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name: membrane-pretreatment description: 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. user-invocable: false

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)

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§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.

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§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).

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§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

1. Setup: 6 paddle jars × 1 L; coagulant in dilute solution (1–10 % w/v)
2. Flash mix: 100–200 rpm × 30–60 s (G ≈ 700–1 000 s⁻¹)
3. Flocculation: 30 rpm × 15–30 min (G ≈ 30–80 s⁻¹, tapered if possible)
4. Settling: 30 min undisturbed
5. 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

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§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:

1. 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.
2. LSI-driven empirical: target concentrate LSI ≤ +1.8 (conservative) or ≤ +2.5 (premium antiscalant). Typical dose 2–5 mg/L product.
3. 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.

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§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)

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§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.

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§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)

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§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:
  1. Measure time t_i to collect 500 mL at t = 0
  2. Flow at 30 psi for 15 min (or 5/10 min if filter plugs early)
  3. 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.

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§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)

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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