JP7598707B2 - How paper is made - Google Patents

Description

The present invention relates to a method for producing paper.

When producing paper or paperboard in paper and pulp factories, starch is added, sprinkled or applied to the raw materials in order to improve the strength of the resulting paper or paperboard. Also, in various food factories, starch is used as a food ingredient. In factories that use starch, such as these, and starch factories, starch dissolves in wastewater, increasing the chemical oxygen demand (COD) and putting a strain on wastewater treatment.

Generally, anionic or cationic precipitants are used to treat wastewater. However, most of the starch that dissolves in wastewater is nonionic, and it is difficult to remove it using the commonly used anionic or cationic precipitants.

To solve these problems, research is being conducted into flocculants that can aggregate and remove the water-soluble starch contained in raw wastewater.

For example, Patent Document 1 proposes the treatment of organic wastewater using melamine formalin resin acid colloid. Patent Document 2 proposes the treatment of starch-containing water using water-swellable cationic polymer particles. Patent Document 3 proposes treating a liquid containing cationic or nonionic polymer microparticles with a homogenizer for 30 seconds or more before use in wastewater treatment. Patent Document 4 proposes a soluble COD component remover in which the active ingredient is smectite, and the sodium ions contained between the layers of the smectite are at a milliequivalent ratio of 1.8 times or more to calcium ions, or the nontronite contained in the smectite is 6.0 mol% or more relative to the smectite.

Japanese Patent Application Publication No. 59-177190 Japanese Patent Application Publication No. 63-232888 JP 2008-246372 A JP 2007-268357 A

However, all of the treatment agents described in Patent Documents 1 to 4 utilize ionic bonds with trace anions in the nonionic substances to be removed or physical adsorption with the nonionic substances to be removed, and often do not achieve a sufficient removal rate. In particular, water-soluble starch is highly hydrophilic, so it is difficult to insolubilize and aggregate it through physical adsorption.

In addition to using the insolubilization and coagulation mechanisms described above, other methods have been proposed, such as biological treatment, activated carbon treatment, ultraviolet irradiation, ozone treatment, and Fenton treatment, which combines ferrous sulfate and hydrogen peroxide. However, these methods are often not practical due to the large size of the treatment facilities, the high costs of chemicals and electricity, and the difficulty of maintaining and managing operations.

On the other hand, in recent years, due to the shortage of water resources and environmental protection considerations, low discharge standards have been set for the COD concentration in wastewater, and high COD removal capacity is required in wastewater treatment.

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide a wastewater treatment method that can efficiently and simply insolubilize water-soluble starch contained in raw wastewater for treatment, thereby reducing COD.

The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that a wastewater treatment method including an insolubilization treatment step in which a polymer having a polyalkylene oxide moiety with a salt viscosity of 2 mPa·s or more when measured by a specified method is added to raw wastewater to be treated that contains water-soluble starch, thereby insolubilizing the water-soluble starch, and thereby reducing the COD in the wastewater to be treated, thereby completing the present invention. Specifically, the present invention provides the following.

(1) A wastewater treatment method comprising an insolubilization treatment step of adding a polymer having a polyalkylene oxide moiety to raw wastewater containing water-soluble starch to insolubilize the water-soluble starch, wherein the salt viscosity of the polymer is 2 mPa·s or more when measured by the following method.
<Method of measuring salt viscosity>
The viscosity (salt viscosity) of an aqueous solution containing 4 g of sodium chloride and 0.5 g of the polymer per 100 ml of the aqueous solution is measured at 25° C. and a rotor rotation speed of 60 rpm according to JIS K 7117-2:1999.

(2) The wastewater treatment method according to (1) above, in which the ratio of the monomer constituting the polyalkylene oxide moiety to the total monomers constituting the polymer is 10 mol % or more and 100 mol % or less.

(3) The wastewater treatment method according to (1) or (2) above, wherein in the insolubilization treatment step, the raw wastewater further contains suspended solids.

(4) The wastewater treatment method according to (1), (2) or (3) above, in which the concentration of the polymer added to the raw wastewater in the insolubilization treatment step is 0.1 mg/L or more and 100 mg/L or less.

According to the present invention, it is possible to efficiently and simply insolubilize water-soluble starch contained in raw wastewater to be treated, thereby reducing the COD in the raw wastewater to be treated. Furthermore, when the wastewater treatment method of the present invention is carried out in a coagulation and separation process prior to a biological treatment process, it is possible to reduce the load on the biological treatment process (reducing the amount of microorganisms, treatment time, etc.).

Specific embodiments of the present invention are described in detail below, but the present invention is not limited to the following embodiments and can be modified as appropriate within the scope of the invention.

<Wastewater treatment method>
The wastewater treatment method in this embodiment includes an insolubilization treatment step of adding a polymer having a polyalkylene oxide moiety to raw wastewater treatment water containing water-soluble starch to insolubilize the water-soluble starch, and is characterized in that the salt viscosity of the polymer is 2 mPa·s or more when measured by the following method.
<Method of measuring salt viscosity>
The viscosity (salt viscosity) of an aqueous solution containing 4 g of sodium chloride and 0.5 g of a polymer per 100 ml of the aqueous solution is measured at 25° C. and a rotor rotation speed of 60 rpm. The viscosity measurement method is based on JIS K 7117-2:1999.

[Insolubilization treatment process]
[Raw wastewater for treatment]
The term "raw wastewater for wastewater treatment" refers to water containing water-soluble starch, and is intended to subject at least the water-soluble starch to insolubilization treatment. As described above, specific examples of "raw wastewater for wastewater treatment" include papermaking process water discharged from paper and pulp factories and manufacturing process water discharged from food factories.

(Water-soluble starch)
In this embodiment, water-soluble starch refers to starch that passes through a filter paper with a pore size of 7 μm. Examples of such water-soluble starch include starch that has been gelatinized (gelatinized) by heating or the like. On the other hand, ungelatinized granular raw starch aggregates relatively easily and can be separated into solid and liquid forms, for example, by the above-mentioned filter paper. Therefore, granular starch and starch dissolved or dispersed in colloidal form in water can be separated by a 5A filter paper with a pore size of 7 mm.

The concentration of water-soluble starch contained in the raw wastewater is not particularly limited, and may be appropriately selected based on the balance between the COD concentration in the raw wastewater and the COD concentration in the wastewater after the wastewater treatment including the insolubilization treatment step or at the time of discharge, but is usually preferably 0.1 mg/L or more. If the water-soluble starch concentration is too low, the amount of water-soluble starch that is insolubilized will also be small, and the amount of water-soluble starch removed may not be commensurate with the addition of the polymer having a polyalkylene oxide moiety. On the other hand, the concentration of water-soluble starch contained in the raw wastewater is not particularly limited. If the concentration of water-soluble starch is too high, the addition of the polymer having a polyalkylene oxide moiety and the filtration of the insolubilized water-soluble starch (hereinafter sometimes referred to as "insolubilized starch") may be repeated multiple times.

In this embodiment, the concentration of water-soluble starch is measured by the following method. The filtrate obtained by filtering raw wastewater through 5A filter paper is diluted 2 to 20 times by volume using 10-fold diluted hydrochloric acid, and then 5.26% by volume of 0.002N iodine solution is added to 100% by volume of the filtrate, and the absorbance at 580 nm is measured using a spectrophotometer. The measured value is then applied to a calibration curve previously prepared using Kishida Chemical's special grade reagent starch to obtain the concentration of water-soluble starch.

(Suspended solids (SS))
Although not an essential component, the raw wastewater preferably contains suspended solids (SS). In the case of treating raw wastewater from a paper/pulp factory, the suspended solids include pulp fibers, chemical fibers, etc. In the case of treating raw wastewater from a food factory, the suspended solids included in the raw wastewater are insoluble materials, fillers, pigments, etc. In the case of treating raw wastewater from a food factory, the suspended solids included in the raw wastewater are insoluble materials, etc. The starch adheres to the suspended matter, making it easier to separate the insolubilized starch into solid and liquid forms.

The concentration of suspended solids contained in the raw wastewater is preferably 0.1 g/L or more, more preferably 0.2 g/L or more, even more preferably 0.5 g/L or more, and particularly preferably 1 g/L or more. If the concentration of suspended solids is too low, even if the water-soluble starch contained in the raw wastewater is insolubilized with a polymer having a polyalkylene oxide moiety, the amount of suspended solids to which the water-soluble starch adheres may be insufficient, and the effect of increasing the ease of solid-liquid separation of the insolubilized starch may not be obtained. In addition, from the viewpoint of removal processing in the subsequent stage, the concentration of suspended solids is preferably, for example, 200 g/L or less, more preferably 100 g/L or less, and even more preferably 50 g/L or less.

In this embodiment, the concentration of suspended solids contained in the raw wastewater to be treated is measured in accordance with JIS P 8225:2003.

[Polymers Having Polyalkylene Oxide Moieties: Starch Insolubilizers]
The wastewater treatment method according to the present embodiment includes a step of adding a polymer having a polyalkylene oxide moiety to the raw wastewater to be treated. The polymer has a function of insolubilizing water-soluble starch contained in the raw wastewater to be treated.

The polyalkylene oxide moieties in the polymer react with the water-soluble starch contained in the raw wastewater, causing the water-soluble starch to become insoluble. Specifically, it is believed that the insolubilization of water-soluble starch occurs due to molecular association caused by hydrogen bonds of hydroxyl groups. Such molecular associations have strong binding strength, which efficiently insolubilizes the water-soluble starch.

In the wastewater treatment method of this embodiment, the shape of the insolubilized starch is not limited to a completely insolubilized solid particulate matter, but also includes jelly-like and highly viscous liquid forms.

In the wastewater treatment method of this embodiment, the polyalkylene oxide moiety is not particularly limited as long as it has a polyalkylene oxide structure, such as polyethylene oxide, polypropylene oxide, polybutylene oxide, etc. The polyalkylene oxide moiety has a polyether structure with repeating units of polymerized alkylene oxides, and the alkylene oxide may be a single type or a combination of multiple types, and the monomer arrangement may be a random structure or a block structure. The polymer structure may be one in which the polyalkylene oxide moiety is the main chain, or one in which the polyalkylene oxide moiety is added as a side chain to a polymer main chain.

As described above, the salt viscosity of the polymer having a polyalkylene oxide moiety is not particularly limited as long as it is 2 mPa·s or more, but is preferably 3 mPa·s or more, more preferably 5 mPa·s or more, even more preferably 10 mPa·s or more, and particularly preferably 50 mPa·s or more. If the salt viscosity of the polymer is too low, even if the water-soluble starch contained in the raw wastewater is insolubilized with a polymer having a polyalkylene oxide moiety, the aggregation efficiency of the insolubilized starch may be poor and proper solid-liquid separation may not be possible. On the other hand, the salt viscosity of the polymer having a polyalkylene oxide moiety is preferably 2,000 mPa·s or less, more preferably 1,000 mPa·s or less, and even more preferably 500 mPa·s or less, in order to prevent the water solubility in the raw wastewater and the handleability of the polymer from being affected.

In the wastewater treatment method of this embodiment, the salt concentration is measured by the following method. The viscosity (salt viscosity) of an aqueous solution containing 4 g of sodium chloride and 0.5 g of polymer per 100 ml of aqueous solution is measured at 25°C and a rotor rotation speed of 60 rpm. The viscosity measurement method is based on JIS K 7117-2:1999.

The proportion of polyalkylene oxide moieties contained in the polymer constituting the starch insolubilizer is preferably 5 mol% or more, more preferably 10 mol% or more, even more preferably 30 mol% or more, and particularly preferably 50 mol% or more, based on the entire polymer. The upper limit of the proportion of polyalkylene oxide moieties contained in the polymer constituting the starch insolubilizer is not particularly limited as long as the salt viscosity of the polymer is within a suitable range, and may be 100 mol% based on the entire polymer. In other words, "polymers having polyalkylene oxide moieties" also include those composed only of polyalkylene oxide moieties (having no moieties other than polyalkylene oxide moieties). In addition, this polymer may be crosslinked.

Of the functional groups constituting the polymer, the moieties (monomers) other than the polyalkylene oxide moieties are not particularly limited, and may be synthetic or natural products. In addition, since hydrogen bonds formed by the polyalkylene oxide moieties are not inhibited by ionic bonds or hydrogen bonds formed by other functional groups, the moieties other than the polyalkylene oxide moieties may be cationic moieties (monomers), anionic moieties (monomers), amphoteric moieties (monomers), or nonionic moieties (monomers).

Examples of cationic monomers include, but are not limited to, dialkylaminoalkyl acrylates (such as diethylaminoethyl acrylate), dialkylaminoalkyl methacrylates (such as diethylaminoethyl methacrylate), quaternary salts or acid salts of dialkylaminoalkyl acrylates or dialkylaminoalkyl methacrylates (dimethylaminoethyl acrylate methyl chloride quaternary salt, dimethylaminoethyl acrylate methyl sulfonate quaternary salt, dimethylaminoethyl acrylate benzyl chloride quaternary salt, dimethylaminoethyl acrylate sulfonate, dimethylaminoethyl acrylate hydrochloride, dimethylaminoethyl methacrylate methyl chloride quaternary salt, dimethylaminoethyl methacrylate methyl sulfonate quaternary salt, dimethylaminoethyl methacrylate benzyl chloride quaternary salt, dimethylaminoethyl methacrylate sulfonate, dimethylaminoethyl methacrylate hydrochloride, etc.). Also included are dialkylaminoalkyl acrylamides or dialkylaminoalkyl methacrylamides, such as acrylamidopropyl trimethylammonium chloride, dimethylaminopropyl acrylamido methyl sulfonate quaternary salt, dimethylaminopropyl acrylamido sulfonate, dimethylaminopropyl acrylamide hydrochloride, methacrylamidepropyl trimethylammonium chloride, dimethylaminopropyl methacrylamido methyl sulfonate quaternary salt, dimethylaminopropyl methacrylamido sulfonate, and dimethylaminopropyl methacrylamide hydrochloride, and their quaternary salts and acid salts.Further, diallyl dialkyl ammonium halides (such as diallyl diethyl ammonium chloride and diallyl dimethyl ammonium chloride) are also included.

Anionic monomers include, but are not limited to, (meth)acrylic acid, itaconic acid, maleic acid, 2-acrylamido-2-methylpropanesulfonic acid, styrenesulfonic acid, and methylpropenesulfonic acid.

Nonionic monomers include, but are not limited to, water-soluble N-substituted lower (straight-chain or branched-chain, carbon number 1 to 6) alkyl acrylamides such as acrylamide, methacrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, N-isopropylacrylamide, N-methylolacrylamide, and diacetone acrylamide, methacrylonitrile, acrylonitrile, alkyl acrylates, hydroxyacrylates, and vinyl acetate.

As the moiety other than the polyalkylene oxide moiety, one of the above-mentioned cationic moieties (monomers), anionic moieties (monomers), amphoteric moieties (monomers), nonionic moieties (monomers), etc. can be used alone or in combination of two or more.

The amount of polymer to be added is not particularly limited, but for example, it can usually be added in the range of 0.1 mg to 100 mg per 1 L of raw wastewater to be treated. Alternatively, if the wastewater treatment contains suspended solids, it may be added in the range of 10 g to 10,000 g per kg of suspended solids. It may also be added in the range of 1/1000 to 10 times the weight of the water-soluble starch.

[Other Additives]
In order to increase the efficiency of the aggregation and precipitation of the insoluble starch and the subsequent solid-liquid separation, or to remove substances other than water-soluble starch, a conventionally known flocculant or coagulant (hereinafter referred to as "flocculant") other than the polymer having a polyalkylene oxide moiety can be used in combination. Specifically, as such a flocculant, either an inorganic flocculant or an organic flocculant can be used. In addition, as the flocculant, either an anionic flocculant, a cationic flocculant, an amphoteric flocculant, or a nonionic flocculant can be used. Such flocculants can be used alone or in combination. In addition, such a flocculant may be added before, after, or simultaneously with the addition of the polymer having a polyalkylene oxide moiety, but when such a flocculant is used for the purpose of solid-liquid separation, it is preferable to add it after the addition of the polymer having a polyalkylene oxide moiety.

[Solid-liquid separation process]
Although not an essential component, the wastewater treatment method of the present embodiment may include a solid-liquid separation step of performing solid-liquid separation of the insolubilized starch.

Specific methods for solid-liquid separation are not particularly limited, but methods such as sedimentation, pressure flotation, and filtration can be used.

[Other steps]
Although not essential components, the wastewater treatment method of the present embodiment may also use biological treatment, adsorption treatment, oxidation treatment, ultraviolet treatment, etc. in combination depending on the water quality of the raw wastewater to be treated, etc. Generally, these treatments are preferably performed after solid-liquid separation, but are not limited thereto, and the order can be set appropriately depending on, for example, the water quality of the raw wastewater to be treated.

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these.

Polymers A to J shown in Table 1 below and agents K to P shown in Table 2 were prepared as starch insolubilizers.

The salt viscosity shown in Tables 1 and 2 is the viscosity (salt viscosity) of an aqueous solution containing 4 g of sodium chloride and 0.5 g of polymer per 100 ml of aqueous solution at 25°C and a rotor rotation speed of 60 rpm. The viscosity measurement method was based on JIS K 7117-2:1999. For Agent M in Table 2, the swelling power of bentonite was measured in accordance with the Japan Bentonite Industry Association Standard Test Method, Bentonite (Powder) Swelling Test Method (JBAS 104 77), 1977.

Next, the raw wastewater discharged from a paper and pulp factory containing approximately 400 mg/L of water-soluble starch and 1,500 mg/L of suspended solids was stirred while adding the starch insolubilizer sample in the amount shown in Table 3. After stirring for 2 minutes, the mixture was allowed to settle for 1 minute, and the water-soluble starch concentration of the supernatant and the chemical oxygen demand (COD _Mn ) measured using potassium permanganate were measured.

Specifically, the concentration of water-soluble starch was measured by filtering the supernatant through 5A filter paper, diluting the filtrate obtained with 10-fold diluted hydrochloric acid by 2-20-fold volume ratio, adding 5.26% by volume of 0.002N iodine solution to 100% by volume of the filtrate, and measuring the absorbance at 580 nm with a spectrophotometer. The measured value was then compared with a calibration curve prepared in advance using Kishida Chemical's special grade reagent starch to calculate the concentration of water-soluble starch remaining in the supernatant. The measurement results for the remaining concentration of water-soluble starch are also shown in Table 3.

The COD _Mn was measured by the method specified in Section 17 of JIS K 0102:2019. The measurement results are also shown in Table 3.

As shown in Table 3, in Examples 1 to 8 in which a polymer having a polyalkylene oxide moiety with a salt viscosity of 2 mPa·s or more was added to raw wastewater containing water-soluble starch, the water-soluble starch concentration and COD _Mn of the supernatant were lower than those of Comparative Examples 1 to 19.

From the above, it was found that by adding a polymer having a polyalkylene oxide moiety with a salt viscosity of 2 mPa·s or more to raw wastewater to be treated that contains water-soluble starch, the water-soluble starch contained in the raw wastewater to be treated can be efficiently and simply insolubilized, thereby reducing the COD in the raw wastewater to be treated.

Claims (3)

The method includes an insolubilization treatment step of adding a polymer having a polyalkylene oxide moiety to raw wastewater containing water-soluble starch to insolubilize the water-soluble starch,
The polymer has a salt viscosity of 2 mPa·s or more as measured by the following method,
a ratio of the monomer constituting the polyalkylene oxide moiety to all monomers constituting the polymer is 13 mol % or more and 100 mol % or less .
<Method for measuring salt viscosity>
The viscosity (salt viscosity) of an aqueous solution containing 4 g of sodium chloride and 0.5 g of the polymer per 100 ml of the aqueous solution is measured at 25° C. and a rotor rotation speed of 60 rpm according to JIS K 7117-2:1999. The wastewater treatment method according to claim 1 , wherein in the insolubilization treatment step, the raw wastewater for treatment further contains suspended solids. 3. The wastewater treatment method according to claim 1, wherein the concentration of the polymer added to the raw wastewater to be treated in the insolubilization treatment step is 0.1 mg/L or more and 100 mg/L or less.