RO/NF Pretreatment
Reference for what feeds the membrane train, with KPI targets, chemistry, and instrumentation. The 80 % rule: most RO/NF/EDI problems originate upstream.
Convention markers:
⚠️ — likely bug or incorrect assumption in calling code💡 — enhancement opportunity (new alert, new monitoring point, new dosing logic)
§1 — Pretreatment Chain Overview
1.1 SWRO open intake
Sea intake (screen 5–25 mm)
→ Travelling band / drum screen (~1–3 mm)
→ In-line coagulation (FeCl3 0.5–5 mg/L as Fe) + flash mix (G ≈ 700–1000 s⁻¹, 30–60 s)
→ Flocculation tank (G ≈ 30–80 s⁻¹, 15–30 min) [optional with inline coag]
→ DAF (HABs/algae/oil) OR direct media filtration
→ Dual / Multimedia gravity filter (5–15 m/h) OR pressure MMF (15–25 m/h)
→ [Optional] UF 0.02 µm for SDI₁₅ ≤ 2
→ Cartridge filter 5 µm absolute (ΔP fresh < 0.3 bar; change at 1.0–1.5 bar)
→ Antiscalant injection
→ SBS dechlorination (only if Cl2 used pre-filter)
→ Acid dosing (H2SO4 or HCl) for LSI control
→ HPP → SWRO 1st pass
1.2 SWRO beach well (subsurface intake)
Beach well / infiltration gallery (natural aquifer filtration)
→ Cartridge filter 5 µm
→ Antiscalant injection
→ SBS only if shock-chlorination at wellhead
→ Acid dosing
→ HPP → SWRO
Subsurface intakes naturally deliver SDI₁₅ < 3 in most geologies → skip coagulation and media filtration. Yield (m³/h per well) and risk of Fe/Mn intrusion from anoxic strata are the practical limits.
1.3 BWRO well (saline groundwater)
Well pump → aeration / oxidation (Cl2, KMnO4, or O2) if Fe/Mn > 0.05 mg/L
→ Manganese greensand / pyrolusite filter (Fe/Mn removal)
→ MMF polishing
→ Cartridge filter 5 µm
→ SBS (if Cl2 residual present)
→ Antiscalant
→ Acid (for high alkalinity / LSI)
→ BWRO
1.4 BWRO surface water
Same architecture as SWRO open intake but lower TDS, lower turbidity peaks. pH control more critical (CaCO3 dominant scaling vs Mg-sulfate).
1.5 Tertiary reuse → BWRO
Biological secondary effluent → MBR or tertiary UF/MF
→ break tank
→ cartridge filter 5 µm
→ DBNPA continuous low-dose (1 mg/L) optional
→ antiscalant (low-P or P-free)
→ BWRO 1st pass
MBR/tertiary UF typically produces SDI₁₅ 0.5–2.0; secondary effluent may exceed 3 in some months.
§2 — Pretreatment KPI Targets (pre-RO)
| Parameter |
SWRO open |
SWRO beach well |
BWRO well |
BWRO surface |
Tertiary reuse |
| SDI₁₅ target |
≤ 3 |
≤ 2 |
≤ 3 |
≤ 3 |
≤ 3 |
| SDI₁₅ max |
5 |
3 |
4 |
5 |
5 |
| SDI₁₅ with UF |
≤ 2 |
n/a |
≤ 2 |
≤ 2 |
≤ 2 |
| Turbidity (NTU) target |
< 0.1 |
< 0.1 |
< 0.1 |
< 0.1 |
< 0.2 |
| Turbidity (NTU) max |
1.0 |
0.5 |
1.0 |
1.0 |
1.0 |
| Free Cl₂ pre-membrane |
0 ppm |
0 ppm |
0 ppm |
0 ppm |
0 ppm |
| ORP post-dechlor |
< 200 mV |
n/a |
< 200 mV |
< 200 mV |
< 200 mV |
| AOC (µg C/L) |
< 50 excellent / < 100 acceptable |
< 50 |
< 50 |
< 100 |
< 100 |
| BGP (µg/L) |
< 70 |
< 70 |
n/a |
n/a |
n/a |
| Iron (mg/L) |
< 0.05 |
< 0.05 |
< 0.05 |
< 0.05 |
< 0.1 |
| Manganese (mg/L) |
< 0.02 |
< 0.02 |
< 0.02 |
< 0.02 |
< 0.05 |
| Aluminum residual (mg/L) |
< 0.05 |
< 0.05 |
< 0.05 |
< 0.05 |
< 0.05 |
| Operating pH |
6.5–7.5 |
6.5–7.5 |
5.5–7 |
5.5–7.5 |
6–7.5 |
| Cartridge ΔP fresh |
< 0.3 bar |
< 0.3 bar |
< 0.3 bar |
< 0.3 bar |
< 0.3 bar |
| Cartridge ΔP change-out |
0.7–1.0 bar |
0.7–1.0 bar |
0.7–1.0 bar |
0.7–1.0 bar |
0.7–1.0 bar |
⚠️ SDI₁₅ > 5 voids most OEM warranties. SDI₁₅ ≤ 5 is the absolute hard limit.
AOC threshold from Vrouwenvelder / Weinrich biofouling correlation studies. Raw seawater commonly 30–400 µg C/L (Tampa Bay 360 ± 180 µg/L; Monterey 30 ± 20 µg/L).
⚠️ Heavy metals (Fe³⁺, Mn²⁺, Cu²⁺, Co²⁺) catalyse SBS → oxidant conversion under O₂. Keep trace metals low and minimize SBS overdose (see §5.4).
§3 — Coagulation / Flocculation Chemistry
3.1 Coagulant comparison
| Coagulant |
Formula |
Dose (as product) |
Dose (as metal) |
Optimal pH |
Notes |
| Ferric chloride |
FeCl₃·6H₂O |
5–40 mg/L |
1–10 mg/L Fe |
5.0–8.5 (best 5–7) |
Wide pH; dense floc; residual Fe³⁺ accepted < 0.05 ppm; preferred for SWRO |
| Ferrous sulfate |
FeSO₄·7H₂O |
10–50 mg/L |
2–10 mg/L Fe |
8.5–11 + Cl₂ oxidize Fe²⁺→Fe³⁺ |
Cheap; needs alkaline conditions |
| Alum |
Al₂(SO₄)₃·14–18H₂O |
5–60 mg/L |
0.4–5 mg/L Al |
6.0–7.5 (narrow) |
Al residual → AlPO₄/Al(OH)₃ fouling on RO; avoid for RO when possible |
| PAC |
Al_n(OH)m Cl{3n-m} |
5–30 mg/L |
0.5–3 mg/L Al |
5.5–9 (wide) |
Lower residual Al than alum; less pH depression; efficient at cold T |
3.2 Polymer flocculant aids
- Type: anionic / cationic / nonionic polyacrylamide (PAM); also tannin-based natural polymers
- Dose 0.05–1 mg/L. Under-dose → no benefit; over-dose → carryover, RO fouling
- Cationic PAM acts as primary coagulant in low-turbidity surface waters
⚠️ Cationic polymer + anionic antiscalant → precipitate → RO fouling. Always jar-test the combination before deployment.
3.3 Jar test methodology
- Setup: 6 paddle jars × 1 L; coagulant in dilute solution (1–10 % w/v)
- Flash mix: 100–200 rpm × 30–60 s (G ≈ 700–1 000 s⁻¹)
- Flocculation: 30 rpm × 15–30 min (G ≈ 30–80 s⁻¹, tapered if possible)
- Settling: 30 min undisturbed
- Sampling: supernatant at fixed depth; measure turbidity, TOC, SDI, residual coagulant metal, pH
G value:
G = sqrt(P / (μ · V)) # P = power (W), μ = dynamic viscosity (Pa·s), V = volume (m³)
3.4 Pros/cons for downstream RO
| Aspect |
Iron-based (FeCl₃) |
Aluminum-based (alum, PAC) |
| Residual carryover risk |
Fe³⁺ acceptable up to 0.05 ppm |
Al³⁺-phosphate fouling, severe at > 0.05 ppm |
| Floc strength |
Dense, settle easily |
Lighter |
| pH window |
Wide |
Narrow (alum); wider (PAC) |
| Cold water |
Good |
Good (PAC) / poor (alum) |
| TOC removal |
Excellent |
Moderate |
| SWRO compatibility |
Preferred |
Avoid alum; PAC OK with caution |
§4 — Antiscalant Selection and Dose Math
4.1 Chemistry families
| Family |
Examples |
Best for |
Typical dose |
Notes |
| Phosphonate |
HEDP, PBTC, ATMP, DTPMP, BHMTPMP |
CaCO₃, CaSO₄, BaSO₄, SrSO₄, CaF₂ |
2–5 mg/L |
Workhorse; P discharge constraints |
| Polyacrylate / acrylic homo & copolymer |
PAA, AA-MA |
BaSO₄, SrSO₄, suspended solids dispersion |
1–5 mg/L |
P-free; lower Ca tolerance |
| Sulfonated copolymer |
AA-AMPS, AA-AMPS-HPA terpolymer |
High Ca, CaPO₄, ZnCO₃ |
2–5 mg/L |
High Ca + alkaline tolerance |
| Maleic-based |
MA-AA, polymaleic |
CaCO₃, CaSO₄ broad |
2–5 mg/L |
P-free; used in P-restricted discharges |
| Polymeric silica dispersant |
PEG, PEGD, PVA, PAMAM, PEI |
SiO₂, colloidal silica |
1–5 mg/L |
For SiO₂ > 100 ppm in concentrate |
| Dendrimer / latest-gen |
Polyaspartate, dendrimer-based |
Broad multi-species |
1–4 mg/L |
Biodegradable; premium |
4.2 Selection by scaling species
| Scaling species |
Antiscalant of choice |
| CaCO₃ |
Phosphonate (HEDP, PBTC) OR maleic-based (P-free) |
| CaSO₄ (gypsum) |
Phosphonate + polyacrylate blend |
| BaSO₄, SrSO₄ |
Polyacrylate or AA-AMPS (phosphonates can co-precipitate with Ba) |
| CaF₂ |
Phosphonate |
| SiO₂ amorphous |
Polymeric silica inhibitor (PEG/PEGD) + pH adjustment |
| Ca₃(PO₄)₂ |
AA-AMPS terpolymer — NOT phosphonate |
| Fe / Mn fouling |
Specialised dispersant blend |
4.3 Dose calculation
Three approaches:
- Vendor projection software (preferred): DuPont WAVE, Hydranautics IMSDesign, Toray DS-Design, Veolia Winflows, Avista AdvisorCi, Genesys Genesys Member. Software computes LSI / S&DSI / IP-Ksp per stage and recommends dose.
- LSI-driven empirical: target concentrate LSI ≤ +1.8 (conservative) or ≤ +2.5 (premium antiscalant). Typical dose 2–5 mg/L product.
- Jar-tested matrix: when projection unavailable.
4.4 Compatibility traps
⚠️ Cationic biocide + anionic antiscalant → precipitate.
⚠️ Phosphonate + Ba²⁺ → BaSO₄ co-precipitation under some conditions.
⚠️ Phosphate-containing antiscalant + Ca²⁺ at high pH → calcium phosphate scale (unintended).
⚠️ EU/CN P discharge limits → switch to maleic / polyacrylate / sulfonated terpolymer for compliance.
§5 — Dechlorination (SBS / SMBS)
5.1 Chemistry
Na2S2O5 + H2O → 2 NaHSO3
NaHSO3 + HOCl → NaHSO4 + HCl
5.2 Stoichiometry
- Theoretical (DuPont 45-D01569): 1.34 mg SMBS per mg free Cl₂ (≈ 1.46 mg NaHSO₃ per mg Cl₂)
- Practical: 1.5–3.0 mg/mg (50–125 % excess) to account for incomplete mixing, competing reactions (DO, chloramines, NOM), continuous overdose margin
- DuPont rule of thumb: ~3.0 mg SMBS per mg free Cl₂ (≈ 2.2× theoretical)
⚠️ SMBS (Na₂S₂O₅) and SBS (NaHSO₃) have different molar masses — confirm which the dosing pump is metering and adjust stoichiometry accordingly.
5.3 Chloramines
- NH₂Cl, NHCl₂ react more slowly with SBS than free Cl₂
- At feed pH ≥ 8.5, SBS becomes ineffective at fully reducing chloramines
- ORP signal unreliable for chloramines — supplement with DPD analyzer
- Mitigation: acidify before SBS injection, increase dose 2–3×, increase contact time
5.4 The oxidant paradox
Under DO + transition metal traces (Cu²⁺, Co²⁺, Fe³⁺, Mn²⁺):
2 NaHSO3 + O2 --[Cu/Co]--> 2 NaHSO4 + sulfite radicals (SO3•⁻, SO4•⁻)
Sulfite radicals attack polyamide. Residual SBS up to 30 ppm has been reported to convert partially to oxidants in this scenario.
⚠️ Chronic SBS overdose is not safer than just-right dosing. Keep margin small (just enough to neutralize Cl₂); minimise air ingress in suction lines; deaerate where Cu/Co > trace.
5.5 Monitoring
- ORP post-dechlor: < 200 mV (some vendors < 150 mV)
- Pre-Cl₂ ORP typical: +300 to +600 mV
- DPD analyzer redundant safety against breakthrough
- ORP under SBS overdose can read misleadingly low
5.6 SBS solution preparation
- Use RO permeate, not raw water, to avoid sulfate scale in dosing line
- Concentration 5–15 % w/w (higher decomposes faster, especially warm)
- Filter SBS solution through dedicated cartridge before injection (insoluble grit, sulfate)
- Inject downstream of last cartridge filter with static mixer
- Solution shelf life ~7 days; replace weekly
⚠️ Do not store dechlorinated water in tanks — promotes bacterial regrowth between dechlor and HPP.
5.7 Alternative: GAC
| Aspect |
SBS |
GAC |
| CapEx |
Low |
High |
| OpEx |
Recurring chemical |
Periodic media replacement (1–3 yr) |
| Reliability |
Active control loop |
Passive failsafe |
| Bacterial regrowth |
None |
High (bed = bioreactor) |
| ORP control needed |
Yes |
No |
| SWRO use |
Primary |
Rare (footprint, bio) |
| Pharma / USP use |
Sometimes |
Common (chlorine-free + TOC reduction) |
§6 — CO₂ / pH Management
6.1 Why CO₂ matters
- Neutral CO₂ permeates RO (small uncharged molecule)
- HCO₃⁻ is rejected (charged, hydrated)
- For 2nd-pass RO or EDI feed, CO₂ in 1st-pass permeate becomes the dominant ionic load downstream
- FCE (for EDI) penalizes CO₂ at 2.79 µS/cm per ppm
6.2 Strategies
| Strategy |
CO₂ outlet |
Use when |
Notes |
| Interpass NaOH injection |
residual converted to HCO₃⁻, rejected by pass 2 |
2-pass RO present |
Target interpass pH 8.4–8.7; also boosts boron and silica rejection |
| Membrane degasser (PTFE/PP hollow fiber, e.g. Liqui-Cel) |
< 5 ppm typ, < 0.5 ppm achievable |
RO + EDI (no 2nd RO) |
Vacuum and/or sweep gas; CIP every 3–6 months |
| Forced-draft atmospheric tower |
5–10 ppm |
Lower-purity applications |
Open to atmosphere → bacterial ingress risk |
| Vacuum tower degasser |
1–5 ppm |
High-purity preferred |
Higher CapEx than membrane |
6.3 pH control loop
- pH transmitter (glass + Ag/AgCl or solid-state), inline post-injection static mixer
- PID-controlled dosing pump (NaOH 50 % or H₂SO₄ 96–98 %)
- Control band ±0.1 pH typical; alarms at ±0.3
⚠️ Buffer-poor RO permeate → small dose causes large pH swing. Tune integral term carefully or use cascade control.
6.4 pH effects on rejection (2nd pass)
- Boron (pKa 9.14–9.24): at pH 9.5–9.8, H₃BO₃ → B(OH)₄⁻, rejected by RO. Critical for SWRO when boron > 0.5 mg/L target (WHO/EU drinking water)
- Silica (pKa 9.84): higher pH → silicate species → better rejection
- CO₂: see §6.1
Tradeoff: pH > 8 increases CaCO₃ scaling risk on 2nd-pass concentrate if Ca slip from 1st pass.
§7 — Biocide Strategy (Split Pre/On Membrane)
7.1 Pre-RO (oxidising OK, must be removed before membrane)
| Agent |
Typical dose |
Notes |
| NaOCl / Cl₂ |
0.5–2 mg/L continuous; 5–10 mg/L shock |
Must dechlorinate before RO |
| Chloramine (NH₂Cl) |
1–4 mg/L |
Slower kill but persistent; less DBPs |
| ClO₂ |
0.1–0.5 mg/L |
Stronger; damages PA more aggressively than Cl₂ — never let through |
| O₃ |
0.5–2 mg/L |
Strong; full decomposition required before membrane |
| KMnO₄ |
0.5–5 mg/L |
Fe/Mn oxidation; Mn must be filtered out |
7.2 On-membrane (non-oxidising only)
| Biocide |
Active conc. |
Mode |
Contact |
PA compatibility |
| DBNPA (2,2-dibromo-3-nitrilopropionamide) |
10–50 mg/L shock; 1 mg/L continuous |
Shock or continuous |
30 min – 3 h |
Excellent; hydrolyses rapidly (half-life hrs–days) |
| CMIT/MIT (isothiazolinone blend) |
25–100 mg/L |
Shock during CIP |
1–24 h |
Good; optimum pH 6.5–8.5; hydrolyses above pH 9 |
| BIT (1,2-benzisothiazolin-3-one) |
50–100 mg/L |
Shock |
1–4 h |
Good; longer environmental persistence |
| Glutaraldehyde |
100–500 mg/L shock; 1 % storage |
Shock + preservation |
1–4 h |
Compatible; toxicity / handling concerns |
| Formaldehyde |
0.5–1 % (storage only) |
Storage |
n/a |
Legacy; carcinogen — discouraged; banned in some EU member states |
| Quaternary ammonium |
Limited |
Compat varies |
n/a |
Cationic — risk of precipitation with anionic antiscalant; not recommended on PA |
| Peracetic acid (PAA) |
50–200 mg/L |
CIP |
< 1 h |
Some vendors allow; strong oxidant — check OEM approval |
7.3 Shock dosing protocols
- DBNPA online: 10–50 mg/L × 1 h, weekly. Bertheas study: 8.5 mg/L × 3 h more effective than 20 mg/L × 1 h
- DBNPA offline CIP: flush 15–30 min → 20 mg/L active recirculate 30 min – 1 h → rinse 30 min
- CMIT/MIT during CIP: 25–50 mg/L, contact 1–4 h, full rinse
7.4 Storage / preservation
- Short-term (< 30 days): 0.5–1 % SMBS, pH 3–6 (acidify to prevent oxidation)
- Long-term (months – 3 yr): 1–1.5 % SMBS, pH ~7, sealed vacuum bag (new-element vendor practice)
- Alternative: 1 % glutaraldehyde or 0.5 % formaldehyde where allowed
- Monitor SMBS solution pH and re-dose every 30–90 days to prevent oxidation to sulfate (loss of preservative + biological growth)
§8 — Monitoring Instrumentation
8.1 SDI test (ASTM D4189-23)
- Method scope: waters with turbidity < 1.0 NTU; not applicable to RO/UF permeate
- Filter: 0.45 µm pore, 47 mm MCE
- Pressure: 30 psig (2.07 bar) constant ± 0.1
- Procedure:
- Measure time t_i to collect 500 mL at t = 0
- Flow at 30 psi for 15 min (or 5/10 min if filter plugs early)
- Measure t_f to collect 500 mL at t = 15 min
SDI15 = 100 · (1 − t_i / t_f) / 15
Units: % per minute drop in flux.
8.2 Online instrumentation (typical set)
| Parameter |
Sensor |
Location |
Action threshold |
| Turbidity |
Nephelometric (Hach 2100, Endress+Hauser CUS52D, Yokogawa) |
Post media filter, post UF |
Alarm > 0.2 NTU |
| Free Cl₂ |
DPD analyzer (Wallace & Tiernan, E+H CCS51D) |
Post dechlor |
Trip > 0.05 ppm |
| ORP |
Ag/AgCl electrode |
Pre and post dechlor |
Trip > 200 mV post-SBS |
| Conductivity |
Inductive (high TDS) or contacting (RO permeate) |
Feed, permeate, concentrate per stage |
Mass balance, salt passage |
| pH |
Glass or solid-state |
Post acid/caustic injection |
PID control |
| Pressure |
Piezo-resistive |
Each stage feed/perm/conc |
ΔP trending |
| Flow |
Electromagnetic (preferred), Coriolis (high accuracy) |
Feed, perm, conc, recycle |
Recovery, mass balance |
| Iron (optional) |
Colorimetric (Hach) |
Feed |
Monthly grab |
| TOC (high-purity) |
UV oxidation analyzer |
RO permeate, EDI product |
UPW spec |
| SDI |
Automatic (Sterlitech SimpleSDI:Auto) or manual |
Pre-cartridge |
Daily check |
💡 Mass-balance closure check (|Qf − (Qp + Qc)| / Qf < 2 %) on the feed/permeate/concentrate flow set is essential — without it, instrument drift produces phantom KPI alerts.
§9 — Bilingual IT/EN Glossary
| EN |
IT |
| Pretreatment |
Pretrattamento |
| Intake |
Opera di presa |
| Brine outfall |
Scarico salamoia |
| Beach well |
Pozzo costiero |
| Open intake |
Presa diretta a mare |
| Screening (coarse/fine) |
Grigliatura (grossolana/fine) |
| Coagulation |
Coagulazione |
| Flocculation |
Flocculazione |
| Sedimentation |
Sedimentazione |
| Dissolved Air Flotation (DAF) |
Flottazione ad aria disciolta |
| Multimedia filter (MMF) |
Filtro multistrato |
| Dual-media filter (DMF) |
Filtro bistrato |
| Cartridge filter |
Filtro a cartuccia |
| Ultrafiltration (UF) |
Ultrafiltrazione |
| Antiscalant |
Antincrostante |
| Biocide |
Biocida |
| Shock dosing |
Dosaggio shock |
| Dechlorination |
Declorazione |
| Jar test |
Jar test (anglicismo) |
| Break-tank |
Vasca di rottura |
| Backwash |
Controlavaggio |
| Cleaning In Place (CIP) |
Lavaggio in posto |
| High Pressure Pump (HPP) |
Pompa di alta pressione |
| Recovery |
Recupero / resa |
| Reject / concentrate / brine |
Concentrato / salamoia |
| Permeate |
Permeato |
| Free chlorine |
Cloro libero / residuo |
| Combined chlorine / chloramine |
Cloro combinato / cloramine |
| Oxidation-Reduction Potential |
Potenziale redox (ORP) |
| Silt Density Index |
Indice di Densità del Limo (SDI) |
| Assimilable Organic Carbon |
Carbonio Organico Assimilabile (AOC) |
| Biological Growth Potential |
Potenziale di crescita biologica (BGP) |
| Total Organic Carbon |
Carbonio organico totale (TOC) |
| Biodegradable DOC |
DOC biodegradabile (BDOC) |
| Polyamide |
Poliammide (PA / TFC) |
Sources
- ASTM D4189-23 SDI
- DuPont FilmTec 45-D01504 (Tech Manual), 45-D01569 (Chlorination/Dechlorination), 45-D01578 (Feedwater Guidelines)
- Hydranautics TSB 108 (Chlorination), TSB 110 (Biocides), TAB-111 (Chemical Pretreatment)
- AWWA Research Foundation Guidance Manual for Disposal of Chlorinated Water
- Weinrich et al. 2011, AEM 77(3):1148 (AOC bioluminescence method)
- Naidu et al. 2016, Water Research (AOC contribution to SWRO biofouling)
- IWA AQUA 2022 — Non-oxidizing biocides for RO PA
- Bertheas et al., DeSWater 2017 — DBNPA dosage for biofouling
- MDPI Membranes 12(2):170 — Roles of Sulfites in RO (Cu/Co catalysed oxidation)
- SnowPure — Interpass pH 8.4–8.7 for 2-pass RO + EDI
- SigmaDAF / Veolia Spidflow — DAF loading 30–45 m/h for SWRO
- Mann+Hummel TSG-C-012 — Dechlorination Using SMBS
1---2name: membrane-pretreatment3description: RO/NF pretreatment chain — feed type architectures (SWRO open/beach well, BWRO well/surface, tertiary reuse), pretreatment KPI targets (SDI, turbidity, AOC, Fe/Mn, free Cl2), coagulation chemistry (FeCl3/PAC/alum dose math), antiscalant selection by scaling species, dechlorination (SBS stoichiometry + chloramine handling), CO2/pH management for 2-pass RO and EDI feed, biocide strategy (oxidizing pre-membrane vs non-oxidizing on-membrane), monitoring instrumentation. Use when designing or auditing what sits upstream of RO/NF/EDI.4---56# RO/NF Pretreatment78Reference for what feeds the membrane train, with KPI targets, chemistry, and instrumentation. The 80 % rule: most RO/NF/EDI problems originate upstream.910Convention markers:11- `⚠️` — likely bug or incorrect assumption in calling code12- `💡` — enhancement opportunity (new alert, new monitoring point, new dosing logic)1314---1516## §1 — Pretreatment Chain Overview1718### 1.1 SWRO open intake1920```21Sea intake (screen 5–25 mm)22 → Travelling band / drum screen (~1–3 mm)23 → In-line coagulation (FeCl3 0.5–5 mg/L as Fe) + flash mix (G ≈ 700–1000 s⁻¹, 30–60 s)24 → Flocculation tank (G ≈ 30–80 s⁻¹, 15–30 min) [optional with inline coag]25 → DAF (HABs/algae/oil) OR direct media filtration26 → Dual / Multimedia gravity filter (5–15 m/h) OR pressure MMF (15–25 m/h)27 → [Optional] UF 0.02 µm for SDI₁₅ ≤ 228 → Cartridge filter 5 µm absolute (ΔP fresh < 0.3 bar; change at 1.0–1.5 bar)29 → Antiscalant injection30 → SBS dechlorination (only if Cl2 used pre-filter)31 → Acid dosing (H2SO4 or HCl) for LSI control32 → HPP → SWRO 1st pass33```3435### 1.2 SWRO beach well (subsurface intake)3637```38Beach well / infiltration gallery (natural aquifer filtration)39 → Cartridge filter 5 µm40 → Antiscalant injection41 → SBS only if shock-chlorination at wellhead42 → Acid dosing43 → HPP → SWRO44```4546Subsurface intakes naturally deliver SDI₁₅ < 3 in most geologies → skip coagulation and media filtration. Yield (m³/h per well) and risk of Fe/Mn intrusion from anoxic strata are the practical limits.4748### 1.3 BWRO well (saline groundwater)4950```51Well pump → aeration / oxidation (Cl2, KMnO4, or O2) if Fe/Mn > 0.05 mg/L52 → Manganese greensand / pyrolusite filter (Fe/Mn removal)53 → MMF polishing54 → Cartridge filter 5 µm55 → SBS (if Cl2 residual present)56 → Antiscalant57 → Acid (for high alkalinity / LSI)58 → BWRO59```6061### 1.4 BWRO surface water6263Same architecture as SWRO open intake but lower TDS, lower turbidity peaks. pH control more critical (CaCO3 dominant scaling vs Mg-sulfate).6465### 1.5 Tertiary reuse → BWRO6667```68Biological secondary effluent → MBR or tertiary UF/MF69 → break tank70 → cartridge filter 5 µm71 → DBNPA continuous low-dose (1 mg/L) optional72 → antiscalant (low-P or P-free)73 → BWRO 1st pass74```7576MBR/tertiary UF typically produces SDI₁₅ 0.5–2.0; secondary effluent may exceed 3 in some months.7778---7980## §2 — Pretreatment KPI Targets (pre-RO)8182| Parameter | SWRO open | SWRO beach well | BWRO well | BWRO surface | Tertiary reuse |83|---|---|---|---|---|---|84| SDI₁₅ target | ≤ 3 | ≤ 2 | ≤ 3 | ≤ 3 | ≤ 3 |85| SDI₁₅ max | 5 | 3 | 4 | 5 | 5 |86| SDI₁₅ with UF | ≤ 2 | n/a | ≤ 2 | ≤ 2 | ≤ 2 |87| Turbidity (NTU) target | < 0.1 | < 0.1 | < 0.1 | < 0.1 | < 0.2 |88| Turbidity (NTU) max | 1.0 | 0.5 | 1.0 | 1.0 | 1.0 |89| Free Cl₂ pre-membrane | 0 ppm | 0 ppm | 0 ppm | 0 ppm | 0 ppm |90| ORP post-dechlor | < 200 mV | n/a | < 200 mV | < 200 mV | < 200 mV |91| AOC (µg C/L) | < 50 excellent / < 100 acceptable | < 50 | < 50 | < 100 | < 100 |92| BGP (µg/L) | < 70 | < 70 | n/a | n/a | n/a |93| Iron (mg/L) | < 0.05 | < 0.05 | < 0.05 | < 0.05 | < 0.1 |94| Manganese (mg/L) | < 0.02 | < 0.02 | < 0.02 | < 0.02 | < 0.05 |95| Aluminum residual (mg/L) | < 0.05 | < 0.05 | < 0.05 | < 0.05 | < 0.05 |96| Operating pH | 6.5–7.5 | 6.5–7.5 | 5.5–7 | 5.5–7.5 | 6–7.5 |97| Cartridge ΔP fresh | < 0.3 bar | < 0.3 bar | < 0.3 bar | < 0.3 bar | < 0.3 bar |98| Cartridge ΔP change-out | 0.7–1.0 bar | 0.7–1.0 bar | 0.7–1.0 bar | 0.7–1.0 bar | 0.7–1.0 bar |99100⚠️ SDI₁₅ > 5 voids most OEM warranties. SDI₁₅ ≤ 5 is the absolute hard limit.101102AOC threshold from Vrouwenvelder / Weinrich biofouling correlation studies. Raw seawater commonly 30–400 µg C/L (Tampa Bay 360 ± 180 µg/L; Monterey 30 ± 20 µg/L).103104⚠️ Heavy metals (Fe³⁺, Mn²⁺, Cu²⁺, Co²⁺) catalyse SBS → oxidant conversion under O₂. Keep trace metals low and minimize SBS overdose (see §5.4).105106---107108## §3 — Coagulation / Flocculation Chemistry109110### 3.1 Coagulant comparison111112| Coagulant | Formula | Dose (as product) | Dose (as metal) | Optimal pH | Notes |113|---|---|---|---|---|---|114| Ferric chloride | FeCl₃·6H₂O | 5–40 mg/L | 1–10 mg/L Fe | 5.0–8.5 (best 5–7) | Wide pH; dense floc; residual Fe³⁺ accepted < 0.05 ppm; preferred for SWRO |115| Ferrous sulfate | FeSO₄·7H₂O | 10–50 mg/L | 2–10 mg/L Fe | 8.5–11 + Cl₂ oxidize Fe²⁺→Fe³⁺ | Cheap; needs alkaline conditions |116| Alum | Al₂(SO₄)₃·14–18H₂O | 5–60 mg/L | 0.4–5 mg/L Al | 6.0–7.5 (narrow) | Al residual → AlPO₄/Al(OH)₃ fouling on RO; avoid for RO when possible |117| PAC | Al_n(OH)_m Cl_{3n-m} | 5–30 mg/L | 0.5–3 mg/L Al | 5.5–9 (wide) | Lower residual Al than alum; less pH depression; efficient at cold T |118119### 3.2 Polymer flocculant aids120121- Type: anionic / cationic / nonionic polyacrylamide (PAM); also tannin-based natural polymers122- Dose 0.05–1 mg/L. Under-dose → no benefit; over-dose → carryover, RO fouling123- Cationic PAM acts as primary coagulant in low-turbidity surface waters124125⚠️ Cationic polymer + anionic antiscalant → precipitate → RO fouling. Always jar-test the combination before deployment.126127### 3.3 Jar test methodology1281291. **Setup**: 6 paddle jars × 1 L; coagulant in dilute solution (1–10 % w/v)1302. **Flash mix**: 100–200 rpm × 30–60 s (G ≈ 700–1 000 s⁻¹)1313. **Flocculation**: 30 rpm × 15–30 min (G ≈ 30–80 s⁻¹, tapered if possible)1324. **Settling**: 30 min undisturbed1335. **Sampling**: supernatant at fixed depth; measure turbidity, TOC, SDI, residual coagulant metal, pH134135G value:136```137G = sqrt(P / (μ · V)) # P = power (W), μ = dynamic viscosity (Pa·s), V = volume (m³)138```139140### 3.4 Pros/cons for downstream RO141142| Aspect | Iron-based (FeCl₃) | Aluminum-based (alum, PAC) |143|---|---|---|144| Residual carryover risk | Fe³⁺ acceptable up to 0.05 ppm | Al³⁺-phosphate fouling, severe at > 0.05 ppm |145| Floc strength | Dense, settle easily | Lighter |146| pH window | Wide | Narrow (alum); wider (PAC) |147| Cold water | Good | Good (PAC) / poor (alum) |148| TOC removal | Excellent | Moderate |149| SWRO compatibility | Preferred | Avoid alum; PAC OK with caution |150151---152153## §4 — Antiscalant Selection and Dose Math154155### 4.1 Chemistry families156157| Family | Examples | Best for | Typical dose | Notes |158|---|---|---|---|---|159| Phosphonate | HEDP, PBTC, ATMP, DTPMP, BHMTPMP | CaCO₃, CaSO₄, BaSO₄, SrSO₄, CaF₂ | 2–5 mg/L | Workhorse; P discharge constraints |160| Polyacrylate / acrylic homo & copolymer | PAA, AA-MA | BaSO₄, SrSO₄, suspended solids dispersion | 1–5 mg/L | P-free; lower Ca tolerance |161| Sulfonated copolymer | AA-AMPS, AA-AMPS-HPA terpolymer | High Ca, CaPO₄, ZnCO₃ | 2–5 mg/L | High Ca + alkaline tolerance |162| Maleic-based | MA-AA, polymaleic | CaCO₃, CaSO₄ broad | 2–5 mg/L | P-free; used in P-restricted discharges |163| Polymeric silica dispersant | PEG, PEGD, PVA, PAMAM, PEI | SiO₂, colloidal silica | 1–5 mg/L | For SiO₂ > 100 ppm in concentrate |164| Dendrimer / latest-gen | Polyaspartate, dendrimer-based | Broad multi-species | 1–4 mg/L | Biodegradable; premium |165166### 4.2 Selection by scaling species167168| Scaling species | Antiscalant of choice |169|---|---|170| CaCO₃ | Phosphonate (HEDP, PBTC) OR maleic-based (P-free) |171| CaSO₄ (gypsum) | Phosphonate + polyacrylate blend |172| BaSO₄, SrSO₄ | Polyacrylate or AA-AMPS (phosphonates can co-precipitate with Ba) |173| CaF₂ | Phosphonate |174| SiO₂ amorphous | Polymeric silica inhibitor (PEG/PEGD) + pH adjustment |175| Ca₃(PO₄)₂ | AA-AMPS terpolymer — **NOT** phosphonate |176| Fe / Mn fouling | Specialised dispersant blend |177178### 4.3 Dose calculation179180Three approaches:1811. **Vendor projection software** (preferred): DuPont WAVE, Hydranautics IMSDesign, Toray DS-Design, Veolia Winflows, Avista AdvisorCi, Genesys Genesys Member. Software computes LSI / S&DSI / IP-Ksp per stage and recommends dose.1822. **LSI-driven empirical**: target concentrate LSI ≤ +1.8 (conservative) or ≤ +2.5 (premium antiscalant). Typical dose 2–5 mg/L product.1833. **Jar-tested matrix**: when projection unavailable.184185### 4.4 Compatibility traps186187⚠️ Cationic biocide + anionic antiscalant → precipitate.188⚠️ Phosphonate + Ba²⁺ → BaSO₄ co-precipitation under some conditions.189⚠️ Phosphate-containing antiscalant + Ca²⁺ at high pH → calcium phosphate scale (unintended).190⚠️ EU/CN P discharge limits → switch to maleic / polyacrylate / sulfonated terpolymer for compliance.191192---193194## §5 — Dechlorination (SBS / SMBS)195196### 5.1 Chemistry197198```199Na2S2O5 + H2O → 2 NaHSO3200NaHSO3 + HOCl → NaHSO4 + HCl201```202203### 5.2 Stoichiometry204205- **Theoretical** (DuPont 45-D01569): **1.34 mg SMBS per mg free Cl₂** (≈ 1.46 mg NaHSO₃ per mg Cl₂)206- **Practical**: **1.5–3.0 mg/mg** (50–125 % excess) to account for incomplete mixing, competing reactions (DO, chloramines, NOM), continuous overdose margin207- **DuPont rule of thumb**: ~3.0 mg SMBS per mg free Cl₂ (≈ 2.2× theoretical)208209⚠️ SMBS (Na₂S₂O₅) and SBS (NaHSO₃) have different molar masses — confirm which the dosing pump is metering and adjust stoichiometry accordingly.210211### 5.3 Chloramines212213- NH₂Cl, NHCl₂ react more slowly with SBS than free Cl₂214- At feed pH ≥ 8.5, SBS becomes ineffective at fully reducing chloramines215- **ORP signal unreliable** for chloramines — supplement with DPD analyzer216- **Mitigation**: acidify before SBS injection, increase dose 2–3×, increase contact time217218### 5.4 The oxidant paradox219220Under DO + transition metal traces (Cu²⁺, Co²⁺, Fe³⁺, Mn²⁺):221```2222 NaHSO3 + O2 --[Cu/Co]--> 2 NaHSO4 + sulfite radicals (SO3•⁻, SO4•⁻)223```224225Sulfite radicals attack polyamide. Residual SBS up to 30 ppm has been reported to convert partially to oxidants in this scenario.226227⚠️ Chronic SBS overdose is **not** safer than just-right dosing. Keep margin small (just enough to neutralize Cl₂); minimise air ingress in suction lines; deaerate where Cu/Co > trace.228229### 5.5 Monitoring230231- **ORP post-dechlor**: < 200 mV (some vendors < 150 mV)232- **Pre-Cl₂ ORP** typical: +300 to +600 mV233- **DPD analyzer** redundant safety against breakthrough234- ORP under SBS overdose can read misleadingly low235236### 5.6 SBS solution preparation237238- Use **RO permeate**, not raw water, to avoid sulfate scale in dosing line239- Concentration 5–15 % w/w (higher decomposes faster, especially warm)240- **Filter SBS solution through dedicated cartridge before injection** (insoluble grit, sulfate)241- Inject downstream of last cartridge filter with static mixer242- Solution shelf life ~7 days; replace weekly243244⚠️ Do not store dechlorinated water in tanks — promotes bacterial regrowth between dechlor and HPP.245246### 5.7 Alternative: GAC247248| Aspect | SBS | GAC |249|---|---|---|250| CapEx | Low | High |251| OpEx | Recurring chemical | Periodic media replacement (1–3 yr) |252| Reliability | Active control loop | Passive failsafe |253| Bacterial regrowth | None | High (bed = bioreactor) |254| ORP control needed | Yes | No |255| SWRO use | Primary | Rare (footprint, bio) |256| Pharma / USP use | Sometimes | Common (chlorine-free + TOC reduction) |257258---259260## §6 — CO₂ / pH Management261262### 6.1 Why CO₂ matters263264- Neutral CO₂ permeates RO (small uncharged molecule)265- HCO₃⁻ is rejected (charged, hydrated)266- For 2nd-pass RO or EDI feed, CO₂ in 1st-pass permeate becomes the dominant ionic load downstream267- FCE (for EDI) penalizes CO₂ at 2.79 µS/cm per ppm268269### 6.2 Strategies270271| Strategy | CO₂ outlet | Use when | Notes |272|---|---|---|---|273| Interpass NaOH injection | residual converted to HCO₃⁻, rejected by pass 2 | 2-pass RO present | Target interpass pH 8.4–8.7; also boosts boron and silica rejection |274| Membrane degasser (PTFE/PP hollow fiber, e.g. Liqui-Cel) | < 5 ppm typ, < 0.5 ppm achievable | RO + EDI (no 2nd RO) | Vacuum and/or sweep gas; CIP every 3–6 months |275| Forced-draft atmospheric tower | 5–10 ppm | Lower-purity applications | Open to atmosphere → bacterial ingress risk |276| Vacuum tower degasser | 1–5 ppm | High-purity preferred | Higher CapEx than membrane |277278### 6.3 pH control loop279280- pH transmitter (glass + Ag/AgCl or solid-state), inline post-injection static mixer281- PID-controlled dosing pump (NaOH 50 % or H₂SO₄ 96–98 %)282- Control band ±0.1 pH typical; alarms at ±0.3283284⚠️ Buffer-poor RO permeate → small dose causes large pH swing. Tune integral term carefully or use cascade control.285286### 6.4 pH effects on rejection (2nd pass)287288- **Boron** (pKa 9.14–9.24): at pH 9.5–9.8, H₃BO₃ → B(OH)₄⁻, rejected by RO. Critical for SWRO when boron > 0.5 mg/L target (WHO/EU drinking water)289- **Silica** (pKa 9.84): higher pH → silicate species → better rejection290- **CO₂**: see §6.1291292Tradeoff: pH > 8 increases CaCO₃ scaling risk on 2nd-pass concentrate if Ca slip from 1st pass.293294---295296## §7 — Biocide Strategy (Split Pre/On Membrane)297298### 7.1 Pre-RO (oxidising OK, must be removed before membrane)299300| Agent | Typical dose | Notes |301|---|---|---|302| NaOCl / Cl₂ | 0.5–2 mg/L continuous; 5–10 mg/L shock | Must dechlorinate before RO |303| Chloramine (NH₂Cl) | 1–4 mg/L | Slower kill but persistent; less DBPs |304| ClO₂ | 0.1–0.5 mg/L | Stronger; **damages PA more aggressively than Cl₂** — never let through |305| O₃ | 0.5–2 mg/L | Strong; full decomposition required before membrane |306| KMnO₄ | 0.5–5 mg/L | Fe/Mn oxidation; Mn must be filtered out |307308### 7.2 On-membrane (non-oxidising only)309310| Biocide | Active conc. | Mode | Contact | PA compatibility |311|---|---|---|---|---|312| **DBNPA** (2,2-dibromo-3-nitrilopropionamide) | 10–50 mg/L shock; 1 mg/L continuous | Shock or continuous | 30 min – 3 h | Excellent; hydrolyses rapidly (half-life hrs–days) |313| **CMIT/MIT** (isothiazolinone blend) | 25–100 mg/L | Shock during CIP | 1–24 h | Good; optimum pH 6.5–8.5; hydrolyses above pH 9 |314| **BIT** (1,2-benzisothiazolin-3-one) | 50–100 mg/L | Shock | 1–4 h | Good; longer environmental persistence |315| **Glutaraldehyde** | 100–500 mg/L shock; 1 % storage | Shock + preservation | 1–4 h | Compatible; toxicity / handling concerns |316| **Formaldehyde** | 0.5–1 % (storage only) | Storage | n/a | Legacy; carcinogen — discouraged; banned in some EU member states |317| **Quaternary ammonium** | Limited | Compat varies | n/a | Cationic — **risk of precipitation with anionic antiscalant**; not recommended on PA |318| **Peracetic acid** (PAA) | 50–200 mg/L | CIP | < 1 h | Some vendors allow; strong oxidant — check OEM approval |319320### 7.3 Shock dosing protocols321322- **DBNPA online**: 10–50 mg/L × 1 h, weekly. Bertheas study: 8.5 mg/L × 3 h more effective than 20 mg/L × 1 h323- **DBNPA offline CIP**: flush 15–30 min → 20 mg/L active recirculate 30 min – 1 h → rinse 30 min324- **CMIT/MIT during CIP**: 25–50 mg/L, contact 1–4 h, full rinse325326### 7.4 Storage / preservation327328- **Short-term (< 30 days)**: 0.5–1 % SMBS, pH 3–6 (acidify to prevent oxidation)329- **Long-term (months – 3 yr)**: 1–1.5 % SMBS, pH ~7, sealed vacuum bag (new-element vendor practice)330- **Alternative**: 1 % glutaraldehyde or 0.5 % formaldehyde where allowed331- Monitor SMBS solution pH and re-dose every 30–90 days to prevent oxidation to sulfate (loss of preservative + biological growth)332333---334335## §8 — Monitoring Instrumentation336337### 8.1 SDI test (ASTM D4189-23)338339- Method scope: waters with turbidity < 1.0 NTU; not applicable to RO/UF permeate340- Filter: 0.45 µm pore, 47 mm MCE341- Pressure: 30 psig (2.07 bar) constant ± 0.1342- Procedure:343 1. Measure time t_i to collect 500 mL at t = 0344 2. Flow at 30 psi for 15 min (or 5/10 min if filter plugs early)345 3. Measure t_f to collect 500 mL at t = 15 min346347```348SDI15 = 100 · (1 − t_i / t_f) / 15349```350351Units: % per minute drop in flux.352353### 8.2 Online instrumentation (typical set)354355| Parameter | Sensor | Location | Action threshold |356|---|---|---|---|357| Turbidity | Nephelometric (Hach 2100, Endress+Hauser CUS52D, Yokogawa) | Post media filter, post UF | Alarm > 0.2 NTU |358| Free Cl₂ | DPD analyzer (Wallace & Tiernan, E+H CCS51D) | Post dechlor | Trip > 0.05 ppm |359| ORP | Ag/AgCl electrode | Pre and post dechlor | Trip > 200 mV post-SBS |360| Conductivity | Inductive (high TDS) or contacting (RO permeate) | Feed, permeate, concentrate per stage | Mass balance, salt passage |361| pH | Glass or solid-state | Post acid/caustic injection | PID control |362| Pressure | Piezo-resistive | Each stage feed/perm/conc | ΔP trending |363| Flow | Electromagnetic (preferred), Coriolis (high accuracy) | Feed, perm, conc, recycle | Recovery, mass balance |364| Iron (optional) | Colorimetric (Hach) | Feed | Monthly grab |365| TOC (high-purity) | UV oxidation analyzer | RO permeate, EDI product | UPW spec |366| SDI | Automatic (Sterlitech SimpleSDI:Auto) or manual | Pre-cartridge | Daily check |367368💡 Mass-balance closure check (`|Qf − (Qp + Qc)| / Qf < 2 %`) on the feed/permeate/concentrate flow set is essential — without it, instrument drift produces phantom KPI alerts.369370---371372## §9 — Bilingual IT/EN Glossary373374| EN | IT |375|---|---|376| Pretreatment | Pretrattamento |377| Intake | Opera di presa |378| Brine outfall | Scarico salamoia |379| Beach well | Pozzo costiero |380| Open intake | Presa diretta a mare |381| Screening (coarse/fine) | Grigliatura (grossolana/fine) |382| Coagulation | Coagulazione |383| Flocculation | Flocculazione |384| Sedimentation | Sedimentazione |385| Dissolved Air Flotation (DAF) | Flottazione ad aria disciolta |386| Multimedia filter (MMF) | Filtro multistrato |387| Dual-media filter (DMF) | Filtro bistrato |388| Cartridge filter | Filtro a cartuccia |389| Ultrafiltration (UF) | Ultrafiltrazione |390| Antiscalant | Antincrostante |391| Biocide | Biocida |392| Shock dosing | Dosaggio shock |393| Dechlorination | Declorazione |394| Jar test | Jar test (anglicismo) |395| Break-tank | Vasca di rottura |396| Backwash | Controlavaggio |397| Cleaning In Place (CIP) | Lavaggio in posto |398| High Pressure Pump (HPP) | Pompa di alta pressione |399| Recovery | Recupero / resa |400| Reject / concentrate / brine | Concentrato / salamoia |401| Permeate | Permeato |402| Free chlorine | Cloro libero / residuo |403| Combined chlorine / chloramine | Cloro combinato / cloramine |404| Oxidation-Reduction Potential | Potenziale redox (ORP) |405| Silt Density Index | Indice di Densità del Limo (SDI) |406| Assimilable Organic Carbon | Carbonio Organico Assimilabile (AOC) |407| Biological Growth Potential | Potenziale di crescita biologica (BGP) |408| Total Organic Carbon | Carbonio organico totale (TOC) |409| Biodegradable DOC | DOC biodegradabile (BDOC) |410| Polyamide | Poliammide (PA / TFC) |411412---413414## Sources415416- **ASTM D4189-23** SDI417- **DuPont FilmTec** 45-D01504 (Tech Manual), 45-D01569 (Chlorination/Dechlorination), 45-D01578 (Feedwater Guidelines)418- **Hydranautics** TSB 108 (Chlorination), TSB 110 (Biocides), TAB-111 (Chemical Pretreatment)419- **AWWA Research Foundation** Guidance Manual for Disposal of Chlorinated Water420- Weinrich et al. 2011, AEM 77(3):1148 (AOC bioluminescence method)421- Naidu et al. 2016, Water Research (AOC contribution to SWRO biofouling)422- IWA AQUA 2022 — Non-oxidizing biocides for RO PA423- Bertheas et al., DeSWater 2017 — DBNPA dosage for biofouling424- MDPI Membranes 12(2):170 — Roles of Sulfites in RO (Cu/Co catalysed oxidation)425- SnowPure — Interpass pH 8.4–8.7 for 2-pass RO + EDI426- SigmaDAF / Veolia Spidflow — DAF loading 30–45 m/h for SWRO427- Mann+Hummel TSG-C-012 — Dechlorination Using SMBS
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