Power plants are redesigning cooling‑water blowdown treatment to strip out zinc and phosphorus and, in arid regions, to flirt with zero liquid discharge — even as studies warn the water savings can be only ~20% of intake.
Cooling towers concentrate minerals as water evaporates. Typical blowdown total dissolved solids (TDS) is relatively low — often less than 10,000 mg/L — and dominated by sodium chloride/sodium sulfate (NaCl/Na₂SO₄), with lesser calcium and magnesium ions (Power Magazine).
The chemistry programs that protect towers add their own signature: industrial reports note common dosing adds roughly 5–15 mg/L phosphate (PO₄³⁻) and 0.5–2.5 mg/L zinc; in practice blowdown may carry several mg/L phosphate and about 1 mg/L zinc — levels that can exceed strict limits by a wide margin (ChemEngOnline).
Regulators are not aligned. Indonesia’s effluent rules for cooling-water blowdown limit temperature (≤40 °C) and free chlorine (≤0.5 mg/L) but set no explicit zinc or phosphate caps (Permen LH 8/2009), a gap noted by local analysts (Mongabay Indonesia). In the U.S., proposals would cap total zinc at 1.0 ppm and chromium at 0.2 ppm in cooling‑tower discharge (POWER Engineering), and several states seek to eliminate virtually all phosphorus to curb algal blooms (Water Technology) (POWER Engineering). Designs increasingly assume the need to remove zinc, phosphate, and sometimes copper and iron to meet tight limits (e.g., <0.1 mg/L P in sensitive waters).
Blowdown chemistry and permit context
Cooling‑tower blowdown is a mineral concentrate with TDS often below 10,000 mg/L, chiefly NaCl/Na₂SO₄, and conditioned by corrosion inhibitors that routinely add 5–15 mg/L phosphate and 0.5–2.5 mg/L zinc — explaining measured levels of several mg/L PO₄ and ~1 mg/L Zn in discharge (Power Magazine) (ChemEngOnline).
Indonesia’s current cooling‑water blowdown parameters address temperature (≤40 °C) and free chlorine (≤0.5 mg/L), but do not prescribe zinc or phosphate limits (Permen LH 8/2009), a gap highlighted by environmental advocates concerned about coastal and marine ecosystems (Mongabay Indonesia). By contrast, U.S. trends include a 1.0 ppm limit for zinc and 0.2 ppm for chromium in new permits (POWER Engineering), and state‑level pushes to slash phosphorus (Water Technology) (POWER Engineering).
pH equalization and chemical precipitation
Designs start with a mixed or equalization tank to stabilize flow and pH, followed by pH neutralization. If blowdown is too alkaline (common where phosphate has replaced older chemistries), dilute sulfuric acid (H₂SO₄) is added to near‑neutral pH≈7. If too low, sodium hydroxide or lime raises pH. Real‑time pH control typically holds effluent within 6–9, the range often required; Indonesian guidance for many effluents is 6–9 pH, though not explicitly set for cooling blowdown [38†L492-L499]. Automated dosing is commonly executed via a dosing pump.
Phosphate removal is achieved by chemical precipitation/coagulation: ferric chloride or alum (aluminum sulfate) is dosed so ferric/aluminum ions form insoluble iron‑ or aluminum‑phosphate. Bench tests and field experience show >90% phosphate removal; industry examples have reduced 5–15 mg/L orthophosphate to well below 0.1 mg/L after coagulation (Water Technology). Coagulant addition fits with commodity programs such as inorganic coagulants.
Metals are then removed by hydroxide precipitation: raising pH to roughly 9–10 (with lime or caustic) drives Zn²⁺ (and Cu²⁺, Fe³⁺) to form metal hydroxides. In practice, >95% zinc removal is typical, with residuals below 0.1 mg/L — sufficient for common discharge limits (POWER Engineering) (Water Technology). For extremely tight goals, a sulfide floc (e.g., dosing sodium sulfide, Na₂S) can convert Zn/Cu to highly insoluble sulfides — a method used in mercury service — but at added complexity (Water Technology). Coagulant aids such as flocculants improve settling.
Clarification, filtration, and polishing targets
Settling removes the metal‑phosphate floc as sludge. Plants typically employ a conventional clarifier or a compact lamella settler. The clarified effluent is filtered — often through sand media — to reach total suspended solids (TSS) ideally below 10 mg/L before final pH trim to 6–9. Dual‑media beds such as sand/silica filters are standard for this duty.
Company experience suggests well‑designed precipitation trains routinely achieve >90% removal of zinc and phosphate. For example, removing 2 mg/L Zn (hydroxide precipitation) and 10 mg/L PO₄ (ferric coagulation) can yield final concentrations below 0.1 mg/L for each, with verification by ICP (inductively coupled plasma) and colorimetric assays (ChemEngOnline) (Water Technology). A ZLD demonstration recorded residual phosphate stabilized at approximately 0.75 mg/L in an RO concentrate stream (Water Technology case study).
Performance goals typically read: zero discharge violations; zinc <1.0 mg/L; phosphate <0.5 mg/L (or lower); TSS 3–5 mg/L; pH 6–9. Jar testing often sets chemical usage in the range of 10–30 mg/L coagulant and 0.5–2 mg/L polymer. Operating results are tracked toward >95% zinc removal and >99% phosphate removal so that a blowdown at pH 10 can be finished at below 0.1 mg/L Zn and <0.1 mg/L PO₄.
Cycles of concentration and flow sizing
The cycles of concentration (COC) directly shift plant hydraulics: higher COC cuts blowdown volume but pushes contaminant concentrations upward. One example with 400 mg/L makeup TDS and a 1,000 mg/L discharge limit yields COC≈3 (giving ~67% blowdown as fraction of flow) (Water Technology).
Another design comparison sized blowdown at approximately 500 GPM at 8 COC versus 200 GPM at 3 COC — a reminder that treatment plants must be built for peak rates (Water Technology).
Sludge handling and disposal classification
The clarified solids — a sludge of metal (hydr)oxides and phosphate compounds — are removed and dewatered. The precipitated solids will be ~~dense, with heavy metal content~~ and must be disposed per hazardous‑waste rules if metals are significant. Consulting Indonesian regulations (PP63/2019) may classify the sludge as B3 if zinc or other toxic metals exceed thresholds. Dewatering equipment such as a filter press is common.
Modern designs that integrate crystallization can produce pellets exceeding 95% dry solids, minimizing downstream handling; one case reported pellets drained to >95% solids, avoiding thickening or centrifuging and yielding a landfill‑ready solid. Achieving such dryness (>90%) is ideal to reduce disposal volume (Water Technology case study).
High‑recovery membranes and crystallization (ZLD)
In very water‑scarce settings, plants consider zero‑liquid‑discharge (ZLD) layouts to reclaim virtually all blowdown. A common scheme employs ultrafiltration (UF) to remove polymer solids, dechlorination via bisulfite, softening to strip calcium/magnesium, pH elevation to approximately 10 to keep silica soluble, then two‑pass reverse osmosis. Vendors style these high‑recovery RO variants as HERO/Opus. UF pretreatment is often implemented with ultrafiltration modules, while dechlorination can use a dechlorination agent. Softening is typically delivered by ion exchange, and the RO platform is commonly brackish‑water RO for maximum TDS around 10,000 mg/L (Water Technology).
This approach can recover about 90% of blowdown as permeate; the remaining 10% is RO brine sent to a thermal evaporator or crystallizer. Electrical mechanical vapor recompression (MVR) or falling‑film evaporators concentrate brine to solids, and pilot programs have demonstrated >90% water recovery. In a water‑stressed Chilean plant, an advanced RO plus fluidized‑bed precipitator achieved 96% blowdown recovery, with crystalline pellets draining to >95% solids and phosphate/silica removed early to enable very high recovery (Water Technology case study) (Water Technology case study). Integrated membrane systems are central to these trains.
Water savings, costs, and energy penalty
Modeling warns the freshwater gain can be modest because evaporative losses remain irrecoverable. Studies suggest ZLD can reduce freshwater use by only ~18–20% compared to standard towers (ACS ES&T Engineering) (NS Energy). One analysis projected a modern NGCC plant uses about 590 m³/h raw water and wastes 124 m³/h (21% loss). Incorporating ZLD could cut that waste to approximately 0.1% of intake, but net intake savings are only ~20% — even a “perfect” ZLD recovers the 21%, leaving total consumption roughly 79% of baseline (NS Energy).
The same case study found ZLD roughly doubled the levelized water cost for the plant, with an energy penalty below 0.1% of generation for RO‑only ZLD and around 0.8% if a brine concentrator is used (ACS ES&T Engineering). Complexity compounds costs: ZLD plants must run at tightly controlled flow and chemistry, requiring continuous monitoring (NS Energy). Even after 90–99% recovery, the residual brine (~1–5% of inflow) must be managed — either with zero‑discharge crystallizers producing solids or via permitted evaporation ponds. Deep‑well injection is rarely permitted for cooling‑tower discharge; trucking off‑site is possible but expensive (Water Technology).
Site conditions and technology selection
In arid regions (e.g., the Middle East, U.S. Southwest), reuse of high‑quality RO permeate as makeup can relieve local stresses, but alternatives may outperform ZLD on water savings: dry‑cooling or air‑cooled condensers reduce nearly 100% of cooling losses, albeit with efficiency and capital penalties (NS Energy).
For plants pursuing ZLD, piloting is recommended. A two‑step pathway is common: conventional RO to recover ~80–90% of water, followed by an evaporator or crystallizer for the remaining concentrate (Water Technology). Advanced designs integrate a salt‑precipitation clarifier ahead of RO to remove silica/calcium and enable >96% recovery (Water Technology case study). Several vendors have packaged power‑specific systems (e.g., Aquatech’s HERO, Veolia’s Opus) combining UF → softening → pH control → high‑recovery RO → evaporation/crystallization, with reported discharge cuts >90% and demonstrations at 96–97% water recovery (Water Technology case study) — though with high system complexity and cost (ACS ES&T Engineering).
Design parameters and system choices
For an HRSG plant, a robust blowdown treatment uses staged clarification: equilibrate pH, then precipitate phosphate (e.g., ferric chloride flocculation) and metals (via high‑pH hydroxide precipitation), followed by sedimentation and filtration (Water Technology) (POWER Engineering). Treated effluent should aim to meet the strictest plausible standards — zinc ≤1.0 mg/L and phosphate near zero — even where local law lacks such limits; this protects sensitive waters and anticipates future rules (Water Technology) (POWER Engineering). Supporting gear such as water‑treatment ancillaries underpins monitoring and control.
For arid‑zone or regulation‑driven projects, a ZLD‑enhanced layout — UF + two‑pass RO + evaporators/crystallizers — can lift overall blowdown recovery to roughly 95–96% (Water Technology case study) (ACS ES&T Engineering). Still, data show ZLD roughly doubles water‑treatment cost while saving only ~20% of intake — suggesting tropical Indonesia would adopt it only if mandated or if local water pricing forces the issue; in very arid markets, it may be cost‑justified (ACS ES&T Engineering) (NS Energy). All final decisions should be guided by site‑specific blowdown chemistry, water availability, discharge standards, and proven pilot studies or vendor guarantees that cite performance statistics.
