The short answer. Before comparing prices, make sure every proposal is designed for the same water. Then check six things: the mass balance, the energy per cubic metre at your tariff, where the solids and any purge go, how scaling and foaming risks were tested, what the performance guarantee actually covers, and the lifecycle cost. Large differences between zero liquid discharge (ZLD) quotes usually come from different assumptions, not from better technology.
Start by fixing the design basis
A ZLD plant is designed around its feed. If one supplier designed for your average water and another for your worst week, their quotes are not comparable, and the cheaper one is often the one that assumed the easier water.
Give every supplier the same design basis in writing:
- Normal, design and worst-case flows, including cleaning, start-up and batch peaks
- A full ion analysis that closes on charge balance, plus silica, organics (TOC or COD), oil and any substances of concern
- The variability you have measured, not a single sample
- Your recovery target, the uses for the recovered water and its required quality
- Your energy sources and tariffs, available steam or waste heat, and site constraints
If you are not sure your analysis is good enough, check it first with our free Water Analysis Validator.
The 12-point ZLD vendor proposal checklist
| # | Question to ask | Why it matters | Red flag |
|---|---|---|---|
| 1 | Does the proposal state the design basis it used, and does it match yours? | Every other number depends on it | Design feed missing, or simplified to TDS only |
| 2 | Is there a full mass and water balance for each stage? | Shows where every cubic metre and every kilogram of salt goes | Only inlet flow and “zero discharge” are shown |
| 3 | What recovery is stated, per stage and overall, and over what period? | Instantaneous recovery hides cleaning, downtime and purge | One headline recovery figure with no basis |
| 4 | What is the electrical and thermal energy per cubic metre of feed and per cubic metre evaporated, at design and turndown? | Energy is usually the largest operating cost in the thermal stages | Energy given only at ideal conditions, or with no units |
| 5 | How many tonnes of solids per year, at what moisture, and how will they be classified? | Disposal cost depends on classification and moisture | “Salt for reuse” assumed with no buyer or specification |
| 6 | Is there a mother liquor or purge stream, how large, and where does it go? | Impurities that do not crystallise build up and must leave somewhere | “Fully closed loop” with no purge at all |
| 7 | How were scaling, fouling and foaming limits determined: model, lab test or pilot? | These set the achievable recovery and the cleaning frequency | Recovery claimed without any test on your water |
| 8 | Are the materials of construction specified for your chloride levels and temperatures? | Corrosion in hot brine is a common cause of downtime | Generic “stainless steel” with no grade or justification |
| 9 | What availability is assumed, what is redundant, and how much storage covers an outage of the final stage? | Production continues while the ZLD block is down | No answer to “what happens when the crystallizer stops?” |
| 10 | What exactly does the performance guarantee cover: which parameters, over how long a test, on which feed, with what remedy? | A guarantee is only as good as its test conditions | Guarantee tested on clean design water, or a short test only |
| 11 | Can the supplier show references with comparable feed chemistry, operating for at least a year? | Similar flow is not similar chemistry | References are pilots, or plants that have not run a full year |
| 12 | What is the lifecycle cost over 15 to 20 years, including energy, chemicals, spares, membranes, labour and solids disposal? | Capital cost is often the smaller part | CAPEX quoted with OPEX left vague or excluded |
Our ZLD guide chapter on when ZLD is justified explains why final concentration, crystallization, solids and operations each need their own evidence, which is the logic behind items 6 to 10. For how a ZLD system works stage by stage, see our guide to zero liquid discharge technology.
Normalise ZLD proposal costs before you compare
Put every proposal into the same units: cost per cubic metre treated over the life of the plant, with solids disposal as a separate line. The example below uses hypothetical figures to show the method.
Two suppliers quote for a plant treating 100 m³ per day, running 350 days a year, so 35,000 m³ a year.
| Supplier A | Supplier B | |
|---|---|---|
| Capital cost | EUR 3.2 million | EUR 2.6 million |
| Operating cost, excluding solids | EUR 6.00 per m³ | EUR 8.00 per m³ |
| Operating cost per year | EUR 210,000 | EUR 280,000 |
| Total over 15 years, undiscounted | EUR 6.35 million | EUR 6.80 million |
Supplier B is EUR 600,000 cheaper to build and EUR 450,000 more expensive to own. Now look at the solids. Supplier B assumed its 300 tonnes a year of salt would be classified as non-hazardous, at EUR 100 per tonne. If the authority classifies it as hazardous and disposal costs EUR 400 per tonne, that adds EUR 90,000 a year, or EUR 1.35 million over 15 years. That single assumption is worth more than the whole capital cost difference.
For a real decision, discount the cash flows and test the sensitivity to energy price, availability and solids classification. Our ZLD cost guide sets out the main cost drivers.
What goes wrong in operating plants
Problems in ZLD plants tend to appear at the back end, long after the proposal stage. A WorleyParsons case study of a 720 MW power plant, filed in the US EPA record for Merrimack Station, is a clear example. The evaporator was designed for 105 gallons per minute but processed about 60 on average, and the back end was roughly 70% of the size needed for continuous operation. The acceptance test had been run at low cooling tower cycles rather than on design feedwater, so it proved the pumps and pipes could carry the flow but not that the system could handle the design salt load. The author notes that this happens on almost all ZLD performance tests, and that buyers tend to choose the cheapest system whose nameplate capacity equals the required flow.
The same paper shows why redundancy and storage matter: six process steps in series, each 90% reliable, give an overall reliability of about 54%. These lessons map straight onto items 1, 7, 9 and 10 of the checklist: the real design basis, tested limits, availability with storage, and a guarantee tested on representative feed.
Red flags at a glance
- No written design basis, or one that differs from yours
- A single recovery figure with no stage-by-stage balance
- Revenue from recovered salt with no named buyer or specification
- No purge stream in a “closed loop” design
- A performance test on clean water or for a few days only
- Operating costs that leave out solids disposal, labour or spares
When to bring in an independent review
A second opinion is most useful when proposals differ widely in price or recovery, when the solids route is unclear, or when the decision will commit capital for 15 years or more. Brine Consulting sells no treatment equipment, so our review has no stake in which supplier wins.
Send us your vendor proposals for an independent second opinion. We check each one against your design basis and this checklist, normalise the costs, and show you where the real differences lie.
“…highly knowledgeable in the field of desalination and brine management.”
Stan Lueck, President at RODI Systems Corp., in a testimonial published on the Brine Consulting website.
Frequently asked questions
How many proposals should we get?
Enough to compare at least two technically different approaches, for example a membrane-heavy train against a thermal-heavy one. Three quotes for the same design tell you less than two quotes for different designs.
Should we insist on a pilot test?
Where scaling, foaming or organics are uncertain, a pilot or at least a laboratory evaporation test on your real water is the best protection. It costs far less than redesigning a crystallizer after start-up.
Is a performance guarantee enough protection?
Only if it names the parameters, the feed it applies to, the test duration and the remedy. A guarantee tested on design water you cannot actually supply protects the supplier, not you.
Who is responsible for the solids?
Usually the owner, which is why the classification, moisture and disposal route belong in your comparison, not only in the supplier’s scope.
Can recovered salt pay for the plant?
Rarely on its own. Count product revenue only when you have a qualified buyer, a specification and a route for off-spec material.
Related tools and guides
- MLD vs ZLD Decision Screener: check whether full ZLD is the right endpoint before comparing quotes
- Industrial Wastewater Scaling & Concentration Tool: screen scaling limits as the brine concentrates
- Zero liquid discharge feasibility study: what a study should deliver before procurement
Sources
- Brine Consulting: ZLD guide, chapter 13, When ZLD is justified
- Brine Consulting: Zero liquid discharge cost
- US EPA: Merrimack Station NPDES permit record (document AR-960: Sampson, No Easy Answers: ZLD Improvement Options for a 720-MW Power Generation Facility, IWC 12-13)
- BHEL: ZLD technical specification and pre-qualification requirements
Written by the Brine Consulting advisory team. The Brine Consulting advisory practice includes Osis G. Kalache, P.Eng., a senior wastewater process engineer with more than 17 years in industrial wastewater treatment and process optimization, and Cesar J.M. Chu Ortega, a water-tech strategist with expertise in water reuse, zero liquid discharge and industrial wastewater treatment. Meet our experts.