Mass balance, recovery and salt balance
A mass balance asks whether everything that goes in comes out. Applied to water and to salt separately, it is the check that makes every other engineering number either meaningful or obviously wrong. This hub explains the relationships and links to the tools that apply them.
Why a mass balance comes first
Every other number depends on it closing.
A mass balance answers one question: does everything that goes in come out? Applied to water and to the salt in it separately, it is the check that tells you whether a design is internally consistent before you ask whether it is any good.
It is the first thing to calculate and the last thing to trust. If the flows do not conserve, no subsequent number means anything — the equipment, the schedule and the report are all derived from these quantities. This is why the platform computes the balance from the train rather than accepting a stated answer, and why a missing input produces a missing result rather than a zero.
Flow conservation
The simplest statement in the discipline, and the one that catches most errors.
Water conservation across a treatment plant is a single statement: total flow in equals total flow out. For a reverse-osmosis stage that is:
For every stream, in and out. Written for the stage itself it becomes:
The second form is the one worth using, because it names both product streams instead of leaving “reject” implicit. Both are the same statement; the second just refuses to hide half the answer.
Because it is so simple, a failure here is diagnostic. If a balance will not close, the usual causes are a stream that was declared but never given a flow, a unit conversion applied twice, or a recovery stated as a percentage where the engine expects a fraction. Those three account for most of it.
Q_feed = Q_product + Q_concentraterecovery = Q_product / Q_feedRecovery, and what it costs you
Recovery is the fraction of the feed that leaves as product. It is a design variable with real trade-offs on both sides, which is why it cannot simply be maximised.
Recovery fixes the feed. To make a given product flow at a given recovery you must abstract a proportional amount of feed — so raising recovery reduces the feed you need, and therefore the intake, the pretreatment and the pumping duty. Recovery also fixes the concentrate volume: whatever is not product leaves as concentrate, and less product per unit of feed means more concentrate per unit of product.
The limit is not hydraulic. It is chemical. As water leaves as permeate, the salt stays behind and the concentrate concentrates, approaching the solubility limits of the sparingly soluble salts. The recovery you can defend is the recovery at which the concentrate is still stable — and beyond it, scaling is not a performance problem but a plant shutdown.
The salt balance
Why concentrate quality is not a design choice.
The second balance is on the salt rather than the water, and it is what fixes the concentrate quality:
This is a conservation statement, not a design choice. Because the salt has to end up somewhere, the concentrate quality is determined by the feed quality, the recovery and the rejection — you do not get to specify it independently. That is why concentrate TDS is an output of the design rather than an input to it.
It is also why the balance must close on both streams independently. A water balance can close while the salt balance does not, and the usual cause is a stated rejection being treated as the product quality instead of being derived from the salt conservation. EngiMetric derives the product quality from salt rejection rather than the other way round, because that is the direction the physics runs.
Where a figure cannot be derived — because a head is missing, or a duty basis was never supplied — the platform shows it as data required and the balance reports incomplete. It does not close the gap with an assumption, because a balance that closes on invented inputs is worse than one that visibly does not close.
Q_feed x TDS_feed = Q_product x TDS_product + Q_concentrate x TDS_concentrateEvery product and concentrate TDS on the calculators and in the plant designer comes from this statement.
Mass balance calculators
Each tool calculates one thing and shows its working. They are the right next step once the principle makes sense.
- Plant Mass Balance
The general flow and mass balance engine: streams in, streams out, and the components that have to add up on both sides.
- 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 Salt Passage Calculator
Determine salt passage percentage through RO membranes and verify complementary rejection balance.
- 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.
Demo cases with a full calculated balance
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
- Brackish groundwater — single RO, higher recovery Quantitative case
Put it into a plant
The Plant Designer performs this balance for the whole train, not one stage. The mass-balance section of a design reports feed, product and concentrate flows and the concentrate quality, and every downstream output — requirements, equipment duties, schedule and report — is derived from those figures.