A chemical precipitation train—equalization, pH shift, ferric/PAC coagulation, clarification, and polishing—cuts zinc and phosphate by ≥90% before blowdown enters the main effluent plant. The approach targets pH 9–10 for metals capture, then returns to neutral for discharge compliance.
Cooling towers in pulp and paper mills concentrate salts and treatment chemicals by evaporation, leaving a blowdown stream loaded with hardness ions, silica, and metal salts from corrosion inhibitors—commonly phosphates and zinc (MDPI) (MDPI). Literature tables cite typical cooling water blowdown (CWBD) containing ~1.2 mg/L Zn and 6.6 mg/L PO₄³⁻ (MDPI).
Those nutrients and metals are problematic—phosphate is a key eutrophication nutrient and zinc is toxic to aquatic life. Cooling water ponds are often described as “phosphorus‑impaired,” and new discharge rules increasingly limit P (and Zn) in blowdown (Power Engineering) (Power Engineering).
Freshwater scarcity is also reshaping make‑up water: mills in the Gulf Cooperation Council (GCC) now use treated sewage for cooling make‑up due to water shortage (MDPI). In practice, mills must purge 10–50% of make‑up as blowdown to prevent scale (ScienceDirect), making this high‑strength stream a priority.
Effluent limits and target setpoints
Indonesian effluent regulations (Permen LHK) allow up to 10 mg/L Zn and 10 mg P/L in industrial wastewater to sewers (ESCAP/UN). By contrast, tighter guidelines (proposed US NPDES, a discharge permitting system) target Zn <1 mg/L and Cr <0.2 mg/L (Power Engineering). pH must be neutral—6–9 per local standards (ESCAP/UN).
Against that backdrop, the treatment objective is straightforward: adjust pH and remove ≥90% of Zn and PO₄ so the blowdown can reliably pass either regime. The design targets effluent at <1–2 mg/L Zn and <0.5–1 mg/L PO₄ while avoiding large flow or energy penalties.
Process train and operating logic
The flowsheet is conventional but tuned to cooling chemistry: (1) Equalization/Neutralization → (2) Chemical Precipitation/Coagulation → (3) Clarification/Solids Separation → (4) Polishing Filter → (5) Discharge to the mill effluent treatment plant. Equalization receives continuous blowdown flow—10–50% of cooling make‑up (ScienceDirect)—to buffer pH and concentration swings, with stirring and pH probes for automated dosing.
pH elevation for zinc precipitation
In the precipitation reactor, dosing an alkali (lime, Ca(OH)₂, or NaOH) raises pH to ~9.0–10.0, where Zn²⁺ precipitates as Zn(OH)₂ (solubility minimum around pH 9–10) and calcium/phosphate begin forming Ca‑phosphates (JRTPPI) (MDPI). Reaction time of ~10–20 minutes allows complete precipitation. Automated alkali feed is commonly handled with dosing pumps for stable pH control.
Evidence is strong: in one study, seeding at pH 9.5 removed 95.3% of ~80 mg/L Zn, reducing it to ~3.7 mg/L (JRTPPI).
Ferric/PAC coagulants for phosphate capture
Coagulant dosing—ferric chloride (FeCl₃) or polyaluminum chloride (PAC)—forms Fe(OH)₃ “sweep flocs” that scavenge dissolved PO₄³⁻ (as FePO₄) and fine Zn(OH)₂. Typical FeCl₃ dose is 50–200 mg/L depending on phosphate load, with a stoichiometric Fe:P ≈ 3:1 by mass; adding ~30–50 mg/L FeCl₃ will precipitate ~10 mg/L PO₄. The combination of high pH and Fe coagulation often removes >95% of PO₄ and Zn, while also clearing fine suspended solids (Power Engineering) (MDPI). PAC is a standard option within mill programs, and PAC coagulants are straightforward to integrate alongside iron salts; many mills also standardize via general coagulant packages.
Clarification and solids separation
After gentle flocculation, clarification with 30–60 minutes retention time allows precipitated solids to settle; typical clarifiers achieve >90% TSS removal (Lenntech). A conventional clarifier suits most mills, while space‑constrained sites often opt for a lamella unit; in the sizing example below, a lamella clarifier settles >95% of precipitate, and a lamella settler can be slotted into compact footprints.
The resulting sludge—mostly zinc and iron phosphates/hydroxides—is periodically wasted.
Polishing filtration and effluent stability
Polishing filtration ensures particle‑free effluent: a multimedia bed is commonly deployed, and mills standardize around sand filters for this duty. Where finer cutoffs are needed, cartridge filters deliver sub‑micron polishing after clarification.
Once pH is nudged back toward neutral, the treated blowdown combines with the main sewage/effluent stream. The pH is stabilized to ~7–8 and Zn/PO₄ are well below 10 mg/L—ideally <1–2 mg/L Zn and <0.5–1 mg/L PO₄—avoiding upset to the main ETP. Remaining TDS and hardness behave like normal cooling water concentrates downstream and are handled by conventional clarifiers or softening; many mills already maintain a softener step in their broader water circuits.
Sizing example and expected removals
For 1,000 m³/day of blowdown with initial Zn≈5 mg/L and PO₄≈10 mg/L, dosing Ca(OH)₂ to raise pH from ~8 to ~10 (roughly 0.1 kg Ca(OH)₂ per m³) plus ~50 mg/L FeCl₃ coagulation yields >90% removal, leaving <0.5 mg/L Zn and <1 mg/L PO₄ in the settled water (JRTPPI) (MDPI). Fed to a lamella clarifier, >95% of precipitate settles, yielding a sludge of ~3–5% solids (hauled away). Effluent turbidity after clarification may be <10 NTU, with polishing filtration reducing this to <1 NTU.
If future reuse is pursued, Lenntech describes RO‑based blowdown reuse pathways (including ZLD) and requisite pre‑treatment (Lenntech); the same precipitation/clarification train provides essential pretreatment ahead of membranes within broader membrane systems.
Operations, energy, and compliance margin
Chemical consumption is moderate: on the order of 0.1–0.3 kg Ca(OH)₂ and 0.05–0.2 kg FeCl₃ per m³ treated. The main byproduct—metal hydroxide sludge—is small, at <2% of blowdown volume, and is readily handled by the mill’s sludge processing. No specialized membranes or high‑voltage equipment are needed; energy use is small (primarily pumps and mixers) compared with electrocoagulation or thermal processes (MDPI).
Performance is robust: leaving final Zn <1 mg/L and PO₄ <0.5–1 mg/L readily meets typical effluent limits (JRTPPI) (ESCAP/UN). Dissolved solids and hardness remain elevated, but are addressed by the existing mill effluent plant via conventional clarifiers or softening. Final pH lands in the 6–9 compliance window (Indonesia’s required range is 6–9; ESCAP/UN), and overall the train removes inhibitors and nutrients from blowdown, reducing eutrophication risk and heavy‑metal load by over 90%.
Alternatives and literature signals
Conventional coagulation routinely delivers ≥90% Zn and P removal in pilot tests (JRTPPI) (MDPI). Electrocoagulation can achieve 99.5% silica removal but at energy >0.2–3.0 kWh/m³ (MDPI), whereas this chemical approach trades some efficiency for lower cost and complexity.
For scale, global pulp/paper effluent has been estimated at ≈40×10⁹ m³/yr (FAO/Unasylva), underscoring the significance of mainstreaming such blowdown treatment. Cooling tower water programs and discharge rules continue to evolve (Power Engineering) (Power Engineering), and reuse trends in water‑stressed regions continue to push pretreatment and membrane adoption (MDPI).
