Conductive biological hydrogel as well as preparation method and application thereof

Abstract

The invention discloses conductive biological hydrogel as well as a preparation method and application thereof, and belongs to the technical field of biological materials. Anaerobic activated sludge, sodium alginate, polyvinyl alcohol and carbon black are used as raw materials, an injector is used for injecting into a boric acid-CaCl2-chitosan mixed solution, continuous stirring is carried out, and by means of the sodium alginate-Ca < 2 + > cross-linking effect of a Ca < 2 + > diffusion mechanism and a polyvinyl alcohol-boric acid polymer cross-linked network, the composite material is prepared. The conductive biological hydrogel with uniform pore distribution and a multi-dimensional network interpenetrating structure is obtained. According to the invention, the anaerobic activated sludge and the activated carbon are encapsulated in the hydrogel material, so that the resistance activity of the anaerobic activated sludge to toxic environmental factors is improved, and efficient biodegradation of refractory pollutants is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of biomaterials, and in particular to a conductive biohydrogel and a preparation method and application thereof. Background Art

[0002] The treatment of high-concentration industrial wastewater based on anaerobic biotechnology is the mainstream wastewater treatment method. At present, this traditional treatment method faces the following defects: the process of anaerobic microorganisms co-treating high-concentration wastewater such as pharmaceutical wastewater and generating green energy usually requires a long start-up time for the granulation process, that is, it has the defect of taking a long time to form granular sludge; high-concentration pharmaceutical wastewater often has a toxic environment such as high antibiotic pressure, the microbial activity in traditional granular sludge is easily affected by wastewater fluctuations, the activity of secreted enzymes to treat antibiotics is unstable, it is difficult to maintain efficient wastewater treatment and energy conversion processes, and it has the defect of being difficult to regulate microbial activity.

[0003] In recent years, the research on the formation of biogels by fixing microorganisms based on the cross-linking effect of hydrophilic polymer materials has attracted widespread attention. The reaction of polymer materials and cross-linking agents can produce a network structure, which can wrap microorganisms in the gel network, thereby efficiently forming biogel particles similar to granular sludge. At present, the research and application of polymer gel structures are mostly concentrated in the biomedical field, and the research on gel-anaerobic microorganism coupling water-energy regeneration technology has not been reported. Summary of the invention

[0004] The primary purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and provide a method for preparing a conductive biohydrogel.

[0005] Another object of the present invention is to provide a conductive bio-hydrogel.

[0006] Another object of the present invention is to provide applications of the conductive bio-hydrogel.

[0007] The present invention proposes to encapsulate anaerobic microorganisms with a polymer biogel structure to improve their ability to inhibit and mitigate toxic factors. The conductive biohydrogel prepared by the present invention includes a hydrogel with a multiple cross-linked network that encapsulates anaerobic activated sludge and conductive material carbon black; the cross-linked network includes sodium alginate-Ca 2+ The conductive biogel provided by the invention has the characteristics of fast reaction start-up, short treatment cycle, strong environmental adaptability, etc. It can be used in high-concentration wastewater treatment and provide technical support for the corresponding wastewater treatment policy and resource utilization.

[0008] The gel particles prepared by the present invention can provide a good microenvironment and shelter for the growth and enrichment of microorganisms, improve the interception rate, cell density and anti-toxicity of anaerobic microorganisms, and thus improve the microbial treatment effect. The anaerobic energy regeneration process carried out by the gel particles prepared by the present invention is mainly completed by three key microorganisms such as hydrolytic acidification bacteria, acetogenic bacteria and methanogenic bacteria, involving reactions such as hydrolysis acidification, hydrogen production, acetic acid production and methanogenesis: hydrolytic acidification bacteria degrade complex organic pollutants into simple organic matter such as monosaccharides and amino acids through the hydrolysis and fermentation process; then microorganisms such as acetogenic bacteria decompose simple organic matter into small molecular products such as formic acid, acetic acid, H2, CO2, etc.; finally, methanogenic bacteria effectively convert it into methane (CH4) to become a renewable energy material.

[0009] Experimental measurements have shown that the conductive biogel prepared by the present invention has good antibiotic degradation ability and can maintain high methanogenic activity under antibiotic inhibition conditions; the gel structure has good mechanical properties of resistance to compression and stretching and resistance to external forces.

[0010] Specifically, the purpose of the present invention is achieved through the following technical solutions: A method for preparing a conductive biohydrogel comprises the following steps: (1) preparing an aqueous solution containing polyvinyl alcohol (PVA) and sodium alginate (SA), sterilizing the solution, and mixing the solution to obtain a PVA-SA mixed solution; (2) preparing an aqueous solution containing CaCl2, H3BO3, and chitosan (CS), adjusting the pH value of the solution to 5, and mixing to obtain a CaCl2-H3BO3-CS mixed solution; (3) adding the anaerobic activated sludge to the PVA-SA mixed solution prepared in step (1), adding carbon black, and mixing and stirring uniformly to obtain a biomass solution; (4) The biomass solution prepared in step (3) is drawn into the syringe and evenly dripped into the CaCl2-H3BO3-CS mixed solution in step (2) to obtain the conductive bio-hydrogel.

[0011] Furthermore, in the PVA-SA mixed solution described in step (1), the concentration of polyvinyl alcohol is 5 wt % to 10 wt %, preferably 7.5 wt % to 9 wt %, and more preferably 7.5 wt %; the concentration of sodium alginate is 0.5 wt % to 2 wt %, and more preferably 1 wt %.

[0012] Furthermore, the sterilization in step (1) is performed by high pressure sterilization at 121° C. in an autoclave for 60±10 minutes.

[0013] Furthermore, in the CaCl2-H3BO3-CS mixed solution described in step (2), the concentration of CaCl2 is 1 wt% to 3 wt%, preferably 2 wt%; the concentration of the H3BO3 aqueous solution is 2 wt% to 4 wt%, preferably 3 wt%; and the concentration of chitosan is 0.05 wt% to 0.2 wt%, preferably 0.1 wt%.

[0014] Furthermore, the pH value of the solution is adjusted in step (2) by using HCl.

[0015] Furthermore, the amount of anaerobic activated sludge added in step (3) is calculated based on a concentration of 2 to 4 g / L in the system, preferably a concentration of 3 g / L in the system.

[0016] Furthermore, the amount of carbon black added in step (3) is calculated based on its concentration in the system being 0.5 to 2 g / L, preferably based on its concentration in the system being 1 g / L.

[0017] Furthermore, the needle diameter of the syringe in step (4) is 3±0.5 mm.

[0018] Furthermore, the dripping speed in step (4) is 100±10 drops / min.

[0019] Furthermore, while the CaCl2-H3BO3-CS mixed solution is being dripped in step (4), the rotating speed is 330-350 rpm.

[0020] A conductive biohydrogel is obtained by the preparation method.

[0021] Application of the above conductive bio-hydrogel in wastewater treatment.

[0022] Furthermore, the wastewater is wastewater containing COD and / or antibiotic drugs.

[0023] Furthermore, the COD concentration is not less than 3 g / L.

[0024] Furthermore, the antibiotic drug includes sulfamethoxazole.

[0025] Furthermore, the concentration of sulfamethoxazole is not less than 6 mg / L.

[0026] The present invention uses anaerobic activated sludge, sodium alginate (SA), polyvinyl alcohol (PVA), and carbon black as raw materials, injects boric acid-CaCl2-chitosan mixed solution with a 3 mm diameter syringe, and continuously stirs to obtain a mixture of boric acid and CaCl2. 2+ Diffusion mechanism of sodium alginate-Ca 2+The cross-linking effect and the cross-linking network of polyvinyl alcohol-boric acid polymer obtain a conductive bio-hydrogel with uniform pore distribution and a multi-dimensional network interpenetrating structure. The present invention encapsulates anaerobic activated sludge and activated carbon in the hydrogel material to improve the resistance of anaerobic activated sludge to toxic environmental factors and achieve efficient biodegradation of difficult-to-degrade pollutants.

[0027] The anaerobic energy regeneration process of the gel particles prepared by the present invention is mainly completed by the collaboration of three key microorganisms, namely hydrolytic acidifying bacteria, acetogenic bacteria and methanogenic bacteria, involving reactions such as hydrolytic acidification, hydrogen production, acetic acid production and methanogenesis: hydrolytic acidifying bacteria degrade complex organic pollutants into simple organic matter such as monosaccharides and amino acids through the hydrolysis and fermentation process; then microorganisms such as acetogenic bacteria decompose simple organic matter into small molecule products such as formic acid, acetic acid, H2, CO2, etc.; finally, methanogenic bacteria effectively convert it into methane (CH4) to become a renewable energy substance.

[0028] Compared with the prior art, the present invention has the following advantages and effects: 1) The multi-dimensional interpenetrating network structure can effectively prevent toxic substances from directly contacting anaerobic microorganisms and protect anaerobic activated sludge; 2) Activated carbon promotes indirect interspecies electron transfer in anaerobic reactions, promotes the degradation of COD and antibiotics, and improves the efficiency of methane production; 3) The conductive biohydrogel constructs a reaction microenvironment containing activated carbon, which is conducive to the adsorption and degradation of antibiotic substances such as sulfamethoxazole (SMX) produced by pharmaceutical wastewater; 4) This gel technology has good biocompatibility and can be used not only in anaerobic processes, but also has the same effect when the activated sludge contains other functional bacteria such as anaerobic ammonia-oxidizing bacteria. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the structure of conductive biogel encapsulating anaerobic sludge and activated carbon based on a multi-dimensional interpenetrating network structure; Figure 2 Flow chart for the preparation of conductive biogels; Figure 3 It is the methane cumulative production curve and final production bar chart; Figure 4 It is the COD degradation curve and degradation rate bar graph; Figure 5 It is the degradation curve and degradation rate bar graph of SMX; Figure 6 This is a cryo-scanning electron microscopy image of a multi-dimensional network interpenetrating conductive biohydrogel at 100 μm; Figure 7 This is a cryo-scanning electron microscopy image of a multi-dimensional network interpenetrating structure conductive biohydrogel at 10 μm (pore changes); Figure 8 This is a cryo-scanning electron microscopy image of a multi-dimensional network interpenetrating structure conductive biohydrogel at 10 μm (bacterial species variation); Fig. 9 This is a microscope image of a 7.5% PVA + 1% SA gel slice; Fig.10 This is a microscope image of a 9% PVA + 1% SA gel slice; Fig.11 This is a physical picture of the gel prepared in Example 1; Fig.12 This is a physical picture of the gel prepared in Comparative Example 1. DETAILED DESCRIPTION

[0030] The following is a detailed description of the embodiments of the present invention. It should be emphasized that the following description is only exemplary and is not intended to limit the scope and application of the present invention.

[0031] The anaerobic activated sludge in the following examples comes from the secondary sedimentation tank of Qianshan Sewage Purification Plant, which contains a variety of anaerobic microorganisms such as hydrolytic acidifying bacteria, acetogenic bacteria and methanogenic bacteria.

[0032] The carbon black used in the following examples was purchased from STREM and had an average particle size of 9 μm.

[0033] The polyvinyl alcohol in the following examples has a purity of 95% and a molecular weight of 89,000 to 98,000.

[0034] Unless otherwise specified, the percentages in the following examples are by mass.

[0035] Example 1 (1) Solution preparation: First, prepare an aqueous solution containing 7.5% polyvinyl alcohol and 1% sodium alginate (SA), place it in a sterilizer and autoclave it at 121°C for 60 min, cool it to room temperature, and mix it to obtain a PVA-SA mixed solution; then prepare an aqueous solution containing 2% CaCl2, 3% H3BO3, and 0.1% chitosan (CS), and add HCl to make the solution pH 5. After mixing, dissolve it by ultrasonication to obtain a CaCl2-H3BO3-CS mixed solution.

[0036] (2) Preparation of biomass solution: Wash the anaerobic activated sludge in the influent water without carbon source for at least 3 times, and centrifuge it at 7000 rpm for 2 min. The purpose of centrifugation is to remove the upper liquid, thereby increasing the concentration of anaerobic activated sludge. Add 3 g / L of centrifuged anaerobic activated sludge to the PVA-SA mixed solution, add 1 g / L of carbon black, mix and stir evenly to obtain a biomass solution. The biomass solution should be stirred slowly to avoid entraining bubbles and affecting the hardness of the gel. In order to maintain the anaerobic environment and the activity of anaerobic microorganisms, the biomass solution should be placed in an anaerobic operating table that is constantly filled with nitrogen.

[0037] (3) Preparation of biogel: Use a syringe to absorb the biomass solution described in step (2), and then use a specially made syringe needle with a diameter of about 3 mm to evenly drop it into the CaCl2-H3BO3-CS mixed solution described in step (1). If the needle is blocked, the needle should be replaced in time. Each time the biomass solution is extracted with a syringe, it should be stirred slowly to ensure that the activated carbon is evenly mixed. The syringe requires "low, edge, and uniform", that is, the distance between the needle mouth and the liquid surface is about 5 cm, the injection speed is about 100 drops / min, and the injection is at the edge of the liquid surface. To prevent the beads from gelling, while injecting at the edge of the liquid surface, the mixed solution is rotated at a constant speed of 330 to 350 rpm, and the gel particles gathered in the center of the liquid surface are stirred from time to time. After the gel is prepared, it should be continuously consolidated in the CaCl2-H3BO3-CS mixed solution for 1 hour.

[0038] Furthermore, to ensure uniform injection, an automatic sample injector can be constructed, and in combination with a gas filling device, the injection rate of the syringe can be controlled by adjusting a fixed gas flow rate.

[0039] (4) Gel curing: The hydrogel particles are soaked in a Na2SO4 mixed solution for curing. The biogel particles are finally washed with pure water and placed in an anaerobic reactor. Samples are taken regularly to monitor the operation. The curing time of the hydrogel beads soaked in the Na2SO4 mixed solution is about 5 hours.

[0040] Example 2 (1) Solution preparation: First, prepare an aqueous solution containing 9% polyvinyl alcohol (PVA) and 1% sodium alginate (SA), place it in a sterilizer and autoclave it at 121°C for 60 minutes, cool it to room temperature, and mix it in a volume ratio of 1:1 to obtain a PVA-SA mixed solution; then prepare a solution containing 2% CaCl2 and 3% H3BO 3， A 0.1% chitosan (CS) aqueous solution was prepared, and HCl was added dropwise to make the solution pH 5. After mixing, ultrasonic dissolution was performed to obtain a CaCl2-H3BO3-CS mixed solution.

[0041] (2) Preparation of biomass solution: Wash the methanogenic sludge containing anaerobic sludge in the influent water without carbon source for at least 3 times, centrifuge it at 7000 rpm for 2 min. Add 3 g / L of the centrifuged anaerobic activated sludge to the PVA-SA mixed solution, add 1 g / L of carbon black, mix and stir evenly to obtain a biomass solution.

[0042] (3) Preparation of biogel: Use a syringe to absorb the biomass solution in step (2), and then use a specially made syringe needle with a diameter of about 3 mm to evenly drip it into the CaCl2-H3BO3-CS mixed solution in step (1). After the gel is prepared, it should be continuously consolidated in the CaCl2-H3BO3-CS mixed solution for 1 hour.

[0043] (4) Gel curing: The hydrogel particles are immersed in a Na2SO4 mixed solution for curing. The biogel particles are finally washed with pure water and placed in an anaerobic reactor. Samples are taken regularly to monitor the operation status.

[0044] Comparative Example 1 The hydrogel was prepared by referring to Example 1, except that the aqueous solution prepared in step (1) contained 6% polyvinyl alcohol and 1% sodium alginate.

[0045] The above configuration results showed that on the third day, there was obvious leakage, the structure of the gel was damaged, and the gel ball structure was too soft ( Fig.12 ).

[0046] Comparative Example 2 The hydrogel was prepared by referring to Example 1, except that the aqueous solution prepared in step (1) contained 10% polyvinyl alcohol and 1% sodium alginate.

[0047] The above configuration results show that the gel ball structure is too rigid and the permeation test results are poor, which is speculated to affect the mass transfer of the reaction.

[0048] Comparative Example 3 The hydrogel was prepared with reference to Example 1, except that in step (3), the gel was not continuously consolidated in the CaCl2-H3BO3-CS mixed solution for 1 hour after preparation. As a result, obvious adhesion occurred.

[0049] Comparative Example 4 The hydrogel was prepared by referring to Example 1, except that the mixed solution was not rotated while being injected at the edge of the liquid surface in step (3). As a result, obvious adhesion occurred.

[0050] Effect Example A laboratory-scale biogel particle batch reaction bottle was built. The effective volume of the reaction bottle device was 200 mL. After the prepared biogel particles were placed in the reaction bottle, 6.0 mg / L SMX artificial water (specific composition see Table 1, chemical oxygen demand (COD) is 3000 mg / L) was added as the reactor inlet water. The reaction bottle regularly collects methane gas produced by the degradation of wastewater organic matter by biogel particles through the top air pipe. The experiment set up a control group, that is, the corresponding anaerobic activated sludge was added, and the pollutant removal reaction was carried out in the same environment.

[0051] Water and gas samples were taken at regular intervals to measure the degradation of SMX, the removal of COD and the generation of methane. The final results are as follows: Table 2 is a schematic diagram of the methane production of the biogel particles of the present invention in an activity test. Figure 3 The cumulative methane production curve and final production bar graph under a 25-day reaction cycle. In order to quantitatively analyze the methane production under different conditions, this study applied the modified Gompertz model:

[0052] Where P(t) is the cumulative methane production at time t (mLL -1 );P max is the maximum methane potential (mLL) at the end of the culture -1 ); t is time (day); R max is the maximum methane production rate (mLL -1 day -1 )); λ is the lag period (day); e = 2.71828.

[0053] The fitting results of the Compertz model parameters for methane are shown in Table 2. 2 The maximum methane yields (mLL) of the three experimental groups were 0.993, 0.985, and 0.991. -1 day -1 ) and lag period (day) were 28.99±0.50, 31.48±0.42, 24.54±0.11 and 6.50±0.20, 8.33±0.49, 11.48±0.54, respectively. It can be seen that the maximum methane yield of the 7.5%PVA+1%SA gel group was better than that of the other groups, and the maximum methane potential of the gel group at the end of culture was better than that of the sludge group.

[0054] Table 3 is a schematic diagram of COD removal measured by the biogel particles of the present invention in an activity experiment. Figure 4The degradation curve and degradation rate bar graph of COD under a 25-day reaction cycle; the experiment set the COD inlet concentration to 3 g / L. The COD concentration of the three groups of reactors decreased over time. Under steady state, the COD removal rates of the three groups of experimental objects were 67.74%, 51.79% and 43.64%, respectively. The traditional method reached a maximum removal rate of 406.67 (mgL-1) on the first day. -1 day -1 The gel group reached the maximum removal rate on the 15th and 13th days, which were 608.33 (mgL -1 day -1 ) and 601.67 (mgL -1 day -1 ). Therefore, the maximum COD removal rate of the gel group was better than that of the control group, increasing by 49.59% and 47.95%, respectively.

[0055] Table 4 is a schematic diagram of the removal of SMX by the biogel particles of the present invention in the activity test. Figure 5 The figure shows the degradation curve of SMX over time under a 25-day reaction cycle and the removal rate of SMX in each group before and after the reaction. The experiment set the SMX influent concentration to be 6 mg / L. The SMX concentrations of the three groups of reactors decreased over time. From 0 to 13 days, the SMX removal of the three groups of reactors was most significant (>60%), and the decrease in the effluent SMX concentration of the 7.5% PVA+1% SA gel group and the 9% PVA+1% SA gel group was greater than that of the sludge group (blank group). The gel structure promotes the degradation of SMX. The degradation of SMX in the three groups of reactors tended to be stable at 13 days. Under the stable state, the SMX removal rates of the sludge group (blank group), the 7.5% PVA+1% SA gel group and the 9% PVA+1% SA gel group were 67.74%, 90.23% and 87.74%, respectively. Compared with the blank group, the degradation efficiency of SMX in the gel group was significantly improved.

[0056] Figure 6 , Figure 7 , Figure 8 The cross-sectional SEM images of the 7.5% PVA+1% SA gel group and the 9% PVA+1% SA gel group before and after the reaction (25 days) are shown. After the reaction, a large number of aggregates were found in the cross section of the beads. The pore size of the beads was about 300 μm before the reaction, and the pore size of the beads was reduced to about 150 μm after 25 days of reaction. The microorganisms in the beads changed from the original surface attachment state to the embedded state, and secreted a large amount of extracellular secretions, indicating that anaerobic microorganisms can be enriched and grown in the beads.

[0057] Fig. 9 , Fig.10These are cross-sectional microscope images of the 7.5% PVA + 1% SA gel group and the 9% PVA + 1% SA gel group. The black dots are activated carbon encapsulated in the cross-linked network structure.

[0058] Table 1 Contents of various substances in artificial water

[0059] Table 2 Compertz model fitting result parameter table

[0060] Table 3 COD degradation status

[0061] Table 4 SMX degradation status

[0062] The above contents are further detailed descriptions of the present invention in combination with specific / preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, they can also make several substitutions or modifications to these described embodiments without departing from the concept of the present invention, and these substitutions or modifications should be regarded as belonging to the protection scope of the present invention.

Claims

1. A method for preparing a conductive biohydrogel, characterized in that: The steps include: (1) preparing an aqueous solution containing polyvinyl alcohol and sodium alginate, sterilizing the solution, and mixing the solution to obtain a PVA-SA mixed solution; (2) preparing an aqueous solution containing CaCl2, H3BO3, and chitosan, adjusting the pH value of the solution to 5, and mixing to obtain a CaCl2-H3BO3-CS mixed solution; (3) adding the anaerobic activated sludge to the PVA-SA mixed solution prepared in step (1), adding carbon black, and mixing and stirring uniformly to obtain a biomass solution; (4) The biomass solution prepared in step (3) is drawn into the syringe and evenly dripped into the CaCl2-H3BO3-CS mixed solution in step (2) to obtain the conductive bio-hydrogel.

2. The method for preparing the conductive bio-hydrogel according to claim 1, characterized in that: In the PVA-SA mixed solution described in step (1), the concentration of polyvinyl alcohol is 5 wt % to 10 wt %; the concentration of the sodium alginate aqueous solution is 0.5 wt % to 2 wt %; In the CaCl2-H3BO3-CS mixed solution described in step (2), the concentration of CaCl2 is 1 wt% to 3 wt%; the concentration of the H3BO3 aqueous solution is 2 wt% to 4 wt%; and the concentration of chitosan is 0.05 wt% to 0.2 wt%.

3. The method for preparing the conductive bio-hydrogel according to claim 2, characterized in that: In the PVA-SA mixed solution described in step (1), the concentration of polyvinyl alcohol is 7.5 wt % to 9 wt %; the concentration of sodium alginate is 1 wt %; In the CaCl2-H3BO3-CS mixed solution described in step (2), the concentration of CaCl2 is 2 wt %; the concentration of the H3BO3 aqueous solution is 3 wt %; and the concentration of chitosan is 0.1 wt %.

4. The method for preparing the conductive bio-hydrogel according to claim 3, characterized in that: The sterilization in step (1) is performed by high pressure sterilization at 121°C in an autoclave for 60±10 minutes; The pH value of the solution is adjusted in step (2) by using HCl.

5. The method for preparing the conductive bio-hydrogel according to any one of claims 1 to 4, characterized in that: The amount of anaerobic activated sludge added in step (3) is calculated based on its concentration in the system of 2 to 4 g / L; The amount of carbon black added in step (3) is calculated based on its concentration in the system of 0.5 to 2 g / L.

6. The method for preparing the conductive bio-hydrogel according to claim 5, characterized in that: The amount of anaerobic activated sludge added in step (3) is calculated based on its concentration in the system being 3 g / L; The amount of carbon black added in step (3) is calculated based on its concentration in the system being 1 g / L; The needle diameter of the syringe described in step (4) is 3±0.5 mm; The dripping speed in step (4) is 100±10 drops / min; While the CaCl2-H3BO3-CS mixed solution is being dripped in step (4), it is rotated at a speed of 330-350 rpm.

7. A conductive biohydrogel, characterized in that: The method is obtained by the preparation method described in any one of claims 1 to 6.

8. Use of the conductive bio-hydrogel described in claim 7 in wastewater treatment.

9. The use according to claim 8, characterized in that: The wastewater is wastewater containing COD and / or antibiotic drugs.

10. The use according to claim 9, characterized in that: The concentration of COD is not less than 3 g / L; The antibiotic drugs include sulfamethoxazole; The concentration of sulfamethoxazole is not less than 6 mg / L.

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