Introduction to Paper-Making Wastewater Treatment Methods
Paper-making wastewater mainly comes from two production processes in paper-making industry: pulping and papermaking. Pulping separates fibers from plant raw materials to make pulp, then bleaches it; this process generates large amounts of paper-making wastewater. Papermaking dilutes, shapes, presses, and dries the pulp to make paper; this process also easily produces paper-making wastewater.
The paper-making wastewater from pulping causes the most serious pollution. The wastewater discharged during pulp washing is blackish-brown, called black liquor, with very high pollutant concentration-BOD up to 5-40 g/L-containing large amounts of fiber, inorganic salts, and pigments. The paper-making wastewater discharged from the bleaching process also contains large amounts of acid-base substances. The paper-making wastewater discharged from the paper machine, called white water, contains large amounts of fiber and fillers and sizing agents added during production.
Paper-making wastewater treatment should focus on improving the circulating-water reuse rate, reducing water consumption and wastewater discharge, and actively exploring various reliable, economical wastewater-treatment methods that can fully utilize useful resources in the wastewater. For example: the flotation wastewater-treatment method can recover fibrous solid matter from white water, with a recovery rate up to 95%, and the clarified water can be reused; the combustion wastewater-treatment method can recover sodium hydroxide, sodium sulfide, sodium sulfate, and other sodium salts combined with organic matter from black liquor. The neutralization wastewater-treatment method adjusts wastewater pH; coagulation sedimentation or flotation can remove suspended solids from wastewater; the chemical-precipitation method can decolorize; the biological-treatment method can remove BOD and is more effective for kraft wastewater; the wet-oxidation method is fairly successful in treating sulfite pulp wastewater. In addition, reverse osmosis, ultrafiltration, electrodialysis, and other paper-making wastewater-treatment methods are also used at home and abroad.
Superconducting high-gradient magnetic treatment method:
High-gradient magnetic separation purification technology is a new method for treating wastewater, because it produces no impurities such as flocs, making it feasible to treat large amounts of wastewater in a short time.
Researchers from the Department of Energy and Environmental Engineering at Osaka University, Japan, studied the use of a magnetic-separation system to treat paper-mill wastewater. The pilot plant treated 2,000 t/d of paper-making wastewater in cyclic operation, achieving chemical oxygen demand (CODCr) <40 mg/L in the water after magnetic separation. The superconducting magnetic tube, an NbTi spiral tube, is 680 mm long with a 400 mm inner diameter.
The system mainly consists of a mixing tank (magnetic seed tank, with seeds being organic matter, pulp, and dyes), a sedimentation tank, and a superconducting magnetic tube. Through the magnetic action inside the superconducting magnetic-separation tube, magnetic particles and organic polymers such as pulp and pigments are captured, and magnetic short fibers and fillers are floated out; part of the magnetic short fibers and fillers settle in the sedimentation tank by gravity, helping reduce the amount of short fibers and fillers in the circulating water passing through the magnetic tube. The system has run successfully for several months with fairly satisfactory results.
Polyacrylamide is widely used in the paper-making field as a retention aid, filter aid, and uniformity agent. Its effect is to improve paper quality, improve pulp dewatering performance, increase the retention rate of fine fibers and fillers, and reduce raw-material consumption and environmental pollution. The effect of polyacrylamide in paper-making depends on its average molecular weight, ionic nature, ionic strength, and the activity of other copolymers. Non-ionic polyacrylamide is mainly used to improve pulp filterability and increase dry-paper strength and the retention of fibers and fillers; anionic copolymers are mainly used as dry/wet paper strength enhancers and retention aids; cationic copolymers are mainly used in paper-making wastewater treatment and as filter aids, and also have a good effect on improving filler retention. In addition, polyacrylamide is also applied in paper-making wastewater treatment and fiber recovery.
Aluminum sulfate is extremely soluble in water; it cannot dissolve in pure sulfuric acid (it merely coexists), and in sulfuric-acid solution it dissolves in water together with sulfuric acid, so the solubility of aluminum sulfate in sulfuric acid is its solubility in water. At room temperature it crystallizes with 18 molecules of water of crystallization, i.e., aluminum sulfate octadecahydrate, which is mostly produced industrially as the octadecahydrate. It contains 51.3% anhydrous aluminum sulfate and does not self-dissolve even at 100 degC (it dissolves in its own water of crystallization). It is not prone to weathering and loss of water of crystallization, is relatively stable, loses water upon heating, and decomposes into alumina and sulfur oxides at high temperature. It begins to decompose into alumina, sulfur trioxide, sulfur dioxide, and water vapor at 770 degC. It is soluble in water, acids, and alkalis, but insoluble in ethanol. Its aqueous solution is acidic, and hydrolysis produces aluminum hydroxide. Long boiling of the aqueous solution can produce basic aluminum sulfate. The industrial product is grayish-white flaky, granular, or lumpy, appears light green due to low-iron salts, and the surface yellows due to oxidation of low-valent iron salts. The crude product is a grayish-white fine-crystal porous substance. It is non-toxic, but the dust irritates the eyes.
Sodium metabisulfite is a white or yellow crystalline powder or small crystals with a strong SO2 odor, specific gravity 1.4, soluble in water, aqueous solution acidic; it releases SO2 upon contact with strong acid to form the corresponding salt. Stored in air for a long time, it oxidizes into Na2S2O6, so the product cannot be stored for long. Above 150 degC it decomposes to release SO2.
Sodium metabisulfite is used in producing sodium hydrosulfite, sulfadimidine, metamizole, caprolactam, etc., as well as in purifying chloroform, phenylpropanesulfone, and benzaldehyde. In the photographic industry it is used as an ingredient of the fixing agent. In the perfume industry it is used to produce vanillin. It is used as a preservative in brewing, a rubber coagulant, and a dechlorinating agent after cotton bleaching. As an organic intermediate, dye, and in tanning it is used as a reducing agent; it is used in electroplating and oilfield wastewater treatment, and as an ore-dressing agent in mining. Industrially it is used in printing and dyeing, organic synthesis, printing, tanning, and pharmaceutical sectors; in food processing it is used as a preservative, bleaching agent, and leavening agent, as a bleaching agent, mordant, reducing agent, rubber coagulant, and also in organic synthesis, pharmaceuticals, and perfumes.
Therefore, the membrane biological fluidized bed (MBFB) process is adopted, using specially treated ceramic membranes to combine the membrane-separation system with a high-load biological fluidized-bed process to obtain stable treatment water quality. This process has been promoted and applied in the sewage and wastewater treatment fields in the United States, Japan, the United Kingdom, Germany, South Africa, Australia, and other countries and regions.
Paper-making wastewater is an industrial wastewater of considerable treatment difficulty, generally degraded of its pollutants through physicochemical + biochemical methods. Because the pollutants in the wastewater itself are very complex, although the effluent after treatment can basically meet discharge standards, it is far from the water-quality requirements for wastewater reuse. Traditional sand filtration, activated-carbon filtration, multi-media filtration, and other treatment processes for wastewater reuse only reduce the effluent suspended-solids concentration to a certain extent, and cannot further remove dissolved pollutants such as COD, ammonia nitrogen, and salinity. If reused, it will directly affect paper quality. The paper-making industry generally reuses reclaimed water only in production processes with low water-quality requirements such as slag removal, pulp washing, and bleaching; moreover, these sections have certain requirements for indicators such as COD, turbidity, and iron, which existing filtration technologies cannot meet, and the traditional multi-stage filtration process has drawbacks such as long flow, large footprint, and unstable produced-water quality. An advanced reclaimed-water reuse treatment process must be adopted to further reduce iron and COD concentration in water on the basis of the original compliant discharge, so that on the one hand it can be directly used as reclaimed water in sections with low water-quality requirements such as slag removal, pulp washing, and bleaching; on the other hand, the treated reclaimed water can directly enter reverse osmosis or ion exchange for desalination, eliminating the multi-stage security filtration and ultrafiltration steps in the reverse-osmosis process, reducing pre-treatment costs and extending RO-membrane service life.
This process starts from sand-filter effluent with COD about 110 mg/L, first using the AFF asymmetric fiber filter for precision filtration. AFF is a high-efficiency filtration equipment integrating dosing, micro-flocculation, sedimentation, and filtration, characterized by fast filtration speed (more than 10 times that of sand filtration), high filtration precision (filtration precision 5 um, 4 times that of general sand filtration), and easy backwashing and convenient management. In this project, AFF is mainly used as equipment for further iron removal and suspended-solids removal from reclaimed water.
The COD indicator of reclaimed water after AFF filtration is still about 100 mg/L, and it is mainly soluble COD (SCOD), which directly affects the reuse value of reclaimed water; at the same time, organic matter greatly affects the service life of reverse-osmosis membranes, so it must be reduced below 30 mg/L through an appropriate treatment process.
The waste-paper recycling paper-making process can be divided into two parts: pulping and papermaking. In the slag removal, pulp washing, bleaching, and other processes of the pulping part, large amounts of washing wastewater are generated. According to the source of waste paper and the difference in production process, the characteristics of washing wastewater differ; its pollutant content is roughly: CODCr 600-2400 mg/L, BOD5 125-585 mg/L, SS 650-2400 mg/L, colority 450-900 times, appearing blackish-gray. The washing wastewater volume is 100-200 t per ton of paper; like the general papermaking process, in the papermaking part of waste-paper recycling paper-making, 'white water' containing fiber, fillers, and chemicals is also generated, and the flotation method is often used to treat this wastewater, recovering fiber and fillers and enabling the treated 'white water' to be recycled.
The effluent treated by the MBFB process, except for the conductivity indicator, can meet the industry standard for workshop reuse water in the paper-making industry, and can be directly used in workshops such as slag removal, pulp washing, and bleaching, achieving a reuse rate of about 60%. At the same time, the MBFB process can also serve as the pre-treatment section of the reverse-osmosis process; MBFB can directly enter the reverse-osmosis membrane for desalination without going through the complex security-filtration and ultrafiltration sections.
AFF asymmetric fiber filtration
The Dijing asymmetric fiber filter (AFF) is a rapid purification equipment for solid waste in reclaimed-water reuse developed by Seattle Environmental Technology Co., Ltd. of the United States. The equipment can be used alone or together with flocculants to remove solid waste from reclaimed water and purify water quality.
In the sewage reclaimed-water reuse system, the filtration equipment is key; through physical filtration, solid particulate matter in the water body is removed and effluent suspended solids are reduced. At present, most of China's reclaimed-water reuse filtration systems use simple equipment such as sand filters; the filtration equipment is mainly sand tanks, a typical granular-filtration method using sand and gravel as filter media, achieving filtration through the adsorption of granular filter media and the interception of solid suspended matter in water by pores between sand grains. It has small specific surface area, small pollutant-holding capacity, slow filtration speed, and low filtration precision, and is not suitable for rapid filtration of suspended solids in reclaimed-water reuse systems.
AFF uses asymmetric fiber-bundle material as filter media, combining the advantages of granular and fiber filter media, such as high-efficiency fiber-ball filter media and suspended-ball fillers. Through a special structure, the filter bed porosity quickly forms a gradient density of large on top and small on bottom, making the filter fast, with large pollutant-holding capacity and easy backwashing, especially suitable for solid-suspended-matter filtration in reclaimed-water reuse systems.
The membrane biological fluidized bed process is based on the biological fluidized bed, using powdered activated carbon (Powdered Activated Carbon, PAC for short) as the carrier, combined with the solid-liquid separation technology of the membrane bioreactor process (Membrane Bioreactor, MBR for short), so that the reactor integrates the physical adsorption of activated carbon, microbial degradation, and the high-efficiency separation of the membrane, allowing small-molecule organic matter difficult to degrade in water to fully transfer and mix with the powdered activated carbon in a fluidized state under aeration conditions, being adsorbed and enriched on the activated-carbon surface, so that a local pollutant-concentration area forms on the activated-carbon surface; the powdered activated carbon also provides a special surface for microbial reproduction, and its porous surface adsorbs a large number of microbial flora, especially those whose metabolic substrate is the target pollutant; at the same time, the powdered activated carbon has a strong adsorption capacity for dissolved oxygen in water. Under high dissolved-oxygen conditions, microorganisms oxidize and decompose the small-molecule organic matter enriched on the activated-carbon surface, and then the ceramic-membrane separation system separates the water from suspended particles such as the powdered activated carbon that has adsorbed the organic matter, further purifying the sewage through cross-flow filtration to meet the reclaimed-water reuse standard. Research shows that MBFB can effectively remove ammonia nitrogen, COD, and other refractory small-molecule toxic organic matter from micro-polluted water bodies.
MBFB is currently mainly used in two aspects in water-treatment systems: one is the deep treatment of micro-polluted water bodies, and the other is the efficient treatment of municipal sewage.
