Design Scheme for Disc-Tube Ultrafiltration Membrane System for Municipal Water Supply

1.0 Design Basis and System Positioning

• Water Source: Surface water (rivers, reservoirs, lakes) or pre-treated groundwater.

• Treatment Objective: Produce municipal tap water compliant with the Standards for Drinking Water Quality(GB 5749-2022), or reclaimed water meeting requirements for landscape, irrigation, or industrial cooling.

• System Positioning: Serves as an enhanced alternative or advanced treatment unit to the traditional "coagulation-sedimentation-sand filtration" process, realizing a "membrane-based water plant" process with physical sieving as its core.

• Core Advantages:

  • Absolute Barrier: Effectively removes bacteria, viruses, parasites (e.g., Giardia, Cryptosporidium), algae, and suspended solids, ensuring biological safety.

  • Response to Sudden Pollution: Strong adaptability to water quality fluctuations (e.g., high turbidity, algae blooms), ensuring stable effluent quality.

  • Chemical Savings: Can reduce coagulant dosage, lowering the formation potential of disinfection by-products.

  • Compact Footprint: Simplified process reduces land use to approximately 1/3 - 1/2 of conventional systems.

2.0 Design Water Quality and Capacity

• Raw Water Quality (Typical Surface Water):

  • Turbidity: 1-100 NTU (can exceed 200 NTU during flood season)

  • COD_Mn: 2-6 mg/L

  • Algae: 10⁴-10⁷cells/L

  • Total Bacteria Count: 10²-10⁴CFU/mL

• Product Water Quality:

  • Turbidity: < 0.1 NTU

  • Cryptosporidium/Giardia: Complete removal

  • Bacterial Log Removal: > 6 log

  • Viral Log Removal: > 4 log

• Design Capacity: 5,000 - 100,000 m³/d (modularly combinable)

3.0 Process Flow Diagram and Explanation

The diagram below illustrates the complete municipal water treatment process centered around the Disc-Tube Ultrafiltration (DTUF) system, including key stages such as pretreatment, core membrane filtration, post-disinfection, and concentrate handling.

Step-by-Step Process Explanation:

1. Pretreatment Section:

   • Raw water first passes through coarse and fine screens to remove large debris.

   • A small amount of coagulant is dosed and rapidly mixed via a pipeline mixer. This destabilizes fine colloids and organics, forming micro-flocs to reduce the fouling load on the downstream membrane. This step can be intelligently dosed or bypassed based on raw water turbidity.

   • The self-cleaning filter acts as the final safety barrier for the membrane system, removing particles >100 µm to absolutely protect the DTUF membrane elements.

2. DTUF Membrane Core Section:

   • Membrane System Composition: Consists of multiple Disc-Tube Ultrafiltration membrane modules in parallel. Each module contains numerous hollow fiber membrane strands and unique flow distribution discs. The system operates intermittently in a "Filtration/Backwash" mode.

   • Filtration Process: Under low pressure (typically 0.5-2.0 bar), feed water flows on the outside of the membrane fibers. Purified water passes through the membrane wall into the central permeate pipe via siphoning or slight negative pressure. Contaminants are retained on the outer surface of the fibers.

   • Operation Mode: Primarily operates in "dead-end filtration" mode, periodically switching to a "cross-flow flushing" mode, using concentrate discharge to scour the membrane surface.

3. Post-Treatment and Disinfection:

   • UF permeate enters the clearwell. Its turbidity and microbiological indicators are already extremely low.

   • Sodium hypochlorite is dosed to maintain a residual chlorine level in the distribution network, or Ultraviolet (UV) light is used for final disinfection, ensuring water safety during conveyance.

4. Backwashing and Chemically Enhanced Backwashing:

   • Hydraulic Backwash: Every 30-60 minutes, UF permeate is used to backflush the membrane fibers, forcefully dislodging the fouling layer. Backwash water is discharged to the concentrate tank.

   • Chemically Enhanced Backwash (CEB): Every 24-48 hours, low-concentration chemicals (e.g., sodium hypochlorite for disinfection/algae control, citric acid for inorganic scale) are dosed into the backwash water to recover membrane performance online.

5. Concentrate/Wastewater Treatment:

   • The DTUF system concentrate primarily contains retained suspended solids, algae, and some organics. It can be discharged to a wastewater treatment plant, or after sedimentation, the supernatant can be recycled to the system inlet to improve recovery.

4.0 Core Design Parameters and Equipment Selection

5.0 Techno-Economic Analysis (Estimate for 50,000 m³/day scale)

6.0 Scheme Characteristics Summary

1. Safe and Reliable: Physical sieving mechanism provides an absolute barrier against microorganisms, a core technology for new-generation drinking water safety.

2. High Adaptability: Disc-tube design offers good fouling resistance, capable of handling seasonal fluctuations in surface water quality and sudden high-turbidity events.

3. Automated and Intelligent: Fully automatic operation and monitoring, with early warning and intelligent cleaning functions, enabling unattended operation.

4. Green and Economical: Short process flow, low chemical usage, low energy consumption, competitive lifecycle costs.

Implementation Recommendations:

• A one-year pilot test is recommended to accurately determine the optimal flux, recovery rate, cleaning frequency, and pretreatment scheme for the specific water source.

• The system design should reserve space for expansion. Membrane racks should be arranged in series for easy future capacity addition.

• Establish a comprehensive membrane integrity monitoring and emergency response plan. This is paramount for ensuring the safe operation of large municipal water supply projects.

This scheme provides a complete and feasible technical pathway for upgrading or constructing new municipal water plants using Disc-Tube Ultrafiltration membrane technology, representing a reliable choice for achieving the goal of "high-quality drinking water."