How to design an RO plant
An RO design is not a series of independent calculations. It is one coupled sequence: a product requirement drives a feed, the feed drives pretreatment, the recovery closes a mass balance, and every equipment duty downstream follows from that. This page walks the sequence, states plainly what the platform calculates and what it deliberately does not, then hands you a working designer.
Design an RO plant See a worked example Verify a single calculation
What RO plant design involves
Nine decisions, in this order. Moving one of them late is the most common way an RO design turns out to be unbuildable.
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Define the product requirement
State the product flow you have to deliver and the water quality it has to meet. Everything downstream is sized backwards from this, and it is a requirement rather than a calculation — the software must not quietly substitute a derived figure for it.
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Establish the source and feedwater
Seawater intake, beach wellfield or brackish groundwater are different engineering problems, not variations of one. The source fixes the feed flow, the salinity and what pretreatment the membranes will need.
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Review the feedwater analysis
Temperature, TDS and the ion chemistry that governs scaling risk. This is measured data, and the Designer treats it as engineer-review rather than as something software can verify.
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Select pretreatment
Media filtration, cartridge filtration, ultrafiltration and dosing exist to protect the membrane. This is usually the decision with the largest effect on whether the plant performs as designed.
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Define the RO configuration
Membrane area, elements per vessel and feed pressure. Flux and array arrangement are an engineer or supplier basis — the platform sizes from what you state rather than choosing for you.
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Establish recovery and the mass balance
Recovery fixes the feed you must abstract and the concentrate you will produce. Concentrate quality follows from salt conservation rather than being specified, and the recovery you can defend is set by the scaling envelope.
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Derive engineering requirements
Duties, flows and pressures for each item of equipment, each labelled as input, derived, calculated or data-required. A missing head stays missing rather than defaulting to zero.
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Select equipment
Products are matched against the requirements using the verified manufacturer catalog, with the evidence for each match shown. This is an engineer action, and the outputs stay empty until you perform it.
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Produce schedule, BOQ and report
Once equipment is selected, the schedule, bill of quantities and engineering report follow from the same model — including the plant flow diagram, which is a projection of the treatment train rather than a separate drawing.
The workflow the platform actually runs
EngiMetric is a workflow, not a calculator collection. Each stage below is computed from the one before it, and the outputs are projections of one engineering model rather than separate results.
- Requirements
- Source
- Feedwater
- Treatment train
- Mass balance
- Engineering requirements
- Equipment
- Schedule
- BOQ
- Report
The interactive plant flow, the equipment requirements and the diagram in the engineering report are all projections of that same model. They are drawn from it rather than reconciled against it, which is why they cannot disagree.
What EngiMetric models, and what it does not
Quantitatively modelled
Computed from the inputs you state, with the basis shown:
- A full salt and water balance across the whole treatment train — feed, product and concentrate flows, with concentrate quality derived by salt conservation.
- Feedwater analysis carried from stated chemistry into the design basis.
- Membrane area and vessel/element counts from a stated design flux and element area.
- Pretreatment sizing for media filtration, cartridge filtration and ultrafiltration.
- Engineering requirements with every value labelled input, derived, calculated or data-required.
- Energy and specific-power figures from stated pressure, recovery and pump efficiency.
- An interactive plant flow projected from the treatment train, and the same projection in the report.
Not quantitatively modelled
Stated plainly, because an over-claim here costs real money:
- Per-element array staging and concentration-polarisation profiling
- The Designer sizes at vessel and train level. Element-by-element flux, pressure and TDS profiles along the array — what a vendor projection package resolves — are not computed here. Use the membrane manufacturer’s own projection software for that.
- Fouling prediction
- Scaling risk is evaluated, because it sets the defensible recovery. Fouling is not modelled, and no normalised-performance or fouling-rate prediction is claimed.
- Second-pass and interstage quantitative treatment
- Additional RO barriers can be placed in the train and the topology is shown correctly, but second-pass and two-stage interstage transfer are not quantitatively evaluated. Read those trains as topology, not as verified balances.
- Thermal desalination sizing
- Multi-effect distillation, multi-stage flash and zero-liquid-discharge arrangements are described as reference technology. The platform will not size them.
- Brine disposal economics
- Concentrate quantity and quality are computed, because they come out of the balance. Disposal cost, route selection and permitting need site-specific data and are not modelled.
Start from a worked example
Reading the sequence is one thing; seeing a whole train balance is another. The case below opens in the Plant Designer already filled in, so you can run it, inspect the diagram and change any input to see what moves.
Seawater — open intake, single RO stage
Baseline seawater case: an open seawater intake feeding a media filter, antiscalant and a single RO stage with remineralisation and a product tank. Start here to see a full seawater chain run end to end.
- Product basis
- 1000 m³/day
- Recovery
- 45%
- Feed TDS
- 35000 mg/L
- Source
- seawater-intake
Synthetic demonstration / educational example — not project design. These are illustrative starting bases to replace with your own project inputs. They are not a reference design, a vendor recommendation or a compliance case.
Open this case in the Designer See it worked through in full All demo cases
Verifying individual decisions
A design workflow does not remove the need to check a single number. These cover the checks worth doing independently — grouped by the decision each one supports, because that is how you actually reach for them. The RO calculator gateway has the complete set.
Check the balance
Before trusting a duty or a price, confirm the flows and the salt actually close.
- RO Recovery CalculatorCalculate RO permeate and concentrate flow from feed flow and system recovery rate.
- RO Concentrate Flow CalculatorCompute brine/reject flow rates and volumetric stream balances for disposal or ZLD design.
- RO Mass Balance CalculatorSimplified TDS-based dissolved-solids balance for RO feed, permeate, and concentrate flow and quality checks.
Check the water quality
Product quality is a consequence of rejection, not a free input.
- RO Salt Rejection CalculatorCalculate membrane salt rejection percentage from feed and permeate TDS concentrations.
- RO Salt Passage CalculatorDetermine salt passage percentage through RO membranes and verify complementary rejection balance.
- RO Permeate TDS CalculatorProject expected permeate total dissolved solids based on feed water salinity and rejection specs.
Check the pressure and energy
The pressure the membranes need, and what that costs in power.
- RO Osmotic Pressure CalculatorEstimate RO feed osmotic pressure using a simplified van’t Hoff model from TDS, temperature, and solute properties.
- RO Feed Pressure & Specific Energy CalculatorEstimate required RO feed pressure (permeate osmotic gap + net driving pressure) and specific energy consumption with optional energy recovery.
- RO Energy & Specific Power CalculatorDetermine specific energy consumption (kWh/m³) based on operating pressure and pump efficiency, with optional energy recovery device savings.
- RO Operating Cost (OPEX) & SEC CalculatorEstimate RO operating expenditure: pumping energy (specific energy consumption), chemical cleaning, antiscalant, and membrane replacement.
Check the hardware and the risk
Whether the membrane area and the scaling envelope hold up.
- RO Membrane & Vessel Element Count CalculatorCalculate number of RO membrane elements and pressure vessels from permeate flow, design flux, and element area.
- RO Scaling Risk & Pretreatment AnalysisScreen CaCO3 scaling risk in RO feed and concentrate using the Langelier Saturation Index (ASTM D3739) and the Stiff & Davis Stability Index (S&DSI) for high-TDS and seawater feeds.
RO workspace tools
Where a design needs to be worked through interactively rather than calculated one figure at a time.
- RO Engineering Workbench The full RO calculation set in one workspace, for working a design through without moving between pages.
- RO Treatment Train Builder Compose a train stage by stage and watch the streams resolve.
- Desalination Plant Design Basis template A worked design-basis document structure to record a project design against.
- Desalination Plant Design Guide The twelve-stage walkthrough, with real captured screens of the running designer.
Where this becomes a design
Individual calculators answer one engineering question well. A plant design needs those answers to stay consistent with each other as the design changes — which is what the Designer is for. Load a case or enter your own figures, and the balance, requirements and outputs are derived from one model rather than recalculated by hand each time.