Updated: July 2002

1. Outline of equipment

Recently, the problem of eutrophication of closed bodies of water is caused by the nutrient salts of nitrogen and phosphorus that can not be removed by such traditional methods as conventional activated sludge process without a large-scale renovation to the existing facilities. The step-feed anoxic/aerobic activated sludge process has been developed to enable efficient denitrification without a major renovation.
With the new system, the primary effluent is introduced by step-feed, providing effective distribution of BOD and reduction of nitrogen load. In addition, the inside of the reaction tank is made anoxic/aerobic to accelerate nitrification/denitrification, achieving effective and stable nitrogen removal.

2. Features

The step-feed anoxic/aerobic activated sludge process has the following features.
1. It has a high rate of denitrification.
By optimizing the conditions, BOD and T-N removal of 60-80% is possible.
2. It is of a compact design.
With the reduction in hydraulic retention time, the tank capacity is reduced to half that of a conventional circulating process.
3. It is energy-efficient.
As it no longer needs circulation of nitrification liquid, energy consumption has be cut to about 70% of the circulating process.
4. Existing facilities can be modified.
Modification to this advanced sewage treatment process can be carried out without sacrificing existing treatment capacity.
5. It is easy to operate.
It does not require complicated controls, and its maintenance is simple.

3. Performance

(1) Design hydrological retention time: 8 hours
(2) Denitrification Rate: 60-80%

4. Applications

1. Advanced treatment of sewage
The step-feed anoxic/aerobic activated sludge process can be built anew, or added to the existing conventional activated sludge plant by:

- installing partitions in the reaction tank, creating anoxic and aerobic zones, and
- adding distribution weir for the step-feed of the primary effluent

These modification will achieve high-efficiency nitrogen removal. If yet higher level of treatment is desired, we suggest adding an absolute anaerobic zone ahead of the reaction tank to biologically remove phosphorus, or installing movable bed type upward-flow continuous sand filter and flocculant dispensing unit downstream of final settling pond. This will carry out in-line coagulation filtration, removing phosphorus and SS.

Schematic Flow of Step-feed Anoxic/Aerobic Activated Sludge Process