Cooling‑tower blowdown is a small stream with big consequences: concentrated salts, metals, and treatment chemicals that must hit tight discharge limits — or be recycled entirely. New designs show how pH tuning, precipitation, and membrane concentration can close the loop.
Start with the math: most evaporative cooling towers (CTs) run at 3–4 cycles of concentration (COC, the ratio of dissolved solids in recirculating water to makeup). That means blowdown total dissolved solids (TDS) rises to roughly 3–4× the source water level (Power).
What’s in that stream? One study found ~1.2 mg/L zinc (Zn) and ~6.6 mg/L phosphate (PO₄) (MDPI), with scale‑forming species like iron oxide, calcium phosphate, calcium carbonate, magnesium silicate, and other silica/hardness compounds (MDPI).
Regulators are paying attention. Indonesia’s Permen LH No.8/2009 caps blowdown at pH 6–9 and free Cl₂ ≤1 mg/L (Permen LH 08/2009), with Zn ≤1 mg/L and PO₄ ≤10 mg/L (Permen LH 08/2009). U.S. proposals likewise target Zn ≲1 ppm and sharply restrict phosphorus (Power Engineering; Power Engineering). Advanced rules also consider metals like Cu and Cr, ammonia, and TDS (Power Engineering).
Design basis and pH control
The workhorse of blowdown treatment is pH adjustment (acidity/alkalinity level on a 0–14 scale) to neutralize and selectively precipitate contaminants. Plants target ~6.5–8.5 for neutralization, then often raise pH above ~9 to push metals and hardness into insoluble hydroxides and carbonates. In one case, shifting from pH 8.6 to ~10 lifted total hardness removal from ~40–64% to ~72–95% — a major boost attributed to high‑9s operation (Power). Zinc removal benefits too, as Zn(OH)₂ forms around pH ~9–10.
Accurate chemical feeds anchor this step; many facilities pair automated controls with a dosing pump to meter lime or caustic soda.
Metal precipitation and clarification
Once pH is set, dissolved transition metals (e.g., Zn²⁺, Cu²⁺) precipitate and can be swept out with coagulants. Aluminum or ferric salts build robust metal‑hydroxide flocs that settle quickly (Power Engineering). Electrocoagulation (EC, an electrochemical floc formation method) has also been reported to remove Zn — and silica — very effectively in the literature (MDPI). Conventional chemical precipitation with Ca(OH)₂ or NaOH remains common to meet the Zn ≤1 mg/L limit (Power Engineering; Permen LH 08/2009).
For chemical aids, plants often standardize on a coagulants program; solids are then separated in a clarifier before filtration.
Phosphate removal strategy
Because corrosion inhibitors can contain phosphate, PO₄ removal is essential to hit caps like ≤10 mg/L (Permen LH 08/2009). The standard playbook is precipitation: ferric chloride or aluminum sulfate to form insoluble Fe/Al phosphates, or lime to form calcium phosphate. Some systems tune pH and add magnesium to crystallize struvite (magnesium–ammonium–phosphate) for phosphorus recovery (HM Cooling Tower). In parallel, some plants evaluate phosphate‑free chemical programs to shrink the load at the source (Power Engineering).
Where phosphate originates from inhibitor packages, specifiers also review options like corrosion inhibitors with alternative chemistry.
Solids separation and filtration
Hydroxide and phosphate precipitates, along with colloids, are removed by settling and filtration. After clarification, filters polish remaining turbidity and help plants align with total suspended solids (TSS) norms — Indonesia allows TSS ≤100 mg/L for main processes (blowdown rules focus more on solubles) (Permen LH 08/2009).
Common trains include a sand filter followed by a cartridge filter to capture fines before discharge or reuse.
Final polishing and disinfection
Where lower residuals or reuse are targeted, advanced polishing comes in. Microfiltration/ultrafiltration (UF, pressure‑driven membrane sieving) removes fine colloids; ion‑exchange or adsorption can pick up trace metals. In practice, physicochemical steps above generally meet pH and key ion standards for most permits. A UF skid often fits as pretreatment if downstream recovery is planned.
For targeted polishing, plants deploy ion‑exchange systems or media beds. Prior to outfall, final neutralization/ammoniation and dechlorination are applied as needed; a dechlorination agent is used to bring free Cl₂ to specification.
Performance snapshots and compliance
Pilot data backs the sequence: electrocoagulation plus clarification at high pH removed silica by 67–98% (Power) and achieved Zn/PO₄ reductions well above 90%. Facilities following this approach meet — and often exceed — Indonesia’s pH 6–9, free Cl₂ ≤1 mg/L, Zn ≤1 mg/L, and PO₄ ≤10 mg/L guidelines (Permen LH 08/2009; Permen LH 08/2009) and track with tightening international trends (Power Engineering; Power Engineering).
Zero‑liquid‑discharge pathway in arid regions
In water‑scarce locales, plants are pushing toward zero liquid discharge (ZLD, a process that eliminates liquid effluents by recovering water and crystallizing salts). A typical ZLD train concentrates treated blowdown via high‑recovery reverse osmosis (RO, a pressure‑driven membrane process) or evaporators, then evaporates/crystallizes the brine for solids disposal — enabling near‑100% water recovery for reuse.
An NREL analysis at combined‑cycle plants found that implementing ZLD — RO plus a brine concentrator and evaporation ponds — reduced raw water withdrawal by ~18% (ACS Publications). Switching to dry cooling saved even more water in that comparison, but ZLD matched the benefit of simply increasing CT cycles. The trade‑off is cost and energy: the levelized cost of water approximately doubled with high‑recovery RO, and energy burden — still under 1% of plant generation — is non‑trivial (ACS Publications).
One pilot using vacuum‑assisted electro‑distillation (VAED, a low‑pressure thermal separation) to treat RO brine removed 98–99% of salts, yielding clean condensate, at 1,750–620 kWh per 1,000 gal (≈460–160 kWh/m³) for feeds at 100,000–186,000 mg/L TDS (Power). The same pilot hit ~97% total water recovery in stable operation (Power).
At the simple end, arid sites with land sometimes adopt lined evaporation ponds or even deep‑well injection, though practitioners flag scale buildup and permitting challenges (Power Engineering).
For membrane‑based concentration, a brackish‑water RO train often sits at the front of ZLD, supported by pretreatment packages under a membrane systems standard.
From permit compliance to reuse
Conventional blowdown treatment — pH adjustment, targeted precipitation, and clarification/filtration — reliably meets current effluent rules (Permen LH 08/2009; Power Engineering). But as water scarcity intensifies and discharge limits tighten, near‑ZLD and full ZLD become more attractive, letting facilities reuse treated blowdown for boiler feed or service water and effectively close the loop on cooling‑cycle makeup (Power Engineering).
