Updated: July 2002

1. Purpose

The conventional municipal sewage treatment equipment primarily contemplates to remove suspended solids and organic substance. However, as pollution loading increase and awareness to environmental problems grows intense, removal of nitrogen and phosphorus has been investigated. This new process has been applied to part of municipal sewage treatment, particularly in closed water bodies.
As such an advanced treatment process, an activated-sludge process with recirculation of the nitrified mixed liquor has been put into practical use. However, this advanced activated-sludge process has disadvantages such as long retention time and requirement to circulate a large amount of nitrifying liquid. To solve these problems, Kobe Steel, Ltd. offers two kinds of fluidized bed type biological treatment processes for efficient removal of organic substance and nitrogen as well as for ease of operation control.

2. BIOL Process

2.1 Features
1) Using inorganic immobilized carrier, microorganisms are maintained to a high concentration, thereby enabling efficient treatment with compact equipment.
2) Return sludge which is indispensable for the activated-sludge process is not required and circulation of nitrifying liquid is not required, achieving extremely easy maintenance control.
3) Stable treated water is obtained even against shock loads such as load fluctuation.
4) Excess microorganisms spontaneously exfoliate from the carrier and flow outside of the system. There is no trouble caused by excessive growth of microbial layer.

2.2 Capabilities and Operating Principle
The flow of the BIOL Process is illustrated as below.
The BIOL Process floats up and suspends minute inorganic carrier in the reaction tank by aeration or by agitator, and removes organic substance and nitrogen in wastewater by the action of microorganisms adhering to the media.

[Nitrification]

Nitrifying bacteria oxidizes ammonia to nitrate through the following reactions:
NH4+2O2 -> NO3^-+H2O+2H⁺

[Denitrification]

Denitrifying bacteria reduces nitrate to nitrogen gas through the following reactions:
2NO3^-+10H -> N2+4H2O+2OH^-

3. Fluidized Bed Bio-reactor with Intermittent Aeration

3.1 Features
1) Organic matter and nitrogen can be removed simultaneously in one single reaction tank by intermittent aeration.
2) Return sludge which is indispensable for the activated-sludge process is not required and circulation of nitrifying liquid is not required, enabling extremely easy maintenance control.
3) Using inorganic immobilized carrier, microorganisms are maintained to a high concentration, thereby enabling efficient treatment with compact equipment.
4) Excess microorganisms spontaneously exfoliate from the carrier and flow outside of the system. There is no trouble caused by excessive growth of microbial layer.
5) Both condensability and dewaterability of generated sludge are far better than those of the standard activated-sludge process.

3.2 Construction and Capabilities
The outline of reactor is illustrated as below. The process flow is described as follows:
1) Raw water is continuously injected to the bottom of the reaction section.
2) Air is fed to the lower part of the draft tube. This fluidizes minute inorganic carrier, bringing the reaction tank inside to the aerobic condition. Then, organic substance is oxidized and decomposes by the action of microorganisms adhering to the media, and ammonia is nitrified at the same time.
3) When air is stopped after specified time, dissolved oxygen in the reaction tank is depleted, while nitrate is removed through reduction to nitrogen gas by utilizing of organic substance in the raw water flowing into the tank.
4) Finally, at the settling section, the carrier and treated water are separated.