JP7683155B2 - Excess sludge treatment method and excess sludge treatment system - Google Patents

Description

The present invention relates to a method and system for treating excess sludge containing cyclic ethers.

1,4-dioxane is a cyclic ether represented by the following formula (1): 1,4-dioxane has excellent compatibility with water and organic solvents and is mainly used as a reaction solvent in organic synthesis.

In fiscal year 2010, the amount of 1,4-dioxane produced and imported in Japan was approximately 4,500 tons per year, and it is estimated that approximately 300 tons per year was released into the environment. 1,4-dioxane is water-soluble, so if it is released into the aquatic environment, it will diffuse over a wide area. In addition, it is difficult to remove from water because it has low volatility, adsorption to solids, photodegradability, hydrolysis, and biodegradability. 1,4-dioxane is acutely and chronically toxic, and has also been identified as carcinogenic, so there are concerns that contamination of the aquatic environment by 1,4-dioxane will have adverse effects on humans, animals, and plants. For this reason, 1,4-dioxane is regulated in Japan by drinking water quality standards (0.05 mg/L or less), environmental standards (0.05 mg/L or less), and wastewater standards (0.5 mg/L or less).

Non-Patent Document 1 reports that industrial wastewater containing 1,4-dioxane may contain various cyclic ether compounds such as 1,3-dioxolane and 2-methyl-1,3-dioxolane in addition to 1,4-dioxane. In particular, 1,3-dioxolane has been confirmed to be acutely toxic, and contaminated water containing 1,3-dioxolane must be appropriately treated.
Here, the effectiveness of the treatment of cyclic ethers such as 1,4-dioxane has been confirmed only in the advanced oxidation method. However, the advanced oxidation method has not come into widespread use due to its high initial and running costs. In addition, Non-Patent Document 2 reports that the presence of other organic compounds reduces the treatment efficiency of 1,4-dioxane by the advanced oxidation method.

There is a demand for a method for treating water containing cyclic ether compounds such as 1,4-dioxane stably at low cost. Non-Patent Document 3 proposes treating 1,4-dioxane using 1,4-dioxane-decomposing bacteria.
The present inventors have reported in Patent Document 1 the N23 strain (Accession No. NITE BP-02032), which is a constitutive 1,4-dioxane decomposing bacterium. Among the constitutive 1,4-dioxane decomposing bacteria reported so far, the N23 strain exhibits the highest maximum specific decomposition rate of 1,4-dioxane, and is highly promising for biodegradation of cyclic ether compounds including 1,4-dioxane. In addition, the present inventors have proposed in Patent Document 2 an efficient method for treating contaminated water containing 1,4-dioxane and other organic compounds, in which a pretreatment step for mainly reducing the concentration of other organic compounds and a biodegradation step for 1,4-dioxane using 1,4-dioxane decomposing bacteria are carried out in that order.

When cyclic ethers and other organic compounds are treated by the method of the invention described in Patent Document 2, if the pretreatment process is performed using activated sludge, excess sludge is generated. The excess sludge is precipitated and dehydrated to become a dehydrated cake (dehydrated sludge), which is then treated as industrial waste. Since it is difficult to biodegrade cyclic ethers in treatment using activated sludge, the excess sludge may contain high concentrations of cyclic ethers. If the excess sludge contains high concentrations of cyclic ethers, the dehydrated cake obtained after dehydration also contains high concentrations of cyclic ethers, and therefore must be treated as specially controlled industrial waste rather than as ordinary industrial waste, resulting in significantly increased disposal costs compared to ordinary industrial waste.

Patent No. 6117450 JP 2019-084498 A

CD. Adams, PA. Scanlan and ND. Secrist: Oxidation and biodegradability enhancement of 1,4-dioxane using hydrogen peroxide and ozone, Environ. Sci. Technol., 28(11), pp.1812-1818, 1994. K. KOSAKA, H. YAMADA, S. MATSUI, and K. SHISHIDA: The effects of the co-existing compounds on the decomposition of micropollutants using the ozone/hydrogen peroxide process. Water Sci. Technol., 42, pp.353-361, 2000. Kazunari Kiyomi and Michihiko Ike: Possibility of biological treatment and purification of contaminated groundwater using 1,4-dioxane-degrading bacteria, Water and Wastewater, Vol.53, No.7, 2011.

The objective of the present invention is to provide a method and system for treating excess sludge containing cyclic ethers that is generated during activated sludge treatment of contaminated water containing cyclic ethers and other organic compounds.

The means for solving the problems of the present invention are as follows.
1. Excess sludge containing cyclic ethers generated in the activated sludge treatment tank is
A method for treating excess sludge, comprising biodegrading cyclic ethers in an excess sludge treatment tank using an N23 strain (accession number NITE BP-02032) under conditions of a BOD of 350 mg/L or less and a ratio of the N23 strain concentration to the excess sludge concentration (N23 strain/excess sludge) of 0.008 to 0.17.
2. The method for treating excess sludge according to 1., wherein the excess sludge concentration in the excess sludge treatment tank is 1,000 mg/L or more and 30,000 mg/L or less.
3. The method for treating excess sludge according to 1. or 2., wherein the N23 strain is supplied to the excess sludge treatment tank in the form of N23 strain-containing sludge generated in a cyclic ether treatment tank located downstream of the activated sludge treatment tank.
4. The method for treating excess sludge according to any one of 1. to 3., wherein the cyclic ether contains 1,4-dioxane.
5. An activated sludge treatment tank for treating the contaminated water containing cyclic ethers and other organic compounds mainly with other organic compounds using activated sludge;
a cyclic ether treatment tank located downstream of the activated sludge treatment tank for treating a cyclic ether;
an excess sludge treatment tank for treating excess sludge containing cyclic ethers generated in the activated sludge treatment tank,
The excess sludge treatment system is characterized in that in the excess sludge treatment tank, cyclic ethers are biodegraded using an N23 strain (accession number NITE BP-02032) under conditions of a BOD of 350 mg/L or less and a ratio of the N23 strain concentration to the excess sludge concentration (N23 strain/excess sludge) of 0.008 to 0.17.
6. The cyclic ether treatment tank is a tank for performing biodegradation treatment using the N23 strain,
6. The excess sludge treatment system according to 5., wherein the N23 strain-containing sludge generated in the cyclic ether treatment tank is supplied to the excess sludge treatment tank.

The excess sludge treatment method of the present invention can efficiently treat cyclic ethers by suppressing the activity of other microorganisms and activating the N23 strain even under conditions in which many other microorganisms derived from the excess sludge are present. By lowering the cyclic ether concentration, the dehydrated cake obtained by dehydrating the excess sludge can be treated as normal industrial waste, thereby reducing treatment costs.
When cyclic ethers are biodegraded by the N23 strain, sludge containing the N23 strain is generated. By supplying this N23 strain-containing sludge to an excess sludge treatment tank and treating the cyclic ethers in the excess sludge treatment tank with the N23 strain-containing sludge, the N23 strain-containing sludge can be effectively utilized, and the cost of culturing the N23 strain can be reduced compared to when the cultured N23 strain is directly supplied to the excess sludge treatment tank.

FIG. 1 is a configuration diagram of an excess sludge treatment system according to one embodiment of the present invention. 1 is a graph showing the relationship between the ratio of the N23 strain concentration to the excess sludge concentration (N23 strain/excess sludge) and the dioxane decomposition rate (relative value). Graph showing the change in dioxane concentration over time at different BOD concentrations with an initial bacterial cell concentration of 200 mg/L. Graph showing the change in dioxane concentration over time at different BOD concentrations with an initial bacterial cell concentration of 1,000 mg/L.

The present invention will be described in detail below.
The method for treating excess sludge of the present invention comprises treating excess sludge containing cyclic ethers generated in an activated sludge treatment tank by:
In an excess sludge treatment tank, cyclic ethers are biodegraded using the N23 strain (accession number NITE BP-02032) under conditions of a BOD of 350 mg/L or less and a ratio of the N23 strain concentration to the excess sludge concentration (N23 strain/excess sludge) of 0.008 to 0.17. Note that in this specification, the expression "A to B" (A and B are numerical values) means a numerical range including the values of A and B, i.e., A or more and B or less.

The excess sludge treatment method of the present invention will be described with reference to an excess sludge treatment system as an embodiment shown in FIG.
The excess sludge treatment system according to one embodiment comprises an activated sludge treatment tank 1, a cyclic ether treatment tank 2, an excess sludge treatment tank 3, and a dehydrator 4.

The contaminated water treated by the present invention contains cyclic ethers and other organic compounds.
Examples of cyclic ethers include 1,4-dioxane, tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 2-methyl-1,3-dioxolane, etc. Of these, it is preferable to treat contaminated water containing 1,4-dioxane.
There are no limitations on the other organic compounds as long as they are organic compounds that can be biodegraded by activated sludge, and the types vary depending on the chemical plant, former factory site, landfill site, etc. where the contaminated water is generated.

Contaminated water containing cyclic ethers and other organic compounds is treated mainly for the other organic compounds in the activated sludge treatment tank 1. The treatment of the other organic compounds in the activated sludge treatment tank 1 can be carried out according to a standard method. In the activated sludge treatment tank 1, it is preferable to remove 30 wt% or more of the other organic compounds, more preferably 40 wt% or more, even more preferably 50 wt% or more, and most preferably 60 wt% or more.
Since the treatment of cyclic ethers does not progress much in the activated sludge treatment, the contaminated water is treated in the activated sludge treatment tank 1 to become primary treated water containing cyclic ethers, which is then transferred to the cyclic ether treatment tank 2 .

The primary treated water is treated for cyclic ethers by the N23 strain in the cyclic ether treatment tank 2 .
The N23 strain (Pseudonocardia sp. N23) was deposited internationally on April 10, 2015 at the National Institute of Technology and Evaluation, Patent Microorganism Depositary (NPMD) (2-5-8 Kazusa Kamatari, Kisarazu City, Chiba Prefecture, Japan (postal code 292-0818)) under the accession number NITE BP-02032.
The N23 strain is a constitutive 1,4-dioxane decomposing bacterium, and constantly produces decomposing enzymes. The N23 strain has the highest maximum specific decomposition rate of 1,4-dioxane, 0.216 mg-1,4-dioxane/mg-protein·h, among constitutive 1,4-dioxane decomposing bacteria reported so far. The N23 strain can decompose 1,4-dioxane to a very low concentration of 0.017 mg/L or less, and can treat 1,4-dioxane at a high concentration of about 5200 mg/L. The N23 strain has an optimum pH of about neutral pH 6 to 8, but the decrease in activity under acidic conditions is small, and the decomposition activity can be maintained at 90% or more at pH 5.0 and 80% or more at pH 3.8 compared to the decomposition activity at pH 7.0. Therefore, the N23 strain can be suitably used in the biodegradation treatment process of 1,4-dioxane.

The primary treated water has a low concentration of other organic compounds, so cyclic ether treatment using the N23 strain can be carried out efficiently. Cyclic ether treatment using the N23 strain in the cyclic ether treatment tank 2 is preferably carried out at a pH of 4 to 8, and more preferably at a pH of 4.5 to 5.5. By keeping the pH within this range, it is possible to suppress the proliferation of miscellaneous bacteria while maintaining a certain level of decomposition by the N23 strain.

Excess sludge is generated in activated sludge treatment tank 1, but since biodegradation of cyclic ethers does not progress much within activated sludge treatment tank 1, the excess sludge contains cyclic ethers. This excess sludge containing cyclic ethers and the N23 strain-containing sludge generated in cyclic ether treatment tank 2 are sent to excess sludge treatment tank 3. The cyclic ethers contained in the excess sludge are then treated by the N23 strains in the N23 strain-containing sludge under conditions of a BOD (biochemical oxygen demand) of 350 mg/L or less and a ratio of the N23 strain concentration to the excess sludge concentration (N23 strains/excess sludge) of 0.008 to 0.17.

In the excess sludge treatment tank 3, various microorganisms derived from the excess sludge and the N23 strain derived from the N23-containing sludge live. The N23 strain is less active and proliferative than other microorganisms, but the activity of other microorganisms is suppressed when the BOD is 350 mg/L or less. The N23 strain can metabolize cyclic ethers and remain active even when the BOD is 350 mg/L or less, so it can be active and break down cyclic ethers preferentially over other microorganisms. The BOD is preferably 320 mg/L or less, and more preferably 300 mg/L or less.

Although the reason for this is unclear, when the ratio of the N23 strain concentration to the excess sludge concentration (N23 strain/excess sludge) is 0.008 or more and 0.17 or less, the N23 strain can maintain a cyclic ether decomposition rate of 50% or more compared to the case where there is no excess sludge, and cyclic ethers can be efficiently biodegraded. The ratio of the N23 strain concentration to the excess sludge concentration (N23 strain/excess sludge) is preferably 0.01 or more, more preferably 0.012 or more, and even more preferably 0.015 or more. This ratio is also preferably 0.15 or less, more preferably 0.14 or less, and even more preferably 0.12 or less.

In the excess sludge treatment tank 3, the ratio of the N23 strain concentration to the excess sludge concentration (N23 strain/excess sludge) can be adjusted by the amount of excess sludge drawn from the activated sludge treatment tank 1 and the amount of N23 strain-containing sludge drawn from the cyclic ether treatment tank 2. In this case, it is preferable that the excess sludge concentration in the excess sludge treatment tank 3 is 1,000 mg/L or more and 30,000 mg/L or less from the viewpoint of treatment efficiency. In the present invention, the supply of the N23 strain is not limited to the form of N23 strain-containing sludge, and it can be supplied in any form such as bacterial cells filtered from a culture solution, frozen and stored bacterial cells, dried and stored bacterial cells, freeze-dried bacterial cells, an immobilization carrier in which the N23 strain is immobilized on a resin or the like, or a suspension containing the N23 strain, such as a culture solution or a concentrated solution thereof.

After the cyclic ether is treated in the excess sludge treatment tank 3 until its concentration falls below a predetermined level, the sludge is allowed to settle, and the treated supernatant water is discharged as treated water. The settled sludge is withdrawn and dehydrated using a known dehydrator 4, and separated into a dehydrated cake and desorbed water. The dehydrated cake can be treated as general industrial waste because the concentration of pollutants such as cyclic ethers is sufficiently low. In the system shown in FIG. 1, the desorbed water is returned to the activated sludge treatment tank 1. The desorbed water can be returned to the cyclic ether treatment tank 2 or the excess sludge treatment tank 3, or can be discharged as treated water if the concentration of pollutants is sufficiently low.

Excess sludge: Excess sludge from the activated sludge process was used as seed sludge and the sludge cultivated was used as excess sludge.
The excess sludge, which had been washed twice with distilled water, was suspended in an inorganic salt medium solution containing 500 mg/L of 1,4-dioxane (medium composition: K ₂ HPO ₄ : 1 g/L, (NH ₄ ) ₂ SO ₄ : 1 g/L, NaCl: 50 mg/L, MgSO ₄ ·7H ₂ O: 200 mg/L, FeCl ₃ : 10 mg/L, CaCl ₂ : 50 mg/L, pH: 7.3) to prepare excess sludge containing cyclic ethers.

N23 strain The N23 strain was subjected to rotary shaking culture (28° C., 120 rpm) for 7 days using MGY medium (Malt Extract: 10 g/L, Glucose: 4 g/L, Yeast Extract: 4 g/L, pH 7.3) to which dioxane was added to make the concentration 500 mg/L. The culture solution was centrifuged at 10,000×g and 4° C. for 10 minutes to collect the bacteria, _which were then washed _twice with inorganic salts medium not containing a carbon source (medium composition: _K2HPO4 : 1 g/L, ( _NH4 ) _2SO4 : 1 g/L, NaCl: 50 mg/L, _MgSO4.7H2O : ₂₀₀ mg/L, _FeCl3 : 10 mg/L, _CaCl2 : 50 mg/L, pH: 7.3) and suspended in the same inorganic salts medium to a bacterial cell concentration of 4,000 mg-dry cell/L. This was used as the N23 strain inoculum.

- Bacterial cell concentration The bacterial cell protein concentration of the N23 strain was measured according to a previous report (Meyers et al., Novel method for rapid measurement of growth of mycobacteria in detergent-free media, J. Clin. Microbiol., 36 (9) 2752-2754 (1998)). The dry bacterial cell weight and microbial concentration of the N23 strain were determined by filtering the sample using glass fiber filter paper GF/B (particle retention capacity 1.0 μm, Whatman) and subtracting the filter weight before filtration from the weight after drying at 105 ° C. for 2 hours.

Experiment 1: Ratio of N23 strain concentration to excess sludge concentration (N23 strain/excess sludge)
The N23 strain inoculum was added to a suspension of excess sludge containing a cyclic ether so that the ratio of the N23 strain concentration to the excess sludge concentration (N23 strain/excess sludge) was 0.01, 0.02, 0.05, 0.10, or 0.25, and rotary shaking culture was performed at 25° C. In addition, as a control, a blank test (excess sludge 0 mg/L) was performed in the same manner, except that the inorganic salt medium solution containing 1000 mg/L 1,4-dioxane was adjusted to have the N23 strain cell concentrations of 200, 1,000, and 5,000 mg/L.
During the test period, the dioxane concentration was measured appropriately using a headspace GC/MS (QP2010PLUS, TURBOMATRIX HS40), and the dioxane decomposition rate (mg/L/day) was calculated. In order to evaluate the effect of the concentration ratio of N23 strain to sludge on the dioxane decomposition rate, the dioxane decomposition rate of the blank was divided by the dioxane decomposition rate at a concentration ratio of 0.01 to 0.25 to calculate the relative value of the dioxane decomposition rate. The results are shown in Tables 1 and 2 and FIG. 2.

When the concentration ratio of N23 strain to excess sludge became too high, the decomposition rate decreased despite an increase in the bacterial mass of N23 strain. This demonstrated that there is an optimal value for the ratio of N23 strain concentration to excess sludge concentration (N23 strain/excess sludge), and as shown in Figure 2, it was confirmed that when this concentration ratio is in the range of 0.008 to 0.17, the dioxane decomposition rate can be maintained at a relative value of 50% or more compared to the blank (no excess sludge) case.

Experiment 2: Effect of BOD concentration during excess sludge treatment A given amount of N23 strain inoculum and glucose solution as a BOD component were added to a suspension of excess sludge containing cyclic ethers (final concentration of excess sludge: 10,000 mg/L), and rotary shaking culture was performed at 25°C. The bacterial cell concentrations of N23 strain were 200 mg/L and 1000 mg/L. The final glucose concentration was 94 to 936 mg/L, and 56% of the total oxygen demand at each glucose concentration was taken as the BOD concentration (Hisada Yoshio, Measurement method of TOD and TOC, Environmental Technology, 1980).
During the test period, the dioxane concentration in the solution was measured by headspace GC/MS (QP2010PLUS, TURBOMATRIX HS40). The results for bacterial cell concentrations of 200 mg/L and 1000 mg/L are shown in Figures 3 and 4, respectively.

In both experimental systems where the bacterial concentration of the N23 strain was 200 mg/L and 1000 mg/L, it was confirmed that the lower the BOD concentration, the faster the decomposition of dioxane. This confirmed that as the BOD concentration increases, the microorganisms in the excess sludge become more active, suppressing the activity of the N23 strain and slowing down the biodegradation of cyclic ethers.

1 Activated sludge treatment tank 2 Cyclic ether treatment tank 3 Excess sludge treatment tank 4 Dehydrator

Claims (6)

The excess sludge containing cyclic ethers generated in the activated sludge treatment tank is
A method for treating excess sludge, comprising biodegrading cyclic ethers in an excess sludge treatment tank using an N23 strain (accession number NITE BP-02032) under conditions of a BOD of 350 mg/L or less and a ratio of the N23 strain concentration to the excess sludge concentration (N23 strain/excess sludge) of 0.008 to 0.17. The excess sludge treatment method according to claim 1, characterized in that the excess sludge concentration in the excess sludge treatment tank is 1,000 mg/L or more and 30,000 mg/L or less. The excess sludge treatment method according to claim 1 or 2, characterized in that the N23 strain is supplied to the excess sludge treatment tank in the form of N23 strain-containing sludge generated in a cyclic ether treatment tank located downstream of the activated sludge treatment tank. The excess sludge treatment method according to any one of claims 1 to 3, characterized in that the cyclic ether contains 1,4-dioxane. an activated sludge treatment tank for treating the contaminated water containing cyclic ethers and other organic compounds mainly by using activated sludge;
a cyclic ether treatment tank located downstream of the activated sludge treatment tank for treating a cyclic ether;
an excess sludge treatment tank for treating excess sludge containing cyclic ethers generated in the activated sludge treatment tank,
The excess sludge treatment system is characterized in that in the excess sludge treatment tank, cyclic ethers are biodegraded using an N23 strain (accession number NITE BP-02032) under conditions of a BOD of 350 mg/L or less and a ratio of the N23 strain concentration to the excess sludge concentration (N23 strain/excess sludge) of 0.008 to 0.17. the cyclic ether treatment tank is a tank for performing biodegradation treatment using the N23 strain,
6. The excess sludge treatment system according to claim 5, wherein the N23 strain-containing sludge generated in the cyclic ether treatment tank is supplied to the excess sludge treatment tank.