| The Heihe Reservoir is a typical canyon-shaped, stratified reservoir and it is the most important raw water source for Xi’an City, it supplies 8.0×105 m3 water to this city almost every day. However, in recent years, seasonal anoxia at the bottom, endogenous pollution, alga bloom and extreme rainfall events have great influence on the water quality. Based on frequently field monitoring, Computational Fluid Dynamics and theoretical analysis have been used to study:(1) Seasonal variation of water quality in the Heihe Reservoir.(2) The evolution characteristics of storm runoff and its effect on reservoir water quality.(3) The relationship between storm runoff and undercurrent and the selective water withdraw scheme in flooding season.(4) The improvement of the water-lifting and aeration technology.(5) The in-situ water quality improvement under different operation conditions and operation optimization. The main conclusions are as following:(1) Temperature stratification in the Heihe Reservoir began in April to May with a rapid increase of the surface water temperature and Diatom began to thrive, it became stable from June to September, the surface water temperature increased and the hypolimnion became anoxic at the end of June, thus, the internal pollutants such as N, P, Fe, Mn, Sulfide released from the sediment, at the same time the Blue alga increased. The storm runoff has significant impact on the thermal stratification and reservoir water quality during the flooding season. The thermal stratification weakened with the decrease of air temperature in October. The nature mixing period was at the end of December or early January the following year, water quality deteriorated at the early mixing period and the reservoir kept mixing state until March.(2) High inflow(>1500m3/s) entered the reservoir as a plunging underflow, leading the bottom water turbidity exceeded 3000 NTU. High inflow volumes broke the stratification structure and it carried a large amount of solids and organic matter which increased the Ammonia, total phosphorus, total organic carbon and permanganate in the undercurrent effect area. Inflow(500-1500m3/s) entered the reservoir as interflow, the density undercurrent decayed along the interflow process, and it had little effect on main reservoir. Most notably, storm runoff leading to increased dissolved oxygen in the bottom water could effectively inhibit the release of endogenous pollution. However, high inflow volumes carrying a large amount of solids and organic matter increased the oxygen consumption rate in the hypolimnion, and anoxic conditions reappeared after the storm runoff ended and accelerated the release of pollutants.(3) We have established the relationship between inflow, inflow sediment concentration, inflow water temperature and inflow density. We have ensured the undercurrent position at different season and different inflow and proposed the selective water withdraw scheme which can ensure storing the clear water and releasing the turbid water in the flooding season to ensure the safety of raw water. Finding the relationship between inflow Q and inflow sediment concentration S according to the data at Hydrological station: S=0.4655×e0.0024Q(R2=0.7114). Finding the relationship between inflow sediment concentration S and the sediment concentration at the main reservoir Smain according to the measured date: Smain=0.034×e0.233S(R2=0.768). Summarize the inflow temperature at different season under different inflow. With inflow temperature and sediment concentration at the main reservoir we can calculate the inflow density, then we can calculate undercurrent position according to the vertical distribution of temperature / density. Water withdraw in flooding season should avoid undercurrent position, but flood discharge should make full use of the undercurrent position at elevation 545 m to achieve store clear water and release turbid water.(4) The improvement of the water-lifting and aeration system: Decrease the elevation of the top of the equipment to 550 m to adapt the variation of reservoir water quality according to Computational Fluid Dynamics to ensure the ability of mixing and controlling alga. Add air pipe pressure regulator system according to my invented patent to solve the air pipe problems. Decrease the bottom install height from 5m to 3m according to Computational Fluid Dynamics to ensure the operation of water-lifting and aeration system cannot bring the sediment up, replacing the microporous aeration device at the same time to improve the oxygen transfer rate. In in-situ experiments, we added 35 holes, each with a diameter of 2 mm, to the outside of the air vessel, which can ensure that the large air piston does not form under a small volume of gas.(5) The height of hypolimnion anoxic layer reduced from 17 m to 3.2m after the water-lifting and aeration system operated 15 days. The alga number at the surface reduced from 90 million per liter to less than 3 million per liter after the system operated 40 days and the algae number reduced by 96.6%. The water-lifting and aeration system operated from the end of September to October every year during 2013-2015, the whole reservoir mixed after the system operated about 35 days. The bottom oxygen concentration increased to over 10mg/L and the release of pollutants inhibited during the operation period, the concentrations of TP and Fe in the vertical water column decreased to lower than 0.015mg/L and 0.1mg/L, which decreased by 74% and 80%, respectively. The operating of the system enhanced the activity of water microbial metabolic and the capacity of self purification, thus enhancing the effect of nitrogen denitrification bacteria, and the total nitrogen in the bottom reduced by 20.5% after the system operation. The operation of the water-lifting and aeration system led the reservoir to be mixed in advance with an increased vertical mixing temperature. The low temperature, high density and oxygen rich surface water would continuously be transferred to the lower layer, forming a continuous mixing process in the natural state. Thus, good water quality could also be maintained after the system stopped running. |
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