The new water source for wet scrubbers: cooling tower blowdown

When a coal plant bolts on a wet FGD scrubber—a system that uses water and lime/limestone to pull SO₂ out of flue gas—it also bolts on a new thirst. For a 500 MW unit, FGD typically needs about 110–130 m³/hour (≈0.2–0.3 m³/MWh), and adding a wet scrubber often raises plant water use by ~10–15% (powerline.net.in) (powerline.net.in).

Regulators have noticed: India now allows ~3–3.5 m³/MWh water use to accommodate FGD (powerline.net.in). In Indonesia, coal plants collectively consume on the order of 2.22×10^8 kL/year, so any auxiliary reduction is material (ije-pyc.org).

Yes, semi-dry scrubbers can trim water needs by ~60%, and dry sorbent injection uses virtually no water, but many fleets are already committed to wet retrofits (powerline.net.in) (powerline.net.in).

Internal wastewater reuse as makeup

A growing tactic is to recycle in‑plant streams as scrubber makeup. One assessment notes that shutdown waters—“FGD systems, ash handling systems, cooling tower blowdown, etc.”—“may be options for reusing certain plant wastewater streams” (watertechonline.com).

There’s a practical reason: wet scrubbers can accept “relatively poor quality water” to cover evaporative losses, so long as chemistry is managed (watertechonline.com).

Cooling tower blowdown (CTBD) as a source

Cooling towers bleed roughly 10–30% of makeup water to control mineral buildup, producing CTBD that’s high in hardness, alkalinity, and dissolved solids. At the Gaojing plant in China, reported CTBD chemistry was: Cl⁻ ≈182–336 mg/L, SO₄²⁻ ≈186–407 mg/L, SiO₂ ≈11.8–33.4 mg/L, and total hardness ~10–16 mmol/L (100–160 mg/L as CaCO₃) (paperzz.com).

Directly piping that into a limestone scrubber would invite scaling and chloride buildup. But targeted treatment works. In Gaojing’s case, a train of multimedia filtration, ultrafiltration (UF, a membrane pretreatment) and two‑pass reverse osmosis (RO) produced water clean enough for the boiler, with electrodeionization (EDI) as the final polish (paperzz.com). Plants commonly deploy UF via ultrafiltration modules to protect downstream membranes, then apply high‑rejection units such as brackish-water RO before finishing with EDI when ultra‑low conductivity is needed.

Specialized high‑pH RO processes have demonstrated >1500 mg/L silica tolerance and a 20–30× reduction in blowdown volume (wateronline.com). In a recent pilot in Chile, a single‑stage RO plus a salt‑precipitation unit treated CTBD at 96% recovery, enabling near‑closed‑loop cooling (watertechonline.com). In practice, feeding treated CTBD to the FGD absorber can displace fresh water roughly equal to the blowdown flow (often 5–15% of plant cooling intake).

To support such recoveries, operators often add antiscalant programs alongside RO; in those cases, membrane health can benefit from targeted chemistry like membrane antiscalants without changing the underlying process described in the case studies.

Boiler blowdown and flue-gas condensate

Boiler blowdown—typically 1–5% of boiler feed—is smaller but hotter and more concentrated, with treatment chemicals like phosphates and sulfites. One scheme sends the stream through a feedwater preheater and a Pelton turbine for heat recovery, then into an RO unit to yield “clean water” (ije-pyc.org). Once cooled and deoxygenated, that product can be directed to scrubber makeup; the blowdown first needs neutralization and filtration, and if fully recycled to FGD, impurities such as sodium and chloride may accumulate—necessitating periodic bleed or polishing.

Another niche source is flue‑gas condensate from a flue‑gas condenser or a wet ESP (electrostatic precipitator). Because it’s essentially condensed steam and SO₂, it’s very pure; it generally only requires pH neutralization with lime or caustic to be compatible with the absorber slurry. The volumes are modest but can offset fresh water on a one‑for‑one basis.

Other streams are more limited. Treated plant wash‑down or stormwater could, in principle, supply FGD makeup, but they demand extensive treatment and are often reserved for non‑contact uses. (For example, Indonesian regulations do not explicitly govern industrial reuse【‡】, so any reused water must still meet general effluent standards.)

Pretreatment and polishing requirements

Whether the source is CTBD, boiler blowdown, or condensate, the treatment targets are the same: remove suspended solids and scale‑forming ions before the absorber.

Solid removal comes first. Multimedia filtration and ultrafiltration knock out suspended particulates and colloids; in Gaojing, turbidity dropped from >20 NTU to <0.4 NTU before RO (paperzz.com). Plants typically use granular beds such as sand/silica media, with top layers of anthracite to improve depth filtration.

Precipitative softening follows for high‑hardness streams. Lime–soda or caustic softening at ~pH 9–10 precipitates calcium and magnesium, and removes a large fraction of silica and phosphate—slashing hardness and alkalinity (wateronline.com). Standard guidance also notes that FGD bleed can be treated “by chemical precipitation (lime/soda, filtration…)” to make it reusable (nepis.epa.gov). Raising CTBD to pH ~10, for instance, has precipitated ≈50–70% of silica alongside hardness (wateronline.com). Softening trains frequently include automated chemical feeds via a dosing pump, and where permanent softening is desired, a packaged softener system can be integrated without changing the underlying chemistry described.

After softening, sludge is removed by settling and fine filtration. Conventional clarification—using a gravity clarifier—is common; coagulants and flocculants are dosed to aggregate fine colloids and then clarified out. Plants typically reach for a primary coagulant from ranges like PAC/ACH coagulants, followed by a high‑molecular‑weight aid from flocculants for faster settling.

Polishing steps target dissolved ions. If further demineralization is required, a weak‑acid cation resin can remove residual Ca/SO₄ using ion exchange equipment charged with the appropriate ion-exchange resin. Alternatively, nanofiltration or RO strips dissolved salts; if chloride exceeds ~5000 mg/L—a corrosion threshold—EPA guidance recommends post‑softening RO (nepis.epa.gov). Many blowdown reuse systems rely on RO (as at Gaojing), with advanced designs using high‑shear RO and a downstream precipitant achieving >90% salt recovery in demonstrations (watertechonline.com). For selective hardness removal at lower pressures, plants may also consider nano-filtration without altering the process intent described in these references.

Chemical conditioning closes the loop: adjust pH to ~7–8 and remove residual oxidants or biocides so FGD chemistry stays stable. Because CTBD often contains antiscalants or biocides that persist as organics, these can be oxidized or adsorbed if necessary; adsorptive beds such as activated carbon are frequently paired with a cartridge filter for fine capture without changing any of the treatment steps cited.

Combining these steps yields recycled water that meets FGD needs. The Gaojing sequence—filtration, UF, dual‑pass RO, and EDI—raised CTBD all the way to boiler‑feed quality (paperzz.com). At a minimum, many plants will deploy media filtration and lime softening before routing CTBD to the absorber. The governing principle is water‑quality limits: FGD can run on “poor quality” makeup, but each source must be checked—chloride, silica, and more—and treated to avoid excessive corrosion or bleeding (watertechonline.com) (nepis.epa.gov).

Water balance impacts and case outcomes

Blowdown reuse can move the needle on fresh intake. As a rule of thumb, each 10× increase in cooling tower cycles cuts blowdown by ~90%; in the Chile RO case, boosting cycles from 2 to 10 slashed blowdown ~96% (watertechonline.com). On a 500 MW unit with ≈120 m³/h FGD water demand (powerline.net.in), even a 25% reuse of the ~20% CT blowdown stream saves ~24 m³/h (~576 m³/day) of fresh water. For a 2×660 MW plant like Sumsel‑8, roughly 396 m³/h FGD flow is expected; reusing, say, 20% from CT blowdown cuts fresh intake by ~80 m³/h (~1900 m³/day).

The dollars add up. One EPA analysis values recycled cooling water at about $1 per 1000 gallons (≈$0.26/m³), so hundreds of cubic meters per day translate directly into lower procurement and discharge costs (nepis.epa.gov).

Real‑world outcomes bear this out. At Gaojing, CT blowdown reuse supplied 40–100% of boiler makeup, effectively eliminating the need for new water (paperzz.com). In Chile, 96% CTBD recovery practically halted freshwater use for cooling (watertechonline.com). In each case, treatment enabled near‑closed‑loop operation. Although FGD wastewater still requires treatment (scrubber bleed and resulting gypsum), recycling makeup water trims overall plant effluent and reduces the need to mine fresh supplies.

As regulations tighten, plants are adopting holistic water balances where CT blowdown, FGD bleed, ash pond water, and treatment‑plant effluents are all reused or mitigated as part of one plan (watertechonline.com) (watertechonline.com). Where space is constrained, compact settlers like a lamella settler can reduce clarifier footprint without changing any of the steps or outcomes documented above; and where membrane assets are critical, specifying materials—such as Filmtec RO elements or Toray membranes—aligns with the UF/RO/EDI trains cited in these case studies without altering their performance claims.