With heat loads soaring and blowdown limits tightening, power plants are rewriting cooling‑tower chemistry. The data show that a coordinated blend of scale control, multimetal corrosion protection, and an assertive biocide regime can recover 6.7 MW and cut water losses — without breaching discharge caps.
Power plants with HRSGs (heat recovery steam generators) push millions of gallons of cooling water through surface condensers and cavernous cooling towers every day. One 550 MW combined‑cycle site ran ~3 million gal/day (11.4×10^6 L/d) of reclaimed water, cycling just ~2.5× before blowdown limits hit — roughly 900,000 gal/day to meet discharge TDS (total dissolved solids)/nitrite regulations — for ~70% overall recovery (Power Magazine) (Power Magazine).
The economics are stark: 70% of incoming cooling water was reused (2.1 Mgal/d), saving ~2.0 Mgal/d from discharge (Power Magazine). Every added cycle of concentration (CoC — the ratio of dissolved solids in recirculating water to makeup) slashes bleed; taking cycles from 2.5 to 5× would roughly halve blowdown. Conversely, a mere 1–2 mm of scale or biofilm can clip heat‑exchanger overall heat‑transfer coefficient (U) by 5–15%, eroding output; one cleanup overcame a 0.85″ Hg vacuum loss, boosted condenser tube cleanliness by about 15%, and recovered 6.7 MW of steam‑turbine output — worth ~$1.6 million/year (Power Magazine).
Water quality baselines and cycles
Start with chemistry. Indonesian waters vary widely; for example, Java groundwater often exceeds 180 ppm as CaCO₃ (a measure of hardness) (beta.co.id). Untreated 150 ppm calcium hardness concentrated to 10× translates to >1.5 g/L CaCO₃ deposition potential on hot surfaces. Plants commonly target 3–6× cycles for moderate hardness (50–150 mg/L) and use blowdown to cap TDS and other ions; in the cited case, limiting cycles to ~2.5× held TDS and nitrite within permit (Power Magazine).
Routine bleed control — conductivity‑based or periodic — stabilizes chemistry and flushes biofoulants. Typical practice: maintain conductivity at ~<2000–3000 µS/cm (dependent on makeup), keep alkalinity around 50–150 ppm as CaCO₃ to moderate Langelier stability, and validate with pilot “softening curves” to pinpoint incipient CaCO₃ formation. As hardness rises, operators often deploy partial softening; options include ion exchange via a softener or hardness trimming with nano‑filtration when the goal is more cycles at lower pressure than RO.
Where makeup is from surface sources, pretreatment to reduce suspended solids and organic load helps extend cycles; some plants pair media filtration with membranes as a pretreatment to RO, and utilities often use ultrafiltration to intercept fine particulates before the tower.
Scale control under high heat flux
Primary foulants are calcium carbonate, sulfates, silicates, and iron/hydroxides. Elevated tube‑side temperatures promote reverse‑solubility scale (e.g., CaCO₃) that accelerates above 2–3× cycles if unmanaged. The backbone is a coordinated blend of threshold inhibitors with dispersants. Modern programs rely on phosphonates or polymeric inhibitors (polyacrylates, polycarboxylates) as “threshold” agents for carbonate salts, while polymeric dispersants keep fines suspended.
Dosing is typically on the order of 5–30 mg/L active inhibitor, with organic copolymer dispersants fed around 10–20 mg/L at 5–6× cycles. In one program, a stabilized phosphate plus polymer held total inorganic phosphate (TIP) at ~4–5 ppm, with post‑installation tube inspections noting “very low” fouling and cycles rising from ~2.5× toward 5–6×; blowdown dropped roughly by half, saving about 500–1000 m³/day of water (Power Magazine) (Power Magazine).
Chemical compatibility matters: some scale inhibitors degrade in the presence of oxidizing biocides; if they break down, scale reappears swiftly (QualiChem). Programs either select oxidizer‑resistant polymers or time “envelope” doses around oxidizer shocks. Plants frequently specify scale inhibitors alongside polymeric dispersant chemicals to hold ions in solution while keeping particulates mobile.
Monitoring follows suit: track residual inhibitor (via specific polymer tests or TOC), calcium, phosphate, and silica; use filtered vs. unfiltered phosphate checks to verify dispersion (Power Magazine). Deposit coupons at the warmest locations and watch tower approach temperature — a rise of >1–2°F often flags under‑treated scaling; one site saw a 1.5°F approach drop after cleaning (Power Magazine).
Multimetal corrosion inhibition
Open cooling loops mix carbon steel, copper alloys, and stainless, demanding a multimetal blend plus pH control. Targets are stringent: keep steel corrosion <0.1 mm/yr (~4 mils per year, mpy) and copper <0.5 mm/yr; one plant set <3 mpy (0.08 mm/yr) as the KPI and met it consistently (Power Magazine).
Chemistry levers include alkaline buffers (pH 8.0–8.5), anodic inhibitors like sodium molybdate or nitrite for copper and orthophosphate for steel, and cathodic inhibitors such as sodium silicate (zinc is often avoided by regulation). One program used tetrakali pyrophosphate (TKPP) as a cathodic inhibitor alongside orthophosphate for film formation (Power Magazine).
Operationally, oxygen scavengers are not applicable in open, aerated towers; instead, protective films are built. Plants feed 2–10 ppm as PO₄ or TIP, managing a saturation threshold below ~20 ppm Ca hardness, and may supplement molybdate at 50–100 ppm Mo if ammonia is present. Blended packages are fed continuously, often split across multiple injection points; LPR (linear polarization resistance) feedback lets teams trim chemical feed to the measured corrosion rate rather than overfeeding to a fixed residual (Power Magazine). Many plants deploy turnkey corrosion inhibitors within these multimetal programs.
Biological control and ORP stability
Biofilm in open towers can cut heat transfer by up to 15% or more and drive MIC (microbiologically influenced corrosion). A robust, layered program pairs oxidizers with penetrants. The workhorse is a continuous low‑level oxidizer (commonly chlorine or bromine) run to a free‑Cl residual ~0.2–0.5 mg/L or ORP of ~600–700 mV; one plant maintained ~650 mV and suppressed colony counts — until high ammonia makeup tied up chlorine as chloramine, a far less effective microbicide (Power Magazine) (Power Magazine). At ~40 ppm ammonia, hypochlorite became inert chloramine, prompting evaluation of chlorine dioxide, which does not form chloramines (Power Magazine).
Periodic shocks matter. Weekly or biweekly oxidizer shocks (2–4 mg/L free Cl for 1–2 hours) disrupt early biofilms; in a documented case, two NaClO shocks plus continuous feed drove Legionella from 10^5.06 to 10^1.77 CFU/L (~98% reduction) and cut total HPC (heterotrophic plate count) by ~1.95 log (~90% reduction). By contrast, a peroxide shock failed and spiked counts to 10^6.14; the optimized regime achieved final HPC ≈10^2 CFU/mL (Log 1.95) (MDPI) (MDPI).
To penetrate stubborn biofilm, non‑oxidizers such as glutaraldehyde are slug‑fed at ~5–10 ppm, often following a biopenetrant and oxidizer shock; this two‑step approach was identified for future implementation in ammonia‑impacted systems (Power Magazine). Plants typically standardize a tiered biocide program to hit these targets.
Auxiliaries help: sunlight exclusion and clarifying makeup to pare nutrients (including phosphate) starve algae; industry reports note growing restrictions on phosphorus because it’s a limiting nutrient for toxic blooms (Chemical Processing) (Chemical Processing).
Performance targets continue to tighten. Spanish rules (RD865/2003) treat HPC ≤10,000 CFU/mL and Legionella ≤100 CFU/L as “safe” thresholds, yet hundreds of positive Legionella cases have been recorded below 10,000 HPC; research identified HPC ≤100 CFU/mL as a more protective sentinel (PMC) (PMC). In practice, targets of HPC <10^3 CFU/mL (ideally <10^2) with undetectable Legionella are applied; in the case above, post‑treatment HPC ~90 cfu/mL confirmed strong control (MDPI). Monthly inspections for visible slime adjust dosing as needed.
Automation, dosing, and feedback
Advanced programs wire chemistry to real‑time controls. Online conductivity (TDS), pH, ORP, and LPR sensors drive inhibitor, dispersant, and oxidizer feeds in closed loops, tied to blowdown and corrosion‑rate signals. One performance‑based setup trimmed chemical cost by about $100k/year while holding corrosion under KPI limits (Power Magazine) (Power Magazine).
ORP controllers steady disinfection, and turbidity plus approach‑temperature analytics flag fouling. Plants often rely on metered injection using a dosing pump to keep residuals on spec, with periodic coupon retrieval and microbiology (HPC, Legionella) validating performance.
Measured returns at the condenser
Data quantify the upside. After optimizing treatment, one plant recorded a 0.85″ Hg drop in condenser backpressure (via removal of biofouling), translating to ~15% gain in tube cleanliness and a 6.7 MW output lift worth ~$1.6 million/year (Power Magazine). Chemical costs fell by 10–20%, with steel corrosion staying well under ~3 mpy on coupons (Power Magazine) (Power Magazine).
Even thin films sting performance: a 1998 Buckman Labs review cited by industry sources noted that just ~1 mil (~25 μm) of biofilm can raise condenser approach by ~3–5°F and trim efficiency by ~0.5–1% — consistent with the observed gains (Power Magazine). On water use, increasing cycles from 2.5 to 5× in a 3 Mgal/d plant would recover ~300,000 gal/day of reuse and avoid over 1100 tons of salt discharge annually.
Where deposits do accrue, targeted cleaning can reset approach; facilities schedule tower and exchanger work with providers of a cooling tower cleaning service in concert with chemistry changes.
Permits, nutrients, and drift
Programs must meet discharge limits, particularly for phosphorus (P), nitrogen (N), and oxidizer residuals. Sensitive areas may cap phosphorus at <0.1 mg/L or ban zinc above ~<0.5 mg/L, pushing adoption of non‑phosphorus scale inhibitors — a trend echoed in industry analyses that call phosphorus a “limiting nutrient for toxic algae” (Chemical Processing). Air permits can constrain drift; defoamers are sometimes used. Plants document all feeds and monitor blowdown for pH 6–9, TSS, chloride, and metals.
Integrated program design
The balanced template is clear. Threshold inhibitors and dispersants fed at ~5–30 mg/L keep CaCO₃, silica, and iron scales in check; mismatched or oxidized inhibitors lead to rapid fouling (QualiChem). Plants often standardize blended cooling‑tower chemicals to allow higher cycles with lower bleed.
Corrosion control pairs pH ~8–9 with film‑formers (orthophosphate, molybdate/nitrite, azoles) to hold steel below 0.1 mm/yr and maintain steel corrosion <3 mpy by coupons/LPR (Power Magazine).
Biological programs combine continuous low‑level oxidizer with periodic shocks and non‑oxidizers (e.g., glutaraldehyde at ~5–10 mg/L slug). Targets are HPC <10²–10³ CFU/mL and no detectable pathogens, accounting for chloramine formation in high‑ammonia makeup and, where indicated, switching to chlorine dioxide (PMC) (MDPI) (Power Magazine).
Automation ties it together: real‑time conductivity/pH/ORP and LPR‑bound control loops ensure inhibitors feed exactly as needed, while periodic sampling (deposit analysis, microbiology) validates conditions. Metered chemical addition through a dosing pump underpins this control strategy.
The payoff is tangible: lower bleed and salt discharge, restored condenser dewpoint, and longer asset life — with case data showing ~15% condenser cleanliness gains and multi‑megawatt output lifts after optimizing water treatment (Power Magazine).
