Wastewater treatment method and sludge settling promoter

Abstract

To provide a method for efficiently achieving both a reduction in treated water COD and an improvement in sludge settling properties in wastewater treatment including settling sludge from a biological treatment tank in a settling tank to obtain treated water as supernatant water.SOLUTION: There is provided a wastewater treatment method including: starting addition of liquid medicine containing calcium ions and iron ions when CST and SV of sludge and COD of treated water are measured and when the CST is 5 to 3,000 seconds, the SV is 90% or more, and the COD is 3.0 to 300 mg / L; and adding the liquid medicine at least once a day for at least five days.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for treating wastewater, more specifically, a method for treating wastewater including precipitating sludge from a biological treatment tank in a precipitating tank to obtain treated water as supernatant water, the method including starting addition of a specific chemical when specific conditions are met and continuing the addition of the chemical for a certain period of time. The present invention also relates to an agent for promoting sludge settling. More specifically, the present invention relates to an agent for promoting sludge settling by improving biological activity in a wastewater treatment including a biological treatment tank and precipitating sludge, as in the above wastewater treatment method. [Background technology]

[0002] In wastewater treatment, the activated sludge process, which has a biological treatment tank, is known as an economically advantageous wastewater treatment method because it does not require special facilities or complicated equipment compared to physical and chemical wastewater treatment methods, and because the organisms in the activated sludge are the main players in the treatment.

[0003] One of the most representative activated sludge processes is the standard activated sludge process (Figure 1). This process is mainly composed of a biological treatment tank and a settling tank. In the biological treatment tank, air is blown in by aeration to activate the microorganisms living in the activated sludge, which decompose and assimilate the organic matter contained in the inflowing wastewater. Meanwhile, in the settling tank, the sludge and the supernatant water are separated into solids and liquids, and the supernatant water is discharged as treated water or transferred to the next treatment process, while the settled sludge is discharged outside the system as excess sludge, or a portion of the sludge is returned to the upper level of the biological treatment tank as return sludge and circulated back into the system as activated sludge. A raw water tank for storing raw water may be installed before the biological treatment tank.

[0004] Organic matter contained in wastewater includes starch, protein, and fats and oils, and the mechanism for their removal depends heavily on the activity of microorganisms (especially aerobic bacteria) living in activated sludge. The microorganisms in activated sludge secrete a wide variety of enzymes, such as starch-decomposing enzymes (amylase), protein-decomposing enzymes (protease), and fat-decomposing enzymes (lipase), to break down the organic matter. Starch, a typical organic matter, is broken down into monosaccharides (glucose), proteins into amino acids, and fats and oils into glycerin and fatty acids.

[0005] The decomposition products of organic matter, such as monosaccharides, amino acids, glycerin, and fatty acids, are relatively low molecular weight and are quickly taken up by microorganisms to become an energy source for the bacterial body. The microorganisms that grow using these energy sources are then eaten by protozoans such as Vorticella that live in activated sludge. These microorganisms and protozoans are then eaten by metazoans such as B. rotifers that also live in activated sludge. In other words, organic matter becomes food for microorganisms, microorganisms become food for protozoans, and microorganisms and protozoans become food for metazoans. As a result, a unique ecological pyramid is constructed in activated sludge, and it is known that the food chain continues to progress.

[0006] As described above, it is the enzymes secreted by microorganisms that break down the organic matter in wastewater into smaller molecules, and it is the microorganisms that feed on the decomposition products broken down into smaller molecules by the enzymes.

[0007] On the other hand, if the organic matter contained in the wastewater contains difficult-to-decompose substances such as high molecular weight natural organic matter, chemically synthesized organic matter, or color components, the decomposition rate by microbial enzymes will be slow, which may increase the COD (Chemical Oxygen Demand) of the treated water.

[0008] In addition, when substances that are difficult to decompose or that are toxic to microorganisms (washing wastewater, acids, alkalis, etc.) flow in with raw water, the bacterial flora in the activated sludge is disturbed, making it difficult for the microorganisms to form good activated sludge with good compactibility, which can worsen the sludge settling property in the settling tank. When the sludge settling property deteriorates, it is often necessary to add a polymer flocculant (also called a polymer) to settle the sludge in the settling tank. However, for example, in facilities that use a settling tank in combination with a membrane bioreactor (also called an MBR), polymer flocculants cannot be used because of the increased risk of membrane clogging. Therefore, it is desirable to improve the sludge settling property by a method other than adding a polymer flocculant.

[0009] JP 2021-20138 A (Patent Document 1) describes a wastewater biological activity promoter for promoting starch decomposition by Bacillus bacteria in a biological treatment tank, which is an agent capable of promoting starch decomposition by Bacillus bacteria in a biological treatment tank, and which contains at least one selected from calcium ions and iron ions and is liquid, and a wastewater treatment method using the same. In addition, JP 2018-153713 A (Patent Document 2) describes a biological activator and a biological treatment method characterized by having an iron salt as the main component for removing persistent COD by improving biological activity. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2021-20138 A [Patent Document 2] JP 2018-153713 A Summary of the Invention [Problem to be solved by the invention]

[0011] Patent Documents 1 and 2 describe the improvement of biological activity in a biological treatment tank. However, Patent Document 1 does not describe the improvement of biological activity of microorganisms other than Bacillus bacteria, or the effect on the settling property of sludge. In addition, Patent Document 1 does not describe the time when the effect is exhibited, the tendency of the oxygen utilization rate coefficient, or the optimal range of the treated water COD or the activated sludge CST to obtain the effect.

[0012] Patent Document 2 also does not describe the effect on sludge settling, nor does it describe the tendency of the oxygen utilization rate coefficient, or the optimal range of the treated water COD or the activated sludge CST for obtaining the effect. In addition, Patent Document 2 suggests that the iron salt product preferably contains manganese, zinc, molybdenum, copper, nickel, and cobalt as impurities, but does not mention calcium. In facilities using biological treatment tanks, the BOD and COD of the treated water are important indicators, but sludge settling is also one of the important indicators. If the sludge settling is deteriorated, SS may leak from the interface of the settling tank, which may violate the wastewater standards. In addition, the sludge compaction is also reduced, which leads to a decrease in the dewatering efficiency of the sludge.

[0013] Conventionally, attempts have been made to use nutrients and microbial preparations as described in Patent Documents 1 and 2 as a means for reducing the COD of treated water, while polymer flocculants and the like have often been used as a means for improving sludge settling properties. There have been few attempts to achieve both a reduction in the COD of treated water and an improvement in sludge settling properties by improving biological activity. Achieving both a reduction in the COD of treated water and an improvement in sludge settling properties by improving biological activity without the need for the use of polymer flocculants is preferable from the standpoint of cost and also in facilities that also use membrane separation such as MBR.

[0014] The present invention aims to provide a method for treating wastewater, which includes settling sludge from a biological treatment tank in a settling tank to obtain treated water as supernatant water, which can achieve both a reduction in treated water COD and an improvement in sludge settleability by improving biological activity. [Means for solving the problem]

[0015] As a result of intensive research into the above object, the inventors have found that when the CST and SV of sludge are within specific ranges and the COD of the treated water (supernatant water in the settling tank) is also within specific ranges, it is possible to effectively achieve both a reduction in the COD of the treated water and an improvement in the settling properties of the sludge by starting the addition of a liquid agent containing calcium ions and iron ions and continuing this addition for a certain period of time. The present invention includes, but is not limited to, the following: [1] A method for treating wastewater, comprising settling sludge from a biological treatment tank in a settling tank to obtain treated water as supernatant water, Measure the CST and SV of the sludge and the COD of the treated water, and start adding a liquid agent containing calcium ions and iron ions when the CST is 5 to 3,000 seconds, the SV is 90% or more, and the COD is 3.0 to 300 mg / L; and The drug is added at least once per day for at least 5 days; The method for treating wastewater, further comprising: [2] The method for treating wastewater according to [1], wherein the agent has a calcium ion concentration of 1 to 20 mass % and an iron ion concentration of 1 to 20 mass %. [3] The method for treating wastewater according to [1] or [2], further comprising adding the agent to a biological treatment tank or upstream thereof. [4] The method for treating wastewater described in [1] or [2], further comprising adjusting the amount of chemical added so as to satisfy BA≧5, where A% is the SV on the 5th day after the start of chemical addition, and B% is the SV at the start of chemical addition. [5] The method for treating wastewater described in [1] or [2], further comprising adjusting the amount of chemical added so that the COD of the treated water on the 5th day after the start of addition of the chemical is reduced by 10% or more compared to the COD of the treated water at the start of addition of the chemical. [6] Measuring the oxygen utilization rate coefficient of the sludge at the start of the addition of the agent; and The method for treating wastewater according to [1] or [2], further comprising adjusting the amount of the chemical added so that the oxygen utilization rate coefficient of the sludge on the fifth day after the start of the addition of the chemical increases by 10% or more compared to the oxygen utilization rate coefficient of the sludge at the start of the addition of the chemical. [7] A liquid sludge settling promoter containing 1 to 20% by mass of calcium ions and 1 to 20% by mass of iron ions. [8] The sludge settling promoter according to [7], wherein the calcium ions are derived from calcium chloride and the iron ions are derived from ferric chloride. Effect of the Invention

[0016] According to the present invention, when the CST and SV of the sludge are within specific ranges and the COD of the treated water is also within specific ranges, the addition of a liquid chemical containing calcium ions and iron ions is started, and this addition is continued for a certain period of time, thereby effectively achieving both a reduction in the COD of the treated water and an improvement in the settling properties of the sludge.

[0017] The liquid agent containing calcium ions and iron ions used in the present invention is considered to have a certain degree of coagulation action due to calcium and iron, but the main contributor to the above-mentioned effect is the improvement of biological activity. The agent improves the activity of the microorganisms living in the biological treatment tank, thereby reducing the COD of the treated water, and the active flocculation by the microorganisms increases the sludge compaction and improves the sludge settling properties.

[0018] Therefore, unlike the flocculation action of flocculants, which takes effect in a short time, the effect of the present invention begins to take effect 5 days after the start of addition of the drug. As evidence supporting the improvement of biological activity by the drug, the present inventors have confirmed that the oxygen utilization rate coefficient tends to increase 5 days after the start of addition of the drug (see Table 2 in Example 1). [Brief description of the drawings]

[0019] [Figure 1]FIG. 1 is a flow diagram of a general standard activated sludge process, showing examples (A to E) of the locations where the chemicals used in the present invention are added. [Diagram 2] FIG. 1 is a flow diagram illustrating an example of a method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The method of the present invention can be used for wastewater treatment in which a biological treatment tank, which is a tank for decomposing organic matter in wastewater with microorganisms, and a sedimentation tank, which is a tank for settling sludge, are used, and treated water is obtained as supernatant water by settling the sludge from the biological treatment tank in the sedimentation tank. The biological treatment tank and the sedimentation tank do not necessarily need to be directly connected, and tanks with different purposes may be interposed between them, such as biological treatment tank → coagulation tank → sedimentation tank.

[0021] The facilities and methods in which the method of the present invention can be used are not particularly limited as long as they use a biological treatment tank and a settling tank, and examples of such facilities and methods include aerobic activated sludge processes such as the standard activated sludge process using a biological treatment tank and a settling tank, the oxidation ditch process, the deep tank aeration process, and the step aeration process, aerobic biofilm processes such as the fixed-bed submerged filter process, the fluidized-bed submerged filter process, the fluidized carrier process, and the rotating disk process, as well as nitrification denitrification processes such as the circulation-type nitrification denitrification process, the step-type nitrification denitrification process, and the direct dehydration (pre-dehydration) type denitrification treatment method. The method can also be used appropriately in anaerobic treatment tanks, methane fermentation tanks, etc.

[0022] The method of the present invention can also be used appropriately in facilities using MBRs where there is a risk of membrane blockage when using conventional sludge settling agents (polymer flocculants). For example, it can be used in facilities where membrane separation by MBR is used in combination with a settling tank, or when it is desired to temporarily settle sludge in an MBR by dredging or the like. With the present invention, it is possible to obtain both the effects of reducing treated water COD and promoting sludge settling, even in facilities using MBRs where it was previously not possible to use polymer flocculants and it was difficult to promote sludge settling.

[0023] The raw water (wastewater) to be treated contains at least organic matter. For example, but not limited to, it may be wastewater from various manufacturing factories such as food manufacturing factories, chemical manufacturing factories, equipment manufacturing factories, and pharmaceutical manufacturing factories, or wastewater from slaughterhouses, water purification plants, sewage treatment plants, sewage treatment plants, farms, etc.

[0024] The raw water may be directly introduced into the biological treatment tank, or a raw water tank may be installed upstream of the biological treatment tank. The raw water tank may be installed for the purposes of measuring the quality of the raw water, adjusting the treatment flow rate, and stabilizing (uniformizing) the quality of the inflowing water.

[0025] In this specification, the treated water refers to the supernatant water obtained from the supernatant of the settling tank located downstream of the biological treatment tank. The treated water does not necessarily directly refer to the effluent water discharged into a river or the like. For example, a further treatment tank such as a sand filter tank may be installed after the settling tank, and then the treated water may be discharged into a river or the like. Even in this case, the treated water in this specification refers not to the effluent water, but to the supernatant water obtained from the supernatant of the settling tank.

[0026] In the method of the present invention, it is necessary to start the addition of the chemical agent described in detail below when certain conditions are met. Specifically, the addition of the chemical agent is started when the sludge CST (Capillary Suction Time) is 5 to 3000 seconds, the SV (Sludge Volume) is 90% or more, and the COD of the treated water is 3.0 to 300 mg / L. By starting the addition of the chemical agent when such conditions are met, it is possible to efficiently obtain the effect of reducing the COD of the treated water after the addition of the chemical agent and the effect of promoting the settling of the sludge.

[0027] To determine the timing to start adding chemicals, it is first necessary to measure the CST and SV of the sludge and the COD of the treated water.

[0028] Furthermore, in the method of the present invention, it is necessary to add the agent at least once a day for at least five days. The agent used in the present invention improves the activity of microorganisms living in the biological treatment tank, thereby obtaining the effects of reducing the COD of the treated water and promoting the settling of sludge. In addition to obtaining the effect of reducing the COD of the treated water by improving the biological activity by adding the agent, the formation of biopolymers (polymers such as polysaccharides and proteins) by the microorganisms in the sludge is promoted, which forms flocs with high compactibility, increases the compactibility of the sludge, and improves the settling property of the sludge.

[0029] Therefore, unlike polymer flocculants, which achieve a flocculation effect shortly after addition, the effect of the present invention is not obtained immediately after the start of addition of the agent, but after the agent is added for about five days. The improvement of biological activity by the agent has been confirmed by the improvement of the oxygen utilization rate coefficient of the sludge from the fifth day after the start of addition of the agent (see Table 2 in Example 1). It has also been confirmed that these effects are maintained, for example, on the seventh or ninth day after the start of addition of the agent.

[0030] Although the coagulation action of the agent containing calcium and iron may have some effect on the settling of sludge, the main mechanism by which the agent used in the present invention exerts its effect is the improvement of biological activity, as described above. If the mechanism by which the agent exerts its effect is only the coagulation action, an increase in the oxygen utilization rate coefficient cannot be confirmed.

[0031] CST is a value that is usually used as an index of sludge dewaterability. In the present invention, CST is considered to correlate with the amount of biopolymer formed by microorganisms in sludge, and is used as one of the conditions for determining the start of addition of an agent. Sludge settling property depends on many factors (e.g., the amount of filamentous bacteria and sludge concentration) in addition to biopolymers, but the agent used in the present invention is considered to contribute to improving sludge settling property by promoting the formation of biopolymers and forming highly compactible flocs.

[0032] However, in sludge in which a large amount of biopolymers already exists, it is difficult to promote flocculation further, and as a result, sufficient sludge settling properties cannot be obtained. Therefore, in order to improve sludge settling properties by the method of the present invention, the addition of the agent is started when the CST of the sludge is in the range of 5 to 3000 seconds, preferably in the range of 5 to 1000 seconds. When the CST is high enough to exceed 3000 seconds, it is considered that a large amount of biopolymers is already present in the sludge, and even if the addition of the agent is started, the effect of improving sludge settling properties cannot be sufficiently obtained.

[0033] CST can be measured by putting a small amount of sludge into a cylinder on filter paper, and measuring the time it takes for the water in the sludge to pass between two points as it is sucked into the filter paper by capillary suction and expands in the circumferential direction. Specifically, in this specification, CST is measured according to the method described in "Sewage Testing Methods, Vol. 1, p. 729, 2012 Edition," using a CST meter (Central Science Trading Co., Ltd., 304M CST), a stainless steel cylinder (inner diameter 18 mm x height 25 mm) as the cylinder, and cellulose chromatography paper (69 mm x 90 mm) as the filter paper, by inserting an appropriate amount of sample (sludge) into the cylinder and reading the time it takes for the sample to pass between two specified points on the CST meter (CST value, unit: seconds).

[0034] The settling property of sludge can be evaluated by SV (activated sludge volume fraction). The sludge generated in the biological treatment tank is separated into solid and liquid in the downstream settling tank. By adding the chemical used in the present invention, biological activity is improved, and the flocculation is promoted by the biopolymer produced by the microorganisms in the sludge, increasing the sludge compaction property, and improving the sludge settling property in the settling tank. However, when the sludge settling property is already very good (when the SV is sufficiently small), it can be said that there is little merit in further improving the sludge settling property by adding the chemical, considering the cost-effectiveness. Therefore, in the method of the present invention, the addition of the chemical is started when the SV is 90% or more, preferably 93% or more.

[0035] In this specification, SV is the volume of settled sludge when sludge is left to stand for 30 minutes in a 1-L measuring cylinder, expressed as a percentage (%) relative to the amount of sludge placed in the measuring cylinder (1 L). Specifically, according to the method described in "Sewage Testing Methods, Vol. 1, p. 685, 2012 Edition," 1 L of sample (sludge) is placed in a measuring cylinder with a capacity of 1 L (inner diameter approximately 6.5 cm), gently stirred so that MLSS (Mixed Liquor Suspended Solids, activated sludge suspended solids) is uniform, and the settled sludge volume (a mL) after standing for 30 minutes is read, and SV is calculated by the following formula: SV(vol%)=(a / 1000)×100=a / 10

[0036] The same sludge can be used for measuring CST and SV. The sludge is preferably collected from the rear of the biological treatment tank, where the sludge's biota is mature and where the sludge settling property in the settling tank can be accurately reflected. The front of the tank refers to the vicinity where the treated water flows into the tank, the rear of the tank refers to the vicinity where the treated water flows out from the tank to the next tank (for example, in the case of a biological treatment tank, the vicinity where the treated water flows out from the biological treatment tank to the next tank (for example, a settling tank)), and the middle of the tank refers to the intermediate portion between them.

[0037] COD is calculated by oxidizing oxidizable substances in water with an oxidizing agent and calculating the amount of oxygen required for oxidation from the amount of oxidizing agent used, and is used as an index to measure pollution by organic matter in water. By adding the chemical used in the present invention, biological activity is improved and the COD of the treated water obtained as the supernatant water of the settling tank is reduced. However, when the COD of the raw water flowing into the biological treatment tank is very high, or when the load is too high and the COD of the treated water before the addition of the chemical is too high, the addition of the chemical may not be effective enough. In addition, since trace amounts of persistent substances and the like usually remain in the treated water, the COD of the treated water never becomes zero.

[0038] Therefore, in the method of the present invention, in order to obtain a sufficient effect of reducing the COD of the treated water, the addition of the agent is started when the COD of the treated water is in the range of 3.0 to 300 mg / L, preferably 10.0 to 100 mg / L. If the COD of the treated water before the start of the addition of the agent is so high as to exceed 300 mg / L, the load is too high and the effect of reducing the COD of the treated water cannot be obtained sufficiently even if the addition of the agent is started.

[0039] It is preferable to use the supernatant water from the settling tank to measure the COD of the treated water, but it is also possible to use the supernatant water in a measuring cylinder after measuring the SV of sludge collected from the downstream of the biological treatment tank, or the filtrate after filtering such sludge, as appropriate.

[0040] In this specification, COD refers to the oxidizing agent potassium permanganate (COD Mn )

[0041] The chemical used in the method of the present invention is a liquid agent containing calcium ions and iron ions, which has the effect of reducing the COD of the treated water and improving the settling properties of sludge by increasing biological activity in the biological treatment tank.

[0042] The term "liquid" refers to a chemical that is in a liquid form without forming any insoluble precipitate at room temperature (20°C). The chemical used in the present invention can be produced by dissolving water-soluble calcium compounds and iron compounds in pure water or wastewater from the site.

[0043] Calcium compounds that can be used in the preparation of the medicament include, but are not limited to, water-soluble calcium compounds such as calcium chloride, calcium hydroxide, calcium oxide, and calcium nitrate, and iron compounds that can be used in the preparation of the medicament include, but are not limited to, water-soluble iron compounds such as ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, ferrous nitrate, and ferric nitrate.

[0044] The calcium compound and the iron compound may each be a combination of two or more compounds (for example, ferrous chloride and ferric chloride as iron compounds), or a single compound may be used. The calcium compound and the iron compound may be a hydrate or an anhydrate. As these compounds, commercially available powder compounds or commercially available liquid compounds (for example, aqueous solutions) may be used, but it is preferable to use liquid compounds because they do not require a powder dissolving step. If the agent contains poorly soluble materials such as minerals, the addition of the agent may cause an increase in the amount of sludge generated due to the accumulation of the poorly soluble materials themselves, or cause pipe blockage. Therefore, when producing the agent used in the present invention, it is necessary to use a compound that dissolves completely in pure water.

[0045] The drug used in the present invention contains calcium ions derived from calcium compounds and iron ions derived from iron compounds. 2+ "Iron ion" refers to Fe 2+ and Fe 3+ "Containing iron ions" refers to Fe 2+ and Fe 3+ By containing both calcium ions and iron ions, the effect of reducing the COD of the treated water and the effect of improving the settling property of the sludge can be obtained more efficiently than when the agent contains only calcium ions or only iron ions.

[0046] When manufacturing the drug, it is sufficient to dissolve both the calcium compound and the iron compound in water. However, care must be taken to avoid the formation of insoluble precipitates by considering the type of compound, the concentration of each compound, and the pH of the liquid.

[0047] For example, calcium ions react with sulfate ions to form insoluble calcium sulfate, so when using ferrous sulfate, ferric sulfate, or polyferric sulfate as the iron compound, attention must be paid to the mixing ratio. Alternatively, it is preferable not to use sulfates such as ferrous sulfate, ferric sulfate, and polyferric sulfate. In addition, for example, when iron ions (Fe 2+and Fe 3+ ) reacts with hydroxide ions to produce a precipitate, so it is preferable not to use calcium hydroxide or calcium oxide.

[0048] From the viewpoint of low risk of insoluble precipitation, it is preferable to use calcium chloride (CaCl2) or its hydrate as the calcium ion source in combination with ferrous chloride (FeCl2) and / or ferric chloride (FeCl3) as the iron ion source. In addition, since ferrous chloride is easily oxidized by oxygen in the air, it is more preferable to use ferric chloride as the iron ion source. In other words, it is most preferable that the calcium ions are derived from calcium chloride and the iron ions are derived from ferric chloride.

[0049] There is no limitation on the mixing method or mixing order when mixing the calcium compound and the iron compound, and any method can be used as long as it can completely dissolve both compounds to produce a liquid drug. For example, if calcium chloride and ferric chloride powder compounds are used as raw materials, they can be mixed in a normal manner, such as adding calcium chloride to pure water and mixing with a mixer, and then adding ferric chloride and mixing with a mixer. Also, if liquid products such as a 35% aqueous solution of calcium chloride and a 37% aqueous solution of ferric chloride are used as raw materials, a predetermined amount of liquid product raw material can be added to a production tank, and then water for dilution can be added vigorously and stirred with a water flow.

[0050] There is no limitation on the temperature at which the calcium compound and the iron compound are dissolved. They may be dissolved at room temperature, or may be dissolved while being appropriately heated using an immersion heater, a jacket heater or the like provided in a dissolution tank.

[0051] The concentrations of calcium ions and iron ions in the agent are preferably 1 to 20% by mass, and more preferably 2 to 18% by mass, of the agent. In such a range, for example, precipitation of the agent during refrigerated storage can be suppressed, while the concentrations are not too low, so that the agent can be added in practical amounts to wastewater treatment equipment to reduce treated water COD and improve sludge settling properties.

[0052] Calcium ions and iron ions (Fe 2+ Fe 3+ The concentration of the compound (the sum of the above) may be calculated from the mass of the compound used and the mass of the entire liquid, or may be measured by flame atomic absorption spectrometry or ICP atomic emission spectrometry (both methods are JIS K0102:2019).

[0053] The mixing ratio of calcium ions and iron ions is not particularly limited, and they can be mixed at any ratio, for example, calcium ions:iron ions = 1:999 (mass ratio) to 999:1 (mass ratio). From the viewpoint of expecting a synergistic improvement in the reduction of treated water COD and the improvement of sludge settling properties due to the presence of both calcium ions and iron ions, the mixing ratio is not limited to this, but for example, calcium ions:iron ions = 1:10 to 10:1 is preferable, 1:4 to 4:1 is more preferable, and 1:2 to 2:1 is even more preferable.

[0054] The agent used in the present invention improves the activity of not only specific microorganisms but also many microorganisms living in the biological treatment tank. For example, in a facility such as a factory that discharges wastewater containing difficult-to-degrade substances (such as color components and non-natural chemically synthesized substances), when high load continues due to continuous operation, the treatment is insufficient with the difficult-to-degrade substance decomposition bacteria that usually exist in small numbers in the biological treatment tank, and there is a concern that the COD of the treated water will increase and the sludge sedimentation property will deteriorate. Since it is difficult for general microorganisms such as Bacillus bacteria to decompose difficult-to-degrade substances, it is difficult to promote the decomposition of difficult-to-degrade substances by simply applying a microbial preparation containing these general microorganisms alive. However, in the present invention, the activity of all indigenous microorganisms that can decompose difficult-to-degrade substances is improved, so that it is considered that it is possible to promote the biological activity of difficult-to-degrade substance decomposition bacteria, leading to a reduction in the COD of the treated water and an improvement in the sludge sedimentation property, even in a site where there are relatively many difficult-to-degrade substances as described above.

[0055] The chemical used in the present invention promotes biological activity with calcium ions and iron ions, thereby reducing the COD of the treated water and improving the settling ability of the sludge, but it is also believed to have secondary effects other than promoting biological activity. For example, calcium ions react with sulfate ions in the raw water to produce calcium sulfate (gypsum), thereby reducing sulfate ions, and iron ions reduce hydrogen sulfide.

[0056] In addition to the calcium ion source and iron ion source, the agent used in the present invention may contain, as appropriate, components for promoting the growth of microorganisms or enhancing biological activity, such as carbohydrates such as glucose, starch, molasses, etc.; proteins such as yeast extract and peptone; lipids such as animal oil, vegetable oil, and waste oil; magnesium compounds such as magnesium sulfate and magnesium chloride; manganese compounds such as manganese chloride, manganese sulfate, and manganese nitrate; silicon compounds such as sodium silicate and calcium silicate; and chlorine compounds such as sodium chloride and potassium chloride, in an amount that does not cause precipitation.

[0057] In addition, since the pH may become acidic due to the influence of iron compounds, the pH may be appropriately adjusted using sodium hydroxide, potassium hydroxide, etc. In this case, care must be taken to prevent the formation of precipitates derived from iron ions or calcium ions.

[0058] When components other than these calcium ion sources and iron ion sources are mixed, the mixing ratio of these components is preferably 50 mass % or less of the drug, more preferably 20 mass % or less, even more preferably 10 mass % or less, and even more preferably 5 mass % or less.

[0059] The chemical used in the present invention may be charged into the wastewater treatment facility in the form as produced, or may be mixed with water and diluted before being charged. Alternatively, a chemical that has been produced in advance and stored may be charged into the wastewater treatment facility, or a calcium compound that serves as a calcium ion source and an iron compound that serves as an iron ion source may be prepared at the wastewater treatment site, and these compounds may be added to wastewater or pure water collected at the site and completely dissolved in the liquid to produce a liquid agent, which may then be charged into the wastewater treatment facility.

[0060] The location of the chemical agent of the present invention in a wastewater treatment facility may be any location where the chemical agent flows into the biological treatment tank. For example, the chemical agent may be added to the biological treatment tank or upstream thereof. More specifically, in a facility having a raw water tank (a tank into which raw water (wastewater containing organic matter) first flows in during the wastewater treatment process), the chemical agent may be added to any location from the upstream of the raw water tank to the biological treatment tank, more preferably from the upstream of the raw water tank to the upstream of the biological treatment tank, even more preferably from the downstream of the raw water tank to the upstream of the biological treatment tank, and even more preferably from the upstream of the biological treatment tank. The chemical agent may also be added from multiple locations, without being limited to one location.

[0061] FIG. 1 shows an example of the location where the chemical is added. In FIG. 1, arrow A indicates the upstream of the raw water tank, B indicates the middle of the raw water tank, C indicates the downstream of the raw water tank, D indicates the upstream of the biological treatment tank, and E indicates the middle of the biological treatment tank. The chemical may be added at one or more of the locations A to E of the arrows shown in FIG. 1. In addition, when the addition of the chemical used in the present invention causes the pH of the water to be treated to fluctuate excessively, it is preferable to add the chemical before the pH adjustment location. For example, when C is the pH adjustment location, the chemical may be added at A or B upstream of C.

[0062] The method of adding the drug of the present invention is not particularly limited, and may be manual addition or automatic addition using a drug injection pump. It may also be one-time addition, multiple intermittent addition, or continuous addition. However, since the drug used in the present invention exerts its effect from the fifth day after the start of addition, regardless of the addition method, the drug is added at least once a day for at least five days.

[0063] For example, when the agent used in the present invention contains 2.7% by mass of calcium ion and 3.5% by mass of iron ion, the agent is preferably added in an amount of 1 to 300 mg / L (0.027 to 8.1 mg / L as calcium ion, 0.035 to 10.5 mg / L as iron ion) to the raw water, and more preferably added in an amount of 5 to 100 mg / L (0.135 to 2.7 mg / L as calcium ion, 0.175 to 3.5 mg / L as iron ion). By adding such an amount, it becomes easier to obtain both effects of reducing the COD of the treated water and improving the settling property of the sludge.

[0064] It is preferable that the above amount of the chemical is supplied to the biological treatment tank at least once a day. If the amount added is too large, it will be disadvantageous in terms of cost, and may cause the pH of the water to be treated to fluctuate drastically, or inhibit the respiration of the organisms, lowering the oxygen utilization rate coefficient of the sludge, resulting in insufficient effect.

[0065] In the method of the present invention, as described above, the addition of a specific chemical agent is started when specific conditions are met (sludge CST is 5 to 3000 seconds, SV is 90% or more, and COD of treated water is 3.0 to 300 mg / L), and the addition of the chemical agent is carried out at least once a day for 5 days or more. This makes it possible to efficiently obtain both effects of reducing the COD of the treated water and improving the sludge settling property. An example of the method of the present invention is shown in FIG. 2. First, sludge is collected from a predetermined location such as the rear of the biological treatment tank, and treated water is collected from the supernatant of the settling tank. Next, the CST and SV of the sludge are measured, and the COD of the treated water is measured. Then, when it is confirmed that the values ​​of CST, SV, and COD are within the above ranges, the addition of a liquid chemical agent containing calcium ions and iron ions is started, and the chemical agent is added at least once a day for 5 days or more.

[0066] The amount of the chemical to be added may be adjusted so that BA≧5 is satisfied, assuming that the SV on the fifth day after the start of the chemical addition is A% and the SV at the start of the chemical addition is B%. In this case, the SV of the sludge is measured on the fifth day after the start of the chemical addition in the same manner as above. If the SV of the sludge on the fifth day after the start of the chemical addition does not satisfy the desired range with respect to the SV at the start of the chemical addition, the amount of the chemical to be added may be appropriately increased or decreased on the fifth day after the start of the chemical addition to obtain a desired reduction in SV. Alternatively, if it is known in advance how much of the chemical should be added at the start of the chemical addition to obtain a desired SV on the fifth day after the chemical addition, the chemical may be added in such an amount from the start of the chemical addition. Alternatively, not only the amount of the chemical to be added but also the frequency, timing, and location of addition may be adjusted.

[0067] The amount of the chemical agent added may be adjusted so that the COD of the treated water on the fifth day after the start of the chemical agent addition is reduced by 10% or more compared to the COD of the treated water at the start of the chemical agent addition. In this case, the COD of the treated water is measured by the same method as described above on the fifth day after the start of the chemical agent addition. If the COD of the treated water on the fifth day after the start of the chemical agent addition does not satisfy the desired range with respect to the COD of the treated water at the start of the chemical agent addition, the desired COD may be obtained by adjusting the amount of the chemical agent added on the fifth day after the start of the chemical agent addition, for example by increasing or decreasing it appropriately. Alternatively, if it is known in advance how much of the chemical agent should be added at the start of the chemical agent addition to obtain the desired COD on the fifth day after the chemical agent addition, the chemical agent adjusted to such an amount may be added from the start of the chemical agent addition. Alternatively, not only the amount of the chemical agent added but also the frequency, timing, and location of the chemical agent addition may be adjusted.

[0068] The amount of the agent added may be adjusted so that the oxygen utilization rate coefficient of the sludge on the fifth day after the start of the addition of the agent is increased by 10% or more compared to the oxygen utilization rate coefficient of the sludge at the start of the addition of the agent. In this case, the oxygen utilization rate coefficient of the sludge is measured at the start of the addition of the agent and on the fifth day after the start of the addition. If the oxygen utilization rate coefficient on the fifth day after the start of the addition of the agent does not satisfy the desired range with respect to the oxygen utilization rate coefficient at the start of the addition of the agent, the amount of the agent added may be appropriately increased or decreased on the fifth day after the start of the addition of the agent so as to obtain the desired oxygen utilization rate coefficient. Alternatively, if it is known in advance how much of the agent should be added at the start of the addition of the agent to obtain the desired oxygen utilization rate coefficient on the fifth day after the addition of the agent, the agent adjusted to such an amount may be added from the start of the addition of the agent. Alternatively, not only the amount of the agent added but also the frequency, timing, and location of the addition may be adjusted.

[0069] The oxygen utilization rate coefficient is expressed as the amount of oxygen (mg / g·h) utilized by a unit weight of activated sludge in a unit time. The larger the oxygen utilization rate coefficient, the faster the respiration rate utilizing oxygen, i.e., the higher the activity of aerobic microorganisms in activated sludge. In this specification, the oxygen utilization rate coefficient is specifically measured by the following method using an oxygen utilization rate measuring device according to the method described in "Sewage Testing Methods, Vol. 1, pp. 688-689, 2012 Edition": (1) Transfer 1 L of the sludge mixture from the biological treatment tank to a 1 L narrow-mouth bottle, leave it for 10 to 20 minutes, and then transfer the supernatant liquid to another narrow-mouth bottle using a siphon or other means. (2) The supernatant is vigorously aerated for 5 to 10 minutes using an aeration device so that the DO concentration is approximately 5 mg / L or higher, and then mixed well with the settled sludge from (1). (3) Fill a 300 mL Erlenmeyer flask with the mixture of (2) and (3), and insert the dissolved oxygen sensor into the flask without allowing air to enter. (4) After the procedure in (3), immediately begin stirring with a magnetic stirrer while measuring the change in DO over time for approximately 5 to 10 minutes. (5) From the amount of oxygen available in the linear portion of the recorded decrease curve, the oxygen availability rate per unit time of the sludge mixed liquid in the biological treatment tank r r Calculate the amount (mg / L h). r r = (mg / L h) = {oxygen utilization (mg / L)} / {elapsed time (h)} In addition, the oxygen utilization rate coefficient K r Calculate the concentration (mg / g h). K r (mg / g·h)={r r (mg / L·h) × 1000} / {MLSS or MLVSS (mg / L)}.

[0070] As described above, the liquid agent containing calcium ions and iron ions used in the present invention has the effect of improving the activity of microorganisms in a biological treatment tank, thereby promoting the formation of biopolymers by the microorganisms, which in turn forms flocs with high compactibility, increasing the compactibility of the sludge, and improving the settling property of the sludge. Therefore, this agent can be used as a settling promoter for sludge.

[0071] As described above for the agent, this sludge settling promoter preferably contains calcium ions and iron ions in an amount of 1 to 20 mass % of the agent, and preferably 2 to 18 mass %. As described above, it can be used in wastewater treatment in which sludge from a biological treatment tank is settled in a settling tank to obtain treated water as supernatant water, and can also be used appropriately in facilities using MBR (membrane separation activated sludge process) where there is a risk of membrane clogging with conventional sludge settling agents (polymer flocculants). For example, it can be used in facilities where membrane separation by MBR and a settling tank are used in combination, or when sludge is to be temporarily settled in MBR by dredging, etc.

[0072] As described above, the sludge settling promoter is preferably added when the sludge CST is 5 to 3000 seconds, the SV is 90% or more, and the COD of the treated water is 3.0 to 300 mg / L, and the addition is preferably carried out at least once a day for 5 days or more. As described above, the amount of the sludge settling promoter added to the raw water is preferably 1 to 300 mg / L (0.027 to 8.1 mg / L as calcium ion, 0.035 to 10.5 mg / L as iron ion), and more preferably 5 to 100 mg / L (0.135 to 2.7 mg / L as calcium ion, 0.175 to 3.5 mg / L as iron ion). It is preferable to add the amount described above at least once a day.

[0073] The calcium ions and iron ions of the sludge settling promoter are most preferably derived from calcium chloride and ferric chloride, respectively, as described above for the chemicals. The sludge settling promoter may consist of water and a calcium ion source and an iron ion source, or may contain other components as described above as appropriate.

[0074] As described above for the chemicals, the sludge settling promoter may be added to the wastewater treatment facility in the form of a liquid preparation as produced, or, depending on the method of addition, the liquid preparation may be mixed with water and appropriately diluted (e.g., two-fold or more) before addition.

[0075] Other conditions for the sludge settling promoter (production method, addition location, addition method, etc.) are the same as those described above for the chemicals. EXAMPLES

[0076] The present invention will be specifically described below using examples, but the present invention is not limited to these.

[0077] [Example 1] A laboratory test apparatus for the standard activated sludge process shown in Figure 1 was created and a continuous water flow test was carried out. The laboratory test apparatus consisted of a raw water tank (effective volume 40 L), a biological treatment tank (aeration tank, effective volume 10 L), and a sedimentation tank (conical shape, effective volume 5 L), and the dissolved oxygen in the biological treatment tank was constantly maintained at 1-3 mg / L using an air pump and air stones. A pH electrode was placed after the raw water tank, and the pH was constantly maintained in the range of 6.5-7.5 using sulfuric acid and sodium hydroxide.

[0078] Raw water and activated sludge (biological treatment tank sludge) were collected from wastewater treatment facilities of food factories and used after appropriate dilution. Mn was adjusted to 150 mg / L (hereafter, all COD Mn). The BOD was 150mg / L, the same as the COD. The MLSS of the activated sludge was maintained at 2000mg / L by adjusting the amount of sludge returned and the amount of sludge withdrawn, and raw water was continuously passed through at 40L / day. The COD of the treated water (supernatant water from the settling tank) from the fifth day after the start of operation under these conditions (stable period) was 10.0mg / L, the CST of the activated sludge sampled from the rear of the biological treatment tank was 200 seconds, and the sludge settleability was SV95%.

[0079] The agent used was CaCl2·2H2O (Tokuyama Corporation) as a calcium compound and FeCl3 (37% aqueous solution, Shin-Nihon Kasei Co., Ltd.) as an iron compound, and both compounds were dissolved in pure water to give a concentration of 10 mass% each, to prepare the agent of the present invention (corresponding to 2.7 mass% calcium ions and 3.5 mass% iron ions). Continuous injection of the agent was started on the fifth day after the start of operation.

[0080] The dosage of the chemical was 30mg / L of the raw water. In other words, the daily dosage was 1.2g / day, so the chemical was diluted 1000 times with pure water and constantly added at about 0.83g / min by a chemical injection pump to the middle of the raw water tank. Separately, a control was prepared in which the same treatment was carried out except that no chemical was added.

[0081] The COD of the treated water was measured when no chemicals were added (no chemicals added) and when chemical addition was started (added). The results are shown in Table 1. The start of chemical addition was considered to be day 1, and compared to the COD of the treated water on day 1, which was 10.0 mg / L, a reduction of 10% or more (9.0 mg / L or less) was evaluated as preferable, and a reduction of 25% or more (7.5 mg / L or less) was evaluated as more preferable.

[0082] [Table 1]

[0083] Although the effect of reducing COD was hardly observed 1-2 days after the start of the addition of the chemical, a significant reduction in COD was observed from the 5th day, and a large reduction in COD was confirmed from the 7th day onwards. From this result, it can be inferred that the addition of the chemical contributes to improving biological activity, and that by continuing the addition of the chemical to a certain extent, the improvement in biological activity will reach a level that contributes to reducing COD. Therefore, in order to reduce the COD of the treated water by adding the chemical, it is preferable to continue the addition of the chemical for 5 days or more, more preferably for 7 days or more.

[0084] The oxygen utilization rate coefficient of sludge was also measured when no chemicals were added (no chemicals added) and when chemical addition was started (added). The results are shown in Table 2. The time when chemical addition began was considered to be day 1, and compared to the oxygen utilization rate coefficient of 2.0 mg / g h on day 1, an increase of 10% or more (2.2 mg / g h or more) was evaluated as favorable, and an increase of 25% or more (2.5 mg / g h or more) was evaluated as more favorable.

[0085] [Table 2]

[0086] As with the transition of COD of the treated water, the oxygen utilization rate coefficient showed little change in the first and second days after the start of the addition of the chemical, but it began to increase significantly from the fifth day, and a significant increase was confirmed from the seventh day onwards. From these results, it can be confirmed that the addition of the chemical improves the biological activity of aerobic microorganisms living in activated sludge, and that by continuing the addition of the chemical to a certain extent, as described above, the improvement in biological activity reaches a level that contributes to the reduction of the COD of the treated water. Therefore, as described above, it can be seen that the addition of the chemical should be continued for preferably 5 days or more, more preferably 7 days or more. The sludge settleability SV at the start of the addition of the chemical was 95%, but the sludge settleability from the fifth day after the start of the addition of the chemical decreased to SV 85% or less.

[0087] [Example 2] The same conditions as in Example 1 were used, except that the MLSS of the activated sludge was maintained at 2000 mg / L by adjusting the amount of sludge returned and the amount of sludge withdrawn, and the load was adjusted by appropriately changing the dilution rate of the raw water to change the COD of the treated water at the start of the addition of the chemical. As in Example 1, the chemical was constantly injected into the raw water so that the amount was 30 mg / L. Under these conditions, the COD of the treated water after the addition of the chemical was started was measured. The results are shown in Table 3. The start of the addition of the chemical was the first day, and it was evaluated that the COD of the treated water on the 7th day was reduced by 10% or more compared to the COD of the treated water on the 1st day, and that the COD of the treated water on the 7th day was reduced by 25% or more was more preferable.

[0088] [Table 3]

[0089] When the COD of the treated water was 1.0 mg / L when the addition of the chemicals began (day 1), almost no reduction in COD was observed even after day 5. This was thought to be because persistent substances that could not be decomposed remained even though the biological activity was improved by the chemicals. On the other hand, when the COD of the treated water was 3.0-300 mg / L when the addition of the chemicals began, the COD began to decrease significantly after day 5, and a significant reduction in COD was confirmed after day 7.

[0090] In particular, a high COD reduction effect was observed when the COD of the treated water was in the range of 10.0 to 100 mg / L at the start of the addition of the chemical. On the other hand, when the COD of the treated water reached 1000 mg / L at the start of the addition of the chemical, no reduction in the COD of the treated water was confirmed. This was presumed to be because, even if the biological activity was improved by the chemical, the load was too high and the COD could not be treated satisfactorily. From the above results, it can be seen that it is preferable to start adding the chemical when the COD of the treated water is in the range of 3.0 to 300 mg / L, more preferably in the range of 10 to 100 mg / L.

[0091] In the test areas where the COD of the treated water at the start of the addition of the chemical was 3.0-300mg / L, the oxygen utilization rate coefficient increased by 25% or more from the fifth day after the start of the addition compared to the start of the addition. In addition, the sludge settling property was BA≧10, where SV at the start of the addition of the chemical was B% and SV on the fifth day after the start of the addition was A%.

[0092] [Example 3] Activated sludge with different CSTs was prepared, and the chemical was constantly injected to the raw water at 30 mg / L under the same conditions as in Example 1 (COD of the treated water at the start of chemical addition was 10.0 mg / L). Under these conditions, the sludge settleability SV was measured after chemical addition was started. The results are shown in Table 4.

[0093] When the start of chemical addition was considered the first day, the sludge settleability SV value on the first day was 95%. SV values ​​of 90% or less on the seventh day after the start of chemical addition were evaluated as favorable, and values ​​of 85% or less were evaluated as more favorable. When no chemicals were added, the SV remained in the range of 94-96% (no change in sludge settleability).

[0094] [Table 4]

[0095] When the CST at the start of chemical addition was too high (6000 seconds), the effect of improving sludge settling (reducing SV) could not be confirmed. On the other hand, when the CST at the start of chemical addition was appropriately low (5 to 3000 seconds), the effect of improving sludge settling was confirmed.

[0096] This is presumably because, when the CST at the start of pesticide addition is appropriately low, the addition of the pesticide improves biological activity, biopolymers are gradually formed, and sludge compaction improves, thereby improving sludge settling properties. However, if the CST at the start of pesticide addition is too high from the beginning, even if biological activity improves by adding the pesticide, no more biopolymers are formed, so sludge compaction does not increase and sludge settling properties do not improve any further.

[0097] Therefore, in order to improve sludge settling properties by adding chemicals, it is recommended to start adding chemicals to sludge with a CST of 5 to 3000 seconds, more preferably 5 to 1000 seconds. In the test area where the sludge CST at the time of starting chemical addition was 5 to 3000 seconds, the oxygen utilization rate coefficient increased by 25% or more and the treated water COD decreased by 10% or more after the fifth day after the start of chemical addition, compared to when the addition started.

Claims

1. A method for treating wastewater, comprising: settling sludge from a biological treatment tank in a settling tank to obtain treated water as supernatant water, Measuring the CST and SV of the sludge and the COD of the treated water, and starting the addition of a liquid agent containing calcium ions and iron ions when the CST is 5 to 3,000 seconds, the SV is 90% or more, and the COD is 3.0 to 300 mg / L; The drug is added at least once per day for at least five days; The method for treating wastewater, further comprising:

2. 2. The method for treating wastewater according to claim 1, wherein the agent has a calcium ion concentration of 1 to 20 mass % and an iron ion concentration of 1 to 20 mass %.

3. The method for treating wastewater according to claim 1 or 2, further comprising adding the chemical to a biological treatment tank or upstream of the biological treatment tank.

4. The wastewater treatment method according to claim 1 or 2, further comprising adjusting the amount of the chemical to be added so as to satisfy B-A≧5, where A% is the SV on the fifth day after the start of the addition of the chemical and B% is the SV at the start of the addition of the chemical.

5. 3. The wastewater treatment method according to claim 1 or 2, further comprising adjusting the amount of chemical added so that the COD of the treated water on the fifth day after the start of the addition of the chemical is reduced by 10% or more compared to the COD of the treated water at the start of the addition of the chemical.

6. Measuring the oxygen utilization rate coefficient of the sludge at the start of the addition of the agent; and 3. The method for treating wastewater according to claim 1 or 2, further comprising adjusting the amount of the chemical added so that the oxygen utilization rate coefficient of the sludge on the fifth day after the start of the addition of the chemical increases by 10% or more compared to the oxygen utilization rate coefficient of the sludge at the start of the addition of the chemical.

7. A liquid sludge settling promoter containing 1 to 20% by mass of calcium ions and 1 to 20% by mass of iron ions.

8. The sludge settling promoter according to claim 7, wherein the calcium ions are derived from calcium chloride and the iron ions are derived from ferric chloride.

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