first, the working principle
the deep layer (well) aeration technology was originally developed by british ici in the 1970s. this new sewage treatment process uses an underground deep well or ground high tower as an aeration tank for an efficient activated sludge system. when it is made into a deep underground well, its diameter is 1~6m, and the well depth is 30~100m; when it is made into a ground tower, its diameter is 5~30m, and the tower height is 10~30m. the activated sludge system is mainly composed of two parts: a riser pipe and a downflow pipe. the mixture of raw sewage and return sludge flows along the downflow tube and the upflow tube. air is injected into the downcomer as an oxygen source for biological oxidation, and air is also injected into the uprising tube as a power source for lifting sewage. it also serves as a source of supplemental oxygen for biological oxidation and as a source of aeration for blowing off biochemical waste gas. its working principle diagram is shown as in fig. 1.
figure 1
second, oxygenation ability
according to the mass transfer formula of oxygen to water:
the oxygenation ability of deep aeration is much higher than that of other aeration methods. the main reasons are:
1.using deep hydrostatic pressure to greatly increase the oxygen-promoting force (cs-c), the energy required to reduce the air bubbles in the downcomer, and the lift effect from the bubbles released from the riser the offset is obtained. therefore, the energy consumed for obtaining the high gas partial pressure is not large, so the oxygenation power efficiency is high.
2. the kla value is increased several times in the descending tube at a high flow rate of 0.6 to 1.5 m/s.
3, the contact time of bubbles and liquids or the action time of bubbles in the liquid phase can be as long as 3 to 5 minutes, while ordinary aeration (ie, shallow aeration) is under 15s, the former is ten to dozens of the latter times, which greatly improves the utilization of oxygen.
according to aerobic requirements, the oxygenation capacity of deep aeration can generally reach 0.5-1.0kg/m3•h, and its power efficiency is still higher than other aeration methods.
third, the deep aeration process operating mode
due to the aeration and oxygenation in a very deep liquid layer, its operation mode and hydrodynamic characteristics are very different from the traditional (shallow aeration) method.
1. the circulating power of the mixture can be compressed air or mechanical-pump.
when compressed air is used as the power, it is called the gas extraction circulation method. the method uses the buoyancy of compressed air to act on the liquid phase and then drives the mixture to rise together. the air is both the circulation power and the oxygen source of the biochemical reaction. group of power machinery.
when a pump is used as a power, it is called a mechanical circulation method. at this time, a separate source of compressed air is still required to inject air into the downflow tube as an oxygen source for biochemical reactions. two sets of power machines are used.
therefore, the use of pneumatic lift has obvious advantages over mechanical lift.
2, fluid mechanics characteristics
deep aeration is when the sewage rises and falls in the deep cylinder while circulating back into the air. therefore, it is necessary to overcome water resistance and air resistance at the same time in operation to provide the total driving force required for operation. this is a unique hydrodynamic problem in deep aeration.
3. in the zhongping industrial company sanping sewage treatment project, due to the large amount of circulating water, if the pump is used to raise, it will not only make the valve configuration difficult, but also increase the power consumption, increase the maintenance workload, and the system configuration and operation will also more complicated, this will lead to increased investment and operating costs. it is therefore advisable to use compressed air to lift, so that only one set of power electronics and gas distribution lines is needed to make the system more concise. this not only reduces the amount of operation and maintenance work, it is very economical in terms of power consumption, and it also makes the investment and operating costs better. a substantial decline.
fourth, the biological characteristics of deep aeration kinetics
1. the aerated aeration tower adopting the deep aeration method is a highly dispersed biological reaction system. the gas-liquid two-phase exhibits high-speed relative motion in the tower container, and the sequential fluid flow and the disordered fluid flow simultaneously. coexistence, gas-liquid mixing, so that solids - activated sludge is not only highly dispersed, microbial organisms can directly breath oxygen uptake from the gas phase and uptake of dissolved oxygen in the liquid phase, the microbial internal and external respiratory rate exponentially accelerated, accelerated microbial growth rate increased geometrically, metabolically prosperous, faster generational renewal, generation chain and micro-ecological chain extension, bio-rich phase, enhanced biological activity, strong appetite, the organic material uptake rate has also grown geometrically. the broadening of recipes for organic species has resulted in efficient degradation and rapid removal of organic compounds of various molecular weights in sewage. this excellent biological reaction kinetics characteristic of the deep aeration method is very suitable for the treatment of high-concentration sewage and difficult-to-degrade organic, toxic, and deep-colored polymer organic wastewater.
2. the concentration of activated sludge in the deep aeration method is high, and the activated sludge concentration is usually 5~10gmlvss/l, which is 2 to several times that of the conventional activated sludge method (shallow aeration) 0.15~0.3gmlvss/l. therefore, the organic material removal rate can reach the 1.5 to 3 power of the conventional activated sludge process or higher.
fifth, deep aeration (including deep well aeration) method of process combination
the deep (well) aeration activated sludge process and the conventional (shallow) aerated activated sludge process are aeration, oxygenation and aerobic oxidation. the combination process still requires subsequent precipitation (solid-liquid separation) and sludge recirculation. because the deep (well) aeration process is performed at a pressure several or even tens of times higher than conventional (shallow) aeration, supersaturated air is dissolved in the mixture and the bio-oxidation process occurs. the exhaust gas (n2, co2, etc.) therefore needs a degassing tank at the top to remove the supersaturated air and exhaust gas dissolved in the mixture so as to be recycled into the downcomer and refilled with fresh air (oxygen). )gas. if the sedimentation tank is subsequent, the microbubbles must be removed before entering the sedimentation tank to ensure that the sedimentation process is not disturbed by the floating of the microbubbles, and the sludge can be normally settled and separated from the mixture.
however, the floating of microbubbles would have been able to carry sludge together, which is spontaneous air flotation. therefore, we can take advantage of this feature of the deep aeration process. after its subsequent flotation separator is used to float the sludge, it is possible to omit the sedimentation tank and do more with less.
in a sewage treatment project in xinjiang, using a set of air flotation separation devices originally built to meet the treatment scale can be used to form the best combination of technology with deep aeration and to fully utilize the processing capabilities of the original facilities. it can be described as a combination of the advantages of two facilities.
source: cic investment advisory network
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