Most of the RO plants we walk into — especially the ones where we've been called because membranes are fouling faster than they should — are dosing antiscalant at the vendor's default rate and blaming the membrane. That's almost never the real problem.
Pakistani feedwater has distinct scaling profiles depending on whether you're drawing from the Indus, tubewells in Punjab, or a coastal intake in Karachi. A dosing rate that works on one can be wildly wrong on another.
Here's a short guide to getting RO antiscalant dose right for Pakistani feedwater.
Start with LSI (or better, S&DSI)
The Langelier Saturation Index (LSI) tells you whether the feedwater — in the reject-side concentrate, not the feed side — is likely to scale with calcium carbonate. For Pakistani RO plants:
- Indus-river-fed plants (industrial clusters near Kotri, Hyderabad): typical LSI at 75% recovery runs +1.5 to +2.5. Manageable with 3–5 ppm UN-307.
- Punjab tubewell feed (textile cluster, Faisalabad / Lahore): variable — LSI at 75% recovery can swing from +0.8 to +2.8 seasonally. Dose 4–6 ppm UN-307 and revisit the feed TDS quarterly.
- Karachi coastal / brackish intake: LSI usually modest (seawater buffering) but sulfate scaling (BaSO₄, SrSO₄) becomes the constraint. 4 ppm UN-307 minimum, verify barium/strontium residuals against solubility index.
- Karachi ground-water brackish (boreholes): high silica is the trap here — can hit 45–60 mg/L. UN-307 manages silica to ~120% solubility; above that you need a silica-specific blend.
Stiff & Davis Saturation Index (S&DSI) is more accurate for brackish/seawater RO and is what we actually specify against. LSI is fine for fresh feed up to ~5000 mg/L TDS.
Common dosing pitfalls we see
- Dosing at the factory-default 2 ppm regardless of feed. That's a labelled "safe starting dose" from the vendor; on 75%-recovery Indus-fed systems running at LSI +2.0, it's underdosing by roughly half.
- Not accounting for dilution water. Some plants inject antiscalant post-chlorination into a high-flow stream, diluting the actual membrane-facing dose to below effective. Always specify dose per total feed flow and verify pump output against operating pressure.
- Running antiscalant in presence of residual free chlorine. Organophosphonate antiscalants like UN-307 tolerate chlorine poorly; if your pretreatment hasn't fully dechlorinated (SMBS dosing underperforming, or feed chlorine spiking during municipal breakpoint events), the antiscalant degrades and membranes see both chlorine attack and unchecked scale. Fix the pretreatment first.
- Forgetting sulfate. LSI-only thinking misses BaSO₄ and SrSO₄. Coastal intakes and some Punjab groundwaters have enough Ba/Sr to bottle-neck your recovery long before calcium carbonate becomes the issue. Request a full cation scan, not just alkalinity + hardness.
Practical starting points (with a caveat)
For a new plant or an unreviewed existing plant, start here and adjust based on the first 30 days of operational data:
- Brackish RO, Indus feed, 75% recovery → UN-307 at 4 ppm
- Brackish RO, Punjab tubewell, 70–75% recovery → UN-307 at 5 ppm
- SWRO, Karachi coast, 45% recovery → UN-307 at 5 ppm + consider silica-specific blend if borehole
- Second-pass RO on demin-grade feed, 85% recovery → UN-307 at 3 ppm, verify downstream CT ionic strength
These are starting points. Monitor normalised differential pressure, salt passage, and permeate flow; if they drift, first check pretreatment integrity, then adjust antiscalant dose, then consider a CIP — in that order.
When to call us (free desk review)
If you're seeing membrane performance degradation and your current chemistry vendor can't tell you why — send us the feed analysis, the plant operating data (last 60 days of NDP, SP, permeate flow), and the current chemistry specification. We'll do a desk review at no cost and come back with a written opinion. If we agree with your current vendor, we'll say so.
Looking for a specified RO chemistry programme, or an autopsy on already-fouled membranes? Get in touch.