Desalination engineering

What desalination is

The engineering problem, stated plainly.

Desalination is the separation of dissolved salts from water so the product can be used. Everything else — the choice of process, the recovery you can defend, the pretreatment the membranes need — follows from the salt concentration in the feed and the purity you have to deliver.

That framing matters because it rules out a common mistake: treating desalination as a single technology. It is a family of processes, and the right one depends on the feed salinity, the product specification, the scale, and what you can do with the concentrate. The Plant Designer treats it that way: you state the source and the requirement, and the train is built to match.

The main approaches, and where EngiMetric draws its line

Two families dominate. Membrane processes push water through a semi-permeable barrier under pressure; thermal processes boil water and condense the vapour, separating salt by phase change rather than by membrane selectivity.

EngiMetric is explicit about which of these it actually calculates. Reverse osmosis, nanofiltration and ultrafiltration are modelled quantitatively — the mass balance, the recovery and the duties are computed from the inputs you state. Thermal desalination (multi-effect distillation, multi-stage flash), brine and zero-liquid-discharge arrangements are described as reference technology: they are here so you can understand the option and its trade-offs, not because the platform will size them for you.

That distinction is shown on every technology card below. Where an entry is marked reference-only, the engineering figures come from the process vendor or a specialist calculation, not from this platform.

Seawater and brackish water are different jobs

Seawater carries roughly ten times the salt concentration of brackish groundwater, and that single fact changes the whole plant. Feed osmotic pressure is an order of magnitude higher, so the operating pressure and the energy demand rise with it. Scale formation risk rises, because the concentrate approaches solubility limits for the sparingly soluble salts. Pretreatment has to work harder, and the concentrate volume is a larger fraction of the feed.

Brackish water needs less pressure and can usually run at a much higher recovery, because the salt load is lower. That makes the membrane area per unit of product smaller and the energy demand lower, but it does not make the engineering easier — the recovery is high enough that the concentrate chemistry is usually the governing constraint.

Both cases are in the demo gallery below with their real figures, so you can compare a seawater and a brackish balance directly rather than taking the difference on trust.

What decides the design

A desalination design is decided by a short list of coupled quantities. The feed salinity sets the osmotic pressure and therefore the pressure the membranes need. The recovery sets how much feed you must abstract for a given product flow, and therefore how much concentrate you produce. The product specification sets the required rejection. Pretreatment exists to protect the membranes from whatever the feed carries.

Because these are coupled, you cannot pick them one at a time. That is why the platform computes them together: a change to the product flow or the recovery re-derives the feed, the concentrate and every downstream duty. It is also why a single-number calculator is useful for understanding one relationship but not sufficient for a plant — the balance has to close across the whole train.

Two cases in the gallery (Two-Stage SWRO — staged array and Two-Pass SWRO — permeate polishing) show multi-stage and multi-pass arrangements. They build a real train, but the interstage and second-pass transfer is <strong>not quantitatively modelled</strong>, so read them as topology rather than as verified balances.

Desalination technologies

Each entry states its own modelling level. Anything EngiMetric does not calculate is marked, rather than implied to be engineered.

• Reverse Osmosis Quantitatively modelled

  The core desalination technology of the platform: a semi-permeable membrane driven by pressure, with recovery, salt rejection, energy and concentrate all derived by real engines.

• Seawater Reverse Osmosis Quantitatively modelled

  RO applied to seawater: high feed salinity, single-pass configuration, and a recovery limited by osmotic pressure and scaling rather than by membrane rejection.

• Brackish Water Reverse Osmosis Quantitatively modelled

  RO applied to low-salinity groundwater or reuse water: lower osmotic pressure allows much higher recovery and lower pressure than seawater.

• Two-Pass Reverse Osmosis Quantitatively modelled

  A second RO pass on the first permeate, optionally with interstage recycle, to reach a permeate quality a single pass cannot deliver economically.

• Multi-Effect Distillation Reference only

  Vapour-compression multi-stage evaporation, usually tied to a waste-heat source. Catalogued for orientation; EngiMetric models no MED thermal or performance figures.

• Multi-Stage Flash Reference only

  Heated brine flashed through successive pressure stages. Catalogued for orientation; EngiMetric models no MSF thermal or performance figures.

• Hybrid RO + Thermal Quantitatively modelled

  Combining SWRO with a thermal stage, usually to treat the RO concentrate thermally. EngiMetric models the RO half of this arrangement only.

• Brine Management Reference only

  Handling the reject stream: its flow and salinity are derived by the RO engines, but its disposal or reuse route is not modelled.

• Zero Liquid Discharge Reference only

  Concentrating a brine to a dry or near-dry residue. Catalogued for orientation; EngiMetric has no ZLD engine and states no thermal or crystalliser figures.

• Mineral Recovery Reference only

  Recovering salts such as sodium chloride from a concentrate. Orientation only; EngiMetric models no crystallization or product-yield calculation.

• Evaporation Ponds Reference only

  Solar evaporation in lined ponds, the lowest-energy concentrate volume reduction available. Orientation only.

Desalination and RO calculators

Each tool calculates one thing and shows its working. They are the right next step once the principle makes sense.

• RO Recovery Calculator

  Calculate RO permeate and concentrate flow from feed flow and system recovery rate.

• RO Salt Rejection Calculator

  Calculate membrane salt rejection percentage from feed and permeate TDS concentrations.

• RO Permeate TDS Calculator

  Project expected permeate total dissolved solids based on feed water salinity and rejection specs.

• RO Concentrate Flow Calculator

  Compute brine/reject flow rates and volumetric stream balances for disposal or ZLD design.

• RO Osmotic Pressure Calculator

  Estimate RO feed osmotic pressure using a simplified van’t Hoff model from TDS, temperature, and solute properties.

• RO Energy & Specific Power Calculator

  Determine specific energy consumption (kWh/m³) based on operating pressure and pump efficiency, with optional energy recovery device savings.

• Small & Modular Desalination Sizing

  Size a small or modular desalination plant from a demand basis forward: community population to product flow, then feed and concentrate by the plant mass balance, then membrane area, vessels and staging. Composes engines that already exist; chooses no recovery, staging ratio, flux or package envelope.

• RO Feed Pressure & Specific Energy Calculator

  Estimate required RO feed pressure (permeate osmotic gap + net driving pressure) and specific energy consumption with optional energy recovery.

• RO Membrane & Vessel Element Count Calculator

  Calculate number of RO membrane elements and pressure vessels from permeate flow, design flux, and element area.

• Desalination Plant Designer

  Compose a whole desalination plant as an ordered chain of unit operations from raw intake to product and waste: design basis, stage recoveries and make-up, whole-plant water and salt balance checks, and concentrate TDS derived by salt conservation — every value labelled input, derived or missing.

• Volumetric → Mass Flow Converter

  Convert volumetric flow to mass flow (and reverse) using fluid density, with common water/TDS basis.

• Brine Volume and Mass Calculator

  Annualise the brine volume and salt mass from the concentrate stream, with optional disposal cost and the dilution arithmetic against a stated permit limit. No compliance verdict is issued.

Seawater and brackish demo cases

Each case opens in the Plant Designer as a working, editable design. They are synthetic demonstration bases — not project data.

• Seawater — open intake, single RO stage Quantitative case
• Seawater — beach wellfield, lower design flux Quantitative case
• Brackish groundwater — single RO, higher recovery Quantitative case
• Municipal seawater — larger flow, multi-vessel RO Quantitative case