Last updated: November 7, 2025 · Reading time: 12–15 minutes · Audience: UF/MBR/RO operators, process engineers, EPCs
Understanding UF feed-water quality helps you prevent fouling, stabilize flux, and protect downstream RO/NF. This guide summarizes the parameters that matter—particles/colloids (turbidity, TSS, SDI), organics (TOC/DOC, UV254/SUVA), oils, Fe/Mn, hardness, conductivity/TDS, pH, and silica—plus sampling notes, practical cues, and acceptance ranges to tune for your project.
Why UF Feed-Water Quality Matters
- Fouling control: particulate/colloidal, organic/biofouling, oil/grease, metals, and silica can all shorten run length.
- Operations: clean data lets you set flux, backwash/CIP frequency, chemical windows, and protect downstream RO/NF recovery.
- Документация: log units, methods, sampling points, and stabilization times for traceability.
UF feed-water quality Particles & Colloids — Turbidity, TSS, SDI
Turbidity (NTU)
- Optical measure of suspended/colloidal matter; quick and continuous.
- Rule-of-thumb cue: UF feed ≤ 5 NTU for stable operation; many drinking-water sites target ≤ 1 NTU.
Total Suspended Solids (TSS, mg/L)
- Gravimetric mass of solids retained on a glass-fiber filter (laboratory standard). More specific than turbidity for fine solids.
- Use when turbidity is unstable or dominated by color/fines that distort optics.
Silt Density Index (SDI)
Quantifies fouling tendency using a 47 mm / 0.45 µm membrane at 30 psi. Record initial time ti to collect 500 mL, then time tf at test end; compute SDI for total time T (min):
SDI = ( 1 − t_i / t_f ) × ( 100 / T ) ← common practice: T = 15 minutes (SDI₁₅)
- Lower is better; for UF/RO pretreatment, SDI₁₅ < 5 is a typical design cue (project-specific).
- Stir/feed recirculation during the test to avoid settling artifacts; temperature affects viscosity—log it.
UF feed-water quality Organics & Biofouling Risk — TOC/DOC, COD/BOD, UV254/SUVA
TOC & DOC (mg/L)
- TOC is total organic carbon (dissolved + particulate); DOC is the ≤0.45 µm fraction.
- Cue: UF feed TOC > 2 mg/L flags higher biofouling likelihood—tighten backwash/CIP or add coagulant/oxidant control as permitted.
UV254 & SUVA
UV absorbance at 254 nm tracks humic/aromatic organics. SUVA (specific UV absorbance) characterizes organic type:
SUVA (L·mg⁻¹·m⁻¹) = UV254 (cm⁻¹) × 100 ÷ DOC (mg/L)- SUVA > 4 → humic-rich; SUVA < 2 → algal/low-aromatic organics.
- Cue: UV254 > 0.5 cm⁻¹ suggests notable biofouling tendency; plan for enhanced cleaning or upstream coagulation.
COD / BOD5
- COD is fast chemical oxygen demand; BOD5 is 5-day biological oxygen demand.
- For UF operations, trend TOC/DOC + UV254 first; use COD/BOD to interpret pollution episodes.
Oils & Hydrocarbons (O&G)
- Even trace oil can create persistent fouling on UF fibers/sheets. Keep below the method detection limit and install oil alarms where relevant.
- Cue: aim < 0.05 mg/L (site-specific; verify method).
Iron & Manganese
- Oxidized Fe/Mn are well retained by UF but deposit as solids; sources include raw well water, corrosion, coagulant residuals.
- Control oxidation state upstream; maintain filters/backwash so particulates don’t overload UF.
Hardness, Conductivity/TDS & pH
Hardness (as CaCO3)
- Soft ≤ 60 mg/L, moderate 60–120, hard 120–180, very hard > 180 mg/L (design context for downstream scaling control).
Conductivity / TDS
- Conductivity (µS/cm) correlates with dissolved salts (TDS). Track temperature or convert to 25 °C.
pH
- Controls corrosion/scaling chemistry and coagulant performance. Note any seasonal drift; calibrate probes regularly.
Silica — Reactive vs Colloidal
- Reactive (dissolved) silica passes UF and limits RO recovery; colloidal silica is retained but fouls the surface if poorly controlled.
- Use filtration (≤0.45 µm) to separate dissolved vs colloidal fraction for accurate diagnosis.
Quick Acceptance Cues (Tune Per Project)
| Category | Параметр | Typical cue | Примечания |
|---|---|---|---|
| Particles | Turbidity | ≤ 5 NTU (≤ 1 NTU for high-spec) | Stabilize upstream filters/backwash |
| Particles | SDI15 | < 5 | 47 mm / 0.45 µm, 30 psi, temperature logged |
| Organics | TOC | < 2 mg/L | Higher values → biofouling risk; combine with UV254/SUVA |
| Organics | UV254 | < 0.5 cm⁻¹ | Humic/aromatic indicator |
| Organics | SUVA | 2–4 L·mg⁻¹·m⁻¹ | >4 humic-rich; <2 algal-type |
| Oil | O&G | Not detected (target < 0.05 mg/L) | Install oil alarms where applicable |
| Metals | Fe/Mn | Low/controlled | Prevent oxidized solids overloading UF |
| Ions | Hardness | Context only | Sets downstream scaling limits (RO/boiler/cooling) |
| Ions | Conductivity/TDS | Trend | Normalize to 25 °C |
| Chemistry | pH | Stable, project window | Affects coagulants, scaling, corrosion |
| Кремнезем | Reactive vs colloidal | Split & trend | Colloids foul UF; dissolved limits RO recovery |
Disclaimer: These are practical cues. Final limits depend on the UF module/vendor, flux, backwash/CIP strategy, and your downstream process targets.
Sampling & QA/QC (Operator Workflow)
- Sample at the UF feed tank or inlet after stabilization; purge lines to avoid dead-leg water.
- Duplicate runs for TOC/DOC and UV254; log filter pore size (≤0.45 µm) wherever “dissolved” is referenced.
- Maintain a spreadsheet with time, flow/pressure, temperature, NTU, TSS, SDI, TOC/DOC, UV254/SUVA, O&G, Fe/Mn, hardness, EC/TDS, pH, and silica.
Visuals & Downloadables
- UF Feed Dashboard (online tool, coming soon)
- UF feed quality checklist (PDF) (placeholder path)
- Water Treatment Solutions - Case Studies - Блог
References & Further Reading
General resources for water quality methods and health context:
Turn Lab Numbers into a Pretreatment Plan
Upload your UF feed dataset (NTU/TSS/SDI, TOC/DOC, UV254/SUVA, O&G, Fe/Mn, hardness, EC/pH, silica). We’ll return a fouling-risk scorecard and a tuned operating window (flux, backwash/CIP).
About the Author
Stark Water — Process engineers focused on UF/RO pretreatment design, membrane fouling control, and compliance-driven operations.
FAQs — UF Feed-Water Quality
1) SDI₁₅ vs SDI₅ or SDI₁₀ — how do I adjust?
Use the same formula SDI = (1 − t_i/t_f) × 100 / T with your chosen total time T (5, 10, or 15 min). Report the time in the subscript.
2) When should I run TSS instead of turbidity?
When turbidity is dominated by color or fines that skew optics. TSS provides gravimetric confirmation of particulate loading.
3) TOC/DOC vs COD/BOD — which should I prioritize?
For UF fouling, trend TOC/DOC and UV254 first. Use COD/BOD to interpret pollution events or permit compliance.
4) What SUVA ranges imply organic character?
SUVA > 4 indicates humic-rich water; SUVA < 2 suggests algal or low-aromatic organics. Pair with DOC.
5) How do I screen for trace oil?
Use O&G methods with low detection limits; install upstream oil alarms if industrial discharges are possible.