Total Dissolved Gas Saturation Research Articles

TDG prediction model improvement by analysis and validation of experiments on a dam model.

The combination of aerated flows and a high-pressure environment in a stilling basin can result in the supersaturation of total dissolved gas (TDG) downstream of hydraulic projects, posing an ecological risk to aquatic populations by inducing gas bubble disease (GBD) or other negative effects. There is limited literature reporting TDG mass transfer experiments on a complete physical dam model; most existing research is based on measurements in prototype tailwaters. In this study, TDG mass transfer experiments were conducted on a physical model of an under-constructed dam, with TDG-supersaturated water as the inflow, and TDG concentrations were meticulously monitored within the stilling basin. The measurements indicate that the TDG saturation at the outlet of the stilling basin decreased by 13.7% and 10.6% compared to the inlet for the two cases, respectively. Subsequently, an improved TDG prediction model was developed by incorporating a sub-grid air entrainment model and a phase-constrained scalar model. The numerical simulation results were compared with experimental data, indicating a maximum error in TDG saturation at all measured points of less than ± 3%. Moreover, the TDG saturation showed an error of only ± 0.3% at the outlet of the stilling basin. This model has broad applicability to various flow types for obtaining TDG mass transfer results and evaluating mitigation measures of TDG supersaturation to reduce the harmful effects on aquatic ecosystems.

Research on the adsorption rule of porous media on supersaturated total dissolved gas

AbstractDischarge of high dams may result in supersaturated total dissolved gas (TDG) in water, which could cause fish that live downstream river to suffer from gas bubble disease. If supersaturated TDG water was taken as a water source for breeding and proliferation stations, farmed fish was confined and may face more severe problems than fish living in the river. Therefore, it is critical to develop strategies to reduce the negative effects of supersaturated TDG. In this research, the adsorption effect of porous media on supersaturated TDG was explored, including biofilter adsorption experiments and previously existing activated carbon adsorption experiments. The experimental results showed that adding porous media to the water effectively accelerated the dissipation of supersaturated TDG, and the adsorption effect was associated with the specific surface area, mass density, and initial TDG saturation. To quantitively evaluate the adsorption effects of the porous media, the porous adsorption coefficient was proposed to express the adsorption rate of porous media on supersaturated TDG. The porous adsorption coefficient was related to the initial TDG saturation and the specific surface area of the porous media. The porous adsorption coefficient increased with increasing the specific surface area and decreasing initial TDG saturation. Based on this, an equation related to the specific surface area, initial TDG saturation, and the porous adsorption coefficient was developed. This equation may be used to evaluate the adsorption effect of porous media on supersaturated TDG. This study could serve as a crucial resource in reducing the adverse impacts of supersaturated TDG.

Experiments about the removal of supersaturated total dissolved gas from water environment by activated carbon adsorption.

Water environment conditions directly support aquatic life. It is important to maintain a suitable water environment to improve the efficient use of water resources. Supersaturation of total dissolved gas (TDG) in the water will cause fish suffer from gas bubble disease and even mortalities. Measures should be taken to mitigate the adverse effect of supersaturated TDG. Considering the adsorption effect of porous medium, activated carbon (AC) was utilized in this experiment to explore the effect of AC on supersaturated TDG removal. The effects of AC properties, AC dosage, and initial TDG saturation were investigated. The results showed that adding AC in the water could effectively accelerate the supersaturated TDG removal rate, which was positively correlated with the AC specific surface area and dosage. Meanwhile, the average dissipation rate of TDG increased and then decreased with increasing initial TDG saturation. The adsorption characteristics of AC on supersaturated TDG were also explored. The maximum equilibrium adsorption capacity and removal rate were 0.262 mg/g and 48.5% respectively. It was concluded that the adsorption process of AC on supersaturated TDG conformed to the Langmuir equation and pseudo-first-order kinetic model. Recycling test indicated that the used AC could be reused after drying. It was hoped that this research could contribute to improving water environment and ensuring the healthy development of the aquatic livings.

Prediction model and application of machine learning for supersaturated total dissolved gas generation in high dam discharge

Supersaturation of total dissolved gas (TDG) caused by high dam discharge is an ecological risk that cannot be ignored in the operation of hydropower stations. The establishment of an efficient and concise TDG generation prediction model is of great significance to the water ecology and water environment protection of hydropower development reaches. The flow conditions and the process of water-gas mass transfer in discharge and energy dissipation are very complicated and difficult to observe in the field, bringing difficulties to the establishment of prediction model and parameter calibration. Increasingly abundant observations make it possible to establish an efficient machine learning prediction model for supersaturated TDG. In this study, extreme learning machine (ELM) and support vector regression (SVR) were used to establish the prediction model. The main influencing factors of supersaturated TDG, obtained by the analysis of the physical process of the generation of supersaturated TDG, were used as the input of the machine learning model. Then, this research took Dagangshan hydropower station and Xiluodu hydropower station as objects, and established machine learning prediction model for supersaturated TDG with several years of observation data in different discharge scenarios. Four models, including ELM, SVR, GA-ELM and GA-SVR, were obtained through genetic algorithm optimization. The relative errors of the simulation results of each model are mostly less than 5%, mean absolute error (MAE) values less than 1.6%, and root mean square error (RMSE) values less than 2.5%. The results showed that these models are highly accurate and time-saving. Based on this, TDG saturation in downstream of Dagangshan hydropower station with different discharge scenarios was simulated by machine learning model, on which the discharge optimization scheme was put forward. The proposed models, as an important supplement to the prediction of supersaturated TDG, enjoy practical significance and engineering value.

Numerical study on the cumulative effect of supersaturated TDG through the spillway

The cascade development of water resource will lead to cumulative effect of supersaturated total dissolved gas (TDG) on the ecological environment, which will cause lethal disease to fish. The TDG dissipates slowly in deep reservoir areas and if supersaturated TDG can't recover through the spillway then it will lead to continuous accumulation in the stilling basin. In order to investigate the cumulative effect of supersaturated TDG of hydropower cascades, a validated TDG numerical model with the VOF method and DES model was used to simulate a spillway discharge of Tongjiezi dam with supersaturated TDG and 100% TDG inflow. In this model, bubble size change is calculated by bubble number density equation and TDG is simulated by transport equation considering the gas-liquid mass transfer. Comparing with the TDG 100% inflow case, it indicated forebay TDG level caused by the upstream dams should be considered and involved in the downstream TDG evaluation. This paper also proposed an engineering measure with baffle blocks in the spillway which can reduce entrained bubble mass and TDG saturation level at the entry point than the case without blocks. It shows baffle blocks in the spillway can effectively reduce the cumulative effect of supersaturated TDG. It is a critical step for the mitigation of accumulation of supersaturated TDG through the spillway and the results provide reference about mitigation measure of supersaturated TDG for hydropower cascades.

Experimental investigations on the dissipation process of supersaturated total dissolved gas: Focus on the adsorption effect of solid walls

Hydropower, which utilizes energy from fast-moving water, can help alleviate the energy crisis and promote economic development. For safety and ecological purposes, dams must discharge periodically. This discharge process, which occurs with the supersaturation of total dissolved gas (TDG), affects the fish and other aquatic organisms living in downstream river areas. Previous studies have shown that the supersaturated TDG dissipation process is closely related to hydraulic properties such as the flow rate, water depth and turbulent kinetic energy. Additionally, the presence of solid walls such as vegetation leaves in water can adsorb dissolved gases in water, thereby promoting the supersaturated TDG dissipation process, and the adsorption effect is closely related to the solid wall material. However, systematic studies on how solid walls in water can quickly absorb dissolved gases from water and rapidly reduce the TDG saturation of water are lacking. Herein, a series of experiments was conducted to study the dissipation process of supersaturated TDG under the action of solid walls in water and to reveal the adsorption effect of solid wall surface properties on dissolved gas. The results showed that the surface roughness and hydrophobicity (contact angle) are the key factors related to the adsorption effect of solid walls on the TDG in water. Generally, the dissipation rate of supersaturated TDG first increased and then decreased with increasing surface roughness. The supersaturated TDG dissipation rate increased monotonically with increasing contact angle. Based on the experimental data, the adsorption coefficient, which represents the adsorption effect of the solid wall for dissolved gas in water, was proposed, and a prediction formula between the adsorption coefficient and the contact angle of the solid wall was established. These results can provide theoretical support for the utilization of the solid wall adsorption effect to mitigate the adverse effects of supersaturated TDG and protect fish.

Cumulative effects of cascade hydropower stations on total dissolved gas supersaturation.

Elevated levels of total dissolved gas (TDG) may occur downstream of dams during the spill process. These high levels would increase the incidence of gas bubble disease in fish and cause severe environmental impacts. With increasing numbers of cascade hydropower stations being built or planned, the cumulative effects of TDG supersaturation are becoming increasingly prominent. The TDG saturation distribution in the downstream reaches of the Jinsha River was studied to investigate the cumulative effects of TDG supersaturation resulting from the cascade hydropower stations. A comparison of the effects of the joint operation and the single operation of two hydropower stations (XLD and XJB) was performed to analyze the risk degree to fish posed by TDG supersaturation. The results showed that water with supersaturated TDG generated at the upstream cascade can be transported to the downstream power station, leading to cumulative TDG supersaturation effects. Compared with the single operation of XJB, the joint operation of both stations produced a much higher TDG saturation downstream of XJB, especially during the non-flood discharge period. Moreover, the duration of high TDG saturation and the lengths of the lethal and sub-lethal areas were much higher in the joint operation scenario, posing a greater threat to fish and severely damaging the environment. This work provides a scientific basis for strategies to reduce TDG supersaturation to the permissible level and minimize the potential risk of supersaturated TDG.

Tolerance of Prenant's Schizothoracin Schizothorax prenanti to Total Dissolved Gas Supersaturated Water at Varying Temperature

AbstractHydropower development changes the temporal and spatial distribution of water temperature and causes total dissolved gas (TDG) supersaturation downstream of dams. We conducted laboratory experiments to determine the effects of TDG supersaturation on Prenant's Schizothoracin Schizothorax prenanti at varying water temperatures. The fish were exposed to 120, 130, 135, and 140% TDG‐supersaturated water at different water temperatures within the range of 12–24°C for 96 h. Survival rate of Prenant's Schizothoracins declined with increasing TDG saturation. At 120% TDG, the final survival rate of fish was over 50% at the different temperatures. However, when the TDG level increased to 130% or above, the survival rate decreased sharply. The median survival time of fish ranged from 5.22 to 18.62 h when exposed to 130% TDG, ranged from 2.41 to 9.41 h at 135% TDG, and was less than 5 h at 140% TDG. Under 120% TDG conditions, fish survival rate was not significantly different among water temperatures (12–24°C). However, when the TDG level was 130%, the survival rate of the fish declined with increasing water temperature. In 135% and 140% TDG‐supersaturated water, the survival time of the fish was longer at 16°C than at other temperatures.

Shelter construction for fish at the confluence of a river to avoid the effects of total dissolved gas supersaturation

Total dissolved gas (TDG) supersaturation downstream has a negative environmental effect on fishes. It is caused by discharge from high dams and increases the incidence of gas bubble disease and fish mortality. Downstream of a high dam, there is an area with low TDG saturation due to the gradual mass exchange of gases between the separation zone and the mainstream and the long retention time in the confluence, which contributes to the dissipation of saturated TDG at the confluence of the mainstream and its tributaries. This area can provide a temporary shelter for fish to avoid the effects of TDG supersaturation during dam discharge. A depth-averaged, two-dimensional model of TDG dissipation at a river confluence was established. The concentration field was verified by a flume experiment. A numerical simulation of the TDG at the confluence of the Zumuzu River and its tributary, the Mozigou River, was conducted. The simulation showed that the convergence of the tributary, which had a low TDG saturation level, could reduce the TDG saturation level of the mainstream. However, the low-saturation area was not large enough for fish to avoid the negative effects of TDG saturation due to a sharp river slope and a large flow ratio between the mainstream and its tributary. To expand the suitable shelter area with low TDG supersaturation levels in order to provide a suitable shelter for fish, some engineering measures were explored, including the excavation of the riverbed and the construction of resistance obstacles. After the engineering measures were introduced, we observed a 30-fold increase in the size of the area with low TDG saturation, which was as high as 10005m2 at 110% of the TDG saturation. Combined with the comprehensive analyses of the flow velocity and the water depth, the confluence region was thought to be suitable to protect the fish from the effects of TDG supersaturation. This study provides an important reference for protecting fish during high dam discharge.

First observations of saturopeaking: Characteristics and implications

During the monitoring of total dissolved gas (TDG) saturation in the Vetlefjordelva River in western Norway in 2014–2015, characteristic waves of supersaturated water were discovered. These waves were significantly correlated with hydropower operation, which was run by hydropeaking (R2=0.82, p<0.001). The TDG saturation varied between 99% and 108%, with a median of 105%.The term “saturopeaking” is introduced for these waves, defined as the artificial, rapid, periodic and frequent fluctuation of gas saturation caused by hydropeaking. Hydropeaking is recognized as hydropower operation that rapidly fluctuates according to the electricity market demand. Though the observed TDG saturation levels were moderate and not likely to cause acute effects on biota, we expect that the observed saturopeaking may have significant ecological impacts in general, especially in cases with TDG saturation levels >110^% which is considered as potentially lethal for fish in rivers.

The tolerance threshold of Chinese sucker to total dissolved gas supersaturation

Total dissolved gas (TDG) supersaturation generated in the river may result in gas bubble disease to Chinese sucker (Myxocyprinus asiaticus Bleeker) and cause their mortality. To assess the effects of TDG supersaturation on Chinese suckers in laboratory, juvenile fish were exposed to different levels of TDG supersaturated water. All of the fish died when the TDG saturation reached and exceeded 125%, but none died at TDG of 120% saturation in 96 h. To evaluate the effects of intermittent exposure of TDG supersaturation, Chinese suckers were exposed to TDG of 130% for a specific duration and then provided a recovery period at 100% saturation. The results showed that some remission is possible for intermittent exposure. Because of the characteristics of natural rivers, experiments exploring the TDG supersaturation detection and avoidance abilities of Chinese suckers were conducted and demonstrated the fish can avoid TDG supersaturation in both the horizontal and vertical directions when higher TDG levels were available. The results indicate the tolerance threshold of juvenile Chinese suckers is a TDG level of 120% saturation.

Effect of total dissolved gas supersaturated water on early life of David’s schizothoracin (Schizothorax davidi)

The effect of total dissolved gas (TDG) supersaturation on fish living downstream of dams is one of the main ecological risks of high dam construction. A strategy for mitigating the negative effects is needed urgently since many high dams are under construction in the upper reaches of the Yangtze River in China. Experiments on the hatching process of David's schizothoracin were carried out and the results show that the hatching rate decreased with increasing TDG levels, and that most eggs hatched within a very short time in the higher TDG saturation groups. By using a stereomicroscope, damages to the head, yolk sac, body, anus, etc. were found in larvae which hatched in TDG supersaturated water. Results show that the lesion rate increased with increasing TDG levels. Furthermore, 7-d-old David's schizothoracin were exposed to TDG supersaturated water levels of 100%, 105%, 110%, 115%, 120%, 125%, 130%, 135%, and 140% for testing their tolerance to TDG supersaturation. We found that the median lethal concentrations (LC50) for 13, 14, 20, 35, 52, 73, and 96 h exposure were 138%, 138%, 134%, 130%, 129%, 128%, and 126%, respectively. The median lethal times (LT50) were 7.49, 11.04, 19.25, and 35.38 h for exposure to water with TDG levels of 145%, 140%, 135%, and 130%, respectively.

Stability of Echogenic Liposomes as a Blood Pool Ultrasound Contrast Agent in a Physiologic Flow Phantom

Echogenic liposomes (ELIP) are multifunctional ultrasound contrast agents (UCAs) with a lipid shell encapsulating both air and an aqueous core. ELIP are being developed for molecular imaging and image-guided therapeutic delivery. Stability of the echogenicity of ELIP in physiologic conditions is crucial to their successful translation to clinical use. In this study, we determined the effects of the surrounding media's dissolved air concentration, temperature transition and hydrodynamic pressure on the echogenicity of a chemically modified formulation of ELIP to promote stability and echogenicity. ELIP samples were diluted in porcine plasma or whole blood and pumped through a pulsatile flow system with adjustable hydrodynamic pressures and temperature. B-mode images were acquired using a clinical diagnostic scanner every 5 s for a total duration of 75 s. Echogenicity in porcine plasma was assessed as a function of total dissolved gas saturation. ELIP were added to plasma at room temperature (22 °C) or body temperature (37 °C) and pumped through a system maintained at 22 °C or 37 °C to study the effect of temperature transitions on ELIP echogenicity. Echogenicity at normotensive (120/80 mmHg) and hypertensive pressures (145/90 mmHg) was measured. ELIP were echogenic in plasma and whole blood at body temperature under normotensive to hypertensive pressures. Warming of samples from room temperature to body temperature did not alter echogenicity. However, in plasma cooled rapidly from body temperature to room temperature or in degassed plasma, ELIP lost echogenicity within 20 s at 120/80 mmHg. The stability of echogenicity of a modified ELIP formulation was determined in vitro at body temperature, physiologic gas concentration and throughout the physiologic pressure range. However, proper care should be taken to ensure that ELIP are not cooled rapidly from body temperature to room temperature as they will lose their echogenic properties. Further in vivo investigations will be needed to evaluate the optimal usage of ELIP as blood pool contrast agents.

Simulations of the Three-Dimensional Total Dissolved Gas Saturation Downstream of Spillways Under Unsteady Conditions

Dams are often operated in a way to discharge over the spillways, which would cause a high dissolved concentration of air and be harmful to fish. The bubble transfer and the water surface transfer play an important role in affecting the concentration of the Total Dissolved Gas (TDG). Based on recent numerical simulations of the total dissolved gas saturation, in this article, a two-phase TDG transport equation is adopted to develop an unsteady three-dimensional (3-D) two-phase flow Computational Fluid Dynamics (CFD) model, including a number of parameters such as water depth, pressure and air volume entrained. This model is used to predict the hydrodynamics and the TDG distribution under unsteady discharge conditions. Good agreement between measured and numerical results is obtained for a case study.

Modeling the Effects of Supersaturated Dissolved Gas on Resident Aquatic Biota in the Main-Stem Snake and Columbia Rivers

Abstract Dissolved-gas levels in the Columbia and Snake rivers during the spring freshet often exceed 110% of saturation, the maximum level permitted by the U.S. Environmental Protection Agency. The highest levels of supersaturation result from high springtime river flows and turbine outages, conditions over which there is little control and that cause high volumes of water passing over the dams and through spillways instead of through turbines. During the spring freshets of 1994–1997, we surveyed nonsalmonid fishes and invertebrates for signs of gas bubble disease (GBD) and conducted holding experiments in three river reaches where gas saturation commonly exceeds 120%. We developed a dissolved-gas exposure index for nonsalmonid fishes sampled at specific times and locations; mean daily total dissolved-gas saturation (TDGS) was ranked and then summed over a 7-d period. We analyzed observations of 39,924 nonsalmonid fishes in an iterative process, which led to development of a mathematical equivalence mode...

Efficacy of Packed Column Designs for Reducing Mortality of Rainbow Trout Eggs and Fry

Abstract An experiment was performed to determine the effect of open and closed packed columns on dissolved gas levels and on mortality of eggs and sac fry of rainbow trout (Oncorhynchus mykiss) at Neosho National Fish Hatchery from March 9 to April 9, 1993. Two water treatments were tested: (1) treatment with a sealed column packed with 1 -in Koch rings and oxygen injection, and (2) treatment with an open column packed with 1 -in Koch rings. Untreated springwater was tested for comparison. Statistically, rainbow trout reared in untreated springwater had significantly higher mortality (P < 0.05) than trout reared in both packed column treatments; mortality did not differ significantly between column treatments. Total dissolved gas saturation in all three groups remained below 100% and did not influence mortality. Untreated springwater had the highest mean nitrogen saturation (103.5%), which may have contributed to egg and fry mortality. Water in the open packed column and in the sealed packed column with ...

Communications: Comparison of Two Methods of Oxygen Supplementation for Enhancing Water Quality in Fish Culture

Abstract In this study, we compared sealed column oxygen supplementation with a pipeline oxygenation system at gas-to-liquid flow ratios of 0.7, 1.5, and 2.2%. The sealed columns receiving oxygen at a gas-to-liquid ratio of 2.2% increased dissolved oxygen by 73.5% (to 149.3% oxygen saturation) while decreasing nitrogen saturation and total dissolved gas saturation to 93.1 and 105.0%, respectively. Oxygen added at a gas-to-liquid ratio of 2.2% through the pipeline manifold increased raceway dissolved oxygen by 53.7% (to 134.6% oxygen saturation) and reduced nitrogen and total dissolved gas saturations to 99.0 and 106.1%, respectively. Sealed column oxygen supplementation was most efficient, having an oxygen transfer efficiency of 82.3% when a 0.7% gasto-liquid ratio was used. Oxygen added through the pipeline manifold resulted in a 54.1–62.7% variation from mean column effluent dissolved oxygen. The pipeline manifold also caused column water flow fluctuations that varied by as much as 21%. We recommend oxygen injection directly into sealed columns over use of a pipeline manifold system for adding supplemental oxygen to fish-rearing water.

Effect of Carbon Monoxide Exposure on Gas Bubble Disease in Rainbow Trout (Salmo gairdneri)

Cumulative mortalities of 100, 50, 20, and 0% due to gas bubble disease (GBD) occurred on exposure of rainbow trout (Salmo gairdneri) for 180 min to supersaturated water at 136, 130, 124, and 116% total dissolved gas saturation (TDGS), respectively. At 130% TDGS, a prior exposure to carbon monoxide (CO), which converted 80% of the hemoglobin to carboxyhemoglobin (COHb), significantly prolonged survival time, but cumulative mortality was insignificantly reduced in the same group. Histologically, all supersaturation mortalities had branchial lesions for GBD (characterized by gas displacement of blood from the afferent arterioles of the gill filaments), while 70% had gas emboli in the retinal choroid gland. At a TDGS of 100%, these histological lesions were not observed and the CO 80% COHb conversion did not induce mortality within 120 min. Breathing movements of both control and CO-exposed fish slowed greatly when a TDGS of 130% was imposed; however, the COHb conversion initiated tachypnea and the CO-treated fish maintained a relatively higher respiratory rate when exposed to supersaturation conditions for a period which was proportional to their prolonged survival time. We concluded that the initial formation of gas emboli preceding fatal GBD was delayed as a result of a CO-inhibited function of hemoglobin.