Pre-ultra Filtration(UF)System Selection Based on Raw Water Quality Indicators (Turbidity/SDI/DOC)

Whether it’s pure water design, wastewater recovery, or zero-liquid discharge design, to reasonably select the pretreatment combination process before UF, we must take raw water turbidity, Silt Density Index (SDI), and Dissolved Organic Carbon (DOC) as core indicators. Combined with the contribution of each indicator to UF membrane fouling (e.g., turbidity causes pore blockage, SDI reflects colloid/particle fouling risk, DOC triggers organic membrane fouling), targeted pretreatment processes should be selected to achieve the goals of “precisely removing pollutants, mitigating membrane fouling, and ensuring stable UF operation.”
Now, let’s discuss:

Pre-ultra Filtration System Selection Based on Turbidity

Turbidity is the main source of physical fouling of UF membranes (when turbidity > 5 NTU, particles/colloids easily block membrane pores, causing flux decline). The core goal of pretreatment is to reduce turbidity to a range that allows stable UF operation (typically < 5 NTU, ideally < 1 NTU).

High Turbidity (Turbidity > 10 NTU, e.g., Yellow River water, rainy season surface water)

Problem: High turbidity means a large number of particles/colloids exist. Direct entry into UF System will cause rapid membrane pore blockage and a sharp flux decline.
Pretreatment Selection: Coagulation-Sedimentation-Sand Filtration combination process (or “baffled flocculation + horizontal flow sedimentation + V-filter”).
Principle: Coagulation (using PAC, PAM) destabilizes particles through charge neutralization; sedimentation (horizontal flow/high-density sedimentation tank) removes large particles; sand filtration (quartz sand) further intercepts fine particles, reducing turbidity to < 1 NTU, providing stable feed water for UF.
Alternative: If space is limited, a coagulation-flotation process (e.g., dissolved air flotation) can be used, where bubbles adsorb particles and float them for removal. This is suitable for high turbidity, high algae-laden water (e.g., summer reservoir water).

Medium Turbidity (Turbidity 5-10 NTU, e.g., reservoir water, groundwater)

Problem: Medium turbidity may still cause slow UF pore blockage, requiring enhanced pretreatment to reduce membrane load.
Pretreatment Selection: Coagulation-Sand Filtration combination process.
Principle: Coagulation removes most particles; sand filtration intercepts remaining fine particles, reducing turbidity to < 3 NTU, meeting UF feed requirements.

Low Turbidity (Turbidity < 5 NTU, e.g., winter surface water, high-quality groundwater)

Problem: Low turbidity but may contain colloids (e.g., humic colloids), which can still cause irreversible UF membrane fouling (colloids adsorb inside membrane pores).
Pretreatment Selection: Sand Filtration + Activated Carbon Filtration (or “direct UF pretreatment”).
Principle: Sand filtration removes remaining particles; activated carbon (GAC) adsorbs colloids and dissolved organic matter, further reducing membrane fouling risk.

Pre-ultra Filtration System Selection Based on on SDI (Silt Density Index)

SDI is a key indicator of colloid/particle fouling of UF membranes (when SDI > 5, colloids/particles easily form a cake layer on the membrane surface, causing increased transmembrane pressure; ideal SDI < 3). The core goal of pretreatment is to reduce SDI to < 3.

High SDI (SDI > 5, e.g., Yellow River water, high algae-laden water)

Problem: High SDI means high colloid/particle content. A cake layer easily forms on the UF membrane surface, causing rapid transmembrane pressure increase (e.g., pressure rises from 0.1 MPa to 0.3 MPa within one month of operation).
Pretreatment Selection: Coagulation-Sand Filtration + UF (or “enhanced coagulation + microfiltration”).
Principle: Coagulation removes large colloids; sand filtration removes small colloids; UF further intercepts remaining colloids (molecular weight cut-off 10-100 kDa), reducing SDI to < 2, completely solving the colloid fouling problem.

Medium SDI (SDI 3-5, e.g., reservoir water, groundwater)

Problem: Medium SDI indicates relatively high colloid/particle content, requiring enhanced pretreatment to reduce SDI.
Pretreatment Selection: Sand Filtration + UF (or “coagulation-flotation + UF”).
Principle: Sand filtration removes most colloids; UF intercepts remaining colloids, reducing SDI to < 3, meeting UF feed requirements.

Low SDI (SDI < 3, e.g., high-quality tap water, treated wastewater)

Problem: Low SDI indicates low colloid/particle content, but soluble organic matter (e.g., humic acid) may still be present, causing organic fouling of the UF membrane (organics adsorb inside membrane pores).
Pretreatment Selection: Activated Carbon Filtration + UF (or “ozone-activated carbon + UF”).
Principle: Activated carbon adsorbs soluble organic matter; UF intercepts remaining organics, reducing SDI to < 2 while mitigating organic fouling.

Pre-ultra Filtration System Selection Based on DOC (Dissolved Organic Carbon)

DOC is a major source of organic fouling of UF membranes (when DOC > 3 mg/L, organics easily adsorb inside membrane pores, causing flux decline; ideal DOC < 2 mg/L). The core goal of pretreatment is to reduce DOC to < 3 mg/L.

High DOC (DOC > 8 mg/L, e.g., high algae-laden water, micro-polluted river water)

Problem: High DOC means a large amount of organic matter exists. UF membranes are easily fouled by organics (e.g., humic acid adsorbs inside membrane pores), causing flux decline (e.g., 50% flux decline within 3 months of operation).
Pretreatment Selection: Advanced Oxidation + Activated Carbon Filtration + UF.
Principle: Advanced oxidation generates hydroxyl radicals (·OH) to degrade organics (e.g., humic acid, proteins), reducing DOC to < 5 mg/L; activated carbon further adsorbs remaining organics (e.g., soluble microbial products), reducing DOC to < 3 mg/L; UF intercepts remaining organics, completely solving the organic fouling problem.

Medium DOC (DOC 3-8 mg/L, e.g., reservoir water, groundwater)

Problem: Medium DOC indicates relatively high organic content, requiring enhanced pretreatment to reduce DOC.
Pretreatment Selection: Coagulation-Sand Filtration + Activated Carbon Filtration + UF.
Principle: Coagulation removes macromolecular organics (e.g., humic acid); sand filtration removes small molecular organics (e.g., polysaccharides); activated carbon adsorbs remaining organics (e.g., soluble microbial products), reducing DOC to < 3 mg/L; UF intercepts remaining organics, mitigating organic fouling.

Low DOC (DOC < 3 mg/L, e.g., high-quality tap water, treated wastewater)

Problem: Low DOC indicates low organic content, but microorganisms (e.g., bacteria, viruses) may still be present, causing biofouling of the UF membrane (microorganisms proliferate to form biofilm).
Pretreatment Selection: Disinfection (e.g., ClO₂, UV) + UF.
Principle: Disinfection kills microorganisms; UF intercepts remaining microorganisms, completely solving the biofouling problem.

Recommended Comprehensive Pre-ultra Filtration System Combination Processes

Based on combinations of raw water turbidity, SDI, and DOC, the following pretreatment combination processes before UF are recommended:

Raw Water Indicator Combination	Recommended Pretreatment Combination Process	Core Goal
High Turbidity (>10 NTU) + High SDI (>5) + High DOC (>8 mg/L)	Coagulation-Sedimentation-Sand Filtration + Advanced Oxidation + Activated Carbon Filtration + UF	Reduce turbidity, SDI, and DOC; mitigate membrane fouling
Medium Turbidity (5-10 NTU) + Medium SDI (3-5) + Medium DOC (3-8 mg/L)	Coagulation-Sand Filtration + Activated Carbon Filtration + UF	Reduce turbidity, SDI, and DOC; stabilize UF operation
Low Turbidity (<5 NTU) + Low SDI (<3) + Low DOC (<3 mg/L)	Disinfection (ClO₂) + UF	Control microorganisms; prevent biofouling

Important Considerations of Pre-ultra Filtration System

Synergistic Effects of Pretreatment Processes: Pretreatment processes need to work synergistically (e.g., coagulation + sand filtration + activated carbon), not individually. For example, coagulation removes large particles, sand filtration removes small particles, and activated carbon removes organics. Only through their synergy can membrane fouling be effectively reduced.

Optimization of Operating Parameters: Operating parameters of pretreatment processes (e.g., coagulant dosage, sand filtration rate, activated carbon contact time) need to be optimized based on raw water quality. For example, excessive coagulant dosage can cause membrane fouling (leading to pore blockage); the optimal dosage should be determined through bench-scale tests.

Monitoring of Membrane Fouling: Transmembrane pressure (TMP), membrane resistance (R), and flux (J) of the UF membrane need to be regularly monitored to adjust the pretreatment process in a timely manner (e.g., when the TMP increase rate exceeds 0.01 MPa/day, the coagulant dosage needs to be increased).

Summary

The pretreatment combination process before UF must take raw water turbidity, SDI, and DOC as core indicators, select targeted pretreatment processes (e.g., coagulation-sedimentation-sand filtration for high turbidity, advanced oxidation + activated carbon for high DOC), and maximize pollutant removal through process synergy (e.g., coagulation + sand filtration + activated carbon) to mitigate membrane fouling and ensure stable UF operation. In practical applications, the appropriate pretreatment combination process should be selected based on raw water quality (e.g., Yellow River water, reservoir water, groundwater) and effluent requirements (e.g., industrial water, drinking water), and process stability should be ensured through optimization of operating parameters (e.g., coagulant dosage, sand filtration rate).