Black Liquor, White Water and the Reuse Ceiling in a Paper Mill

Paper mills generate wastewater in two quite different places, and treating them as one problem is where most flowsheets go wrong. Pulping separates fibres from plant raw material to make pulp and then bleaches it. Papermaking dilutes, shapes, presses and dries that pulp into sheet. Both discharge, but pulping discharges far worse.

Black Liquor and White Water

Wash water from pulping runs blackish-brown and is called black liquor. Its pollutant concentration is extreme - BOD reaches 5-40 g/L - and it carries large amounts of fibre, inorganic salts and pigments. Effluent from the bleaching process adds large amounts of acid-base substances. The paper machine discharges what is called white water, loaded with fibre, fillers and the sizing agents added during production.

Recovery First, Treatment Second

The priority order for a mill is raising the circulating-water reuse rate, cutting water consumption and wastewater discharge, and finding reliable, economical ways to use whatever is recoverable. Flotation recovers fibrous solid matter from white water at a rate up to 95%, and the clarified water can be reused. Combustion recovers sodium hydroxide, sodium sulfide, sodium sulfate and other sodium salts bound to organic matter out of black liquor. Neutralisation adjusts pH; coagulation sedimentation or flotation removes suspended solids; chemical precipitation decolorises; biological treatment removes BOD and works well on kraft wastewater; wet oxidation has been fairly successful on sulfite pulp wastewater. Reverse osmosis, ultrafiltration and electrodialysis are also in use.

A Magnetic Route That Was Actually Piloted

High-gradient magnetic separation is interesting because it introduces no flocs of its own, so large volumes can be treated in a short time. Researchers at the Department of Energy and Environmental Engineering, Osaka University, Japan, studied a magnetic-separation system on paper-mill wastewater. The pilot plant treated 2,000 t/d of papermaking 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 combines a mixing tank - the magnetic seed tank, seeded with organic matter, pulp and dyes - a sedimentation tank and the superconducting tube. Magnetic particles and organic polymers such as pulp and pigments are captured inside the tube, and magnetic short fibres and fillers are floated out; part of them settles by gravity in the sedimentation tank, which reduces the load reaching the magnet. The system ran successfully for several months with satisfactory results.

Chemicals in the Wet End

Polyacrylamide is widely used as a retention aid, filter aid and uniformity agent. It improves paper quality and pulp dewatering, raises retention of fine fibres and fillers, and cuts raw-material consumption. Its effect depends on average molecular weight, ionic nature, ionic strength and the activity of other copolymers: non-ionic grades mainly improve pulp filterability and dry-strength and retention, anionic copolymers serve as dry and wet strength enhancers and retention aids, and cationic copolymers are used in wastewater treatment and as filter aids. Aluminium sulfate is extremely soluble in water but cannot dissolve in pure sulfuric acid - it merely coexists - so its solubility in sulfuric acid is really its solubility in water. At room temperature it crystallises with 18 molecules of water of crystallisation as the octadecahydrate, containing 51.3% anhydrous aluminium sulfate, and it does not dissolve in its own water of crystallisation even at 100 degC. It begins to decompose into alumina, sulfur trioxide, SO2 and water vapour at 770 degC. Sodium metabisulfite is a white or yellow crystalline powder with a strong SO2 odour and specific gravity 1.4; it releases SO2 on contact with strong acid, oxidises to Na2S2O6 on long storage, and decomposes above 150 degC to release SO2 again.

The Reuse Ceiling

Papermaking wastewater is degraded through physicochemical plus biochemical methods, and the effluent can basically meet discharge standards - but that is far from the water quality reuse demands. Traditional sand filtration, activated-carbon filtration and multi-media filtration only reduce effluent suspended solids to a degree; they cannot remove dissolved pollutants such as COD, ammonia nitrogen and salinity. Reuse that water and it shows up in paper quality. Mills therefore limit reclaimed water to low-specification sections such as slag removal, pulp washing and bleaching, and even those sections have requirements for COD, turbidity and iron that existing filtration cannot meet, while the traditional multi-stage filtration train is long, large and unstable. Waste-paper recycling makes it harder: washing wastewater from the pulping side runs CODCr 600-2400 mg/L, BOD5 125-585 mg/L, SS 650-2400 mg/L and colourity 450-900 times, appearing blackish-gray, at 100-200 t per ton of paper.

AFF, Then MBFB

The practical answer is a two-step polish. Starting from sand-filter effluent with COD about 110 mg/L, the AFF asymmetric fiber filter does the precision filtration. AFF integrates dosing, micro-flocculation, sedimentation and filtration in one unit: filtration speed is more than ten times that of sand filtration, filtration precision is 5 um, four times that of ordinary sand filtration, and it backwashes easily. It uses asymmetric fiber-bundle media so the bed porosity forms a gradient - large on top, small below - which is what makes it fast and high-capacity.

AFF does not finish the job. Reclaimed water leaving it still carries a COD of around 100 mg/L, and it is mainly soluble COD (SCOD), which directly decides the reuse value of the water. Organics also shorten reverse-osmosis membrane life, so SCOD has to come down below 30 mg/L before the water goes further. That is what the membrane biological fluidized bed (MBFB) is for. MBFB uses specially treated ceramic membranes to combine the membrane-separation system with a high-load biological fluidized bed, and has been promoted in the United States, Japan, the United Kingdom, Germany, South Africa and Australia. It is built on the biological fluidized bed with powdered activated carbon (Powdered Activated Carbon, PAC) as carrier, married to the solid-liquid separation of the membrane bioreactor (Membrane Bioreactor, MBR). Refractory small molecules contact and adsorb onto the fluidised carbon under aeration, a local high-concentration zone forms on the carbon surface, and its porous surface hosts the flora whose metabolic substrate is exactly those pollutants. PAC also adsorbs dissolved oxygen strongly, so under high-DO conditions the enriched organics are oxidised where they sit, and the ceramic membrane separates the water cross-flow from the loaded carbon.

Except for conductivity, MBFB effluent meets the industry standard for workshop reuse water in the papermaking sector and goes straight to slag removal, pulp washing and bleaching, achieving a reuse rate of about 60%. It also works as the pretreatment section of reverse osmosis - MBFB effluent enters the RO membrane directly without security filtration or ultrafiltration, which cuts pretreatment cost and extends membrane service life.