During the production process of carbonless copy paper, various coatings (pigments, developers, adhesives, etc.) need to be added. Some of the coatings enter the coating wastewater during the production process and become the main pollutants in the coating wastewater. The coating wastewater has high concentration of organic pollutants and suspended solids and deep color, making it difficult to treat and causing great pollution to the environment.

Carbonless copy paper (CCP), also called pressure-sensitive recording paper, was invented by the American NCR Company in 1954. After decades of development and improvement, industrial production has reached a certain scale.

The appearance of CCP is no different from ordinary paper, but it is a kind of invisible carbon paper with the function of direct copying and direct color development. It is not like the commonly known carbon paper, which requires indirect transfer of dyeing substances on the paper through external pressure to achieve the copying effect. Instead, it uses the function of pressure sensitivity and electron-donating colorless dyes and electron-accepting dyes. The principle of chemical color development between acidic color-developing materials directly develops color on paper to obtain copies or the same recording material.

CCP has a wide range of applications, mostly on documents. Nowadays, all formal documents with legal effect such as invoices, contracts, and treaties on the market are made of carbonless paper. Traditional documents are just ordinary paper, so a copy layer must be added under the document. Carbonless copy paper is bound with special paper, which is very simple and convenient to use and leaves no traces.

Take the triple carbonless copy paper receipt as an example. Its composition is divided into upper paper, middle paper and lower paper. The top paper is also called back-coated paper (code name CB, or Coated Back). The back of this layer of paper is coated with microcapsules containing force-sensitive pigment oil. The middle paper layer is also called front and back double-coated paper (code name CFB, or Coated Front and Back). ), the front side of the paper is coated with a developer, and the back side is coated with microcapsules containing force-sensitive pigment oil. The lower paper layer is also called top-coated paper (codenamed CF, or Coated Front). The paper surface is only coated with a developer. Self-contained paper (codenamed SC, i.e. Self-Contained) is coated with a microcapsule layer containing force-sensitive pigment oil on the back of the paper, and a color developer and microcapsules containing force-sensitive pigment oil are coated on the front.

The upper paper and the lower paper do not have a copying effect. Only the middle paper sandwiched between them has a copying effect. When using documents printed on carbonless paper, a small piece of cardboard is usually provided to separate the coupons to prevent excessive writing force from being copied on other coupons underneath.

For the treatment of CCP coating wastewater, the traditional method generally adopts a treatment process that combines coagulation sedimentation physicochemical method and anaerobic aerobic biochemical method. The physicochemical process mainly uses chemicals to be added to wastewater to remove pollutants in wastewater through flocculation, sedimentation and the use of equipment capabilities. Traditional physicochemical wastewater treatment equipment includes sedimentation tanks (such as advection, vertical flow, radial flow and inclined plate, inclined tube sedimentation tanks), air flotation devices, filtration devices, centrifugal separation and magnetic separation equipment. Biochemical treatment mainly adopts a single aerobic or a combination of anaerobic and aerobic methods according to the different quality of wastewater. Among them, aerobic treatment includes aerobic pond, activated sludge method, contact oxidation method, biological filter, biological turntable, and SBR method. and many other forms: The oxygen treatment methods mainly include anaerobic ponds, anaerobic filters, ordinary anaerobic ponds, anaerobic contact reactors, UASB, etc., relying on the metabolic ability of microorganisms themselves to ultimately oxidize and decompose pollutants in the water into Carbon dioxide and water to achieve the purpose of cleaning water quality. However, due to the low removal rate of wastewater pollutants in traditional coagulation and sedimentation equipment (the removal rate of COD generally does not exceed 60%), it is not suitable for wastewater with high concentration and complex composition such as paper coating, because The low efficiency of removing wastewater pollutants will lead to high investment and operating costs in the post-process "biochemical section", and it will also be difficult for the final effluent to meet discharge requirements.

For the treatment of wastewater in the middle section of papermaking, the most commonly reported primary physical and chemical treatment is coagulation, neutralization, sedimentation, centrifugation, air flotation, ultrafiltration or a combination of ultrafiltration and centrifugation. There are two methods commonly used in the market to remove the color of wastewater: one is to separate the color-producing organic matter from the wastewater to reduce the color, such as flocculation, adsorption, membrane separation, etc.; the other is to destroy the organic matter. Chromophoric groups and auxiliary chromophore groups reduce the color of wastewater, such as chemical oxidation methods, optical methods, biological methods, electrochemical methods, etc.

The production of carbonless copy paper belongs to the paper processing industry. The wastewater discharged from the production process is mainly the coating wastewater generated during the coating production process and cleaning equipment. The wastewater contains adhesives (starch, latex, PVA, etc.), pigments (kaolin, calcium carbonate) etc.), chromogen microcapsules, chromogen phenolic resin, dyes and other residual chemical materials. The wastewater is characterized by a small amount of wastewater, but the concentration of pollutants such as SS and CODcr is generally relatively high. Due to the different proportions and large concentration changes of various coatings (such as CE and CB coatings) in wastewater, the CODcr concentration of wastewater is generally 1500-7000mg/L, and the CODcr concentration is mostly around 3500mg/L. If it is not treated directly Emissions can cause significant pollution to the environment. The treatment of this type of paint wastewater must adopt a pre-treatment primary physical and chemical treatment process suitable for this type of paint wastewater. First, SS, COD and other pollutants contained in the wastewater are largely removed, and then the wastewater is further treated by a biochemical process.

At present, there are mainly several methods for treating papermaking wastewater on the market, namely chemical methods, biological methods, physical methods and combinations of these three methods.

Physical method refers to the use of mechanical and physical means to remove pollutants in papermaking wastewater. This method is mainly used to remove insoluble impurities with larger particle sizes in wastewater, including mechanical filtration (such as grilles, screens, Microfiltration machine, filter bed), clarification (precipitation) and other methods. The filtration method usually uses a fine screen or a microfilter, but due to the large load, it may cause blockage, so cleaning operations should be considered. Since filtration cannot remove ink, dissolved substances and too fine suspended solids, it can only be used as a pretreatment method. At present, domestic paper mills use more microfiltration treatment equipment, mainly inclined screens or filters. Inclined screens are generally designed and manufactured by each factory. Compared with filters, they save power consumption and require less investment. The mesh size is generally 60 to 100 mesh. At present, inclined screen filtration has been adopted by most small and medium-sized paper mills. In the past, inclined screens were used mainly to collect fine fibers in wastewater, but now the concept of purifying wastewater has been added. The increased area of the inclined screen is beneficial to the removal of SS in wastewater.

Chemical method refers to the method of adding special reactants to wastewater and using chemical reactions to change the form and properties of pollutants in the water to remove dissolved substances or colloidal substances in the wastewater. Common methods include neutralization and precipitation

Redox, catalytic oxidation, photocatalytic oxidation, micro-electrolysis, electrolytic flocculation and other methods. Biochemical treatment mainly uses a single aerobic or anaerobic or a combination of anaerobic and aerobic methods according to the different quality of wastewater. Among them, aerobic treatment includes aerobic pond, activated sludge method, contact oxidation method, and biological filter. , biological turntable, SBR and other forms: The anaerobic treatment methods mainly include anaerobic ponds, anaerobic filters, ordinary anaerobic ponds, anaerobic contact reactors, UASB, etc., relying on the metabolic ability of the microorganisms themselves to remove the chemicals in the water. The pollutants are eventually oxidized and decomposed into carbon dioxide and water to achieve the purpose of cleaning.

Physicochemical treatment mainly involves the removal of slag and suspended solids in wastewater. The methods include: grid method, grid method, inclined mesh method, sedimentation method, coagulation sedimentation method or air flotation, filtration method, etc., mainly using wastewater treatment methods. Chemicals are added in the middle to achieve the purpose of removing pollutants through flocculation and precipitation.