2S concentration in Lake Ogawara. Water samples were collected every 2 m from the surface to the bottom in the center of Lake Ogawara in Sep. and Nov. 2022, and analyzed for H2S concentration. H2S concentrations were almost 0 mgS/L in the low-salinity layer, but increased to 31–37 mg/L below the halocline layer (12–18 m in Sep, 10–18 m in Nov). Total amount of H2S in the lake water was estimated to be 3.3×103 t in Sep. and 2.8×103 t in Nov., with 83% and 89% of the total distributed in the high-salinity layer, respectively.
Second, the effectiveness of nanobubble (NB) technology for H2S decomposition was evaluated. Specifically, H2S was decomposed using ozone (O3-) NB and oxygen (O2-) NB in a laboratory-scale model experiments. Decomposition rate of H2S concentration due to O3-NB and O2-NB aeration was fitted to the first order reaction equation: y=A0×e-kt+c (A0, initial concentration; k, degradation rate; c, constant). The half-lives of H2S in the decomposition experiments using O2-NB and O3-NB were 20 and 4.0 min, respectively. Based on these results and realistic oxygen supply in the field, it was considered that O3-NB can remove H2S from the high-salinity layer at a practical rate.
First, as basic information, it is necessary to clarify the amount of H2S in high-salinity water. Therefore, we surveyed a vertical profile of H2S concentration in Lake Ogawara. Water samples were collected every 2 m from the surface to the bottom in the center of Lake Ogawara in Sep. and Nov. 2022, and analyzed for H2S concentration. H2S concentrations were almost 0 mgS/L in the low-salinity layer, but increased to 31–37 mg/L below the halocline layer (12–18 m in Sep, 10–18 m in Nov). Total amount of H2S in the lake water was estimated to be 3.3×103 t in Sep. and 2.8×103 t in Nov., with 83% and 89% of the total distributed in the high-salinity layer, respectively.
Second, the effectiveness of nanobubble (NB) technology for H2S decomposition was evaluated. Specifically, H2S was decomposed using ozone (O3-) NB and oxygen (O2-) NB in a laboratory-scale model experiments. Decomposition rate of H2S concentration due to O3-NB and O2-NB aeration was fitted to the first order reaction equation: y=A0×e-kt+c (A0, initial concentration; k, degradation rate; c, constant). The half-lives of H2S in the decomposition experiments using O2-NB and O3-NB were 20 and 4.0 min, respectively. Based on these results and realistic oxygen supply in the field, it was considered that O3-NB can remove H2S from the high-salinity layer at a practical rate.