Updated: July 2002

1. Introduction

When nitrogen enters public waters such as rivers, sea areas, lakes and etc., it causes eutrophication and becomes a prime mover for the generation of a red tide or algae. Nitrogen takes various forms such as ammonium nitrogen, nitric nitrogen, organic nitrogen and etc. and a biological denitrifying method is widely used as a means for treating them. However, the biological treating method has had such disadvantages that since it requires an extremely large reaction tank, the area of installation of the tank becomes extremely large and the construction cost becomes enormously high.
In order to solve the above-described problems, our company has developed and produced a high-efficiency denitrifying equipment (USB system).
This system has made it possible to subject nitric nitrogen discharged during the process of metallic surface treatment, inorganic medicine manufacture or catalyst manufacture to a denitrifying treatment by using a reaction tank of a volume of 1/10-20 of that of the conventional reaction tank.

2. Outline of Equipment

The USB system comprises a reaction section filled with a granule obtained by condensing denitrifying bacterial to a high density, a vapor-solid separation section for separating from the granule nitrogen gas generating due to denitrification and a precipitation section for separating the granule from treated water.

Waste water containing nitric acid nitrogen enters the USB system from the lower part of the system and passes through the reaction section as it forms an upward flow. In this process, the nitric nitrogen is reduced to a nitrogen gas by denitrifying bacteria contained in the granule. Then the nitrogen gas flows upward within the reaction tank as it becomes fine air bubbles and moves upward within the tank so that it is separated from the granule by a vapor-solid separator provided at the upper part of the reaction tank. Then the separated nitrogen gas is released into the atmosphere while the granule is separated from the processed water at the precipitation section and is re-circulated through the reaction section.
In the process of denitrification reaction, an organic carbon source is needed as an energy source for the denitrifying bacteria but in case where the waste water contains no organic carbon source, methanol is added.

3. Features

(1) Realization of High-Efficiency Nitrogen Load Factor
The granule used in the UBS system retain high-density denitrifying bacteria and the MLVSS density reaches a level as high as 50000-80000 mg/l.
Therefore, according to the conventional system, the nitrogen load has been limited to 1-2kg-m³N/day. On the contrary, according to the USB system, it is possible to obtain a nitrogen load of 30 kg - m³N/day at the maximum and that of 10-20 kg-m³N/day even normally. This means that the size of the equipment becomes 1/10-30 of that in the case of the conventional method. Thus, the USB system has made it possible to sharply reduce not only the area of installation of the equipment but also the cost of equipment.
(2) High nitrogen treatability
By using a granule retaining denitrifying bacteria to a high density, the USB system is resistant against load fluctuations and has a wide range of adaptability even with respect to waste water having a nitric nitrogen density of several tens-several thousands mg/l and it is possible with this system to treat the target water down to a level of below 10 mg/l in a stabilized manner.
(3) Reduction of running cost
(4) Reduction of the amount of generation of sludge