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Water • Pretreatment
Pretreatment design & engineering tools
Pretreatment is the part of a water or RO plant that decides whether the
expensive part works. Every barrier you put in front of a membrane
changes the flow, the water quality and the recovery that the membrane
is then designed against — so these are engineering calculations, not
rough sizing rules.
This calculator provides preliminary engineering estimates for informational and planning purposes. Actual reverse osmosis / engineering system performance depends on site conditions, feed-water chemistry, membrane characteristics, operating pressure, temperature, recovery limits, fouling/scaling potential, and system design. Verify results using project-specific data, manufacturer projections, and applicable engineering standards before final design or operation.
What pretreatment actually does
Pretreatment establishes the feed basis for everything downstream. It
removes what the next barrier cannot tolerate — suspended solids,
turbidity, colloids and particulate load — and it controls the
chemistry that would otherwise precipitate onto a membrane surface.
That is why it is an engineering decision rather than a box to tick.
A barrier that under-performs does not fail loudly; it costs membrane
life, cleaning frequency and eventually recovery.
Where the model stops
Pretreatment coverage is not uniform across
technologies. Media filtration, cartridge filtration and
ultrafiltration are sized by calculation here. Coagulation and
dosing are supported as inputs to a design basis rather than as
full process simulation, and fouling behaviour itself is not
predicted. Each technology below states its own level.
The pretreatment design workflow
Each step below is somewhere you can actually go. The chain matters
because the order is not cosmetic: you cannot size a barrier against a
feed you have not characterised, and you cannot fix a recovery without
knowing what the concentrate will do.
FeedwaterCharacterise the source before choosing anything.
Pretreatment calculators are grouped here by the question they close. If
you already know what you need, use the
full tool directory instead.
How much filter do I need for this flow?
When you have a design flow and a permitted filtration rate, and need the vessel area or diameter that results.
Filter Sizing CalculatorSize a conventional rapid-gravity multimedia filter from design flow, filtration rate, bed depth and media effective size: required filter area, design area after the engineer's safety factor, and media volume. The unit count needs a stated catalogue unit area, and no clean-water head loss is invented.
Clarifier Sizing CalculatorSize a circular or rectangular clarifier from the surface overflow rate, then check it against the solids loading when you state both. Reports which criterion governs and never predicts a solids removal.
Then: Confirm what the filtrate must achieve before RO — then size the fine barrier ahead of it.
What fine-filtration capacity protects the membranes?
When specifying the barrier immediately upstream of RO, where a defined solids removal is more than a media filter can hold.
Cartridge Filter Sizing CalculatorSize cartridge and bag filter elements and housings from design flow, feed solids and the manufacturer's element flow rating: hourly solids load, element count and housing count. No element size is assumed and no differential pressure is invented.
Then: Test the design against the real feed with a worked case, or move to the design-intent chain below.
Is a membrane barrier ahead of RO justified, and what area does it need?
When you are deciding between conventional pretreatment and UF, or sizing a UF stage from a design flux.
Ultrafiltration (UF) Membrane EngineeringSize UF membrane area from permeate flow and design flux, compute transmembrane pressure from the feed/filtrate/concentrate pressures, and read normalised flux against the reference condition to expose fouling.
Nanofiltration (NF) Membrane EngineeringClose the NF water and solute balance from a stated recovery and rejection to get permeate and concentrate flow and TDS, then size membrane area from the permeate flux.
Ion Exchange (IX / IER) Bed SizingSize an ion exchange bed from service flow, resin volume and empty-bed contact time, and compute stoichiometric regenerant demand from the ionic loading.
What conditioning does the feed need, and what does it cost in flow?
When deciding on coagulation, antiscalant or biocide control — and when the dose affects downstream carry-over.
Coagulant Dosing CalculatorTurn a jar-test coagulant dose into active metal and product mass per day, from the dose basis you measured and the product active content. The dose is your jar test and the pH outcome is not predicted.
Sodium Metabisulphite Dosing CalculatorDose sodium metabisulphite from the chlorine residual it must neutralise or the sulphite level it must hold, using the stoichiometric ratio and your product strength. No chlorine residual and no product strength are assumed.
Disinfection CT Value CalculatorCompute the achieved CT value from residual disinfectant concentration and contact time, compare it against a required CT you state from your disinfection reference, and get the contact time the design needs to reach it.
Then: Check what the water will do to the membrane once conditioned, before you fix the recovery.
Will the feed water behave, and does the basis hold up?
Before committing to a recovery: scaling indices, the demand and storage the design must serve, and cleaning provision.
Water Quality Saturation Indices (LSI & RSI)Evaluate calcium carbonate stability of a water with the Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) from pH, temperature, TDS, calcium, and alkalinity.
Water Analysis & Treatment SelectionEnter a feedwater analysis once: derive hardness, cation–anion charge balance, TDS by summation, ionic strength and saturation indices (LSI/RSI), then compare water-quality drivers against the treatment surfaces already built on the platform — a qualitative start, not a diagnosis.
Water Demand & Flow Requirement CalculatorEstimate community or facility water demand (domestic, commercial, fire) and the design flow requirement for supply and storage sizing.
Water Tank Storage CalculatorSize cylindrical and rectangular water storage tanks from dimensions or daily demand and residence time.
CIP Tank Sizing CalculatorSize CIP cleaning and storage tanks from your cleaning protocol volume per element and the heel the tank retains, with separate alkaline, acid and neutral volumes and optional annual chemical consumption.
Then: Run the whole train, not one number — that is what the Plant Designer is for.
Pretreatment technology options
Each entry states what the platform actually does with it. Reference-only
entries are described but not sized — see the
Technology Library for the full
treatment.
Compares a stated or measured feedwater analysis against the barriers a desalination pretreatment train can actually contain, raising a candidate only from a fact you supplied or the platform derived — and reporting its own confidence rather than choosing for you.
Low-pressure membrane barrier that removes colloids, bacteria and suspended solids ahead of RO or NF, quantified by membrane area, TMP and flux condition.
Disposable or washable depth filters that protect downstream membranes from particles, ahead of any high-pressure stage.
Work it through on a real train
These 9 cases declare a pretreatment sizing field or carry a
pretreatment barrier, so each one is a genuine pretreatment case rather
than an RO case that happens to be nearby. Open one and it loads into the
Plant Designer already filled in — run it, inspect the diagram, change an
input and watch the balance move.
Synthetic demonstration / educational example — not project
design. These are illustrative starting bases, not reference
designs, vendor recommendations or compliance cases.
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.
Same seawater chain, but abstracted from a beach wellfield at a lower RO design flux — the gentler-flux variant an engineer explores when scaling risk or membrane warranty drives the basis down.
A brackish groundwater case: lower feed TDS, a higher stage recovery and a lower feed pressure. Useful for seeing how a non-seawater source changes the balances and the duty the model reports.
A seawater case whose train adds an ultrafiltration stage in front of the RO stage, with UF-specific flux and filtrate-recovery fields. It exercises a different train shape and the separate UF/RO bases.
A larger municipal-scale seawater case (2,000 m³/day product) with a higher element count per vessel, to exercise the multi-vessel RO sizing path and a bigger intake / pressure-pump duty.
The engineering-acceptance dataset re-runnable on the designer default train: 1,000 m³/day product from 3 beach wells via a multimedia filter at 97 % and a single-RO stage at 40 % on a 2,577.32 m³/day intake, with full source, hydraulic and sizing basis. A test input, not a reference design.
A two-stage seawater RO array: the first stage rejects a concentrate that a second stage treats further, so both stages permeate becomes product and plant recovery exceeds a single stage. Demonstrates the concentrate-fed chain.
A two-pass seawater RO train: the first pass permeate is polished by a second pass for a lower product TDS. The first pass contributes no separate plant product, so overall recovery is lower than a single pass at the same feed.
Choose a barrier. Start from the decision groups above.
Run the calculation. Check the number against your own flow.
Review a worked example. The
UF + RO worked example shows a membrane
barrier taking 2,328.0 m³/day and handing the RO stage 2,211.6 m³/day —
the filtrate split that changes the membrane design basis.
Load a demonstration case. Any case above opens in the Designer pre-filled.
Build the treatment train. Place the barriers and derive the rest.
Generate the engineering outputs. Requirements through to report.
Calculations are not the end of the workflow
One calculation answers one question. A plant design has to keep every
answer consistent as the design changes: the pretreatment you sized
changes the RO feed, which changes the recovery, which changes the
concentrate, which changes the equipment duty and the schedule. The Plant
Designer derives all of it from one model.
Equipment, schedule, BOQ and report stay empty until an engineer selects
products from the verified catalog. That is the workflow refusing to
invent an equipment list, and it is worth knowing before you start.