Study On Strengthening Nitrogen Removal In A/A/O Oxidation Ditch Process By Regulation Technology

Nitrogen emission reduction in municipal wastewater has important significancefor the control of eutrophication and improvement of water environment quality. As aresult, efficient denitrification is one of the difficult points of operation regulation inmunicipal wastewater treatment. Considering wastewater treatment is a highenergy-consuming operation, it is possible to make energy saving possible througheasonable energy configuration, which is of strong practical significance. A/A/Ooxidation ditch process has many advantages, such as simple process flow, convenientoperation and management and so on, consequently it is widely used in the municipalwastewater treatment in our country. However, operation regulation involves not onlythe complex ditch hydraulics and biochemical processes, but may also be affected byenvironmental conditions and the water load changes. Because of the lack of effectiveregulation strategy, such consequences may be found such as low nitrogen removalefficiency, unstable operation, easy accumulation of mud ditch and high energyconsumption. Therefore, on basis of environment conditions and dynamiccharacteristics of influent loads in the practical wastewater treatment plant, in this studythe factors of hydrological and biological fields were considered and the combination ofbatch equipment test, pilot test, field test, dynamic and the numerical simulation wasadopted. Adaptive optimization strategy and control method to enhance nitrogenremoval and optimize the energy saving in the A/A/O oxidation ditch are systematicallyand deeply researched.Through analysis on the operation status of wastewater treatment plant adoptingthe A/A/O oxidation ditch process, it was found that the key of process optimization isto enhance the denitrification and to reduce energy consumption. Enhancing thedenitrification capacity of the pre-anoxic zone and promoting SND of the main reactionzone have already been proven to be the important ways of high efficient nitrogenremoval. The key point of energy saving is optimal control of aeration and plug-flowsystem.In this paper, effects of the dissolved oxygen concentration, sludge return ratio,sludge concentration and so on were studied systematically, and the reasonable valueranges of the operation parameters have been optimized. Studies on kinetics of nitrationreaction and the result idicated that DO was decreased appropriately and the NH₄⁺-N concentration of the aeration tank was controlled in the range between2mg/L and4mg/L, which was essential to maintain nitrification in high efficient stage to improve thenitrification rate and reduce the aeration energy consumption. If DO concentration washigher than1mg/L, it would have a significant influence on the denitrification reaction,so the dissolved oxygen concentration should be strictly controlled from the insidecircumfluence to anoxic zone. Effect of NH₃^--N on the denitrification rate was not sosignificant.In the mode of the continuous aeration, the distribution rules of importantcharacteristic parameters such as velocity, dissolved oxygen and sludge concentration inthe main reaction zone were obtained through field test, and the effects of differentcontrol conditions on the distribution of characteristic parameters were investigated.Dissolved oxygen environment of aerobic zone and anoxic zone alternating existed inoxidation ditch through reasonably controlling the operation mode of aeration wheeland promote dissolved oxygen transferring and the big sludge forming, which createdmacro and micro hypoxia environment necessary for SND process.Measures of segmentation optimization that are applied in the main reaction zoneof combination whole and part are put forward based on the regulation of quantity,location and rotational speed of aeration wheel in operation, so as to realize safeoperation, enhanced nitrogen reduction and energy saving. When the control measuresof segmentation optimization are applied in the sewage treatment plant, the effluentquality can meet the requirement of the first level B standard specified inGB18918-2002, the effluent TN was reduced from15.2-26.5mg/L to9.3-13.5mg/L,and the average removal rate increased from39.5%to71.7%, the power consumptionper unit water was decreased by19.8%.In the view of the problem existing excessive aeration led to excessive nitrificationand the high energy-consumption in A/A/O oxidation ditch, denitrifying rate andeffluent water for feasibility criteria to the nitrification rate, the denitrification rate andthe effluent quality are as the criterion of feasibility, and operation condition ofintermittent aeration process are made between the pilot test optimization andproductive commissioning research. The result shows that, when operation mode ofaerated1h stopped aeration2h is adopted, the effluent TN is reduced from16-27.6mg/L to9-15mg/L, and the average removal rate increased from50.5%to72.8%, thepower consumption per unit water is decreased by30.9%on the basis of the effluentstandards. Using the material balance method to analyze the flow situation of carbon, nitrogenand phosphorous in two kinds of the continuous aeration mode （condition two,condition four） and two kinds of the intermittent aeration mode （aerated1h stoppedaeration2h, aerated2h stopped aeration2h） under the A/A/O oxidation ditch system,the SND phenomenon is analyzed quantitatively. The result shows that the aerobic zoneSND could be enhanced through controlling aeration wheel operation, this makes greatcontributions to improving the whole of the nitrogen removal effect, while threeaeration wheels are in operation; when the continuous aeration mode operationsaccording to condition two, anoxic zone SND covers43.6%of total nitrogen removal.The three-dimensional flow pattern model of the A/A/O oxidation ditch wasestablished and checked. From the optimization flow pattern view to studying on therelationship between pond type of the oxidation ditch, facility places and energy saving,that analyzes problems of energy configuration about the mixing plug flow in theoxidation ditch, and optimization measures are proposed and simulation prediction andfield modification are carry out. The area of sludge deposition zone of anoxic zone andthe main reaction zone has reduced after optimization, the energy configuration havebeen optimized in the oxidation ditch.On the basis of the dynamic variation characteristics of the influent in a full scaledyeing wastewater treatment plant, the whole year and the whole day are respectivelydivided into different period and different time. The time-sharing optimal controlstrategy was proposed on the basis of the influent dynamic change and used inproduction control. The control strategy optimized with the variation of influent flowand water quality as feed forward main control index and the change trend of dissolvedoxygen as feedback auxiliary control index. The regulation results show that the effluentCOD, NH₄⁺-N are stable superior to the first level of A standard GB18918-2002; Theeffluent TN is stable superior to the first level of B standard; Power consumptiondecreased by11%,13.2%and16.8%respectively in low temperature period, hightemperature period and temperature period.