Updated: July 2002Oxidation Ditch
The oxidation ditch (OD) is a sort of equipment used for a long-term aeration. It consists of a long channel of an elliptical or circular shape equipped with an aeration equipment called a rotor for generating a water flow and stirring water in the channel to supply oxygen. Thought it requires a relatively large area, it has a simple structure and can be easily operated as well as being able to remove nitrogen easily. Thus, it has recently been widely used in relatively small wastewater treating plants.
2. Advantages and Disadvantages
Advantages of the oxidation ditch are as follows.
1) It can be easily maintained.
2) It is hard to be effected by load fluctuations and forms only a little sludge.
3) OD can be easily controlled by changing the rotation of the rotor and the dipping depth.
4) It requires relatively little energy as the rotor is operated efficiency.
5) It can perform nitrification and denitrification easily.
Disadvantages of the oxidation ditch are as follows.
1) As the tank is large and the depth is small, it requires a large area.
It can be said the OD method is particularly suitable for a small scale equipment.
3. About the Oxidation Ditch Method
Fig. 1 shows a flow diagram of the oxidation ditch method. Though it is not different from a standard OD method, it is characterized by the "aqua-rotor".
The shape of the "aqua-rotor" is shown in Photo. 1. It aerates by rotating a shaft equipped with a number of L-shaped blades. The aqua-rotor generates a stirration flow in the up-down and right-and-left directions. It exerts a high performance with an efficiency of 2-2.5 kgO2/kWh and 2 to 3 kgO2/(mh)
- 1) In the case of BOD removal only
When only BOD removal is performed, the rotations of the inverter and the VS motor and the mechanical transmission in the rotor and the dipping depth of the rotor are adjust to supply the required amount of oxygen. When oxygen supply is excessive due to continuous operation, the oxygen supply is adjusted by making it supplied intermittently.
2) In the case denitrification is also performed
- (1) Continuous method
The downstream portion of the long channel is used as the aerobic zone, while the upstream portion is the anaerobic zone. Raw water is flowed into the anaerobic zone to be denitrified there and then nitrified in the aerobic zone. However, it is difficult to always keep the anaerobic zone and the aerobic zone in the same condition if the raw water concentration fluctuates.
(2) Intermittent method
The period of rotor operation is made to be the aeration period, while the period of rotor stoppage to be the anaerobic period. Thus, nitrification and denitrification are repeated. Unless extreme water quality fluctuations are present, it can provide stable operation
Fig.1 Flow diagram of oxidation ditch method
Photo. 1 aqua-rotor