bag filters for water treatment

Bag filters are commonly used filtration devices in the water treatment field. They intercept suspended solids, colloids, particulate impurities, etc., in water through built-in filter bags, featuring simple structure, high filtration efficiency, and convenient maintenance. Below is key information and application highlights:

I. Core Components and Working Principle

1. Main Structure

Filter Housing: Mostly made of stainless steel (e.g., 304/316L) or engineering plastics, pressure-resistant and corrosion-resistant, with inlet, outlet, and filter bag chamber.
Filter Bag: Constructed from synthetic fibers like polypropylene (PP) or polyester (PET), with precision ranging from 1–500 microns, fixed inside the filter via a top ring.
Support Basket: Secures the filter bag to prevent deformation during filtration and guides uniform water flow through the filter media.

2. Working Principle

Water to be treated flows into the filter through the inlet. As it passes through the filter bag, impurities are trapped inside the bag, while clean water exits through the outlet via filter bag pores.
When the filter bag is saturated with contaminants (judged by inlet-outlet pressure difference, e.g., >0.1MPa), the bag needs replacement without disassembling the entire system.

II. Main Types and Characteristics

1. Classification by Housing Type

Type	Characteristics	Application Scenarios
Single-bag filter	Contains only 1 filter bag, compact structure, small flow rate (5–50 m³/h), low cost.	Small systems, laboratories, household water purification
Multi-bag filter	Can install 2–24 filter bags in parallel, large flow rate (up to 500 m³/h), requires manifold design.	Industrial water treatment, municipal water supply (large systems)
Top-inlet/side-inlet	Different inlet positions; top-inlet ensures more uniform water flow, while side-inlet suits space-constrained scenarios.	Selected based on installation space

2. Classification by Pressure Rating

Low-pressure type: ≤0.3MPa, suitable for normal or low-pressure scenarios (e.g., municipal tap water filtration).
Medium-high pressure type: 0.6–1.6MPa, ideal for high-pressure systems like industrial circulating water or RO pretreatment.

III. Advantages and Limitations

Advantages

Flexible filtration precision: Can meet various needs from coarse to fine filtration by replacing filter bags of different precisions (1–500 microns).
High dirt-holding capacity: Deep filter bag structure retains more impurities, extending replacement cycles (30–50% higher dirt capacity than cartridge filters).
Low maintenance cost: Filter bags can be replaced without professional tools, with low individual bag cost (¥10–200), minimizing downtime.
High compatibility: Suitable for various water qualities (acidic/alkaline liquids, oily wastewater, high-temperature water) via material selection for bags and housings.

Limitations

Higher pressure loss: Deep filtration structure causes greater water flow resistance than precise filtration methods like membrane filtration.
Low automation level: Requires manual monitoring of pressure difference or regular bag replacement, unsuitable for unmanned scenarios (can be improved with automatic pressure alarms).

IV. Typical Application Scenarios

1. Industrial Water Treatment

Circulating cooling water filtration: 截留 algae, sediment, rust, etc., to prevent heat exchanger blockage and extend equipment life.
Chemical wastewater pretreatment: Removes suspended solids and colloids for subsequent biochemical or membrane treatment.
Metalworking fluid filtration: Filters metal chips and sludge from cutting fluids for recycling.

2. Municipal and Civil Applications

Waterworks pretreatment: Further removes fine particles after sedimentation to improve water clarity.
Sewage plant deep filtration: Serves as a post-secondary clarifier unit to reduce effluent SS (suspended solids) concentration for compliance.
Commercial/household water purification: Used in pool filtration, direct drinking water pretreatment (with activated carbon filter bags), etc.

3. Special Industry Applications

Food and beverage industry: Filters impurities from syrups and alcoholic beverages to meet hygiene standards (requires FDA-certified food-grade filter bags).
Pharmaceutical and electronics industry: Prepares high-purity water by intercepting particulate contaminants for reverse osmosis (RO) or EDI systems.

V. Selection and Maintenance Tips

1. Key Selection Parameters

Flow rate and number of filter bags: Choose single or multi-bag filters based on system flow. A single No. 2 filter bag (810mm length) handles ~30 m³/h at 50-micron precision.
Material compatibility:Precision selection: Based on impurity particle size (e.g., ≤5 microns for RO pretreatment, 50–100 microns for coarse filtration).
- Acidic liquids: PP filter bag + stainless steel housing;
- Oily wastewater: PET filter bag + anti-corrosion coated housing;
- High-temperature liquids (>80℃): high-temperature-resistant PET filter bag.

2. Maintenance Recommendations

Replacement cycle:
- Normal water quality: Replace every 1–3 months (pressure difference-dependent);
- High-turbidity water: Inspect weekly and replace immediately when pressure difference reaches 0.1MPa.
Operation notes:Environmental treatment: Prioritize degradable filter bags or centralized recycling for disposable bags to reduce pollution.
- Relieve pressure before replacing bags to avoid high-pressure water impact;
- Ensure the bag’s sealing ring tightly fits the filter to prevent bypass flow.