KR20010038791A - Paper wastewater treatment process and paper manufacturing method using the treated water therefrom - Google Patents

Abstract

PURPOSE: Disclosed is a method for treating pulp/paper wastewater, particularly corrugated cardboard wastewater retaining a large quantity of organics and dissolved electrolytes such as sulfate, calcium ion. CONSTITUTION: First, wastewater flows into an anaerobic reactor (4) wherein organics are decomposed to methane and carbon dioxide by methane-producing bacteria (CH3COOH→CH4+CO2) and sulfate (SO4¬2-) is converted to sulfuretted hydrogen (H2S) by sulfate reducing bacteria£CH3COOH+SO4¬(2-)+2H¬(+)→H2S+2H2O+2CO2|. A portion of discharged wastewater from the anaerobic reactor is returned to the anaerobic reactor after being mixed with raw wastewater (3) through a return pipe (6) and the rest wastewater discharged from the anaerobic reactor, after adding NaOH (5) for increasing pH, is transferred into an aerobic stripping tank (7) for stripping carbon dioxide into atmosphere. As pH of wastewater increases due to carbon dioxide stripping, calcium ion that exists in wastewater reacts with carbonate ion generated from bicarbonate ion. Therefore, insoluble calcium carbonate is formed as follows: H¬(+)+CO3¬(2-) ↔ HCO3¬(-), CO3¬(2-)+Ca¬(2+) ↔ CaCO3(s)↓. Because chemicals are not used in this process, electric conductivity of wastewater may be decreased. Insoluble calcium carbonate travels through a discharge pipe (12) into a settling tank (8) and drained through a sludge draining pipe (13). In case treated wastewater by this process is applied to the manufacturing process of corrugated cardboard, produced paper shows upgraded mechanical strength compared with conventional paper which is manufactured by using white water.

Description

Paper wastewater treatment process and paper manufacturing method using the treated water therefrom}

The present invention relates to a papermaking wastewater treatment method and a papermaking method using the treated water, and more particularly, to a biological and physical treatment method of wastewater discharged from a paper mill and a papermaking method for reuse of the treated water.

Paper mills discharge large amounts of wastewater due to the nature of the papermaking process. Among them, wastewater discharged from factories that manufacture industrial paper such as corrugated cardboard contains high concentrations of organic matter, calcium ions and sulfate ions. Papermaking wastewater treated by the method has a number of problems to recycle into process water.

That is, since calcium ions contained in papermaking wastewater are hardness-induced substances, they are precipitated to form scale when included in the cooling water of the papermaking process, and the hydrogen ions interfere with the swelling of the papermaking fibers and lower the hydrogen bonding force between the fibers. It is a limiting factor in increasing the strength of. In addition, if the electrical conductivity of the process water increases due to the presence of a large amount of calcium ions or sulfate ions, the dehydration rate is lowered because the activity of the retention agent is lowered. Will result.

Specifically, there is a general activated sludge process as a conventional paper wastewater treatment method used. However, in the case of activated sludge process, it has been found that there are many problems in operation such as sludge sedimentation deterioration when wastewater containing high concentration of organic matter is introduced. Study on Characteristics ", Korea Water Conservation Society, 12 (4), 447 ~ 454, 1996].

In order to solve this problem, a process of stably removing high concentrations of organic matter by increasing the concentration of microorganisms by increasing the partial pressure of oxygen, such as the oxygen aeration method, has been used. And improvement of treatment efficiency ", Korean Society of Environmental Engineering, 18 (10), 1195-1208, 1996]. However, since the aerobic process is installed only to remove organic matters, it is impossible to remove hardness-induced substances such as calcium ions in the water, which causes scale formation in the papermaking process when the treated water is reused as process water. have.

These calcium scales inhibit the flow of fluid in many pipes of the papermaking process and prevent the dehydration by blocking the wire screen. In addition, calcium scales grown in crystalline form intensify wear and tear in machines and significantly shorten machine life [AMScallan, "The effect of acidic groups on the swelling of pulps: a review", Tappi Journal, 66 (11). ), 73-75, 1983].

In addition, it was not possible to remove the sulfate ions present in a large amount in the water, it was not possible to reduce the electrical conductivity in the water. Process water with high electrical conductivity should reduce the activity of the additives added to the papermaking process and add an excessive amount of additives [S.D. Alexander and R.J. Dobbins, "The buildup of dissolved electrolytes in a closed paper mill system", Tappi Journal, 60 (12), 1977].

In order to solve the above problems, it has been proposed to use a membrane separation method to remove calcium or electrical conductivity causing substances [L. Webb, "Closing up the water loop without closing down the mill", PPI, June, 43-46, 1997]. Thus, the membrane separation method has high separation and removal efficiency, but the initial investment is very high. Especially, when applied to the papermaking process, the membrane is easily contaminated by foreign matter such as fine powder and fiber, which causes a big problem in operation. Therefore, a significantly expensive pretreatment device is required before the membrane separation process, and a considerable cost is required for the membrane replacement after a certain time of operation.

Therefore, the technical problem to be achieved by the present invention is to provide a method for economically removing not only the organic matter contained in the papermaking wastewater discharged from the papermaking process, but also calcium ions, sulfate ions, hydrogen ions, etc. and recycling them as process water. .

1 is a flowchart of a papermaking wastewater treatment process according to an embodiment of the present invention.

2 is a flowchart of a papermaking process according to an embodiment of the present invention.

Figures 3a to 3c shows the molecular structure change of the polymer used as a retention agent in the papermaking process.

The present invention to achieve the above technical problem,

In the method of treating wastewater of a paper mill containing a large amount of organic matter, sulfate and calcium ions,

a) an anaerobic reaction step of introducing the wastewater into an anaerobic reactor to decompose organic matter and convert it into carbon dioxide and methane gas, and sulfate to hydrogen sulfide and carbon dioxide;

b) the treated water in the anaerobic reactor is introduced into an aerobic carbon dioxide degassing tank to degas the carbon dioxide into the atmosphere to induce an increase in pH, thus increasing the concentration of carbonate ions to convert calcium ions into insoluble solid calcium carbonate. Degassing step; And

c) solids are precipitated and removed from the treated water undergoing the aerobic degassing step and the supernatant is obtained as the final treated water.

According to one embodiment of the present invention, it is preferable to further include a step of recirculating a portion of the treated water subjected to the anaerobic reaction step and supplying it back to the anaerobic reactor together with the incoming wastewater.

According to one embodiment of the invention, the aerobic degassing step is preferably carried out in an aerobic degassing tank equipped with a surface aerator and an internal mixer.

According to another exemplary embodiment of the present invention, the method may further include adding sodium hydroxide before introducing the treated water that has undergone the anaerobic reaction step into the aerobic degassing step.

The present invention also provides a method for reusing the treated water treated by the papermaking wastewater treatment method as the papermaking process water.

According to one embodiment of the present invention, the papermaking wastewater treated by the method of the present invention can be put into a pulp and used for dissociation of the papermaking fee.

According to another embodiment of the present invention, the paper wastewater treated by the method of the present invention may be introduced into the white water tank to reduce the organic matter concentration and the ion concentration in the overall papermaking process water.

The present invention relates to a biological and physical method of papermaking wastewater and a method capable of improving the quality of a product by reusing the treated water.

The method according to the present invention primarily removes organic substances present in high concentrations in wastewater in an anaerobic reactor, and converts sulfate ions in water into hydrogen sulfide to separate them into gaseous state. In the latter stage, excess carbon dioxide contained in the water is removed through surface aeration to react the carbonate and calcium ions in the water to inactivate and remove the calcium ions. Through this two-stage treatment, the concentrations of organic calcium ions and sulfate ions in water are greatly reduced, and the pH value is increased by decreasing the concentration of hydrogen ions in water. Water treated in this way is significantly reduced in hardness-induced substances such as calcium ions and does not cause problems such as scale formation due to precipitation of calcium ions even when used as cooling water during various papermaking processes.

In addition, the hydrogen ion concentration in the treated water is low, the swelling phenomenon of papermaking fiber appears. Swelling of the fibers increases the hydrogen bonding force between the fibers, leading to an effect of increasing the strength of the product. In addition, the ions conductivity in the process water is reduced by removing the sulfate ions, calcium ions and the like through this process. This reduction in ion conductivity may result in an increase in the retention of the retention agent, thereby reducing the amount of retention of the retention agent than when using a process water with high ion conductivity. In addition, increased activity of the retention agent results in an increase in the rate of dehydration resulting in an increase in the production rate. Therefore, the method of the present invention enables the reuse of the treated water in addition to the wastewater treatment function of removing contaminants in the papermaking wastewater and at the same time contributes to the improvement of product quality.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

1 is a flowchart of a papermaking wastewater treatment process according to an embodiment of the present invention.

An anaerobic reactor 4 is installed to remove organic matter in the influent wastewater. In the anaerobic reactor 4, methane producing bacteria for converting organic matter into methane gas and sulfate reducing bacteria for converting sulfate to hydrogen sulfide are present. Such microorganisms can be used as granular sludge and also as microorganisms immobilized on a carrier.

Organic matter contained in wastewater discharged from paper mills, especially from corrugated cardboard manufacturing plants, is a component that can be easily decomposed, and when introduced into an aerobic process such as activated sludge, it causes problems such as sludge settling due to overgrowth of filamentous fungi. On the other hand, in anaerobic processes, there is no possibility of problems such as growth of filamentous fungi, and when organic matter that can be easily decomposed is introduced, it is easily converted into carbon dioxide and methane gas by organic acid production and methane producing bacteria. In particular, in the case of wastewater from corrugated cardboard manufacturing plant, since the organic acid is fermented into the anaerobic reactor in the process, the methane production reaction is performed without installing a separate organic acid fermentation tank. That is, the organic matter in the wastewater 3 introduced into the anaerobic reactor 4 is decomposed as shown in Schemes 1 and 2 to be converted into carbon dioxide and methane gas.

CH ₃ COOH → CH ₄ + CO ₂

CH ₃ COOH + SO ₄ ^2- + 2H ⁺ → H ₂ S + 2H ₂ O + 2CO ₂

The gas (1) generated from the anaerobic reactor contains a significant amount of methane and can therefore also be used as fuel (2) for boilers. In addition, the influent wastewater (3) contains a high concentration of sulfate (SO ₄ ^2- ), such sulfate ions are hydrogen sulfide (H ₂ S) by the sulfate reducing bacteria contained in the anaerobic reactor (4) Is converted to) and vaporized.

As an anaerobic reactor, an upflow anaerobic reaction process (UASB: Upflow Anaerobic Sludge Blanket) using granular sludge in addition to the continuous continuous anaerobic tank, and a process of attaching and operating anaerobic microorganisms using a carrier (AF: Anaerobic Filter) are used. Can be. Since USAB and AF processes are highly efficient, high-load anaerobic reactors, the required area is considerably smaller than conventional anaerobic tanks.

A part of the wastewater 11 treated in the anaerobic reactor 4 is mixed with the inflow wastewater 3 through the conveying path 6 and flows back into the anaerobic reactor 4. The remaining effluent flows into the aerobic carbon dioxide degasser 7 to degas the carbon dioxide in the air. The aerobic carbon dioxide degassing tank 7 is preferably equipped with a surface aerator and an internal mixer to degas carbon dioxide in the water. In this case, it is also preferable to incorporate alkaline chemicals such as sodium hydroxide in order to increase the pH in the degassing process.

The degassing of carbon dioxide causes the pH in the water to increase, which in turn leads to the conversion of bicarbonate ions to carbonate ions. Increasing the concentration of carbonate ions eventually form insoluble solid calcium carbonate by binding with calcium ions present in water as shown in Scheme 3 below.

_{^{H + + CO 3 2- ↔ HCO}} 3 -

CO ₃ ^2- + Ca ²⁺ ↔ CaCO ₃ (s) ↓

In the case of removing calcium by a general chemical precipitation method can not be expected to reduce the electrical conductivity, the method according to the present invention can remove the calcium ions without using a chemical agent can increase the electrical conductivity reduction effect.

The solid insolubilized in the form of calcium carbonate enters the settling tank 8 through the outflow pipe 12 and is sedimented and separated. The calcium carbonate solid separated in the settling tank 8 is discharged through the sludge discharge pipe 13, dehydrated and then discarded. Part of the treated water 14 from which the solids are removed from the settling tank is reused as the papermaking process 10 and the remaining part can be discharged through further treatment.

Figure 2 is a general paper manufacturing process used to produce a corrugated cardboard base paper. The solid line indicates the movement of the stock (fibrin), and the dotted line indicates the flow of water. The paper 21 used as a papermaking raw material is dissociated in the pulp 22, and requires a large amount of water when dissociated. The dissociated raw material is primarily removed from the filter 24 via the raw material selection 23. Through the stock preparation 25, the headbox 26, the water is secondarily separated from the crawler wire screen 27. The feedstock from which the water has been removed is dried in the drying unit 28 and then transferred to the product manufacturing step 29.

On the other hand, the water separated from the paper in the filter 24 and the wire screen 27 on the endless track is collected in the white water tank 30. Most of the white water is fed back to the pulper 22 and the surplus is sent to the wastewater treatment plant 31. The large amount of water required in the pulper 22 stage directly supplies the treated water 32 or untreated white water 33 in the wastewater treatment plant 31 from the white water tank 30.

Currently, most paper mills contain a large amount of calcium ions, sulfate ions, high concentrations of organic matter, and have a relatively low pH value. However, in the case of using the wastewater treatment method according to the present invention, the concentration of calcium ions and sulfate ions as well as organic matters can be effectively lowered, and the pH of the treated water rises relatively, so that the fiber dissociated from the pulp when used as process water Increase the swelling properties, which in turn improves the mechanical strength of the product.

In addition, the overall low ion concentration, ie low electrical conductivity, increases the retention of the stock introduced on the crawler wire screen 27 from the headbox 26. This is possible because the overall contamination level of the water 32 treated in the wastewater treatment plant 31 is considerably low, and as a result, the contamination level of the water throughout the process as well as the water 33 from the white water tank 30 is considerably low. The increase in retention may result in the production of high basis weight products and at the same time reduce the amount of contaminants or suspended solids entering the white water tank 30 as unreserved, thereby lowering the contamination level in the white water tank 30.

Specifically, FIGS. 3A to 3C schematically illustrate the molecular structure of a polymer that is generally used as a retention agent in the papermaking process, and illustrates the effect of changing the pH of the process water on the polymer structure of the retention agent. In the state of low electrical conductivity, the retention agent polymer has a structure as shown in FIG. 3A, but when the concentration of ions increases, and thus the environment has a high electrical conductivity, the structure of the polymer changes as shown in FIG. 3B. When the electrical conductivity becomes very high, the result is a structure as shown in FIG. 3C. In other words, when the ion concentration in the process water is high, the polymer becomes entangled by itself, while the ion concentration is low, but when the ion concentration is low, the polymer has a linear structure to increase the retention of the material.

Hereinafter, the present invention will be described in more detail with reference to examples, which are merely illustrative and the scope of the present invention is not limited thereto.

<Example 1>

The wastewater of the corrugated cardboard factory was treated using an upflow anaerobic reactor (USAB) as the anaerobic reactor 4 of FIG. 1. The COD of the influent wastewater was 5,000 mg / ℓ, the concentration of sulfate was 550 mg / ℓ, and the removal efficiency of organic matter and sulfate according to residence time is shown in Table 1.

Residence time (hrs) Organic matter load (kg COD / m ³ / d) Sulfate Volume Load (kg SO ₄ / m ³ / d) Organic matter removal efficiency (%) Sulfate Removal Efficiency (%) 20 6.0 0.66 85 90 15 8.0 0.88 85 88 12 10.0 1.1 80 88 10 12.0 1.3 78 85 8 15.0 1.65 75 85 6 20.0 2.2 75 83

<Example 2>

As can be seen in the above Scheme 2, the production of one equivalent of hydrogen sulfide consumes two equivalents of hydrogen ions, thereby increasing the alkalinity of the treated water. Table 2 below shows the increase in alkalinity according to the removal of sulfate under the same conditions as in Example 1. The pH of the influent wastewater was about 6.5.

Residence time (hrs) Sulfate Volume Load (kg SO ₄ / m ³ / d) Sulfate Removal Efficiency (%) Alkalinity (mg CaCO ₃ / ℓ) pH 20 0.66 90 1,950 7.6 15 0.88 88 1,800 7.5 12 1.1 88 1,720 7.5 10 1.3 85 1,450 7.4 8 1.65 85 1,400 7.5 6 2.2 83 1,300 7.3

From Table 1 and Table 2, it can be seen that alkalinity has a proportional relationship with sulfate removal efficiency. When the residence time decreases, organic acids that have not been removed flow out to suppress the increase in alkalinity and slow down the increase in pH value. Influent wastewater of about pH 6.5 goes through an anaerobic reactor and increases its alkalinity to about pH 7.5. This increase in alkalinity improves the interfiber bond strength by increasing the ability of water to swell fiber.

<Example 3>

In Examples 1 and 2, the effluent from the anaerobic reactor was introduced into a carbon dioxide degassing tank equipped with a surface aerator and an internal mixer to increase pH due to carbon dioxide degassing, and insoluble solids formed by the above-described reaction formula 3 in the sedimentation tank. To remove the calcium removal efficiency was observed. Table 3 shows the carbon dioxide degassing and calcium removal efficiency according to the water temperature change.

Water temperature (℃) Calcium removal efficiency (%) Electrical conductivity reduction rate (%) Effluent pH 10 70 35 7.9 20 80 40 8.0 30 85 41 8.1 40 87 42 8.1

<Example 4>

The water quality of the treated water obtained in Example 3 is shown in Table 4 in comparison with the white water and industrial water (constant water) of the conventional corrugated paper production plant.

Analysis item Treatment water of Example 3 jobless Industrial water pH 8.3 6.5-7.0 7.3 Calcium Hardness (mg / ℓ) 50 700 4 Conductivity (ms / cm) 3.5 6.7 0.13 COD (mg / l) 400 5,000 20 Sulfate (mg / ℓ) 470 880 10 Total dissolved solids concentration (mg / ℓ) 2,600 6,200 50

As can be seen from Table 4, the paper wastewater treated by the method according to the present invention has a calcium concentration that is less than the level of industrial water, but is significantly lower than that of the current papermaking process, and electrical conductivity values due to sulfate and other ions are existing. It can be seen that even lower than the white water. In addition, the concentration of COD, which acts as an anion trap in the process, was significantly reduced.

<Example 5>

Corrugated cardboard was manufactured using KOCC (Korean Old Corrugated Container) waste paper collected in Korea as a raw material. Industrial water, white water, and treated water of Example 3 were used as the water, and the raw materials were mixed with each of the waters to dissociate, followed by mixing the retention polymer (polyacrylamide) and retention aid (bentonite) to prepare corrugated cardboard. The basis weight of the produced product was kept constant at about 100 g / m ² . The impact of water on product properties is shown in Table 5.

Analysis item Treatment water of Example 3 jobless Industrial water Tensile Strength (Nm / g) 41 35 41 Bursting Strength (kPam ² / g) 2.7 2.3 2.7 Compressive strength (Nm ² / g) 0.72 0.63 0.72 % Hold once 82 71 82 Dehydration Rate (sec) 42 53 42

According to Table 5, the treated water according to the present invention is 17% or more, particularly 30% in the case of the compressive strength, as compared with the conventional white water in the overall papermaking properties such as tensile strength, rupture strength and compressive strength. It can be seen that the above is excellent. This strength property is very important because corrugated cardboard is used as a box or other container for packaging objects. In comparison with the industrial water, in the case of the bursting strength or the tensile strength, it can be seen that the mechanical properties are improved to such an extent that there is no difference between the treated water and the industrial water according to the present invention.

On the other hand, one-time retention was improved by about 15%, which means that the increase in retention could reduce the concentration of contaminants in white water by more than 15%. In addition, the dehydration rate has been improved by about 20%, which increases the overall speed of the machine, which is directly related to the increase in production. The increase in retention and dehydration rate is due to the increase in efficiency of the retention agent and retention aid, and the effect of the ion concentration in the water on the structure of the retention agent polymer has already been described above.

The effect obtained when using the wastewater treatment method according to the present invention can be summarized as follows.

First, the high concentration of organic matter contained in the wastewater of the paper mill, especially the corrugated cardboard manufacturing plant, is treated primarily in the anaerobic reactor to avoid sludge sedimentation degradation problems such as activated sludge process or pure oxygen aeration process. There is no need to have a separate power source to supply.

Second, since the alkali can be removed by insolubilizing calcium in the carbon dioxide degassing tank using the alkalinity generated in the anaerobic reactor, it is not necessary to add a separate chemical to form the precipitate.

Third, unlike the chemical precipitation method, since the chemical is not added, the electrical conductivity of the water can be greatly reduced, and the efficiency of the retention agent and the retention aid is increased, which reduces the amount of use, and increases the one-time retention rate. It can lower the concentration of contaminants and increase the dehydration rate.

Fourth, generally papermaking wastewater is a high temperature, anaerobic reaction tank and carbon dioxide degassing tank is not required because of the high temperature of the wastewater, the removal efficiency of organic matter and calcium increases, unlike the aerobic process does not require a separate cooling device.

Fifth, when the papermaking wastewater treated by the present invention is used as dissociation water in the pulper, mechanical strength such as tensile strength, bursting strength, and compressive strength of the product is greatly improved.

Claims (7)

In the method of treating wastewater of a paper mill containing a large amount of organic matter, sulfate and calcium ions, a) an anaerobic reaction step of introducing the wastewater into an anaerobic reactor to decompose organic matter and convert it into carbon dioxide and methane gas, and sulfate to hydrogen sulfide and carbon dioxide; b) the treated water in the anaerobic reactor is introduced into an aerobic carbon dioxide degassing tank to degas the carbon dioxide into the atmosphere to induce an increase in pH, thus increasing the concentration of carbonate ions to convert calcium ions into insoluble solid calcium carbonate. Degassing step; And c) solids are precipitated and removed from the treated water undergoing the aerobic degassing step and the supernatant is obtained as the final treated water. The method of claim 1, further comprising recycling a portion of the treated water that has undergone the anaerobic reaction step and feeding it back to the anaerobic reactor with the influent wastewater. The method according to claim 1, wherein the aerobic degassing step is performed in an aerobic degassing tank equipped with a surface aerator and an internal mixer. The method of claim 1, further comprising adding sodium hydroxide prior to introducing the treated water that has undergone the anaerobic reaction step into the aerobic degassing step. A method for reusing papermaking wastewater treated by the method according to any one of claims 1 to 4 as papermaking process water. 6. The method according to claim 5, wherein the treated papermaking wastewater is put into a pulper and used for dissociation of the papermaking fee. The method of claim 5, wherein the treated paper wastewater is introduced into a white water tank to reduce organic matter concentration and ion concentration throughout the papermaking process water.