Choosing pretreatment so the membranes survive

Matching filtration and dosing decisions to the fouling and scaling risk of the raw water.

The engineering question

Membranes are fouling before their warranty is used up. What pretreatment does this water actually need?

Worked example

The figures below were produced by running the RO Antiscalant & Chemical Dosing Calculator with the same inputs .

These figures are worked by the published method rather than executed at build time, because this tool is a self-contained page calculator without a programmatic engine.

Water analysis establishes which scalants are actually at risk, which is what decides the pretreatment.
Antiscalant dose is expressed as ppm by mass of feed, so it follows from flow and concentration.
Filtration and cartridge ratings then follow the fouling risk rather than the scaling risk.
The choice itself remains judgement — the tools quantify the consequences, not the decision.

Sanity check

Dose in kg/day should follow flow × ppm ÷ 1000. A figure an order of magnitude out usually means the dose was entered as mg/L and treated as a percentage.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

Engineering review required

This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.

References & standards

DOWEX — pretreatment and membrane fouling
Hydranautics — system design guidelines