Papermaking Effluent: Black Liquor, White Water and the Route Back to Reuse

Two stages, two very different wastes

Papermaking wastewater comes mainly from two production stages, pulping and sheet forming. Pulping separates fibres from plant raw materials to make pulp, which is then bleached, and this stage generates the largest volumes. Sheet forming dilutes, forms, presses and dries the pulp into paper, and it also readily generates wastewater.

The wastewater from pulping is by far the most heavily polluted. The water discharged during pulp washing is dark brown and is called black liquor, with very high pollutant concentrations: BOD reaching 5-40 g/L, plus large amounts of fibre, inorganic salts and pigments. Bleaching effluent carries large amounts of acidic and alkaline substances. The water discharged from the paper machine is called white water and contains large amounts of fibre together with the fillers and sizing agents added during production.

Reduce first, treat second

Treatment priority should always start with raising the water recycling rate and cutting water consumption and discharge, while recovering what is useful. 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 pH; coagulation-sedimentation or flotation removes suspended solids; chemical precipitation can decolorize; biological treatment removes BOD and is fairly effective on kraft pulp wastewater; and wet oxidation has worked well on sulfite pulp wastewater. Reverse osmosis, ultrafiltration and electrodialysis are also used on papermaking wastewater in various places.

One less conventional option comes from the Department of Energy and Environmental Engineering at Osaka University in Japan, which studied a magnetic separation system for paper mill wastewater. The pilot plant treated 2,000 t/d, and 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 tube capture magnetic particles and organic polymers such as pulp and pigments, floating out magnetic short fibres and fillers; some settle in the sedimentation tank by gravity, which reduces the load carried through the magnet tube. The system ran successfully for several months.

Recycled-paper mills: a different profile

Recycled-paper papermaking also splits into pulping and sheet forming. In pulping, screening, pulp washing and rinsing produce large volumes of washing wastewater whose characteristics depend on the waste paper source and the process. Pollutant levels run to BOD5 125-585 mg/L and SS 650-2,400 mg/L, with colour of 450-900 times and a dark gray appearance. Washing wastewater volume is 100-200 t per ton of paper. As in conventional papermaking, sheet forming produces white water containing fibre, filler and chemicals, and this is usually treated by flotation to recover fibre and filler so the treated white water can be recycled.

The chemicals sitting on site

Polyacrylamide is widely used in papermaking as a retention aid, filter aid and leveling agent. It improves paper quality, enhances pulp dewatering, increases retention of fines and fillers, and reduces raw material consumption and environmental pollution. Its performance depends on average molecular weight, ionic character, ionic strength and the activity of other copolymers. Non-ionic polyacrylamide mainly improves pulp drainage, raises dry paper strength and increases fibre and filler retention; anionic copolymers serve as dry and wet strength agents and retention aids; cationic copolymers are used for papermaking wastewater treatment and as filter aids and also work well for improving filler retention.

Aluminium sulfate is highly soluble in water, though it does not dissolve in pure sulfuric acid where 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 aluminium sulfate octadecahydrate, which is the industrial product in most cases. It contains 51.3% anhydrous aluminium sulfate and does not dissolve in its own crystal water even at 100 degrees C. 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 vapour. Its aqueous solution is acidic and hydrolyzes to form aluminium hydroxide, and prolonged boiling can produce basic aluminium 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 have oxidized. 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 odour 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 it cannot be kept long, and above 150 degrees C it decomposes and releases SO2. Its uses sprawl: making sodium hydrosulfite, sulfadimidine, analgin and caprolactam; purifying chloroform, phenylpropyl sulfone and benzaldehyde; photography fixer; vanillin in the flavour industry; preservative in brewing, rubber coagulant and dechlorinating agent for bleached cotton cloth; reducing agent in leather making; electroplating; oilfield wastewater treatment; and flotation agent in mining.

Getting from compliance to reuse

Papermaking wastewater is generally treated by physicochemical plus biochemical methods. The pollutants are highly complex, so treated effluent can basically meet discharge standards but falls far short of reuse quality. Conventional sand filtration, activated carbon filtration and multimedia filtration only reduce suspended solids; they cannot remove dissolved COD, ammonia nitrogen and salts, and reusing such water directly affects paper quality. In the paper industry reclaimed water is generally reused only in screening, pulp washing and rinsing, and even those stages set requirements for COD, turbidity and iron that existing filtration cannot meet. Traditional multi-stage filtration also brings a long process train, a large footprint and unstable product water quality.

The practical route starts from sand filter effluent with a COD of about 110 mg/L and runs it through an AFF asymmetric fiber filter for fine filtration. AFF is a high-efficiency filtration unit integrating chemical dosing, micro-flocculation, sedimentation and filtration. Its filtration rate is more than 10 times that of a sand filter, its precision is 5 um, four times that of an ordinary sand filter, it backwashes easily and it is convenient to manage. In this role AFF mainly removes iron and suspended solids from the reclaimed water. It works because asymmetric fiber bundle material combines the advantages of granular and fibrous media: the bed quickly forms a porosity gradient that is large at the top and small at the bottom, giving high filtration rate, high dirt-holding capacity and easy backwashing. A sand tank, by contrast, has a small specific surface area, low dirt-holding capacity, slow filtration rate and low precision, and does not suit rapid filtration in a reuse system.

After AFF filtration the reclaimed water still has a COD of about 100 mg/L, mostly soluble COD (SCOD), which directly affects reuse value and greatly shortens RO membrane life, so a suitable process must bring COD below 30 mg/L. A membrane biological fluidized bed (MBFB) process does that. Built on a biological fluidized bed using powdered activated carbon (PAC) as carrier and combined with the solid-liquid separation technology of the membrane bioreactor (MBR) process, the reactor integrates physical adsorption of activated carbon, microbial degradation and efficient membrane separation. Refractory small-molecule organics undergo thorough mass transfer and mixing with the fluidized PAC under aeration, adsorb and concentrate on the carbon surface, and create local zones of concentrated pollutants. The porous carbon surface also hosts large microbial populations, especially those using the target pollutants as metabolic substrates, and PAC strongly adsorbs dissolved oxygen, so under high DO the microorganisms oxidize the concentrated small-molecule organics. A ceramic membrane separation system then separates the water from organics-laden PAC, and cross-flow filtration polishes it to reuse standard.

The results justify the complexity. Except for conductivity, MBFB effluent meets the industry standard for workshop reuse water in the paper industry and can be used directly in screening, pulp washing and rinsing, achieving a reuse rate of about 60%. It can also serve as pretreatment for reverse osmosis, feeding the RO membranes directly for desalination without complex cartridge filtration and ultrafiltration, which cuts pretreatment cost and extends membrane life. The Dijing asymmetric fiber filter, developed by an environmental technology company in Seattle, USA, can be used on its own or together with a flocculant for the same duty.