JPH0156840B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION 3-1 Field of Industrial Application The present invention relates to a nitrogen removal device that biologically removes nitrogen compounds contained in raw water for drinking water, municipal sewage, various industrial wastewater, and the like.

3-2 Prior Art Recently, pollution of rivers, lakes, and marshes that are sources of drinking water has progressed, and the content of nitrogen compounds such as ammonia nitrogen in raw water for drinking water has increased. In addition, it is difficult to remove nitrogen compounds from urban sewage and various industrial wastewater in the conventional secondary treatment stage, and currently, in many cases, except in some areas, the water is discharged untreated into public water bodies. In water treatment, if raw water contains a large amount of ammonia nitrogen, a larger amount of chlorine must be injected to ensure free residual chlorine during the water purification process. In this case, 10 times more chlorine than ammonia nitrogen is required.
Furthermore, it is necessary to add an alkaline agent to compensate for the decrease in alkalinity caused by chlorine injection. As a result,
The amount of chemicals used in the water purification process will increase significantly. In addition, nutrient salts including nitrogen compounds derived from urban sewage and various industrial wastewaters cause eutrophication of water bodies, causing various damage and disorders.

Various nitrogen removal devices have been developed and put into practical use due to these current circumstances, but biological nitrogen removal devices are the mainstream because they are capable of large-capacity treatment, have a relatively simple process, and have low operating costs. It is becoming. According to such general biological nitrogen removal equipment, nitrogen compounds (organic compounds, ammonia, etc.) contained in the water to be treated are first oxidized into nitric acid and nitrite by the action of nitrifying bacteria in a nitrification tank. is,
Next, in a denitrification tank, nitric acid and nitrite are reduced to nitrogen gas by the action of denitrifying bacteria using a hydrogen supply source (for example, methanol) under an anaerobic atmosphere. Conventionally, when the ammonia nitrogen concentration in the water to be treated is low, the above-mentioned first-stage treatment, i.e., only biological nitrification, is performed, and the ammonia nitrogen is oxidized to nitrate or nitrite nitrogen. Therefore, there were many cases where relatively stable compounds were released. However, in the field of water supply, nitric acid and nitrous acid produced by nitrification are not desirable for drinking water even if they are mineralized, and are known to cause problems in the gastrointestinal tract. Therefore, the tap water standards stipulate that ammonia nitrogen and nitrite nitrogen cannot be detected at the same time, and that nitrate nitrogen is 10 mg/or less. Nitrite nitrogen is also not desirable for industrial water, and for example, in the dyeing and brewing industries, water containing nitrite nitrogen has no utility value. Due to the above, recently,
After converting nitrogen compounds into nitric acid and nitrite nitrogen in biological nitrification equipment, the process of reducing these compounds to nitrogen gas and releasing it into the atmosphere;
In other words, more and more biological denitrification devices are being added. By this method, harmful nitrogen compounds in the water to be treated are completely removed and returned to the atmosphere as harmless nitrogen gas.

Currently, various methods such as activated sludge method, rotating disk method, and fluidized bed method are in practical use as biological nitrification and denitrification equipment, but this medium rotating disk method is easy to maintain and operate. It is attracting attention because of its low cost. When a rotating disk is used as a biological nitrification device, atmospheric oxygen is dissolved on the disk surface or in the rotating disk tank due to the rotation of the rotating disk, and the microorganisms attached to the disk consume it. Then, a biological nitrification reaction is carried out. Therefore, in this device, oxygen is supplied by the rotation of the rotating disk, and it is difficult to freely control the amount of oxygen supplied. For example, in other activated sludge equipment,
Since oxygen is supplied by a blower such as a blower, the amount of oxygen supplied can be controlled relatively easily by changing the amount of air blown. However, this control is difficult in the rotating disk method, and if the rotational speed is constant, a nearly constant amount of oxygen will be supplied. If the water to be treated is raw water for drinking water, urban sewage, secondary treatment water, etc.,
The content of organic components such as BOD is very low, so most of the oxygen used for biological oxidation is for nitrification of nitrogen compounds. In addition, the content of this nitrogen compound is 10 to 20 mg/or less as nitrogen, and when it is less than 1
There are also examples of less than mg/mg. For this reason, the amount of oxygen utilized within the nitrification device is very small, and as a result, the excess oxygen supplied remains in the water flowing out of the nitrification device and dissolves, resulting in a high value of dissolved oxygen. This becomes a hindrance to the fact that the denitrification equipment installed in the latter stage is operated in an anaerobic atmosphere in principle, and reduces the denitrification efficiency. In addition, in biological denitrification equipment, it is necessary to add an organic hydrocarbon source such as methanol as a hydrogen donor to the denitrifying bacteria that reduce nitric acid and nitrite, but if dissolved oxygen exists in the water to be treated, this Oxygen reacts with the organic hydrocarbon source and consumes the organic hydrocarbon source, resulting in a deficiency in the denitrification reaction. Therefore, in this case, in order for the denitrification reaction to proceed sufficiently, an organic hydrocarbon source must be added in addition to the amount consumed by dissolved oxygen.
The cost of chemicals required for the denitrification reaction will increase. This value will increase as the dissolved oxygen concentration in the water to be treated increases.

3-3 Purpose of the Invention The present invention has been made in view of the above-mentioned problems, and its purpose is to make the biological denitrification reaction proceed more efficiently and to further reduce its operating cost. The object of the present invention is to provide a nitrogen removal device that obtains the desired results.

3-4 Structure of the Invention In other words, the present invention provides a nitrification tank that biologically nitrifies nitrogen compounds in water to be treated using a rotating disk, and a denitrification system that biologically denitrifies the water that has been nitrified in the nitrification tank. This nitrogen removal device is characterized in that a vacuum degassing device for removing oxygen gas contained in the water to be treated is installed between the tank and the tank.

The present invention will be explained in detail below with reference to the drawings.

FIG. 1 is a schematic diagram of a biological nitrogen removal apparatus, which is an embodiment of the present invention, and FIG. 2 is a graph showing the relationship between the dissolved oxygen concentration at the outlet of the denitrification tank and the denitrification rate.

To explain Fig. 1 below, the nitrification tank 1 consists of corrugated plastic discs 2 and flat discs 2, each having a diameter of 1 to 3 m and a thickness of approximately 0.5 to 2.0 mm, stacked alternately. It consists of rotary disks 4 arranged in a rotary manner and a tank 5 in which approximately 50% of the entire disk surface of the rotary disks 4 can be submerged in the water to be treated. A plurality of tanks (four tanks in this case) are sequentially connected in series along the flow direction of the water to be treated W supplied from the tank. The nitrification tanks 1 may be connected in series and/or in parallel.

In addition, the denitrification tank 7 consists of a rotating disk 4' in which a plurality of disks 2' are arranged and supported on a central rotation shaft 3', as in the nitrification tank 1 , and in this case, the entire rotating disk 4' is The denitrification tank 7 consists of a tank 5' whose disk surface is substantially submerged in the water to be treated and which can be sealed. (In this case, 4 tanks) are connected in series. The denitrification tank 7 may be connected in series and/or in parallel like the nitrification tank 1 .

Although the denitrification tank 7 is composed of a rotating disk in the case of FIG. 1, it is not limited to this.
Any biological denitrification device may be used, and even a normal agitated water tank type denitrification tank can be used. Note that the denitrification tank 7 is provided with an injection device 8 that adds an organic hydrocarbon source, such as methanol, as a hydrogen donor just before the water to be treated flows into the tank.

Thus, in the present invention, a vacuum deaerator 9 is installed between the nitrification tank 1 and the denitrification tank 7 in order to remove oxygen dissolved in the water to be treated W. In FIG. 1, the vacuum degassing device 9 is composed of a column 10 in which gas is stored at the top above the liquid level, and a vacuum pump 11 connected via a pipe from near the top of the column. There is. The column 10 may be a cylindrical drum-like structure, the height of which is determined by the set pressure inside. Furthermore, in order to increase deaeration efficiency, it is also possible to provide a shelf inside the column or insert a packing material such as a Raschig ring. The vacuum pump 11 can also be replaced with an ejector or the like.

Next, the operation of the present invention will be described. Water to be treated W containing nitrogen compounds (organic compounds, ammonia, etc.) is supplied to the nitrification tank 1 made up of a rotating disk 4 through the supply path 6. Here, the nitrogen compounds are oxidized to nitric acid or nitrous acid by the action of microorganisms attached to the surface of the rotating disk. An aerobic reaction occurs due to the supply of oxygen from the atmosphere, and oxygen is consumed. However, if the concentration of nitrogen compounds and organic matter in the water to be treated is low, the amount of oxygen supplied exceeds the amount consumed, and the effluent from the nitrification tank 1 (The nitrified water to be treated W) exhibits a high dissolved oxygen concentration. Therefore, in the present invention, this effluent water is not directly led to the denitrification tank 7 , but is once supplied to the reduced pressure deaerator 9 , and the inside of the equipment is maintained at a reduced pressure state below atmospheric pressure by the vacuum pump 11. Dissolved gases and bubbles are released. This removes dissolved oxygen in the water to be treated. The water to be treated W passes through a vacuum degassing device 9 to reduce the dissolved oxygen concentration, while an organic hydrocarbon source is added by a hydrogen donor (methanol etc.) injection device 8 and supplied to the denitrification tank 7. Ru. In the tank 7 , nitric acid and nitrite are reduced by the action of denitrifying bacteria, and nitrogen gas is released. Through the above steps, nitrogen compounds in the water to be treated are decomposed and removed.

Figure 2 is a graph showing the dissolved oxygen concentration at the outlet of the denitrification tank and the denitrification rate when a biological nitrification and denitrification experiment was conducted using the rotating disk method as a pretreatment for the water purification process in water treatment. It is. In this case, no vacuum deaerator is interposed between the nitrification tank and the denitrification tank (that is, it corresponds to the direct connection of the nitrification tank 1 and the denitrification tank 7 in Fig. 1), and the outflow water from the nitrification tank is Dissolved oxygen was present at almost saturation concentration. Methanol, a hydrogen donor, was added to the effluent in various proportions. As a result, the dissolved oxygen concentration at the outlet of the denitrification tank fluctuates, and the denitrification rates of nitric acid and nitrite also change accordingly. As is clear from FIG. 2, the lower the dissolved oxygen concentration, the higher the denitrification rate. In this case, an organic hydrocarbon source as a hydrogen donor is added in excess of the amount necessary for the denitrification reaction to the nitrification tank effluent to reduce dissolved oxygen. However, this method requires reduction of oxygen and a large amount of organic hydrocarbon source, and this amount increases as the denitrification rate increases. As a result, the operating cost of the equipment increases rapidly.

In the present invention, the removal of dissolved oxygen from the organic hydrocarbon source described above is carried out by a vacuum degassing device 9 installed between the nitrification tank 1 and the denitrification tank 7 . The amount of organic hydrocarbon source required for the denitrification reaction is limited to only the power costs for pumps, etc., and the amount of chemicals used can be greatly reduced. Further, according to the present invention, the dissolved oxygen in the nitrification tank effluent is stably maintained at a low value, so that the denitrification reaction in the denitrification tank can be carried out efficiently and stably. For example, according to Figure 2, when the amount of organic hydrocarbon source necessary for the denitrification reaction is added to the denitrification tank, the denitrification rate is only about 30% when the dissolved oxygen in the denitrification tank is 5 mg/. . However, it is clear from the data shown in Figure 2 that according to the present invention, a vacuum degassing device is installed at the rear stage of the nitrification tank consisting of a rotating disk to reduce the dissolved oxygen concentration in the water flowing into the denitrification tank. As such, the denitrification rate gradually increases. The denitrification tank has dissolved oxygen of 0.5 mg/approx.
% denitrification rate will be obtained.

In summary, the present invention provides raw water for drinking water, urban sewage,
In equipment that biologically removes nitrogen compounds contained in various types of industrial wastewater, etc., a vacuum degassing device is installed between the nitrification tank and the denitrification tank, which are made up of rotating disks, to remove oxygen gas dissolved in the water to be treated. It is characterized by intervening.

3-5 Effects of the Invention According to the present invention, dissolved oxygen is mechanically removed by a vacuum deaerator from effluent water with a high dissolved oxygen concentration that has been nitrified in a nitrification tank consisting of a rotating disk, and Supplied to nitrogen tank. As a result, the organic hydrocarbon source, which was conventionally consumed in the reaction with high dissolved oxygen in addition to the denitrification reaction, became unnecessary, and the operating cost of the denitrification tank could be significantly reduced. At the same time, it has become possible to maintain the dissolved oxygen concentration in the denitrification tank at a constant low level, making it possible to obtain a high and stable denitrification rate.

[Brief explanation of drawings]

FIG. 1 is an embodiment of the present invention, which is a schematic diagram of a biological nitrogen removal device, and FIG. 2 is a graph showing the relationship between the dissolved oxygen concentration at the outlet of the denitrification tank and the denitrification rate. 1 ...Nitrification tank, 7 ...Denitrification tank, 9 ...Reducing pressure deaerator.

Claims (1)

[Claims] 1. Water to be treated is placed between a nitrification tank that biologically nitrifies nitrogen compounds in the water to be treated using a rotating disk and a denitrification tank that biologically denitrifies the water to be treated that has been nitrified in the nitrification tank. A nitrogen removal device characterized by being equipped with a vacuum deaeration device for removing oxygen gas contained in the nitrogen gas.