Heat Recovery Steam Generator (HRSG) blowdown carries phosphate and zinc at levels that blow past discharge limits. The data say a high‑pH lime program can remove 95–99% of both — with RO and ion exchange as targeted polishers.
In power plants, HRSG (Heat Recovery Steam Generator) blowdown is a small stream with outsized compliance risk. Sampling cited in a recent patent shows phosphate at 20–30 mg/L PO₄ in HRSG blowdown (and ~10–15 mg/L in boiler blowdown) (patents.google.com). Zinc can also spike; an Indonesian ZnO plant’s effluent clocked Zn at 79 mg/L before treatment (jrtppi.id). Those numbers dwarf typical limits, pushing plants toward phosphate and heavy‑metal removal to head off eutrophication and toxicity.
Indonesia’s MoE Reg. 5/2014, for instance, sets strict metals controls. In one study, treated Zn reached ≈3.7 mg/L — compliant with local limits — after a pH program (jrtppi.id).
Phosphate removal by chemical precipitation
The principal lever on dissolved phosphate is chemical precipitation. Dosing lime (Ca(OH)₂) lifts pH and forms insoluble calcium phosphates (e.g., Ca₃(PO₄)₂). In controlled tests on synthetic wastewater, Hosni et al. found a Ca:P molar ratio ≈2.07 delivered ~98.9% PO₄ removal; removal rose steeply with dose, jumping from ~1.6% to 35% as Ca/P increased from 0.3 to 0.6, and crossing ≥50% once Ca/P>1 (researchgate.net) (researchgate.net).
In practical terms, running pH 9–10 with Ca:P ≈1.5–2 routinely yields ~99% phosphate removal in ≤30 minutes, with bench tests showing residual PO₄ below 0.1–0.5 mg/L after optimization (researchgate.net) (researchgate.net). A greenhouse discharge case reached ~99% removal at Ca:P 1.5 using hydrated lime once pH rose from ~8.6 to 9.0 (pubmed.ncbi.nlm.nih.gov).
Industrial blowdown behaves similarly. Chen et al. showed that dosing Ca(OH)₂ to pH ~10.5 generated abundant CaCO₃ and Ca₃(PO₄)₂ solids with very large flocs and excellent settling; under these conditions, heavy metals and phosphate fell to “extremely low” concentrations (ccsenet.org). Accurate chemical feed helps hit those targets; plants typically rely on an inline pH program with an accurate chemical dosing pump, then settle solids in a downstream clarifier.
Alternative precipitants are common, too. Aluminum or ferric salts form AlPO₄ or FePO₄ and add a coagulant boost in municipal service, though in alkaline or recycle contexts lime is often favored for both phosphate precipitation and pH buffering (activatedsludgeguide.com) (activatedsludgeguide.com).
Zinc and heavy metals via pH adjustment
Heavy metals in blowdown — notably Zn — are most economically removed by hydroxide precipitation after pH adjustment. Zn(OH)₂ forms strongly around pH 8–10; the sweet spot is near pH 9.5. In a controlled study, adjusting a Zn‑bearing wastewater to pH 9.5 with Ca(OH)₂ delivered the best Zn precipitation (researchgate.net) (researchgate.net).
In the Indonesian ZnO case, raising pH to 9.5 with Ca(OH)₂ and adding 50 mg/L of an organic coagulant precipitated Zn from 79 mg/L to 3.71 mg/L — a 95.3% removal — while turbidity and COD dropped by >99% and 70%, respectively (jrtppi.id). Results confirm the steep pH dependence: above ~9, Zn rapidly falls as Zn(OH)₂; above ~10.5, amphoteric behavior can redissolve Zn, making ~9–10 optimal (jrtppi.id).
The upshot: typical Zn removal exceeds 90–95% at pH ≈9.5, meeting even strict standards (many jurisdictions require ≤5 mg/L Zn). Coagulation aids floc formation; plants commonly dose a polymer from a coagulant program and, where needed, add a flocculant to improve settling. Note: once removed, Zn hydroxide sludge must be handled.
Other technologies and integrated approaches
Beyond straight precipitation, membranes and ZLD (zero‑liquid discharge, minimizing liquid waste) can push recovery higher. A power‑plant demonstration achieved 96% recovery of cooling‑tower blowdown using advanced RO paired with a crystallizer (watertechonline.com). For boiler blowdown, high‑temperature RO that can handle ~80 °C feed has shown promise in pilots, yielding low‑salinity permeate while concentrating precipitable solids (scribd.com). Plants often pair precipitation with an RO stage — for example a brackish‑water RO — and use ultrafiltration pretreatment to protect membranes.
Ion exchange or adsorption can polish residuals to very low levels. Chelating resins and specialty media can drive metal concentrations toward <1 mg/L; carbon or granular ferric hydroxide can target trace phosphate or other anions. These are typically secondary steps, trading higher material cost for “finishing” quality. Options include a packaged ion‑exchange system or targeted media in an ion‑exchange resin train; activated organics are addressed with activated carbon.
Where ammonia is present, adding Mg²⁺ can form “struvite” (NH₄MgPO₄·6H₂O), removing PO₄ while recovering a fertilizer product. This is routine in municipal sidestreams; for boiler blowdown it applies only if feedwater treatment leaves significant ammonia and magnesium.
Design targets and operational controls
The design playbook is data‑driven: target ~pH 9–10 with Ca(OH)₂ (or NaOH), keep Ca:P at ≈1.5–2, and provide mixing/settling time of ~20–30 minutes. This co‑precipitates calcium phosphates and metal hydroxides into flocculated sludge (with very large flocs noted at pH ~10.5), which settles in conventional clarification (ccsenet.org). Inline monitoring of pH and residual phosphate or Zn guides chemical dosing. Membrane filtration can then concentrate or recycle blowdown — as shown in the 96%‑recovery RO case (watertechonline.com) — with modular membrane systems configured for RO, NF, or UF.
For Indonesian plants, the evidence is encouraging: MoE discharge limits (e.g., Zn ~5 mg/L in Permen 5/2014) are reachable — one documented case lowered Zn from 79 to 3.7 mg/L at pH 9.5 with lime and a light coagulant dose (jrtppi.id). Phosphate limits are generally more lenient, yet the lime data show similarly robust reductions. These interventions are technically mature and quantifiable.
What the numbers add up to
Across the datasets, high‑pH lime hits both targets at once: ≥95–99% removal of phosphate and zinc. Specifically, Ca:P ≈1.5–2 delivers ~99% P removal (residual ~0.1–0.5 mg/L) (researchgate.net) (researchgate.net) (pubmed.ncbi.nlm.nih.gov), while pH ≈9.5 yields ~95% Zn removal and regulatory compliance (~3.7 mg/L from 79 mg/L) with a small coagulant assist (50 mg/L) (jrtppi.id) (researchgate.net). Fail to hit those pH/Ca:P marks, and removal falls off sharply (researchgate.net) (jrtppi.id).
The bottom line for HRSG blowdown: set pH and calcium right with lime or NaOH, use precipitation as the workhorse, then polish smartly where the economics justify it. The combination has been demonstrated from bench tests to full‑scale case studies — and it’s ready to implement.
