Water‑scarce regions and tougher discharge rules are pushing combined‑cycle gas turbine plants toward zero liquid discharge. Here’s how evaporation ponds, brine concentrators, and crystallizers compare — and when the math pays back.
In arid markets, combined‑cycle gas turbine (CCGT) plants are running out of water and regulatory runway — and zero liquid discharge (ZLD, fully eliminating liquid effluent by converting contaminants to solids and recycling water) is no longer optional. China now mandates ZLD for new thermal plants and U.S. states like Florida already run ZLD on several power stations (westechwater.com) (nurkimyaaritma.com).
Indonesia has drawn a hard line: under Regulation 22/2021, if wastewater violates quality standards it cannot be discharged and must be reused or recycled on‑site (enviliance.com). That pressure bites because Indonesian combined‑cycle plants average ~3,500 L/MWh of water demand (researchgate.net).
Public perception favors closure too: ZLD eliminates liquid effluent and concentrates pollutants into a solid waste stream (pubs.acs.org) (westechwater.com).
Evaporation ponds: low energy, large land
The simplest ZLD option stores blowdown from HRSG (heat recovery steam generator) and cooling systems in lined solar ponds and lets nature do the evaporation. Historically, western U.S. cooling‑water blowdown ponds achieved ZLD with minimal energy use (powermag.com).
The draw is ultra‑low OPEX (≈0 kWh/m³), high reliability in arid climates, and minimal operating complexity (condorchem.com). The catch is footprint and weather risk. One design handling 456 m³/day used two 131×131×1.5 m ponds (~3.4 ha total) (gasprocessingnews.com), and constructing two 228 m³/d ponds cost about $1.8 M — roughly $4,000 per m³/d of capacity (gasprocessingnews.com). Ponds risk overflow in heavy rain and slow down in wet seasons (condorchem.com); climate viability generally demands ambient evaporation >5–10 mm/day, and rain dilutes stored brine (condorchem.com).
Solids must be removed manually, and organics can drive odor or biofouling (condorchem.com). Many sites pre‑concentrate or condition blowdown before ponding: routing through membranes or softeners to control silica and hardness cuts pond volume and scaling risk. In practice, that can be as simple as deploying a softener to knock down calcium and magnesium or adding a modular membrane system upstream.
Brine concentrators (MVC/MVR) performance
Mechanical vapor‑compression (MVC, also called MVR for mechanical vapor recompression) evaporators boil brine under vacuum, compress the vapor, and reuse it to heat incoming brine. They typically recover ~90–95% of incoming water and shrink the residual volume sharply (lenntech.pl).
Energy use runs about 15 kWh per m³ of recovered water (lenntech.pl), and capital costs sit around $1,750 per m³/day of capacity (lenntech.pl). Multi‑effect distillation (MED, steam‑heated multiple stages) is an alternative at about ~$1,375/m³/d if steam is available (lenntech.pl). Some designs push recovery to ~97.5% (lenntech.pl), leaving 5–10% of the original volume to finish.
Real‑world systems pair concentrators with membranes to minimize thermal load. RO (reverse osmosis) is common, and high‑recovery trains benefit from upstream clarification and conditioning to tame silica/hardness scaling. A plant might, for instance, add a clarifier for suspended solids control and run a brackish‑water RO ahead of the evaporator. Plants also deploy membrane antiscalants to extend cleaning intervals, and accurate chemical feed via a dosing pump narrows swings that drive fouling.
Crystallizers: the last 5–10%
Forced‑circulation crystallizers (steam‑driven or vapor‑recompression) push the brine to salt precipitation, removing the final water fraction and yielding solids for landfill or recovery. This step is energy‑intensive: typical specific energy is ~50 kWh per m³ processed (at near 95% recovery) because it supplies latent heat for the last water (lenntech.pl).
Florida plants use steam‑driven forced‑circulation crystallizers, often with a thermocompressor to recycle vapor and improve efficiency (nurkimyaaritma.com) (nurkimyaaritma.com). The solid “salt cake” typically occupies ~1–5% of the initial liquid volume; case data range from ~1 ton/day at a 150 MW gas plant to ~12 tons/day at a 920 MW coal plant (nurkimyaaritma.com) (nurkimyaaritma.com). The condensate is very pure (<50–100 ppm) and is recycled to the plant (nurkimyaaritma.com) (nurkimyaaritma.com).
Membrane hybrids and pretreatment trains
Membrane‑thermal hybrids can blunt energy bills. Aquatech’s HERO™ (a high‑recovery RO process) has been used as an economical ZLD substitute in Arizona, cutting blowdown by 90% at lower energy but constrained by osmotic pressure limits (archive.aquatech.com). High‑recovery RO benefits from robust pretreatment; packaged ultrafiltration is often used upstream to protect membranes, and integrated water‑treatment ancillaries simplify operations.
Capital costs and operating energy
As a rule of thumb, thermal ZLD equipment costs about $1–2k per m³/day of capacity (lenntech.pl). One analysis cites ~$1,750/m³/d for an MVC evaporator (lenntech.pl) and ~$1,800/m³/d for a 3‑effect thermal unit. By contrast, the lined pond example worked out to ~$4,000/m³/d (gasprocessingnews.com).
Scaling up, a 4,000 m³/d blowdown capacity implies MVC‑based ZLD CAPEX of ≈$7–8 M, whereas evaporation ponds could be ~2× that (plus a few hectares of land). Broadly, ZLD capital runs ~40% higher than biological or partial‑discharge approaches for similar flows (power-eng.com).
Operating cost is dominated by energy. MVC evaporation uses ~15 kWh/m³ and a finishing crystallizer ~50 kWh/m³, for a combined ≈65 kWh per m³; at $0.10/kWh that’s ~$6.50/m³ (lenntech.pl). Design optimization matters: a Florida plant added heat‑exchange area to cut power from ~25 to 18 kWh/m³, spending ~$150k extra to save about ~$425k per year in energy (nurkimyaaritma.com).
Avoided hauling and compliance savings
Even with energy costs, on‑site ZLD can be far cheaper than trucking brine. Hauling can run ~$0.50 per US gallon (~$132/m³), and one 250 MW plant facing full ponds estimated $17M to empty a single pond (watertechnologies.com). Installing an on‑site recycling system (membrane recovery plus polishing) recycled ~60 million gallons (227,000 m³) into boiler water over two years, avoiding $17–$20M in hauling costs (watertechnologies.com) (watertechnologies.com).
Water value and byproduct revenues
Closing the loop saves make‑up water. A well‑designed ZLD system can cut a plant’s blowdown loss from ~21% to 0.1% of intake, saving up to ≈20% of make‑up water (nsenergybusiness.com). In some cases, precipitated salts such as CaCO₃ or NaCl can be sold, partially offsetting costs (westechwater.com) (mdpi.com).
Regulatory value is significant: avoiding effluent discharges reduces permitting exposure and fines. Indonesia’s Regulation 22/2021 explicitly directs operators exceeding standards to “utilize” wastewater on‑site (enviliance.com).
Technology trade‑offs at a glance
Evaporation ponds carry negligible energy (~0 kWh/m³) but need vast land and dry climates; one 228 m³/d example required a 131×131 m pond (gasprocessingnews.com) and are climate‑dependent with overflow risk (condorchem.com). Brine concentrators (MVR) draw ≈15 kWh/m³ with ~90–97% recovery and CAPEX of ~$1,750/m³/d (lenntech.pl) (lenntech.pl), leaving ~10% volume. Crystallizers add ~50 kWh/m³ to drive solids formation and achieve true ZLD (~98–99% water recovery overall; without crystallizers recovery is ~90–95%) (lenntech.pl).
Site economics and payback signals
Thermal ZLD typically consumes on the order of 10–50 kWh per m³ of blowdown. The decision hinges on avoided disposal and water costs. Where fees or hauling hit hundreds of dollars per m³ (watertechnologies.com) or where self‑sufficiency is strategic, ZLD can pay back in a few years. One analysis put the levelized cost of ZLD water at ~$7–14/m³ with sale of recovered salts, yielding under‑8‑year payback (mdpi.com). Absent such offsets, ZLD remains a premium choice, usually taken when regulation or scarcity is acute (pubs.acs.org) (nsenergybusiness.com).
A representative comparison underscores the trade‑off: pond CAPEX of ~$1.8 M for 456 m³/d (gasprocessingnews.com) versus ~$1.75 M for an equivalent MVR unit (lenntech.pl), with MVR requiring ≈15 kWh/m³ and crystallization ≈50 kWh/m³ (lenntech.pl). Such systems recover >99% of water and capture nearly all salts in a solid stream.
Case choices follow the money and mandates. One CCGT in Arizona selected a high‑recovery membrane pre‑treatment (HERO) that cut blowdown by 90% at about half the cost of a thermal concentrator (archive.aquatech.com). A U.S. coal‑CCGT plant’s $150k upgrade to an evaporator saved roughly $425k per year in energy (nurkimyaaritma.com). And in Indonesia, Regulation 22/2021 means that if effluent exceeds limits, on‑site reuse is effectively mandatory (enviliance.com).
