Paper mills are rewriting their water playbook with multi‑barrier treatment

The pulp-and-paper business is one of the world’s thirstiest industries, drawing ≈54 m³ of water for every tonne of product produced (Water Technology). About 85% of that is “process water” that quickly loads up with fibers, lignins, resins, chlorine compounds (AOX), BOD/COD and suspended solids (BOD/COD are measures of oxygen‑demanding organics) (Fluence). Globally, mills account for roughly 12% of industrial water withdrawal (TAPPI Paper360).

The sector has already slashed water use, TSS and BOD since the 1970s (IntechOpen), but rising freshwater costs, stricter withdrawal/effluent rules and ESG commitments have put the squeeze on raw water intake and pushed reuse higher (Water Technology; Fluence). One pinch‑analysis study shows solids‑focused pretreatment can cut freshwater intake by 36–93%, depending on reuse configuration (ScienceDirect).

The shift is visible on the ground. An OKI pulp mill in Indonesia reduced intake from 7.0 to 5.8 million m³ per month by switching to higher‑quality source water and closed‑loop reuse, and pushed process‑water intensity down from 2.57 to 2.31 m³/ADT (air‑dried ton) via whitewater recycling (OKI Pulp & Paper; OKI Pulp & Paper). The trend is clear: multi‑barrier treatment is moving from compliance tool to business strategy.

Source intake and first barrier

Design starts at the river or reservoir intake, where quality can swing seasonally. Coarse screens at ≥10–15 mm keep out logs and knots, followed by a grit chamber to drop sand. For continuous debris removal at the headworks, mills often consider an automatic screen. A covered raw‑water basin or equalization tank then buffers flow and quality before treatment.

Coagulation, flocculation, and clarification

The first removal step doses coagulants — alum, ferric chloride, or polyaluminum — and flocculants in rapid‑mix and floc basins. Typical coagulant doses (10–100 mg/L) and retention times (30–60 minutes) can precipitate 50–90% of suspended solids, depending on raw turbidity. Accurate chemical feed is often managed with a dosing pump, and polyaluminum choices include PAC or ACH grades.

Flocculated water flows to a clarifier or to a high‑rate dissolved air flotation (DAF) unit. Modern high‑rate clarifiers and DAF capture woody fibers, colloidal pitch and settleables; well‑engineered clarifiers usually reduce TSS to <20 mg/L (Filtox), with many mills more typically achieving <10 mg/L. A target of 20 mg/L TSS or below is common leaving primary clarifiers (Filtox). Where flotation performance is preferable, a DAF system serves the same role.

Depth filtration polishing

After clarification, multimedia depth filters polish residual turbidity and fine colloids. Many mills specify a sand-silica bed for 5–10 micron capture. Others add an anthracite top layer to improve depth filtration. Properly sized and backwashed filters typically deliver <5 NTU (nephelometric turbidity units) effluent — Filtox engineers report <5 NTU on pulp‑mill loop water (Filtox). At this point, water is largely clear of particles and settleables, with >90% of original TSS removed and “downstream membranes” such as RO/NF/UF systems better protected.

Activated carbon for color and organics

Even after solids removal, dissolved organics — humics, lignins, residual chemicals — can leave color, COD and taste/odor. Granular activated carbon (GAC) beds, typically 1–2 m deep, are the workhorse for adsorption of these compounds. An industrial study found 97% color removal at a 2 g/L carbon dose (International Journal of Research in Chemistry and Environment), while another achieved 94–96% removal of lignin and color under optimized conditions (Springer). Designers can conservatively expect ≥80–90% reduction in dissolved organic color/COD in a well‑operated carbon filter. For media selection and replacement programs, operators typically turn to activated carbon suppliers.

Optional oxidation and disinfection

Some mills add oxidation (ozone or hydrogen peroxide) ahead of carbon to improve COD/color removal, or use UV disinfection after polishing when microbial control is necessary. Such advanced oxidation is not always required at intake, but it is included when pathogen risk is high or when closed‑loop reuse demands microbially safe water. Where UV is preferred, low‑operating‑cost systems such as ultraviolet units are commonly integrated.

Softening and demineralization for boilers

Depending on end use, a softening/demineralization stage follows polishing. For processes or boilers that need very low hardness and salts, mills install ion‑exchange or membrane units. A lime‑soda softener or Na‑zeolite exchanger can remove Ca²⁺/Mg²⁺ to near‑zero (boiler feedwater specs often target <0.1 mg/L as CaCO₃). Typical industrial softeners and deionizers reliably reduce hardness/Ca and Na by >95%. When designing toward these outcomes, teams often select a softener as the first step and expand with an ion-exchange train as requirements tighten.

For very low conductivity, cation–anion systems and mixed‑bed DI produce <10 µS/cm, with modern two‑bed or mixed‑bed setups achieving ~99–100% ion removal and near‑deionized quality suitable for high‑pressure boilers. The choice often includes a demineralizer followed by a mixed‑bed polisher. Resin inventory is sized to run to the next regeneration (often every 1–2 weeks), and operators track ion-exchange resin condition closely.

Reverse osmosis for desalting and recycle

Reverse osmosis (RO) removes 90–99% of dissolved salts and is used for demineralization and for reclaiming process water. Korean pulp mills, for example, use RO to reclaim ∼90% of boiler makeup water. RO is deployed either as the primary desalting step or as polishing after partial softening; separate brine (concentrate) management is required. In mills standardizing on packaged trains, a brackish-water RO can be paired with upstream softening to meet conductivity and silica specifications.

Controls, sizing, and multi‑barrier monitoring

The value of a multi‑barrier train is redundancy. Sensors and controls track turbidity, pH, ORP and conductivity at each stage; if one barrier underperforms, the next compensates. Modern plants routinely ensure turbidity entering the headbox <50 NTU and final circulating‑water conductivity <100 µS/cm (Filtox). Targets like turbidity <5 NTU post‑filter and conductivity <10 µS/cm post‑demineralizer trigger adjustments in chemical dosing or backwash cycles in real time. Digital twins and analytics are increasingly used to optimize the water balance.

Units are sized for peak flow and contaminant loads. Clarifiers typically run at ~1–2 m³/m²·h overflow rate with 1–2 h detention. Dual‑media filters operate at 5–15 m/h flux, keeping filter ΔP under ~0.5 bar before backwash. Activated carbon filters commonly run ~10–20 bed volumes before breakthrough, with GAC beds often swapped out and reactivated offsite. Softener resin volume is designed for expected hardness loading, with salt regeneration commonly every 1–2 weeks.

Performance, cost, and competitive outcomes

Well‑implemented, the train yields stable process water, compliance, and cost savings. Cutting turbidity and suspended solids reduces fouling in paper machines and boilers. Mills report that better particulate removal reduces downtime and chemical usage — translating to “millions of dollars” in savings from fewer sheet breaks, less biocide use, and improved heat‑exchange efficiency (Filtox). In one case, supplemental pretreatment that removed >90% of TSS enabled a mill to cut freshwater purchases by >90% through reuse (ScienceDirect).

Regulatory and ESG momentum is aligned. In Indonesia, the sector faces programs like PROPER and “Green Industry” standards to reduce water intensity and pollution (Kementerian LHK; OKI Pulp & Paper). Meanwhile, global studies estimate that by 2030 around 1.6 billion people will lack safe water, raising scarcity risk for industry (TAPPI Paper360). As one industry guide puts it, “sophisticated turbidity reduction strategies have become a cornerstone of competitive mill operation” (Filtox).

KPIs and specification targets

To document performance, mills track: intake turbidity/COD versus post‑clarifier; post‑filtration turbidity (aim <5 NTU; <a "="" href="https://www.filtox.com/applications/process-water-treatment-for-pulp-and-paper-industry#:~:text=">Filtox); carbon effluent color (Platinum–Cobalt units) or COD (aim >80% reduction); hardness and TDS at softener outlet (e.g., <1 mg/L hardness, conductivity <50 µS/cm); and microbiological indicators when relevant. Benchmarks include >90% color removal by activated carbon (International Journal of Research in Chemistry and Environment; Springer) and <20 mg/L TSS after DAF (Filtox). Financial KPIs — water cost per tonne and effluent‑fee reductions — round out the dashboard.

In summary, a multi‑barrier train — screening/coagulation/clarification, media filtration, granular activated carbon, and, as needed, softening/demineralization — is now the state of the art for pulp‑and‑paper raw water. By removing solids, color and dissolved contaminants in stages, mills achieve stable, high‑quality water and maximum reuse, meeting stringent regulatory and product‑quality demands while delivering measurable wins in water savings, downtime and chemical costs (ScienceDirect; Filtox).

Sources: Industry technical literature, case studies and reports (Water Technology; IntechOpen; Fluence; OKI Pulp & Paper; Springer; Filtox; International Journal of Research in Chemistry and Environment).