Most of the cost and reliability of a zero liquid discharge plant is fixed before any supplier is chosen. It is set by the feed cases the plant must handle, the recovery each stage is asked to reach, and what the specification lets bidders assume. Get those wrong and even good equipment ends up sized for water you do not have.
Brine Consulting, an independent industrial water advisory firm in Amsterdam, writes that owner’s side of the design for ZLD and minimal liquid discharge (MLD) projects: the basis of design, the water and salt balance, the process concept and a performance specification that every bidder prices on the same terms. We sell no treatment equipment, so no line of the specification is written around a product we supply. Detailed engineering, supply and the performance guarantee stay with the supplier you select. If an offer is already on your desk, book a scoping call and we will say which parts of it to check first.
What design consulting delivers, and who uses it
| Deliverable | What it fixes | Who relies on it |
|---|---|---|
| Basis of design | The flows, water analyses and operating cases the plant must meet, including upset and future cases | Every bidder, your permit team, later the acceptance test |
| Water and salt balance | Where each cubic metre and each kilogram of salt goes, stage by stage and case by case, including purge, sludge and cleaning streams | Process design, utility sizing, solids disposal planning |
| Process concept | The train: pretreatment, membrane concentration, thermal concentration, crystallization and solids handling, or an MLD endpoint instead | Budget estimate and the choice of which suppliers to invite |
| Performance specification | What the plant must achieve, measured how, over what test period, with what remedy if it falls short | Bid package and contract |
| Design review | Whether a supplier’s design matches your basis and closes its own balance | You, before you sign |
Whether ZLD is the right target at all is a separate question, answered by a ZLD feasibility study. Design consulting starts once that direction is set, or runs alongside the final decision.
Why the basis of design decides the size of the evaporator
The example below is hypothetical, but the arithmetic is the same one every ZLD design rests on. It shows how one design-basis choice moves the most energy-hungry part of the plant.
A site sends 50 m³/h of pretreated wastewater at 5,000 mg/L dissolved solids to a reverse osmosis (RO) stage, followed by a brine concentrator that takes the RO reject to 200,000 mg/L before a crystallizer.
Case A, designed on the average analysis: the RO runs at 85% recovery.
Case B, designed on the worst analysis the site actually sees: higher silica, or colder water, limits the RO to 75%. This is the normal design rule rather than a pessimistic one. Membrane makers’ design manuals tell designers to lower recovery whenever the calculated concentrate silica exceeds its solubility, which falls with lower temperature and pH (FilmTec RO technical manual, section 2.4.7).
| Case A: RO at 85% | Case B: RO at 75% | |
|---|---|---|
| RO reject to brine concentrator | 7.5 m³/h at 33,300 mg/L | 12.5 m³/h at 20,000 mg/L |
| Water evaporated in the brine concentrator | 6.25 m³/h | 11.25 m³/h |
| Brine to the crystallizer | 1.25 m³/h | 1.25 m³/h |
| Salt to the crystallizer | 250 kg/h | 250 kg/h |
| Evaporator power at 18 to 35 kWh per tonne evaporated | about 113 to 219 kW | about 203 to 394 kW |
Three things follow.
The crystallizer does not care which case you chose. Its load is set by the salt arriving, which is the same 250 kg/h in both cases. Salt load fixes the crystallizer; recovery upstream fixes the evaporator.
The evaporator in case B removes 80% more water. A plant sized on case A that meets case B water for a few weeks cannot keep up. The RO has to be turned down, or the reject tank fills, and either way the site is no longer at zero discharge.
The energy gap is large and permanent. The power range uses the 18 to 35 kWh per tonne of water evaporated that a vapour-compression process engineer gave in POWER magazine in 2011. Use your bidders’ figures for a real project; the ratio between the cases is what matters here.
Assumptions: full salt rejection in the RO, volumes treated as additive, one tonne of water taken as one cubic metre, steady state. These are the same bookkeeping rules as our free Brine Mass Balance Calculator, so you can rerun the example with your own numbers. As the tool itself states, a balance that closes proves the numbers reconcile, not that the separation is physically achievable. That second question is for testing and the supplier’s process design.
What should a ZLD basis of design contain?
The basis of design is the one document every bidder designs against. For ZLD of high-TDS wastewater, it needs more than a flow and a TDS figure.
Flows for each operating case: normal, design, minimum and upset, how often each occurs and how long it lasts. Include the case where production runs harder than planned.
A full ion analysis for each case, with measured variability rather than one sample. Check each analysis before it goes out: positive and negative ions should balance, and the ion sum should match the reported TDS. Our Water Analysis Validator does both checks.
The minor constituents that decide ZLD feasibility: silica, calcium and sulphate, magnesium, organics, and volatile compounds such as ammonia that can carry into the distillate.
Product water: the quality required, and where the water goes.
Residues: whether any liquid purge is allowed, and the solids’ expected classification, moisture and disposal route. The disposal route often decides whether full ZLD or MLD makes more sense. Where the end product is a mixed salt, see our guides to mixed salt disposal and to thin film evaporators for the final drying step.
Utilities and site limits: steam, power, cooling, footprint, and the operator hours actually available.
Availability and storage: how long the thermal stage may be down before production is affected, and how much buffer volume covers it.
Reading a real ZLD specification: what was fixed and what was left open
Public ZLD specifications are rare. One that is online shows what a single-point design basis does and does not pin down. In April 2019 Bharat Heavy Electricals Limited (BHEL) issued a technical specification for a 6 m³/h evaporator and forced-circulation crystallizer with mechanical vapour recompression, for Karnataka Power Corporation’s 370 MW combined-cycle plant at Yelahanka. The feed is reverse osmosis reject.
What the document fixes well:
A complete feed analysis: TDS 27,674 ppm, chloride 14,744 ppm, sodium 8,550 ppm, sulphate 1,963 ppm, silica 124.5 ppm, ammonium 210.5 ppm, pH 8.2.
Materials for the hot chloride service, such as titanium grades for crystallizer heat exchanger tubes and 6% molybdenum steel, Inconel 625 or C-276 for the crystallizer body.
Two guaranteed outputs: distillate below 100 ppm TDS, and salt from the centrifuge at no more than 20% moisture by weight.
What we found when we checked the analysis: we ran the published ions through a charge balance. Cations and anions agree to within 0.02%, and the ions add up to 27,674.2 mg/L, matching the stated TDS to the decimal. Lab results on a real sample seldom close that exactly, so in our reading the analysis looks calculated rather than sampled, for example taken from an RO projection. The document does not say how it was produced. Either way it is one point, with no minimum or maximum, so it tells bidders nothing about how the feed varies.
What the document leaves open: no recovery value, no guaranteed power or steam consumption (the power annexure is a form for bidders to fill in, with no limit attached), and no duration or conditions for the performance guarantee test. The specification refers to tender attachments that are not included, and bidders may have received more there.
In our view the lesson is not about one tender. A specification that guarantees product quality but not recovery, energy or test conditions lets each bidder design for a different plant. A vendor-neutral specification adds a feed envelope instead of a single analysis, a guaranteed recovery and purge, energy and chemical use at design and turndown, and a test of defined length on defined feed, with a remedy if the plant falls short.
How a design engagement runs
Data review. We check your flows and analyses, close the gaps that matter and list what still needs sampling or testing.
Basis of design. Agreed with your operations, permit and project teams, then frozen under change control.
Balances and concept. Water and salt balances for each candidate train and each operating case, with an MLD option where the endpoint allows it.
Specification and bid package. Performance requirements, acceptance test protocol and the information every bidder must return.
Bid evaluation and design review. Offers brought to one baseline, and the selected supplier’s balance and design checked against your basis.
Where a stream’s chemistry is unusual, the data review may show that some limits, such as scaling or foaming during concentration, need a laboratory or pilot test before the basis can be frozen. That work is a separate step, described on our treatability study and pilot testing page, and our guide chapter on laboratory and pilot testing explains which test scale settles which kind of limit.
Most of this work runs on your data and documents, so it does not depend on where the plant is. Dutch and other EU projects we take on directly; for plants in other regions the same design work is done remotely or under a defined engagement.
Already holding a ZLD proposal?
A design review is the quickest way to see whether an offer will work on your water. We rebuild the water and salt balance from your own data, compare it with the supplier’s, and check the basis of design, the purge and solids assumptions and the guarantee test. For the full list of what to check, see our ZLD vendor proposal checklist.
Short answers on ZLD design
Our supplier will do the design. Why write a basis of design ourselves? Because the supplier designs to whatever basis it is given. If you do not define the worst feed case, each bidder picks its own, and the cheapest offer is often the one that assumed the easiest water.
Can the same firm write the specification and supply the plant? It can, but then the specification tends to describe that firm’s technology. We sell no treatment equipment, so the specification describes the outcome you need and leaves the technology open.
What does a ZLD plant cost? It depends heavily on the recovery split and the residue route. Our ZLD cost guide sets out the drivers.
Fix the basis of design before bidders see it
A basis of design costs a document revision to change before the tender is issued, and a variation order after the contract is signed. Book a scoping call to talk through your feed data and tender timetable, or send us your draft specification and analyses. For the wider picture, see our water treatment and ZLD consulting service and our process engineering consultancy.
Written by the Brine Consulting advisory team.
The Brine Consulting advisory practice includes Paolo Mattiuzza, Executive Advisor, Brine Management and ZLD Systems, who specializes in brine management, Zero Liquid Discharge, Minimum Liquid Discharge and industrial resource recovery, with more than 20 years of international leadership experience in the environmental and industrial B2B sectors, and Osis G. Kalache, P.Eng., Senior Consultant, Wastewater Process Engineering and Optimization, with more than 17 years of experience. Meet our experts.