Updated: July 2002

1. Purpose

Sewage treatment system of small communities, first of all, requires the easy maintenance control and inexpensive construction/maintenance control costs. In addition, for large daily/weekly fluctuation in the influent load, the stable treatment performance is obtained, and further as there are many cases that the water-intake is made for the city and agricultural water in the downstream of the river where the effluent is discharged, the high removal performance is also required for the nutrients like nitrogen and phosphorus. Under such a background, the intermittent aeration system with single reactor (generally called the two-beat system) was developed. Hereinafter, the principle and features of the system are introduced.

2. Principle

The intermittent aeration system with single reactor (the intermittent aeration system for short hereafter) is that, a submerged mechanical aerator for both anoxic and aerobic treatment is installed in a reactor of complete mixing type, and by the intermittent air supply, anoxic and aerobic conditions are distinctly produced in a time series in the same reactor for treatment. Fig. 1 shows the basic flow of this system.

Fig. 1 Basic Flow of Treatment

3. Features

1) High nitrogen removal efficiency and energy-saving effect
Also for large fluctuation in the influent load, the secure and distinct anoxic/aerobic conditions is formed by DO control adopted. As a result enables the following: stable high nitrogen removal efficiency, no-bulking and energy-saving effect. As to this method the rotational speed of either aerator or blower, or of both is adjusted by frequency conversion for DO control. In addition, the aerator runs continuously, therefore, the activated sludge can be held in the complete floating condition even at the anoxic time zone.
Fig. 2 shows the example of DO control.

Fig. 2 DO and pH Patterns at DO Control

When the flow rate is max. (at peak) in the morning and the load is low at night, the anoxic condition (DO concentration of less than 0.2 mg/l) and aerobic condition (DO concentration of more than 0.5 mg/l) are respectively secured for one-hour, regardless of large fluctuation in the influent load of sewage, and further the max. DO is controlled at approx. 2.5mg/l, thereby carrying out secure DO control.
2) ASRT control
The organic substance/ammonia nitrogen concentration of the treated water is related to ASRT. If this ASRT is held at the required level for growth of nitrification bacteria according to the reaction water temperature, complete nitrification can be achieved even under the low water temperature in winter. From such a viewpoint, in order to allow the operation of ASRT control to be easy and secure, the excess sludge is withdrawn directly from the reactor in this system.
3) Coping with load fluctuation of water quantity/quality
In sewerage of small communities, the daily fluctuation of the influent flow rate is remarkable, therefore, its adjustment is an important subject for the maintenance control. In this system, the constant flow rate discharge unit (pressure regulating) is provided, that is, by allowing the water level in the tank to change, the flow rate adjustment function is secured in the reaction tank, and the effluent load to the final sedimentation tank is stabilized.
4) Compact facilities
The reaction tank, final sedimentation tank, sludge storage tank, switch room,office, etc. are monolithic construction, and setting in series by three-dimensional layout is possible to do, therefore, the installation space is small, resulting in reduction of drilling volume and construction cost of civil structures. Furthermore, because of compact facilities, taking measures for peripheral environments (odor, noise, etc.) is easy.

4. Examples of Implementation

Small scale sewage treatment, 12 places.