Updated: July 2002

1. Features

1) It is possible to remove the nitrogen/phosphorous content at high efficiency and little energy.
2) No special chemical is required and number of units required is limited, and operation cost is limited.
3) The process can be applied to small-scale plant up to large-capacity plant, and requires narrower area as compared with conventional method.

2. Principle

The nitrogen is removed within the reaction tank by performing four steps (combination of anaerobic/aerobic) repeatedly. On the other hand, the phosphorous is removed by applying such phenomenon that the amount of phosphorous taken in is greatly increased when the phosphorous contained in the sludge under the existence of organic in the inflow sewage, is discharged by adjusting the sludge temporarily to the anaerobic status before the phosphorous is taken in with the microbe within sludge kept at aerobic status, and the sludge is then adjusted to aerobic status. The phosphorous discharged from the sludge within the absolute anaerobic tank is removed as excess sludge which was taken excessively within the reaction tank (arranged at the latter stage) under aerobic status.

3. Mechanism

Step-A
- Reaction tank-I ... Anaerobic
- Reaction tank-II ... Aerobic
The raw sewage passed through the absolute anaerobic tank is supplied in the reaction tank-I which is under anaerobic status, and the BOD content contained in the raw sewage is turned to energy source (hydrogen donor) required for nitrogen removal reaction, by which the nitrate-nitrogen generated in the last step (D) is decomposed to nitrogen gas. On the other hand, the reaction tank-II is maintained at aerobic status, where the BOD content is oxidated/ removed, and the ammonia-nitrogen is nitrated.

Step-B
- Reaction tank-I ... Aerobic
- Reaction tank-II ... Aerobic
The raw sewage is supplied to the reaction tank-I in succession in the same manner as the step-A.
In this step, however, both reaction tanks-I and reaction tank-II are maintained at aerobic status in succession by aeration, in which the BOD content is oxidated/removed, and ammonia-nitrogen is nitrated.

Step-C
- Reaction tank-I ... Aerobic
- Reaction tank-II ... Anaerobic
The raw sewage is supplied to the reaction tank-II in substitute for reaction tank-I by changeover valve.
The reaction tank-II by changeover valve.
The reaction tank-II is maintained at anaerobic status in the same manner as reaction tank-II in step-A, in which the nitrate-nitrogen is nitrated. The aerobic status is maintained in the reaction tank-I continuously in the same manner as step-B, in which the BOD content is deoxidated/removed and ammonia-nitrogen is nitrated.

Step-D
- Reaction tank-I ... Aerobic
- Reaction tank-II ... Aerobic
The raw sewage flows in the reaction tank-II in succession in the same manner as step-C. In this step, both reaction tank-I and reaction tank-II are maintained at aerobic status in the same manner as step-B, in which the BOD content is oxidated/decomposed and ammonia-nitrogen is nitrated. After completion of this step,the step-A is resumed again and the same cycle is performed repeatedly.