Requirements
- Decided
- Product flow, recovery target, product TDS target.
- Calculated
- Nothing yet — these are the stated targets the whole design is measured against.
- Output
- A requirement set the platform will not overwrite with derived values.
Seawater RO plant design — worked example
One complete seawater reverse osmosis design, followed from design basis and feedwater analysis through the treatment train, mass balance, engineering requirements, equipment, schedule, bill of quantities and the engineering report. Every figure below is produced by the same model the Plant Designer runs.
Synthetic demonstration / educational example — not project design. This is an illustrative case built to exercise the workflow. It is not a real plant, not a client project and not a procurement-ready design.
Open this case in Plant Designer View engineering report (PDF)
The treatment train this example produces
Seawater intake, media filtration and antiscalant dosing ahead of a single RO stage, with remineralisation and product storage after it. The high-pressure pump is drawn inline with the train, not as a process stage of its own — a distinction that matters when the diagram is read as a process flow.
- Source / intake Quantitatively modelled
Intake flow and source type are stated inputs; they appear as the train origin.
- Media filtration Quantitatively modelled
Sized from stated design flow and the 97% filtrate recovery stated above.
- Antiscalant dosing Workflow representation
Present in the train as a dosing step. Dose is an engineer/supplier input, not derived here.
- RO high-pressure pump Quantitatively modelled
Duty derived from RO feed and the stated 70% pump efficiency. Rendered inline with the train, not as a process level.
- Reverse osmosis stage Quantitatively modelled
The mass balance, product quality and membrane duty all derive from this single stage.
- Remineralisation Workflow representation
Shown in the train; the re-mineralisation dose is an engineer decision, not derived.
- Product water tank Quantitatively modelled
Volume derives from product flow and a stated residence time.
- Concentrate Quantitatively modelled
A derived stream, not an input. Its quality follows from salt conservation.
Design basis
Required design targets
Stated by the engineer. The platform does not substitute a derived figure for any of these, and a design is judged against them rather than against whatever the model happened to produce.
| Requirement | Target | Why it is a target, not a result |
|---|---|---|
| Required product flow | 1000 m³/day | The design target the plant must deliver. |
| Required recovery | 45% | Engineer decision. Sets the feed that must be abstracted. |
| Required product TDS | 300 mg/L | Product quality target. Distinct from the calculated result below. |
Source and feedwater
Source typeSeawater — open intake
Raw water TDS35000 mg/L
Temperature25 °C
pH7.8
Intake flow2223 m³/day
Media filter filtrate recovery97%
Feedwater analysis stays at engineer review even on a fully-run design. No software can confirm a laboratory analysis or a wellfield.
RO stage basis
The membrane basis is an engineer or supplier input, chosen here so the case is internally consistent and runnable. It is not a catalogue entry and not a recommendation.
RO stage recovery45%
Design flux15 L/m²·h
Membrane element area80 m²
Elements per vessel6
Feed pressure55 bar
Static head / pump efficiency3 m / 70%
Mass balance
Feed is split into product and concentrate, and the concentrate quality follows from salt conservation rather than being specified. The balance closes because it is conservation — not because the numbers were fitted.
RO feed2156.31 m³/day
Product970.34 m³/day
Concentrate1185.97 m³/day
✓ 970.34 + 1185.97 = 2156.31 m³/day — closes against the RO feed
What was calculated, and from what
| Result | Calculated | Derived from |
|---|---|---|
| RO feed flow | 2156.31 m³/day | Intake × media-filter filtrate recovery |
| Product flow | 970.34 m³/day | RO feed × RO stage recovery |
| Concentrate flow | 1185.97 m³/day | RO feed − product |
| Plant recovery achieved | 45.0% | Product ÷ RO feed |
Product quality is derived from rejection and salt conservation. It is not a free input — which is why the calculated product TDS and the required product TDS target above are two different numbers, and why they should be compared rather than conflated.
The engineering journey
The Designer’s own stages, in its own terminology. Note where a stage has nothing to calculate — that is the point of separating stated inputs from derived results.
Source & wells
- Decided
- Seawater open intake, intake flow.
- Calculated
- Feed flow available to the train.
- Output
- Source node and intake duty in the diagram.
Feedwater analysis
- Decided
- TDS, temperature, pH, ion chemistry.
- Calculated
- Osmotic pressure basis and scaling inputs.
- Output
- Feedwater basis carried into the design. Stays engineer-review: software cannot verify a laboratory analysis.
Design basis
- Decided
- Recovery, membrane flux, element area, feed pressure.
- Calculated
- Membrane area, elements, vessels, and the feed the RO stage must see.
- Output
- Sizing basis for the RO stage.
Treatment train
- Decided
- Which barriers sit ahead of, and after, RO.
- Calculated
- Each barrier’s area and duty from its own stated basis.
- Output
- The ordered train, and the diagram projected from it.
Treatment train detail
- Decided
- Dosing and re-mineralisation steps.
- Calculated
- Process identity and stream direction.
- Output
- Complete stream topology including concentrate.
Mass balance
- Decided
- Nothing — this stage is entirely derived.
- Calculated
- Feed, product, concentrate flows and concentrate quality by salt conservation.
- Output
- The balance every downstream output is sized from.
Engineering requirements
- Decided
- Nothing.
- Calculated
- One requirement per duty, each labelled input, derived or data-required.
- Output
- Requirements with an explicit basis. A missing head stays missing rather than defaulting to zero.
Equipment
- Decided
- Product selection against each requirement.
- Calculated
- Match evidence per criterion.
- Output
- Selected equipment. Stays not-started until the engineer performs the selection.
Schedule
- Decided
- Engineer-selected products.
- Calculated
- Duration and sequence relationships.
- Output
- Equipment schedule, each row traceable to a requirement.
Bill of quantities
- Decided
- Selected equipment.
- Calculated
- Quantities from the model.
- Output
- BOQ with no fabricated prices — unpriced lines are marked price unavailable.
Engineering report
- Decided
- Everything above.
- Calculated
- Consolidated results.
- Output
- The engineering report, including the same plant-flow projection shown in the Designer.
Engineering outputs
Outputs are projections of the one engineering model, so they cannot disagree with each other. But they are also downstream of an engineer decision, and that decision has not been made in this example.
- Engineering model complete
- Engineering requirements 3 derived
- Equipment awaiting selection
- Schedule awaiting equipment
- Bill of quantities awaiting equipment
- Engineering report awaiting equipment
Why the last four are not populated here
Equipment selection matches products against each requirement using the verified manufacturer catalog, and it is an engineer action. Until you perform it, the Designer reports Equipment, Schedule, BOQ and Report as not started, and the report says so rather than inventing an equipment list. The report PDF linked below was generated from a session in which that selection had been made — it is a real generated report, not a mock-up.
The generated engineering report
Twenty-one sections and seven tables, including the same plant-flow projection the Designer draws. Dates are formatted, not raw ISO.
Want to change the assumptions and see the design update?
Open this case in the Designer and it arrives already filled in. Change the product flow, the recovery or the feedwater chemistry, run the design, and watch the balance, the requirements and the diagram move together — because they are all derived from one model rather than recalculated by hand each time.
Check this example yourself
The same calculations behind this case, as standalone tools, for when you want to test a figure with your own inputs.
- RO Recovery Calculator
Check the 45% recovery this example is built on, against a feed you actually have.
- RO Concentrate Flow Calculator
Confirm the concentrate volume a given recovery produces — the figure most often misused downstream.
- RO Salt Rejection Calculator
Verify the product quality that the stated rejection produces in the permeate.
- RO Permeate TDS Calculator
Derive product TDS from feed salinity and rejection rather than assuming it.
- RO Energy & Specific Power Calculator
Price the specific power implied by the stated pressure, recovery and efficiency.
Related resources
- RO plant design — the full design sequence, and what the platform does and does not model.
- RO calculator gateway — every RO calculation grouped by the question it answers.
- RO recovery — the concept behind the 45% figure used here.
- Desalination technologies — where reverse osmosis sits against thermal processes.
- Plant Designer guide — the twelve stages with real captured screens.