Power plants are chasing “zero liquid” — here’s the gritty chemistry making it possible

Cooling water is the thirstiest part of a power plant. Industry sources put it at roughly 90% of total use (watertechonline.com). The unavoidable byproduct is cooling-tower “blowdown” — the concentrated bleed-off from recirculating water — which typically runs 3–5% of makeup at 3–5× cycles of concentration. Its total dissolved solids (TDS) can climb to several times the makeup water (e.g., 3–7×).

One model CCGT (combined cycle gas turbine, a gas turbine with a steam cycle recovering waste heat) taking 590 m³/h (2,600 gpm) makeup discharged about 124 m³/h (544 gpm) of blowdown, roughly 21% of intake (nsenergybusiness.com). That stream carries everything concentrated in the tower — salts, hardness, and additives like phosphates and zinc from corrosion control — straight to the permit writer’s desk.

Those permits are tightening. Indonesia’s Permen LH 8/2009 requires pH 6–9, free chlorine ≤1 mg/L, Zn ≤1 mg/L, and phosphate ≤10 mg/L for untreated cooling-tower blowdown (scribd.com). U.S. EPA proposals similarly call for Zn ≤1 mg/L and Cr ≤0.2 mg/L in cooling-water effluent (power-eng.com). One case study even found tertiary-treated makeup arriving with 5–20 mg/L phosphate (power-eng.com). Untreated, that inflates PO₄ and Zn in blowdown, elevating risks like [algae blooms](https://beta.co.id/en/blog/the-algae-reckoning-in-farm-ponds-whats-working-now) or toxicity. The design brief: reduce Zn and PO₄ and tune pH to hit those limits.

Chemical precipitation for metals and phosphate

The baseline train is chemical neutralization and precipitation. Blowdown often arrives slightly alkaline; operators typically [raise pH to about 9–10 with lime](https://beta.co.id/en/blog/ccgts-wastewater-reckoning-lime-for-phosphate-highph-for-zinc) (Ca(OH)₂) or caustic, forming insoluble metal hydroxides such as Zn(OH)₂, Fe(OH)₃, and Cu(OH)₂ for removal (power-eng.com). Dosing iron or calcium — for example, ferric salts or lime — precipitates phosphate as iron or calcium phosphates (power-eng.com).

Coagulants and polymers tighten up fines for settling, with removal typically above 90% for zinc and phosphorus; simply holding pH near ~9.5 can drop most Zn out (power-eng.com). Under these conditions, treated blowdown routinely meets Zn ≤1 mg/L and PO₄ ≤10 mg/L in Indonesia (scribd.com). Some operators avoid phosphate-based cooling chemistry altogether to simplify discharge compliance (power-eng.com).

On the membrane front, plants increasingly slot in high-recovery membrane systems downstream of precipitation to push reuse.

Stepwise primary treatment train

1. Pre-treatment: oil and debris come off first via skimming and coarse separation. For this duty, packaged screens and oil-removal systems in primary separation skids are common. Free chlorine, if present, is quenched before metals recovery using a dechlorination agent.

2. pH adjustment: lime or caustic raises pH to roughly 9.0–9.5 to precipitate Zn²⁺, Cu²⁺, and Fe²⁺/³⁺ as hydroxides. U.S. studies note proposed limits of only 1 ppm (mg/L) zinc in blowdown (power-eng.com), so dosing is controlled with a dosing pump to stay in the sweet spot.

3. Phosphate precipitation: calcium or ferric additions (e.g., Ca(OH)₂ or ferric chloride) bind PO₄³⁻ into insoluble solids (power-eng.com).

4. Flocculation and clarification: polymers consolidate fines for settling; plants often standardize on a primary flocculant program. Clarified effluent leaves a dedicated clarifier, then runs through media filtration to polish turbidity; dual-media beds with sand/silica media are typical.

5. Post-treatment: operators verify pH and nudge it back to a neutral 6.5–8.5 window for discharge. Where trace organics persist, a pass through activated carbon is added; for trace metals or final polish needs, plants deploy ion exchange resins.

Conventional precipitation-plus-clarification trains have repeatedly shown >95% removal of zinc and phosphorus to meet numeric limits (power-eng.com) (power-eng.com). Sludge — a mix of metal hydroxides and phosphates — is then dewatered and either disposed of or recycled separately.

Membranes and reuse pathways

Where water recycling is prioritized, membranes follow the clarifier. Ultrafiltration (UF, a fine physical barrier for colloids) protects downstream desalination; plants typically drop a dedicated UF stage in front of reverse osmosis (RO, a high-pressure membrane that removes dissolved salts). The RO element is commonly a brackish-water RO tailored for industrial salinity.

A recent demonstration by IDE Technologies ran at roughly 96% recovery on cooling-tower blowdown using an advanced single-stage RO with a cascade mode — far above the ~55–60% recovery typical of conventional RO (watertechonline.com) (watertechonline.com). The clean RO permeate is reusable as cooling makeup or even boiler feed with polish.

Concentrated RO brine remains. Many designs send it to a brine concentrator or evaporator. At Indiantown, FL (360 MW), a full-scale system routes 650 gpm of cooling blowdown through twin brine concentrators and a spray dryer; all distillate returns as boiler water and the reject becomes dry solids (watertechonline.com). Thermal concentrators typically raise salt content to about 20–25% solids before final crystallization (nsenergybusiness.com).

Emerging concentration technologies

Electrocoagulation (EC, an electrical method that generates coagulant in situ) has cleared more than 90% of hardness in blowdown ahead of RO, maintaining membrane flux with minimal scaling (powermag.com). Vibration-enhanced membranes (VSEP) and membrane distillation (MD) also appear in pilots; one review reported EC/VSEP achieving about 99.5% dissolved silica removal from concentrated waters, at energy inputs of roughly 0.2–3 kWh/m³ for EC (mdpi.com).

Tying it together, a precipitation→UF→high-recovery RO→thermal concentration train can push liquid waste toward zero: a 600‑ton cooling system pilot used EC+UF+RO+vacuum evaporation to recover virtually all salts (powermag.com) (powermag.com). In that program, RO plus distillation removed roughly 98–99% of dissolved solids even at raw TDS near 100–186 g/L, producing a salt cake of about 80% solids (powermag.com).

ZLD trade-offs in arid regions

In very arid settings, discharge can be prohibited or water prohibitively scarce. Zero liquid discharge (ZLD, a treatment scheme that recycles all wastewater and converts the remainder to solids) becomes a strategic option. An NGCC analysis found ZLD could cut water withdrawals by about 18% relative to a standard setup, but it roughly doubled the levelized cost of water and required about 0.1–0.8% of plant generation for treatment energy (pubs.acs.org).

Operationally, ZLD has been shown to lower plant waste from roughly 21% of intake to under 0.1% (nsenergybusiness.com), though net water savings can be only about 20% because most water is lost across condensers (nsenergybusiness.com). In one model, stacking high-recovery RO with brine concentration drew less than 0.8% of plant electricity yet still doubled water costs; attempts to lean on evaporation ponds actually raised unit water costs, indicating high recovery is economically optimal despite energy use (pubs.acs.org).

Case studies echo the boundaries. An advanced single-stage RO demonstrated about 96–97% blowdown recovery, leaving only 3–4% brine (watertechonline.com), versus ~60% for conventional RO (watertechonline.com). Full ZLD at the Indiantown, FL cogeneration plant (360 MW) sends 650 gpm of blowdown through brine concentrators and a spray dryer; only dry solids leave the boundary (watertechonline.com).

For perspective, air-cooled condensers can save more water overall — greater than 80% of makeup — than ZLD can, but with higher capital costs and efficiency penalties (nsenergybusiness.com). In water-plentiful regions, ZLD often fails the cost test; under severe scarcity or strict zero-discharge permits, it can make sense (pubs.acs.org).