Ultrafiltration and Activated Carbon Systems: Pros and Cons in Ultrapure Water Preparation

Ultrapure water is water from which conductive ions, colloids, organic matter, and other impurities have been completely removed, with a product water resistivity exceeding 18 MΩ·cm (at 25°C). Ultrapure water systems typically employ pretreatment technologies, reverse osmosis (RO), electrodeionization (EDI), and post-treatment processes, often supplemented by auxiliary treatments like UV lamps and TOC (Total Organic Carbon) removal units.

Ultrapure Water Production Process Flow:
The standard process flow is: Tap water → Pretreatment system → Multi-stage high-pressure RO system → EDI system → TOC removal unit → Polishing mixed-bed system → Point of Use (POU). During production, pumps serve as the power source, and storage tanks at various stages ensure safe and stable pump operation. Adding filters of different grades at specific points ensures product water quality.

Additionally, during filter operation, various chemicals can be added to adjust water pH, reduce residual chlorine, and slow down RO membrane scaling, etc. In colder northern regions during winter, heat exchangers may be added at the inlet of the pretreatment system to raise the tap water temperature to meet process requirements.

Furthermore, purging the ultrapure water storage tank with nitrogen gas is crucial to isolate the water from the atmosphere and prevent secondary contamination.

Importance of Pretreatment:
1. RO Membrane Feed Water Requirements:
The RO system is the most critical component in ultrapure water production. Its proper operation ensures the safety of the entire pure water system and the quality of the product water. RO systems have specific feed water requirements:

• Silt Density Index (SDI) < 4.0

• Turbidity (NTU) < 1.0

• Organic Matter Content (COD) < 1.5 mg/L

• Residual Chlorine Content < 0.1 mg/L (ideally controlled at 0 mg/L)

• Iron Content < 0.05 mg/L when Dissolved Oxygen > 5 mg/L

• Silica (SiO₂) in Concentrate < 100 mg/L

• Langelier Saturation Index (LSI): pHb - pHs < 0

• Ions prone to forming insoluble salts (e.g., Sr, Ba): Ionic Product (Ipb) < 0.8 Ksp

2. Role of the Pretreatment System:
The pretreatment system removes residual chlorine, large suspended solids, flocculated colloids, organic matter, oxides, organic compounds, and heavy metals from the water before it enters the RO system, thereby reducing COD and SDI values.

Moreover, ions like calcium, magnesium, barium, sulfates, silicates, and carbonates can be complexed by adding scale inhibitors to form larger particles, which are then discharged as concentrate from the RO unit.

(1) Impact of System Malfunctions on RO Membranes:
As outlined in the pretreatment section, the vast majority of harmful macromolecules are removed by the pretreatment system before reaching the RO membranes. If the pretreatment system is deficient and multiple RO feed water parameters are non-compliant, irreversible physical and chemical damage to the RO membrane elements can occur, significantly shortening their lifespan. Factors affecting RO membrane lifespan include:

• RO membrane scaling

• RO membrane fouling by metal oxides

• RO membrane blockage by suspended solids

• Fouling by colloids, organics, and microorganisms, leading to increased product water COD.

(2) Impact of System Malfunctions on the RO Ultrapure Water System:
Malfunctions in the pretreatment system affect the entire RO ultrapure water system in three main ways:

• Reduces the system's product water yield and quality.

• Increases water and energy consumption of the RO equipment.

• Increases operational costs, including expenses for scale inhibitors and other water treatment chemicals.

Working Principles of UF and Activated Carbon Systems:
Common pretreatment systems in practice include Ultrafiltration (UF) systems and Activated Carbon systems. A UF system typically consists of a disc filter and a UF unit. An Activated Carbon system usually comprises a multi-media filter (MMF) and an activated carbon filter (ACF).

1. Working Principle of UF System:

• Disc Filter: Works by surface and depth filtration using tightly packed plastic discs. Primarily removes large particles like sand and silt.

• Ultrafiltration (UF): A pressurized membrane separation technology. Under pressure, small solute molecules and solvent pass through a membrane with specific pore sizes, while larger solute molecules (molecular weight 10,000 - 30,000 Da) are retained, partially purifying the solution by removing macromolecules. (See schematic Fig 1 for UF principle).

2. Working Principle of Activated Carbon System:

• Multi-Media Filter (MMF): Utilizes one or several filter media (e.g., quartz sand, anthracite, manganese sand) under pressure. Water with high turbidity passes through a bed of granular material, effectively removing suspended solids and clarifying the water. It reduces SDI to below 3 and is used for turbidity removal and as pretreatment for softened/pure water.

• Activated Carbon Filter (ACF): Water from the MMF enters the ACF, which contains a bed of quartz sand supporting activated carbon media. Activated carbon removes residual chlorine, organic matter, and suspended impurities primarily through physical adsorption (van der Waals forces) within its vast network of pores.

Pros and Cons of UF and Activated Carbon Systems:
Each type of pretreatment system has its advantages and disadvantages during operation.

Ultrafiltration (UF) System:

1. Pros:

   • High concentration factor, product water recovery rate can exceed 90%.

   • Excellent product water clarity and quality.

   • As RO pretreatment, UF significantly reduces RO investment costs and extends RO membrane lifespan (to over 3 years).

   • High automation level, simple structure, low operation/maintenance costs; capable of online backwashing and chemical cleaning (CIP).

   • Small footprint.

2. Cons:

   • UF membrane pore sizes (0.002 - 0.1 μm) cover a wide filtration range (colloids ≥0.1 μm, latex ≥0.5 μm, bacteria ≥0.2 μm, particles ≥5 μm). Some components are prone to clogging during operation, affecting flow rate and membrane life.

   • Higher initial investment cost (typically 2-3 times the cost of an MMF+ACF system for the same capacity).

   • Strict requirements on feed water pressure.

   • Cannot be shut down for extended periods; membranes require chemical preservation for long-term idleness.

Activated Carbon System:

1. Pros:

   • Lower initial investment cost.

   • Stable water output; less sensitive to feed water pressure variations.

2. Cons:

   • Large footprint.

   • During operation, the activated carbon and anthracite undergo physical/chemical reactions, leading to natural consumption, adsorption capacity exhaustion, and deterioration of product water quality, negatively impacting the downstream RO system.

   • Backwashing cycle for the ACF is difficult to control and often short. Inadequate backwashing leads to carbon fines leakage, severely contaminating water quality and downstream equipment. Infrequent backwashing causes high differential pressure across the filter, posing operational risks and reducing flow rate.

   • High long-term operation and maintenance costs, significant workload; filter media has a short lifespan (requires annual replacement).