JPH0237240B2 - - Google Patents

Description

[Detailed description of the invention]

[] BACKGROUND TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for removing nitrogen from wastewater using an activated sludge method. More specifically, the present invention utilizes conventional
This paper relates to a method for denitrifying wastewater by controlling the conditions for implementing the activated sludge method for BOD reduction. Among the wastewater denitrification techniques that have been put to practical use to date, biological treatment is the most common method, and one specific example of this method is the nitrification-denitrification method. This nitrification-denitrification method consists of first oxidizing and decomposing nitrogen compounds in wastewater into nitrate nitrogen, and then reducing this to nitrogen gas and dissipating it into the atmosphere. The nitrification process and the reduction or denitrification process are carried out by utilizing the physiological effects of obligate aerobic bacteria (nitrifying bacteria) and facultative anaerobic bacteria (denitrifying bacteria), respectively. This denitrification method has similarities with the activated sludge method, which is widely used for BOD removal, and
It appears to be effective to utilize wastewater with reduced nitrogen content when a further reduction in nitrogen compound content is required. However, as mentioned above, the conventional nitrification-denitrification method basically consists of a nitrification process and a denitrification process, and denitrification equipment is required in addition to nitrification equipment to implement this method. Therefore, even if existing nitrification equipment for BOD removal can be used, it is necessary to install new denitrification equipment at considerable expense, so this method is not necessarily easy to implement. In fact, practical examples of this denitrification method have only been seen in human waste wastewater with a high concentration of nitrogen compounds, and at present it cannot be said that it has been generalized to industrial wastewater treatment. Prior Art There are many reports regarding factors that affect nitrification in conventional nitrification-denitrification methods. For example, the relationship between PH and temperature and nitrification rate (Water Treatment Technology, 18
(8), 753 (1977)), nitrifying bacteria are more susceptible to the load and dissolved oxygen than BOD bacteria (Water and Wastewater, 12 , 1076 (1970)), and nitrification is influenced by water temperature and dissolved oxygen. It is affected by oxygen (``Ministry of Construction Civil Engineering Research Institute, Sewage Department Research Laboratory 5-54''), and nitrate nitrogen decreases as the load increases (``Sanitary Chemistry'', 25 (1) ), 13-18 (1979). However, as a matter of course, based on the denitrification principle of the conventional nitrification-denitrification method, the idea of suppressing nitrification from the viewpoint of denitrification was There are very few studies based on this, and as far as the present inventors know, the only ones that have been reported are Japanese Patent Application Laid-open Nos. 53-45048 and 53-31366. It is understood that the purpose is to multiply bacterial cells for nitrogen, so the concentration of microorganisms in wastewater is kept low at 50 to 200 ppm, and a high load is applied to increase bacterial cell growth. In this prior invention, the total organic carbon treatment weight per day per total organic carbon weight per microorganism weight in the treatment tank (KgTOC/KgMLSS/day) is adjusted to be 3 to 25. [] Summary of the Invention As mentioned above, the mainstream denitrification technology today is the nitrification-denitrification method, which requires complete nitrification as a preliminary step for denitrification. However, the present invention attempts to perform denitrification by preventing nitrification by reusing ammonia nitrogen leaking from raw wastewater and from bacterial cells for bacterial cell synthesis. The method for denitrifying wastewater according to the invention is that when wastewater is treated by the activated sludge method, which consists of treatment with activated sludge made of microorganisms in an aeration tank, the activated sludge method is carried out under the following conditions, and the treated water is (1) The nitrogen concentration of the wastewater to be treated is N/BOD (weight ratio) ≦5/ 100. (2) The aeration tank water temperature, minimum required MLSS load, and dissolved oxygen amount must meet the conditions listed in Table-1.

【table】

[Characteristics of wastewater]

(B) COD cr. 700-1300 (mg/) (B) BOD/COD approx. 0.7 (C) PH 6-10 (D) T-N (E) Nutritional balance COD:N:P=
100:2.9:0.6 BOD:N:P=100:4.1:0.9 (4) Activated sludge Collected from the aeration tank outlet was used. (1) MLSS 7000-11000mg/ (2) MLVSS 4500-7000mg/ (3) VSS/SS 0.65 (5) Analysis method (1) COD JIS-KO102 (2) BOD JIS-KO102 (3) Organic nitrogen JIS- KO102 (4) Nitrate nitrogen JIS-KO102 (5) Dissolved oxygen JIS-KO102 (6) MLSS, MLVSS Sewage test method [2] Experimental conditions A Test using activated sludge experimental equipment

[Table] (Note) Operate at a sludge load of 0.05 until the inside of the aeration tank reaches the test temperature. (2 days)

【table】

[Table] *) Based on standing still.

[Table] B Field test at wastewater treatment plant (B-1) Aeration tank reduction operation method at low load (1) Aeration tank reduction degree calculation (1) Sludge load calculation COD concentration of raw wastewater (C, mg/
), drainage volume (Q, m ³ /d), sludge concentration (MLSS, mg/), and aeration tank volume (V, m ³ ) to calculate the COD sludge load. COD sludge load (KgCOD/Kg・MLSS・d) =C×Q×10
^-3 /MLSS×V×10 ^-3 (2) Comparison with the required sludge load After calculating the COD sludge load, compare it with the required sludge load (described later). When COD sludge load > Required sludge load → Normal operation When COD sludge load and required sludge load → Reduced operation (3) Determining the number of drainage channels (reduction degree) during reduced operation Unit division tank into which wastewater flows during reduced operation (hereinafter referred to as a flow path or path) is determined using the following formula. Number of channels used = SL/ESL x total number of channels in the aeration tank (Note) SL = sludge load. ESL = Required sludge load. (Example) SL = 0.03, ESL (25°) =
0.07, total number of channels in the aeration tank = 5, number of channels used = 0.03/0.07×
5≒2 (passes) Therefore, feed wastewater to 2 of the 5 passes of the aeration tank. (Reduction degree = 2/5) (4) Name of reduction operation The operation method that performs wastewater treatment in only 2 out of 5 passes of the aeration tank is called ``2/5''.
5 reduction operation. Therefore, if the aeration tank has 5 passes, it will be 1/5, 2/5, 3/5, depending on the load amount.
A reduced operation of 4/5 is considered. (2) Reduced operation method (1) Basics of aeration In reduced operation, sufficient dissolved oxygen is given to the channels where wastewater flows in to oxidize organic matter, and in channels where wastewater does not enter, aeration is weakened to remove almost all dissolved oxygen. The basic driving principle is to set it to 0. (2) Conditions for reduced operation The operating conditions are as shown in the table below. The amount of wastewater inflow will be changed depending on the degree of reduction. (Example) 2/5 reduction operation

[Table] (B-2) Field test In the field test, normal operation or reduced operation was performed depending on the amount of wastewater generated. Normally, the experimental results (required load) obtained in A-3 to A-6 are used for selection of reduction. Based on the results of field tests, we will create operational management standards for N control. 2 Test Results Tests using an activated sludge testing device and field tests were conducted and the following results were obtained. (1) The nitrification rate increases as the aeration tank water temperature increases (Figure 1) Nitrification rate at 15℃...0.5 g・NO ₂ −
N+NO ₃ -N/KgMLSS・d Nitrification rate at 25℃…0.6 to 1.2 g・
NO ₂ −N + NO ₃ −N/KgMLSS・d Nitrification rate at 35℃…1.5 to 1.6 g・
NO ₂ −N+NO ₃ −N/KgMLSS·d (2) The higher the MLS concentration, the more NO ₃ −N produced. However, per unit amount of bacterial cells
The amount of NO ₃ -N produced is almost constant regardless of the MLSS concentration. Therefore, when the concentration is high
The reason why NO ₃ -N is also high is simply because the amount of bacterial cells is large (Figure 2).

[Table] (3) Dissolved oxygen and nitrification rate (1) When DO≒0 If there is almost no dissolved oxygen, nitrification will not proceed, and the amount of NO ₃ -N produced will be 1mg/
It will be below (nearly 0). (2) When DO > 0 Nitrification progressed, but the amount of nitrification was not proportional to DO. That is, DO=3
mg/ was higher than when DO=9 mg/. However, in terms of unit bacterial mass, both are almost the same, and the nitrification rate is approximately 0.2g・NO ₃ −N/KgMLSS・d
(at 20℃). From the above, as far as nitrogen control is concerned,
Dissolved oxygen seemed to have little to do with its level, except when it was less than 1 mg/kg (Figure 3). (4) Control of NO ₃ -N by COD load (1) The nitrification suppression effect is greatly affected by water temperature, and when a constant load is applied, NO ₃ -N
The rate of decrease in NO ₃ -N increases as the load increases (Figures 5, 7, 10) (3) The load required for NO ₃ -N suppression differs depending on the water temperature, and the lower the temperature, the smaller the load, and the higher the temperature, the larger the load (5th, 7th,
(Figure 10) (4) The minimum required load to prevent the generation of NO ₃ -N is as follows.

[Table] (5) Field test (1) The minimum required load obtained as a result of the small-scale test was applied to an actual plant, and it was confirmed that the same effect was obtained. (2) Reduced operation did not cause any deterioration in treatment even after one week of continuous operation. (3) Based on the results of small-scale tests and on-site tests, we created "Operation Management Standards for Nitrate Nitrogen Control" (one example). The standard load used in setting this operation management standard was set as follows, with reference to the minimum required load amount and the relationship between NO ₃ -N and load during normal operation at the site. This load includes a safety factor (Table 9, Figure 12).

[Table] (4) "Operational management standards for NO ₃ -N control" considering actual plant operation records
(for 5 passes) was created (Fig. 13A~
B, Table 10).

【table】

【table】

【table】 [Brief explanation of drawings]

1 to 13 are graphs showing experimental results. The contents of each figure are as follows. Figure number Contents 1 Experimental results under the conditions shown in Table 1. The upper row of the horizontal axis showing the number of days elapsed is the number of acclimatization days, and the lower row is the number of days elapsed at the specified temperature. 2 Experimental results under the conditions shown in Table 2. The horizontal axis is the number of days that have passed. 3 Experimental results under the conditions shown in Table 3. 4 Experimental results under the conditions of item A-4 1 in Table 4. 5 Experimental results under the conditions of item A-4, 2 in Table 4. The upper row of the horizontal axis showing the number of elapsed days is the number of days during dry aeration, that is, when the sludge load is 0, and the lower row is the number of days elapsed at the time of sludge charging. 6 Experimental results under the conditions of item A-5 1 in Table 4. 7 Experimental results under the conditions of item A-5, 2 in Table 4. The upper row of the horizontal axis showing the number of elapsed days is the number of days during dry aeration, that is, when the sludge load is 0, and the lower row is the number of days elapsed at the time of sludge charging. 8 Experimental results under the conditions of item A-6 1 in Table 4. In the figure, the subscript of the broken line is the sludge load. 9 Experimental results under the conditions of item A-6, 2 in Table 4. In the figure, the subscript of the broken line is the sludge load. 10 The results shown in Figures 8 and 9 have been rewritten as a function of sludge load. The first time corresponds to Figure 8, and the second time corresponds to Figure 9. 11 The results shown in Figures 4 to 9 have been rewritten as a function of sludge load. The 25°C curve corresponds to the results shown in Figures 4-5, the 30°C curve to Figures 6-7, and the 35°C curve to Figures 8-9. 12 Sludge load and NO ₃ − in an actual plant
Relationship with N production amount. The curves in the figure are copied from FIG. 11. 13 NO ₃ − in treated water at an actual plant
Effect of reduced operation on N concentration. (A) Normal operation (B) Reduced operation

Claims (1)

[Scope of Claims] 1. When treating wastewater by the activated sludge method, which consists of treatment with activated sludge made of microorganisms in an aeration tank, the activated sludge method is carried out under the following conditions, and the treated water is characterized by reducing the total nitrogen content by suppressing the production of nitrate nitrogen,
Wastewater denitrification method. (1) The nitrogen concentration of the wastewater to be treated satisfies the relationship of N/BOD (weight ratio) ≦5/100. (2) The aeration tank water temperature, minimum required MLSS load, and amount of dissolved oxygen must meet the conditions listed in Table-1. [Table] [Table] 2 Wastewater is now distributed into aeration tanks consisting of unit compartment tanks, and the amount of wastewater to be treated has decreased, resulting in an increase in the MLSS load value (KgBOD/KgMLSS/day). The denitrification method according to claim 1, wherein when the minimum required value described in claim 1 is not reached, the activated sludge method is carried out under the following conditions by introducing wastewater into only a part of the unit division tank. [Table] Values listed without wastewater input 0 <1
Ward 3 Wastewater to be treated is already N/BOD≦5/
The denitrification method according to any one of claims 1 to 2, wherein the nitrogen concentration satisfies the relationship of 100.