What comes out of a healthy plant
A brackish element rejecting 99% of salt at standard conditions does not deliver 1% of feed TDS in practice. The field number runs higher, because rejection is measured against the average concentration along the membrane, which recovery pushes well above the feed value. Working expectations for a healthy brackish system: permeate TDS around 1 – 4% of feed TDS: about 1 – 2% with high-rejection elements at moderate recovery, rising toward 4% with low-energy elements or high recovery. A 1 000 mg/L feed typically yields permeate in the tens of mg/L. That is excellent by any drinking standard, but not zero. Where near-zero matters (boilers, electronics, laboratory), the answer is a second pass or polishing (mixed-bed DI, EDI) downstream, not disappointment in the first pass.
Two chemical quirks of permeate surprise people. CO₂ passes straight through the membrane, so permeate pH is typically 5 – 6.5 and the water is unbuffered and slightly aggressive. pH correction or remineralisation ahead of copper pipework or drinking supply is normal design, not a defect. And permeate is hungry water: low-TDS, low-alkalinity water pulls minerals from cementitious materials, another reason storage and pipework materials deserve a thought.
Normal drift — the movements that are not faults
- Temperature. Salt passage rises with temperature even as production rises: summer permeate is more plentiful and slightly saltier. Judge the plant on normalised numbers, not raw seasonal ones.
- TDS creep on start-up. During shutdown, salt diffuses across the idle membrane; the first minutes of production run measurably saltier until it flushes through. Plants that cycle frequently should divert or tolerate the first permeate; a permeate flush at shutdown largely prevents it.
- Recovery drift. Anything that quietly raises recovery (a throttled concentrate valve, a permeate demand change) raises average feed-side concentration and with it permeate TDS. Check the flows before blaming the membranes.
- Ageing. Rejection declines gently over an element's life; a slow multi-year drift is the membrane amortising, not an event.
Faults — the movements that are
A step change in permeate conductivity is mechanical until proven otherwise, and two different faults hide under that heading. A failed interconnector o-ring or a cracked permeate tube opens a direct path from feed into permeate: one small leak moves the number faster than any membrane chemistry can, and probing the vessel (sampling conductivity along the permeate tube) locates it. A brine seal fitted backwards or damaged does something different: feed slips around the outside of the element instead of being driven through it, so rejection falls and the vessel never builds its normal pressure drop, with no direct feed-to-permeate path to find. Probing finds the first; a vessel running soft on ΔP points at the second. A steady climb with rising ΔP in the last stage says scaling. Chlorine damage shows as rejection falling plant-wide while flow increases, as the oxidised membrane starts passing salt like an expensive cartridge filter. Each signature points at a different repair, which is why the weekly log of normalised flow, salt passage and per-stage ΔP is the cheapest instrument on the plant.
Rule of thumb: expect permeate around 1 – 4% of feed TDS, slightly acidic, and seasonally variable. That is a healthy plant. Sudden change means mechanical leak, quality-plus-ΔP change means scale, quality-plus-more-flow means oxidation. Normalise before diagnosing.