Pulp and paper mills are extreme water users — and biofouling doesn’t take days off. Here’s how mills disinfect raw intakes with chlorination, UV, and ozonation, and why most now stack barriers.
Pulp and paper mills are extremely water‑intensive: U.S. operations average 54 m³ of water per tonne of product, with roughly 85% of intake serving as process water (WaterTechOnline) (WaterTechOnline).
Raw river or reservoir intake arrives loaded: suspended solids, dissolved organics (lignin, tannins, resins), nutrients, and a microbial load (bacteria, algae, protozoa). Left alone, these drive biofouling (slimes, pitch spots), raise downtime, and drag quality.
So mills front‑load pretreatment to cut turbidity and biological risk. Typical trains use coarse screening, sedimentation/clarifiers or DAF (dissolved air flotation), and multimedia filters — often pushing turbidity to <5 NTU (nephelometric turbidity units) before fine membranes (Filtox), meeting process targets such as <50 NTU before the headbox (Filtox) and conductivity at ≲100 µS/cm for showers (Filtox). Physical barriers remove most particulates and some microbes. Disinfection still matters — for residual pathogens (coliforms, viruses, fungi) and to hold back biofilm in pipes and tanks.
Coarse screening often starts with equipment like an automatic screen to continuously strip debris >1 mm before settling. Clarification can be handled in a gravity unit or a clarifier, with some mills opting for DAF where loads and footprints demand it.
Chlorination: residual and byproducts
Chlorination (Cl₂ gas or hypochlorite) hydrolyzes to hypochlorous acid, which oxidizes and kills bacteria and viruses. It is cheap and provides a residual — typically ~0.2–2 mg/L free chlorine — that maintains disinfection through tanks and pipes. Chlorine inactivates most bacteria and viruses quickly; for common enteric viruses a CT (concentration×time; mg·min/L) of only a few mg·min/L can deliver 4‑log (99.99%) reduction (PMC) (PMC).
Limitations show up with protozoa: Cryptosporidium oocysts need CT ≈3600 mg·min/L for 4‑log inactivation — effectively impractical (PMC). Efficacy also dips as pH rises, temperature drops, or turbidity climbs — all of which inflate required CT (PMC) (PMC).
Advantages include well‑understood chemistry, easy dosing, and a long‑lived residual. The spectrum is broad; even chlorine‑resistant organisms (like Giardia integers) are largely inert (PMC). Downsides: chlorine forms disinfection byproducts (DBPs). As Arnott notes, “chlorination… generates disinfection byproducts… which may potentially introduce another public health risk” (WaterWorld). Trihalomethanes (THMs) and haloacetic acids (HAAs) form when chlorine meets natural organics; in pulp mills, wood organics can react too, ramping AOX (absorbable organic halides). Chlorine gas handling demands scrubbing; chlorine adds salt and acidity (as NaCl/HCl), raising corrosion risks, and can accelerate scaling on membranes — a reason experts recommend ozone in some cases “to eliminate long‑term residual and make the water less caustic to the membranes” (WaterTechOnline).
Key data: a typical contact tank doses ~1–3 mg/L with 30–60 minutes’ contact. Literature puts CT for 4‑log E. coli at ~0.1–0.3 mg·min/L at 20°C; many viruses fall below 10 mg·min/L (PMC). Guidelines often target 3–4 log coliform reductions, with free chlorine residuals around 0.5 mg/L to inhibit regrowth. Indonesian drinking‑water standards (Permenkes) allow 0/100 mL coliform (E. coli absent); while most P&P water isn’t potable, facilities typically run a separate potable line.
Chemical feed can be straightforward; many plants pair disinfection chemistry with an accurate dosing pump to hold target residuals steady through load swings.
UV sterilization: fast, chemical‑free
Ultraviolet (UV) at 254 nm (UV‑C) inactivates microbes by damaging DNA/RNA. UV irradiates the entire flow without adding chemicals. It is highly effective for bacteria and many viruses when turbidity is low. A typical medium‑pressure lamp delivering about 400 W‑hr/m³ yields doses around 20–40 mJ/cm²; studies report 4‑log bacterial inactivation at ~10 mJ/cm² (PMC). Many common viruses (enteroviruses, adenoviruses) require 10–50 mJ/cm², though some resilient strains may need ~100–140 mJ/cm² (PMC) (PMC). UV is highly effective against chlorine‑resistant protozoa; a 40 mJ/cm² dose is often cited for ~3–4 log Cryptosporidium kill. Residence times are measured in seconds.
Advantages: no toxic residues or byproducts — “only energy has been applied (no chemicals), there is no negative impact to the water” (WaterWorld). UV systems are easy to install/retrofit, with lamps typically lasting ~9,000 hours (≈1 year) and annual cleaning (WaterWorld). Properly dosed, UV reliably achieves ~4‑log kill of bacteria/viruses (WaterWorld).
Disadvantages: no residual means recontamination risk downstream; UV is often paired with a low chlorine dose or another barrier for distribution protection (WaterWorld). Turbidity and color can “shadow” pathogens, so upstream filtration is critical — many designs insist on <5 NTU (and often <1 NTU) before UV (Filtox). Energy and lamp maintenance add costs, and highly UV‑resistant viruses or spores can survive at standard doses.
Designers commonly size for 20–40 mJ/cm², delivering >4‑log bacterial and 2–3 log viral reductions under good water quality, without regulated DBPs or corrosion issues (WaterWorld). In mill applications, UV often polishes after filtration or on a separate drinking‑water line to meet Permenkes and residual norms without dosing chemicals.
For installations prioritizing chemical‑free primary disinfection, compact units such as an ultraviolet system are often placed immediately downstream of fine filtration.
Ozonation: powerful, transient oxidation
Ozone (O₃) — generated on‑site, typically at >10 wt% in oxygen — is a strong oxidant. Sparged or contacted with water, it attacks cell membranes and viral capsids and oxidizes organics (color, phenolics, AOX precursors). Inactivation is generally faster than with chlorine; a 4‑log virus kill can occur at CT <2 mg·min/L (PMC). Many bacteria fall near CT≈10 mg·min/L (PMC). In practice, dose rates around 0.5–1.5 mg/L with minutes of contact are common. Ozone penetrates biofilms better than chlorine and leaves no chlorinated byproducts.
Advantages: extremely broad‑spectrum disinfection with short contact times; oxidation of organics reduces color and odor. For mills, ozone does not form AOX or THMs, and its residual chemistry is mainly dissolved oxygen (and some hydrogen peroxide). Industry experience notes swapping chlorine for ozone can help protect membranes — “using ozone… eliminates long‑term residual and makes water less caustic to the membranes” (WaterTechOnline). Residual is minimal, so a secondary barrier is used if downstream protection is required.
Disadvantages: higher capital and energy (generator, contactor, off‑gas destructor); toxic gas handling; potential bromate formation if bromide is present (bromate is regulated); rapid demand in high‑organics water can push doses up. Key data include virus inactivation within CT <2 mg·min/L (20 mg/L×6 s) (PMC), and applications where 0.5–1.0 mg/L achieved complete Pseudomonas/Staphylococcus inactivation in secondary effluent with ~5–10 min contact (ResearchGate). Advanced oxidation can run roughly 0.2–0.4 kWh/m³ (ozone + UV/H₂O₂) in reuse applications — higher than chlorine but in the range of reverse osmosis costs.
Other oxidizing biocides
Peracetic acid (PAA) is used in some mills. It is a strong oxidizer (bacteria/viruses/fungi/spores) that decomposes to acetic acid and oxygen. Industry notes favor oxidizing biocides like PAA because they “decompose to benign by‑products and maintain efficacy across varying pH” (Filtox). PAA generates no halogenated byproducts, works hot or cold, and can be applied in‑line; drawbacks are cost and handling a corrosive liquid. Chlorine dioxide can also offer broad‑spectrum kill without THMs, though its use in pulp water is noted as less common.
Comparative performance and trade‑offs
Efficacy: all three (Cl₂, UV, O₃) can achieve high log‑kills (>99.9%) under ideal conditions. Chlorine and ozone in bench tests generally inactivate “hypersensitive” viruses at CT ≲10 mg·min/L (PMC), whereas UV dosing of ~10–40 mJ/cm² is needed for 4‑log bacterial kill — and somewhat higher for resilient viruses (PMC) (WaterWorld). UV is very effective on protozoa that chlorine misses; ozone is effective across the board.
Residual: chlorine is unique in providing a lasting residual in piping. In practice, many mills combine UV plus a low chlorine residual to capture both benefits (WaterWorld). Ozone can be paired with occasional chlorine or peracetic dosing to maintain residual protection.
Byproducts: chlorine produces DBPs (THMs, HAAs, AOX) that may affect product quality and carry health concerns; UV and ozone produce no chlorinated DBPs. Ozone can form bromate if bromide is present; PAA leaves only acetate.
Cost & operation: chlorine is inexpensive and easy to feed (with safety and handling considerations). UV carries moderate capital and electricity (lamps ~80 W each) with periodic cleaning/replacement (WaterWorld). Ozone demands the highest equipment/energy (on‑site generator, 10–20 kW‑scale). All have compact footprints relative to other unit operations.
For fine‑solids control ahead of disinfection, many mills rely on robust media beds such as a sand and silica filter, and increasingly, pathogen‑barrier membranes like ultrafiltration to hold pre‑UV turbidity low.
Example outcomes and targets
In a pulp‑mill retrofit, replacing chlorine with an ozone generator (3 mg/L dose) reduced AOX by ~90% and enabled reuse of bleach filtrate (WaterTechOnline). Another case showed high‑efficiency UF/RO combined with UV polishing delivered water <1 NTU to showers (Filtox), enabling 90% white‑water reuse with <50 µS/cm boiler feed conductivity (Filtox). Industry reports estimate multi‑barrier systems (coagulation+UF+UV) can trim overall treatment costs by up to 20–30% versus conventional lines, via chemical savings and higher reuse (Filtox) (Filtox).
Where downstream desalination or polishing is needed, integrated trains — for example, membrane systems that bundle UF and RO — are increasingly paired with UV to stabilize bio‑risk before high‑pressure stages.
Multi‑barrier disinfection strategy
No single process guarantees absolute safety. A true multi‑barrier plan starts with the best available source water protection, then layers treatment and disinfection steps (WaterWorld).
For a mill’s raw water intake, a representative sequence is:
Coarse screening and sedimentation/DAF: remove debris, fish, and settleable solids. Sand or multimedia filtration — sometimes followed by UF — polishes to low turbidity (e.g., <5 NTU) and physically removes >90% of microbes (Filtox) (Filtox). Ultrafilters or microfilters also serve as pathogen barriers (capturing colloidal bacteria, molds).
Primary disinfection (UV or ozone): UV can deliver ~4‑log inactivation of bacteria/viruses as water flows through (WaterWorld), while an ozone contactor achieves similar reductions at ~1–2 mg/L.
Secondary/residual disinfection (chlorine or another oxidant): a small dose maintains a low‑level residual in storage and distribution, polishing any breakthrough microbes and suppressing regrowth. One industry note: “introducing a UV unit as primary, chlorine use can be minimized… while maintaining the benefit of low‑level residual chlorine to protect the water” (WaterWorld).
Settling capacity can be intensified with a DAF unit where oils/fats or fine floatables challenge classical gravity separation, smoothing the load on downstream filters.
Risk compounding and monitoring
Layering compounding removal matters. Example: if raw water carries 10⁵ CFU/100 mL coliform, a first‑pass engine chamber might remove 90%, a filter another 90%, and UV inactivates 99.99% of the remainder — driving an overall >99.9999999% (9‑log) reduction before final chlorination touches the water. WHO and industry guidance frames it bluntly: “filtration will always be necessary to remove suspended particles… and then eliminate any or all harmful organisms” (WaterWorld).
By distributing the disinfection load, multi‑barrier systems stay robust even if one step underperforms (e.g., a UV lamp outage). That buffer is critical in tropical climates (as in Indonesia), where pathogen regrowth accelerates. Monitoring closes the loop: turbidity, chlorine residual, and microbiological tools like ATP/HPC (adenosine triphosphate/heterotrophic plate counts) are key operational anchors (Filtox).
Media choices in the filtration step can tighten those monitors’ job. For example, upgrading to a stacked bed with anthracite media can extend run times while keeping fine solids from shadowing UV light further downstream.
Bottom line
Chlorination, UV, and ozonation each bring distinct strengths. Chlorine is inexpensive and provides residual kill but can form DBPs; UV is chemical‑free and rapid on viruses/cysts but offers no residual; ozone is powerful on pathogens and organics but is costlier and transient. A multi‑barrier strategy — filters plus UV/ozone plus a backup disinfectant — delivers the safest outcome. With design and monitoring (turbidity, chlorine residual, ATP/HPC assays), mills can meet operational targets (preventing biofouling and turbidity spikes) and any process or potable standards (PMC) (WaterWorld) (WaterTechOnline) (WaterTechOnline) (Filtox).
