Characteristics of Papermaking Wastewater and Its Treatment Processes
Papermaking wastewater comes mainly from two production stages in the paper industry: pulping and sheet forming. Pulping separates the fibers from plant raw materials to make pulp, which is then bleached, and this stage generates large volumes of papermaking wastewater. Sheet forming dilutes, forms, presses, and dries the pulp into paper, and it also readily generates papermaking wastewater.
The papermaking wastewater generated by pulping is the most heavily polluted. The wastewater discharged during pulp washing is dark brown and is called black liquor. Pollutant concentrations in black liquor are very high, with BOD reaching 5-40 g/L, and it contains large amounts of fiber, inorganic salts, and pigments. The wastewater discharged from the bleaching stage also contains large amounts of acidic and alkaline substances. The wastewater discharged from the paper machine is called white water and contains large amounts of fiber together with the fillers and sizing agents added during production.
Papermaking wastewater treatment should focus on raising the water recycling rate and reducing water consumption and wastewater discharge, while actively exploring reliable, economical treatment methods that make full use of the useful resources in the wastewater. For example, flotation can recover fibrous solids from white water with a recovery rate of up to 95%, and the clarified water can be reused; combustion can recover sodium hydroxide, sodium sulfide, sodium sulfate, and other sodium salts bound to organics from black liquor. Neutralization adjusts wastewater pH; coagulation-sedimentation or flotation removes suspended solids; chemical precipitation can decolorize; biological treatment removes BOD and is fairly effective for kraft pulp wastewater; and wet oxidation has been quite successful for sulfite pulp wastewater. Reverse osmosis, ultrafiltration, and electrodialysis are also used for papermaking wastewater at home and abroad.
Superconducting high-gradient magnetic separation:
High-gradient magnetic separation is a new method for treating wastewater. Because it does not produce impurities such as flocs, treating large volumes of wastewater in a short time becomes feasible.
Researchers in the Department of Energy and Environmental Engineering at Osaka University in Japan studied the use of a magnetic separation system to treat paper mill wastewater. The pilot plant treated 2,000 t/d of papermaking wastewater; after circulating operation and magnetic separation the chemical oxygen demand (CODCr) in the water was below 40 mg/L. The superconducting NbTi solenoid was 680 mm long with an inner diameter of 400 mm.
The system consists mainly of a mixing tank (a magnetic seeding tank, with seeds of organic matter, pulp, and dye), a sedimentation tank, and a superconducting magnet tube. Magnetic forces inside the superconducting magnetic separation tube capture magnetic particles and organic polymers such as pulp and pigments and float out magnetic short fibers and fillers; some of the magnetic short fibers and fillers settle in the sedimentation tank by gravity, helping to reduce the amount of short fiber and filler carried through the magnet tube by the circulating water. The system has run successfully for several months with fairly satisfactory results.
Polyacrylamide is widely used in papermaking as a retention aid, filter aid, and leveling agent. It improves paper quality, enhances pulp dewatering, increases the retention of fines and fillers, and reduces raw material consumption and environmental pollution. Its performance in papermaking depends on its average molecular weight, ionic character, ionic strength, and the activity of other copolymers. Non-ionic polyacrylamide is mainly used to improve pulp drainage, increase dry paper strength, and raise fiber and filler retention; anionic copolymers are mainly used as dry and wet strength agents and retention aids; cationic copolymers are mainly used for papermaking wastewater treatment and as filter aids, and also work well for improving filler retention. Polyacrylamide is also used in papermaking wastewater treatment and fiber recovery.
Aluminum sulfate is highly soluble in water. It does not dissolve in pure sulfuric acid (the two merely coexist); in sulfuric acid solution it dissolves in water together with the acid, so its solubility in sulfuric acid is its solubility in water. At room temperature it crystallizes with 18 molecules of water as aluminum sulfate octadecahydrate, which is the industrial product in most cases. It contains 51.3% anhydrous aluminum sulfate and does not dissolve in its own crystal water even at 100 degrees C. It does not weather easily or lose crystal water and is fairly stable; heating drives off water, and high temperatures decompose it into alumina and sulfur oxides. Heating to 770 degrees C begins decomposition into alumina, sulfur trioxide, sulfur dioxide, and water vapor. It dissolves in water, acids, and alkalis but not in ethanol. Its aqueous solution is acidic and hydrolyzes to form aluminum hydroxide; prolonged boiling of the solution can produce basic aluminum sulfate. The industrial product is off-white flakes, granules, or lumps, tinged pale green by low-valence iron salts and yellowish on the surface where those iron salts have oxidized. The crude product is an off-white, fine-crystalline porous solid. It is non-toxic, but the dust can irritate the eyes.
Sodium metabisulfite is a white or yellow crystalline powder or small crystals with a strong SO2 odor and a specific gravity of 1.4. It dissolves in water to give an acidic solution and releases SO2 on contact with strong acids to form the corresponding salts. On long storage in air it oxidizes to Na2S2O6, so the product cannot be kept for long. Above 150 degrees C it decomposes and releases SO2.
Sodium metabisulfite is used to make sodium hydrosulfite, sulfadimidine, analgin, and caprolactam, and to purify chloroform, phenylpropyl sulfone, and benzaldehyde. In photography it is used as an ingredient of fixer, and in the flavor industry to make vanillin. It also serves as a preservative in brewing, a rubber coagulant, and a dechlorinating agent for bleached cotton cloth. As an organic intermediate and dye chemical it is used in leather making as a reducing agent, in electroplating, in oilfield wastewater treatment, and as a flotation agent in mining. Industrially it is used in dyeing and printing, organic synthesis, printing, leather, and pharmaceuticals; in food processing it acts as a preservative, bleaching agent, and leavening agent. It is also used as a bleaching agent, mordant, reducing agent, and rubber coagulant (in dyeing, printing, and photography), and in organic synthesis, pharmaceuticals, and flavors.
Recycled-paper papermaking can be divided into pulping and sheet forming. In the pulping stage, during screening, pulp washing, and rinsing, large volumes of washing wastewater are produced. Depending on the source of the waste paper and the production process, the characteristics of this washing wastewater vary; pollutant levels are roughly CODCr 600-2,400 mg/L, BOD5 125-585 mg/L, SS 650-2,400 mg/L, color 450-900 times, and its appearance is dark gray. The washing wastewater volume is 100-200 t per ton of paper. As in conventional papermaking, the sheet-forming stage of recycled-paper production also generates white water containing fiber, filler, and chemicals; this wastewater is usually treated by flotation to recover fiber and filler so that the treated white water can be recycled.
Papermaking wastewater is a rather difficult industrial wastewater, generally treated by physicochemical plus biochemical methods to degrade the pollutants it contains. Because the pollutants are highly complex, treated effluent can basically meet discharge standards but falls far short of the water quality required for reuse. Conventional processes such as sand filtration, activated carbon filtration, and multimedia filtration only reduce effluent suspended solids to some extent and cannot further remove dissolved pollutants such as COD, ammonia nitrogen, and salts; if such water were reused it would directly affect paper quality. In the paper industry, reclaimed water is generally reused only in processes with low water quality requirements such as screening, pulp washing, and rinsing, and even these stages have certain requirements for COD, turbidity, and iron that existing filtration technology cannot meet. Traditional multi-stage filtration also has drawbacks such as a long process train, a large footprint, and unstable product water quality. An advanced reclaimed-water reuse process must therefore be adopted to further reduce iron and COD concentrations on top of compliant discharge. On the one hand the water can then be used directly for screening, pulp washing, and rinsing; on the other hand the treated reclaimed water can go straight to reverse osmosis or ion exchange for desalination, eliminating the multi-stage cartridge filtration and ultrafiltration steps of an RO process, reducing pretreatment costs, and extending RO membrane life.
This process starts from sand filter effluent with a COD of about 110 mg/L. It first uses an AFF asymmetric fiber filter for fine filtration. AFF is a high-efficiency filtration unit that integrates chemical dosing, micro-flocculation, sedimentation, and filtration. Its features are a high filtration rate (more than 10 times that of a sand filter), high filtration precision (5 um, four times that of an ordinary sand filter), easy backwashing, and convenient management. In this project AFF mainly serves to further remove iron and suspended solids from the reclaimed water.
After AFF filtration the reclaimed water still has a COD of about 100 mg/L, mostly soluble COD (SCOD), which directly affects its reuse value; organics also greatly shorten RO membrane life, so a suitable process must bring COD below 30 mg/L.
A membrane biological fluidized bed (MBFB) process is therefore adopted. Using specially treated ceramic membranes, it combines membrane separation with a high-load biological fluidized bed process to obtain stable treated water quality. The process has been promoted and applied in sewage and wastewater treatment in the United States, Japan, the United Kingdom, Germany, South Africa, Australia, and other countries and regions.
Except for conductivity, the effluent from the MBFB process meets the industry standard for workshop reuse water in the paper industry and can be used directly in production areas such as screening, pulp washing, and rinsing, achieving a reuse rate of about 60%. MBFB can also serve as pretreatment for reverse osmosis: its effluent can enter the RO membranes for desalination directly, without complex cartridge filtration and ultrafiltration.
AFF asymmetric fiber filtration
In water treatment systems MBFB is currently used mainly in two areas: the advanced treatment of slightly polluted water bodies and the high-efficiency treatment of municipal sewage.
In wastewater treatment and reclaimed-water reuse systems the filtration equipment is the key: physical filtration removes solid particles from the water and reduces effluent suspended solids. At present most reclaimed-water filtration systems in China use crude equipment such as sand filters, mainly sand tanks. A sand tank is a typical granular filtration device that uses sand and gravel as the filter medium and filters through the adsorption of the granular media and the interception of suspended solids in the pores between the sand grains. It has a small specific surface area, low dirt-holding capacity, slow filtration rate, and low filtration precision, and is not suitable for the rapid filtration of suspended solids in reclaimed-water reuse systems.
AFF uses asymmetric fiber bundle material as its filter medium, combining the advantages of granular and fibrous media such as high-efficiency fiber ball media and suspended ball packing. Thanks to its special structure the filter bed quickly forms a porosity gradient that is large at the top and small at the bottom, giving the filter a high filtration rate, high dirt-holding capacity, and easy backwashing. It is particularly suitable for filtering suspended solids in reclaimed-water reuse systems.
The membrane biological fluidized bed process is based on a biological fluidized bed using powdered activated carbon (PAC) as the carrier, combined with the solid-liquid separation technology of the membrane bioreactor (MBR) process. The reactor thus integrates the physical adsorption of activated carbon, microbial degradation, and efficient membrane separation. Refractory small-molecule organics in the water undergo thorough mass transfer and mixing with the fluidized powdered activated carbon under aeration and are adsorbed and concentrated on the carbon surface, creating local zones of concentrated pollutants. The powdered activated carbon also provides a special surface for microbial growth, its porous surface adsorbing large microbial populations, especially those using the target pollutants as metabolic substrates. At the same time, powdered activated carbon strongly adsorbs dissolved oxygen; under high dissolved oxygen conditions the microorganisms oxidize and decompose the small-molecule organics concentrated on the carbon surface. A ceramic membrane separation system then separates the water from the suspended particles such as organics-laden powdered activated carbon, and cross-flow filtration further purifies the water so that it meets reclaimed-water reuse standards. Studies show that MBFB effectively removes ammonia nitrogen, COD, and other refractory small-molecule toxic organics from slightly polluted water.
The Dijing asymmetric fiber filter (AFF) is a rapid solid-waste purification unit for reclaimed-water reuse developed by an environmental technology company in Seattle, USA. It can be used on its own or together with a flocculant to remove solids from reclaimed water and purify it.
