Brief Introduction of Water Treatment Equipments

Water treatment involves purifying contaminated industrial wastewater or raw water using a range of equipment to meet national water quality standards. Given the close relationship between social production, daily life, and water, water treatment spans a wide array of applications, constituting a significant industrial sector. The process uses physical and chemical methods to remove unwanted substances from water.

Common water treatment includes both wastewater and pure water treatment. This document will focus specifically on various equipment used in pure water treatment.

I. Pre-treatment Equipment

1. Media Filters

1.1 Overview

Media filters use one or more filtration media to remove suspended solids from water under pressure, clarifying water with a turbidity often reaching below 3 NTU. Common filter materials include quartz sand, anthracite coal, and manganese sand. They are used for turbidity removal, water softening, and pre-treatment for demineralization.

The system comprises a filter body, related piping, and valves. The filter body includes components such as the filter vessel, water distribution components, support components, backwash air pipe, filter media, and air vent valve.

1.2 Working Principle

1.2.1 Filtration Process

In filtration mode, raw water passes through a multi-layer water distributor and a specially designed shell, reaching the filter media layer in a near-laminar flow. Impurities are trapped within the media layer, and the filtered water is collected by mushroom-shaped collectors at the bottom of the filter. The laminar flow ensures efficient filtration even at high flow rates.

1.2.2 Backwash Process

As impurities accumulate, the head loss increases. Upon reaching a set limit or after a predetermined time, the system automatically switches to backwash mode to clean accumulated impurities. Regular backwashing is essential. During backwash, clean, filtered water flows backward through the filter unit via a three-way backwash valve. The filter media layer is lifted by the water flow, and impurities are discharged through the valve.

1.3 Common Media Filters

1.3.1 Multi-Media Filters (MMF)

Common types include anthracite-quartz sand-magnetite filters and activated carbon-quartz sand-magnetite filters.

Key filter layer design considerations:

• Density Difference: Distinct density differences among filter materials ensure no mixing after backwash.
• Water Quality & Use: Filter material selection is based on raw water quality and product water use.
• Particle Size: Lower layer media has a smaller particle size than upper layers for effectiveness.

1.3.2 Activated Carbon Filters (ACF)

Using activated carbon media to remove color, taste, residual chlorine, and organic matter through adsorption. It is effective for removing dissolved organic compounds like benzene and phenols, as well as pollutants difficult to remove biologically or chemically. After the activated carbon filter bed, suspended solids in water are <0.1mg/L, COD removal is typically 40%-50%, and free chlorine is <0.1mg/L.

1.3.3 Iron and Manganese Filters

Filled with refined manganese sand and quartz sand to remove iron and manganese from water. Typically used in pre-treatment for water with high iron and manganese content.

2. Disc Filters

2.1 Overview

Compact, with a small footprint and high filtration accuracy. Increasingly replacing multi-media filters as pre-treatment, especially for large-scale ultrafiltration applications.

2.2 Working Principle

Thin plastic discs with micron-sized grooves are stacked and pressed together by springs and liquid pressure within a filter cartridge. The grooves intersect, forming deep filtration channels. During filtration, the discs are tightly compressed, enhancing self-locking and efficient filtration. Liquid flows from the disc edge through the grooves to the center, passing t

hrough multiple filtration points. Backwashing can be performed manually or automatically.

Advantages of Disc Filters:

• Stable filtration effect
• Deep filtration for superior dirt-holding capacity
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• Simple operation and convenient maintenance
• Low system operating costs and reliable performance with a lifespan of over 10 years

3. Precision Filters (Cartridge Filters)

3.1 Overview

Also known as cartridge filters or security filters, these primarily remove fine particles. Cartridges are commonly made of polypropylene melt-blown fiber or wound PP cotton. Suitable for smaller ultrafiltration units, with nominal accuracy between 1-100 microns. 100-micron filters are commonly used.

3.2 Working Principle (PCF Pore Size Adjustable Fiber Filter Example)

During filtration, fiber filaments are compressed to reduce pore size, trapping suspended solids. As the filter becomes clogged, reducing the flow rate and increasing the pressure, it automatically enters the backwash process. During backwash, the compression is released, and compressed air and treated water are used to flush out impurities.

Performance Characteristics:

• High filtration accuracy with uniform pore size
• Low filtration resistance, high flow rate, strong dirt-holding capacity, and long service life
• High purity of filter material with no contamination of the filtration medium
• Resistance to acids, alkalis, and other chemical solvents
• High strength, temperature resistance, and minimal deformation of the filter element
• Low cost, low operating expenses, easy to clean, and replaceable filter elements

4. Ultrafiltration (UF)

4.1 Overview

Ultrafiltration is a physical sieving process that uses pressure to force liquid through a membrane with pore sizes typically between 10-100nm and a molecular weight cut-off (MWCO) of 6,000 to 500,000 Daltons.

4.2 Basic Principle

Under pressure, raw liquid flows across the membrane surface. Water and small molecules pass through the membrane’s micropores to become permeate, while substances larger than the pore size are retained on the feed side as concentrate. This achieves purification, separation, and concentration of the raw liquid.

4.3 UF Classification and Characteristics

UF membranes are primarily classified into spiral wound, plate-and-frame, tubular, and hollow fiber types. Hollow fiber is widely used domestically, characterized by lacking a membrane support structure, relying on the fiber tube’s strength to withstand pressure. Hollow fiber membranes are further divided into inside-out and outside-in configurations.

Inside-out hollow fiber membranes are advantageous due to their large effective membrane area per unit volume and small footprint.

II. Ultrapure Water Treatment Equipment

5. Reverse Osmosis (RO)

5.1 Overview

Reverse osmosis is a membrane process that separates liquid mixtures by selectively passing solvent (typically water) through a membrane while retaining ionic substances. It uses a pressure difference across the membrane to overcome osmotic pressure, allowing the solvent to pass.

5.2 Separation Mechanism

The selective permeability of the RO membrane is related to the dissolution, adsorption, and diffusion of components in the membrane. This depends on membrane pore size, structure, chemical and physical properties, and interactions between the components and the membrane. Chemical factors (membrane characteristics) dominate the separation process.

5.3 Applications

RO is used for brackish water and seawater desalination, pure water production, and separating mixtures difficult to separate by other methods. Industrial applications include:

1. Seawater desalination
2. Drinking water production
3. Pure water production

6. Anion and Cation Exchangers

6.1 Overview

Exchangers resemble multi-media filters in appearance, often with a rubber lining for corrosion protection. They are filled with various anion and cation exchange resins, often using strong alkaline anion exchange resins and strong acid cation exchangers.

6.2 Working Principle

6.2.1 Strong Acid Cation Exchange Resin (001×7)

When the resin contacts water, cations in the water exchange with H+ ions on the resin, removing cations and releasing hydrogen ions.

6.2.2 Strong Base Anion Exchange Resin (201×7)

When the resin contacts water, anions in the water exchange with OH- ions on the resin, removing anions and releasing hydroxide ions.

7. Mixed Bed Ion Exchangers

7.1 Overview

Used to produce high-purity water. Typically placed after anion and cation exchangers, or downstream of electrodialysis or RO. Used in electronics, pharmaceuticals, paper-making, chemicals, and nuclear industries, as well as for purifying sugar solutions, glycerol, and polyols.

7.2 Working Principle

A mixed bed contains anion and cation exchange resins mixed thoroughly in a 2:1 ratio. Anions and cations are adsorbed onto the resins, and the exchanged H+ and OH- ions react to regenerate water, achieving ultrapure water production.

8. Electrodeionization (EDI)

8.1 Overview

EDI, or electrodeionization, is a technology for producing pure water without using acids or alkalis. Relying on electricity, it continuously produces high/ultrapure water without chemical consumption. It’s a next-generation, green technology that replaces traditional mixed beds for deep demineralization.

8.2 Working Principle

In EDI, ions in the feed water are removed by exchange through a resin layer, purifying the water. Voltage applied across the membrane stack electrolyzes water molecules into hydrogen and hydroxide ions to continuously regenerate the resin. Ions exchanged are migrated to the concentrate chamber under the influence of the electric field.

8.3 Advantages Compared to Mixed Bed Technology

8.3.1 Harmless Chemistry

• Continuous regeneration using electricity, eliminating corrosive chemicals.
• Requires minimal cleaning if the upstream RO system operates normally.
• No investment in chemical regeneration equipment such as alloy valves, pipes, pumps, and chemical storage.

8.3.2 Continuous Regeneration

• Eliminates the need for backup ion exchange equipment.

8.3.3 Simple Startup/Operation

• Eliminates complex mixed bed regeneration steps.
• Fewer valves, automated operation, and ease of use.

8.3.4 Convenient Module Replacement

• Typical module lifespan exceeds 3-5 years.
• Easy storage and replacement of EDI modules.

III. Auxiliary Equipment

9. Flocculation

Small colloidal ions (<5 microns) in raw water carry electric charges and repel each other, making sedimentation or pre-filtration difficult. A flocculant is added to neutralize the charge, causing the particles to agglomerate for removal. Common flocculants include ferric chloride, aluminum sulfate, and alum. pH should be 6-8.4 for ferric chloride and 5-7.5 for aluminum sulfate. Small amounts of polymer coagulant aids, such as polyacrylamide, can be added to improve flocculation.

10. Degasifier (Decarbonator)

10.1 Overview

A forced-draft, atmospheric-pressure steel structure device. Typically installed after cation exchangers or RO systems to remove free CO2 decomposed from acidic water using air.

10.2 Working Principle

Acidic water is introduced at the top, sprayed mechanically, and flows over a packing layer with a large surface area. Air passes upward in the opposite direction, stripping free CO2 from the water, which mixes with the air and is discharged at the top. Typically, residual CO2 after this process will not exceed 5mg/L.