Technology and characteristics of three ditch oxidation ditch

Oxidation ditch, also known as oxidation ditch, also known as circulating aeration tank, is a deformation of activated sludge process, which was first designed by PASVEER in the Netherlands in the 1950s. Initially, it is generally used to treat urban sewage below 5000m3.

Three ditch oxidation ditch is a typical structure type of oxidation ditch. The three ditch oxidation ditch process adopted at present is initiated by Denmark on the basis of intermittent operation of oxidation ditch. In fact, it is still a continuous flow activated sludge process. It only integrates the aeration and sedimentation processes, and has the characteristics of alternating operation in time sequence. Its operation cycle can be based on the treatment of water quality So that the operation is more flexible and convenient. This process is simple and does not need another primary and secondary sedimentation tank and sludge reflux device, which greatly reduces the infrastructure investment and operation cost of the oxidation ditch process, and to a certain extent solves the shortcomings of the previous oxidation ditch covering a large area. This process is adopted by Handan East sewage treatment plant in China.

2. Process flow of three ditch oxidation ditch

The three ditch oxidation ditch process is mainly operated in rotation according to the following six stages.

Stage a: the sewage enters into ditch I through the water distribution well, and the brush in the ditch runs at a low speed. The rotating speed is controlled to only maintain the mixing of water and sludge, and promote the circulating flow of water, but it is not enough to supply the oxygen needed for biodegradation of organic matters. At this time, ditch I is in the state of anoxia. The activated sludge in the ditch uses the organic matter in the water as the carbon source, while the denitrifying bacteria in the activated sludge uses the oxygen in the nitrate produced in the previous section to degrade the organic matter, release nitrogen, and complete the denitrification process. At the same time, the outlet weir of ditch I rises automatically, and the mixture of sewage and sludge enters into ditch II. The brush in ditch II operates at a high speed to ensure that there is enough dissolved oxygen in the ditch to degrade the organic matter, and to transform the ammonia nitrogen into nitrate to complete the nitrification process. The treated sewage flows into ditch III, and the brush in ditch III stops running to play the role of sedimentation tank to separate the mud and water. After the treatment of ditch III The water is discharged through the automatically lowered outlet weir.

Stage B: the influent water is changed from ditch II in aerobic state to flow in and is discharged after sedimentation of ditch III. At the same time, the rotary brush in ditch I starts to run at high speed, which changes from anoxic state to aerobic state, and the organics and ammonia nitrogen entering into ditch I in stage a are treated with aerobic treatment. When the dissolved oxygen in the ditch rises to a certain value, the stage ends.

Stage C: the water burst is still injected from ditch II and discharged through ditch III. however, the rotary brush in ditch I stops running and starts to separate the mud and water. When the separation is completed, this stage is over. Stage a, B and C constitute the upper half of the working cycle

Stage D: the inflow is changed from ditch III, the outlet weir of ditch III is increased, and the outlet weir of ditch I is decreased, and the outflow begins. At the same time, the transfer brush of ditch III starts to run at low speed, making it in the state of anoxia. Ditch II is still in the state of aerobic, and ditch I acts as a sedimentation tank. The flooding direction of stage D is opposite to that of stage A. ditch III plays the role of denitrification and the effluent is discharged from ditch I.

Stage e: similar to stage B, the water enters from ditch II, and ditch I still plays a role of sedimentation. The rotary brush in ditch III starts to operate at high speed and changes from anoxic state to aerobic state.

Stage F: similar to stage C, inlet water of trench II and outlet water of sedimentation of trench I. The rotary brush in ditch III stops running and begins to separate mud and water. This completes the cycle.

Generally, a working cycle takes 4-8 hours. During the whole cycle, the middle groove is always in aerobic state, while the rotary brush in the outer two grooves is in alternate operation state. When the rotary brush is running at low speed, the desquamation process is carried out. When the rotary brush is running at high speed, the nitrification process is carried out. When the rotary brush is stopped, the oxidation ditch acts as a sedimentation tank. It is not difficult to see that if the operation time of each stage is adjusted, different treatment effects can be achieved to adapt to the change of water quality and quantity. At present, most of this process is to put the operation time of each stage into the computer program of operation management according to the specific water quality and quantity, so as to make the whole management process run flexibly and operate conveniently.