The membrane's terms of employment
A spiral-wound element is a metre of tightly rolled membrane with feed channels about half a millimetre wide. Everything the feed carries either passes through those channels, deposits in them, or reacts with the membrane surface. Pretreatment exists because three specific enemies are each fatal on their own schedule:
Chlorine kills the chemistry. Polyamide membranes have effectively zero chlorine tolerance — oxidation damage is cumulative and irreversible, measured in ppm-hours. The specification is free chlorine below 0.1 mg/L, treated as zero. Protection is activated carbon ahead of the membrane (or sodium metabisulphite dosing on larger plants), and the discipline is verifying it: a free-chlorine test after the carbon at every service, because a spent carbon bed fails silently and the membrane pays.
Particles and colloids blind the surface. The measure is SDI (silt density index) — the membrane-plugging test that turbidity cannot substitute for. DuPont's ceiling is SDI 5; design for SDI below 3 and the cleaning interval stretches from months to years. Getting there is the job of media filtration or ultrafiltration upstream, finished by a 5 µm (or finer) cartridge immediately before the high-pressure pump — whose real role is insurance: it catches upsets, media carryover and pipework debris, and its ΔP gauge is the early-warning instrument for the whole pretreatment train.
Sparingly soluble salts scale the tail. At 75% recovery the concentrate is four times the feed concentration; calcium carbonate, sulphate scales and silica precipitate in the last elements first. The control is either softening (remove the hardness before the membrane — common on small systems) or antiscalant dosing (hold it in solution through the plant — usual at scale), and the design number is the LSI of the concentrate, not the feed.
The quieter enemies
- Iron and manganese: guideline feed limits are severe (Fe < 0.05 mg/L is the working target) because oxidised metals both foul membranes and catalyse chlorine-type oxidation damage. Borehole iron needs removing before the membrane, not remembering after.
- Biology: membranes are a banquet for biofilm. Warm feeds, carbon fines, stagnant standby periods all feed biofouling — design out dead legs, flush on shutdown, sanitise on schedule.
- Oil and grease: effectively banned (they coat the membrane irreversibly); relevant on reuse and industrial feeds.
The standard train
For a typical borehole or municipal feed, the pattern is settled practice:
- Media filtration (or UF) — bulk solids, SDI reduction
- Activated carbon — chlorine destruction (size for contact time, verify at service)
- Softener or antiscalant — scale control per the concentrate LSI
- 5 µm cartridge — final barrier, gauges either side
- High-pressure pump and membranes
Individual waters rearrange or add steps (iron removal, pH trim), but omissions from this list are how membranes die young. The economics are lopsided in pretreatment's favour: the elements are the expensive consumable, and every stage above exists because it is cheaper than the replacement membranes it protects.
Rule of thumb: zero chlorine (verified, not assumed), SDI under 3 by design, and scale control chosen from the concentrate LSI. If any of the three is missing from a proposal, the membrane warranty conversation will eventually be about that gap.