Pulp and paper wastewater treatment with DAF
Paper mills are the one sector where dissolved air flotation is usually justified on recovered material rather than on compliance. The float is fibre and filler, and it goes back into the furnish.
In a paper mill, a dissolved air flotation unit — known in the industry as a DAF saveall — clarifies whitewater to 10–50 mg/L suspended solids from an inlet of 500–5,000 mg/L, recovering 95–99 % of the fibre and filler for return to the stock system. It runs at high surface loading rates of 8–15 m/h with modest chemical demand: 0.5–3 mg/L of cationic polymer, sometimes with 50–250 mg/L of polyaluminium chloride or alum where anionic trash must be neutralised. Clarified whitewater is reused as shower and dilution water, cutting fresh-water intake substantially.
Two different duties, often confused
A mill typically needs flotation in two distinct places, and they are not interchangeable.
| DAF saveall (internal) | Effluent DAF (end of pipe) | |
|---|---|---|
| Purpose | Recover fibre and filler, produce clarified water for reuse | Meet discharge consent on TSS and colour |
| Inlet TSS | 500 – 5,000 mg/L | 150 – 800 mg/L |
| Outlet TSS | 10 – 50 mg/L | 20 – 60 mg/L |
| Surface loading | 8 – 15 m/h | 5 – 10 m/h |
| Chemicals | Cationic polymer, sometimes PAC/alum | PAC or alum plus anionic polymer |
| Float destination | Back to stock chest | Sludge dewatering and disposal |
| Economic driver | Recovered raw material and water | Compliance |
A saveall pays for itself on recovered fibre. On a recycled-containerboard mill running 120 t/day with 2 % of production reporting to whitewater, recovering 97 % of it returns roughly 2.3 t/day of fibre — at typical OCC-derived furnish values, tens of thousands of dollars a year, before counting the water saved.
Effluent characteristics by mill type
| Mill type | Effluent m³/t | TSS mg/L | COD mg/L | Notes |
|---|---|---|---|---|
| Recycled containerboard / testliner | 3 – 10 | 1,000 – 5,000 | 1,500 – 6,000 | High anionic trash, stickies |
| Tissue from recycled fibre | 10 – 25 | 800 – 3,000 | 1,000 – 3,500 | Deinking load if DIP |
| Fine paper / printing & writing | 15 – 40 | 500 – 2,000 | 800 – 2,500 | High filler content in float |
| Kraft pulp mill | 30 – 70 | 300 – 1,200 | 1,000 – 4,000 | Colour and lignin dominate COD |
| Corrugating / converting only | 1 – 4 | 200 – 900 | 500 – 2,000 | Starch load is soluble |
Chemistry: cationic, not anionic
This is where paper differs from almost every other DAF duty. Cellulose fibre carries a negative surface charge, and recycled furnish carries a heavy load of dissolved anionic material — “anionic trash” from starch, latex, dispersants and deinking chemistry. The correct flocculant is therefore cationic, typically a medium-to-high charge-density polyacrylamide at 0.5–3 mg/L.
- Where anionic trash is high (recycled containerboard, deinked tissue), add a low-molecular-weight cationic fixative or polyaluminium chloride at 50–250 mg/L ahead of the flocculant to neutralise it first. Without this, polymer consumption doubles and results stay poor.
- Where the float returns to the furnish, chemical selection is constrained by product quality: aluminium salts affect sizing and pH control, and some fixatives affect brightness. Coordinate with the wet-end chemist before specifying.
- Do not over-flocculate. Large, weak flocs shear in the recycle nozzle and in the skimmer, releasing fines back into the clarified water. Paper flocs want to be small and tight.
Worked sizing example: 120 t/day recycled containerboard mill
- Whitewater flow to saveall. 120 t/day at 6 m³/t of circulating whitewater requiring clarification: 720 m³/day, continuous over 24 h = 30 m³/h. Many mills run the saveall on a larger recirculating loop; size on the actual clarifier feed, not on net effluent.
- Solids load. Inlet TSS 2,500 mg/L: 30 × 2.5 = 75 kg/h.
- Flotation area. At 12 m/h (high-rate is appropriate for fibre, which floats readily): 30 ÷ 12 = 2.5 m². Select 3.0 m².
- Air demand. A/S of 0.008 kg air/kg solids — low, because fibre is buoyant and easy: 75 × 0.008 = 0.60 kg air/h.
- Recycle flow. At 5 bar and 40 °C (mill whitewater is warm), usable dissolved air ≈ 84 mg/L. Recycle = 0.60 ÷ 0.084 = 7.1 m³/h, a recycle ratio of 24 %.
- Recovered fibre. 97 % of 75 kg/h = 73 kg/h dry, at 4 % consistency ≈ 1.8 m³/h of thickened stock returned to the blend chest. Over 350 days that is roughly 610 t/year of fibre that would otherwise have gone to sludge.
- Clarified water. 25–40 mg/L TSS, suitable for showers, seal water and dilution — typically displacing 400–600 m³/day of fresh water.
Materials and design notes
Whitewater is warm (35–50 °C), often slightly acidic to neutral, and frequently carries chlorides in recycled systems where water is closed up. SS304 is standard; SS316L where the mill runs a closed water loop with chloride above roughly 500 mg/L. Carbon steel with epoxy is acceptable on effluent duty at the end of pipe but a poor choice on a warm saveall.
- Screening ahead of the unit: 2–3 mm, mandatory. Wire fragments, plastic film and staples from OCC will wreck a recycle pump.
- Stickies: tacky contaminants in recycled furnish adhere to tank walls, nozzles and skimmer flights. Specify accessible nozzles and plan a wash-down regime.
- Warm water reduces air solubility. At 45 °C, saturation is roughly 30 % below 20 °C. Correct the recycle ratio for it; see the design parameters reference.
Frequently asked questions
What is a DAF saveall in a paper mill?
A saveall is a clarifier on the whitewater loop whose purpose is to recover fibre and filler for return to the stock system and to produce clarified water for reuse in showers and dilution. A DAF saveall uses dissolved air flotation rather than gravity settling or a disc filter, and achieves 95–99 % solids recovery with clarified water at 10–50 mg/L TSS at surface loading rates of 8–15 m/h.
Should I use cationic or anionic polymer on paper whitewater?
Cationic. Cellulose fibre is negatively charged and recycled furnish carries substantial dissolved anionic material. A medium-to-high charge-density cationic polyacrylamide at 0.5–3 mg/L is the standard flocculant. Where anionic trash is heavy, add a low-molecular-weight cationic fixative or 50–250 mg/L of polyaluminium chloride upstream to neutralise it before flocculating.
Can a DAF replace a disc filter or gravity saveall?
In most cases yes, and with a smaller footprint. A DAF saveall gives cleaner filtrate than a gravity saveall, tolerates variable inlet consistency better than a disc filter, and does not require a sweetener stock layer. Disc filters remain preferable where the mill wants three separated filtrate qualities from one machine, or where chemical use must be zero.
How much water can a paper mill save with a DAF?
Clarified whitewater at 10–50 mg/L TSS is suitable for showers, seal water, chemical make-up and stock dilution. On a mill at 15 m³/t, closing the loop with a saveall commonly reduces fresh-water intake by 30–60 %. The practical limit is not the DAF but the build-up of dissolved salts and calcium in the closed loop, which eventually forces a bleed.