Filtration Options for Closed Loop Systems

How they Apply to Different Types of Closed Loop Filtration

  • 4 décembre 2018
  • Author: Chem-Aqua, Inc
  • Number of views: 19392

Closed loop systems, typically hot and cold water recirculating systems, are often ignored when it comes to water treatment programs. Not only should closed loops be chemically treated to control corrosion and microbiological growth, they should also include a side-stream filter to remove suspended solids. 

Even though closed loop systems are theoretically “closed,” suspended solids can still be introduced as a result of corrosion, new construction, new additions to older systems, leaks, and poor commissioning practices. Turbidity or discoloration can be one sign of suspended solids.

If left unfiltered, suspended solid buildup could lead to:

  • Erosion of metals
  • Corrosion
  • Deposit formation
  • Microbiological growth
  • Increased wear and tear on seals, control valves, piping, and instrumentation
  • Decreased performance of water treatment chemicals

Incorporating a closed loop filtration system into your current water treatment program can help remove these suspended contaminants and their associated problems. Typically side-stream filtration is recommended so flow is not interrupted in the main water stream if the filter becomes blinded, requires replacement, or is backwashing.

The chemical and physical properties of the suspended contaminants can make some approaches to removal more suitable than others. Some factors to consider include:

  • Particle size removal
  • Solids loading on the filter
  • Water losses
  • Temperature
  • Operational costs (e.g., filter bag replacement)
  • Labor required
  • Manual vs. automatic operation

Types of Closed Loop Filtration

Bag and Cartridge Filters

Bag and Cartridge filters are a common method used to help remove suspended solids. These manually operated filters are ideal for smaller closed recirculating loops less than 2,000 gallons. They require an inlet and outlet pressure gauge to know when to change the filter.

There are a range of filtration size options, with 0.35 to 50 microns being typical. When initially cleaning up a system, a series of progressively smaller filters may be required to reduce suspended solids without blinding the filters too quickly.  For example, 50 micron filters may be used to remove the largest particles in the system until the filter will stay online for a week without being blinded. At that point, the 50 micron filter is replaced with 20 micron filters until one can stay online for a week without being blinded, after which a 10 or 5 micron filter is used from that point forward, depending upon the system’s need.

Bag and cartridge filters are not always effective and practical. The downside of these filters is that they are operator dependent for effectiveness. They will need to be manually shutdown, drained, and flushed to change the bag or cartridge. These filters can be expensive to maintain, as well, making them not ideal for every situation. System water loss during the filter change will also need to be considered along with any long-term impact on chemical treatment costs.

Sand Filtration

Sand filters are available in many designs and sizes. The water to be filtered is sprayed over the top of the filter bed (sand). The suspended solids are caught on and in the filter bed, and the clean water exits the bottom. A properly maintained and operated sand filter will remove suspended particles as small as 5 microns.

Sand filters require regular backwashing to remove the accumulated solids. They are usually designed to automatically backwash based on a pressure drop or timer setting. With pressure drop control, it is a good practice to include a backup timer to trigger backwash if it does not occur within a set time period to prevent hard, matted deposits from accumulating. To reduce water and treatment chemical losses from the closed loop system, sand filters should be backwashed with makeup water rather than closed loop water.

High Performance Filters (HPF)

High Performance Filters (HPF) can remove suspended solids down to 5 microns using a unique, high--surface media to allow particulate matter to penetrate deep within the filter bed that can still be easily backwashed. These filters are self-cleaning with automatic backwashing. 

The HPF is designed to efficiently and economically remove suspended solids from recirculating closed water systems that operate at less than 120°F and a maximum of 100 psig. HPFs operate at higher flow rates with longer periods of time between backwashes, saving water usage costs. They can be programmed to automatically backwash based on a user selectable pressure differential switch, preset for 14 psig. The HPF uses fresh makeup water for backwash, which minimizes the amount of treated water lost from the closed loop system to the volume within the filter vessel itself. A mounted digital counter displays the number of backwashes completed.

A hot water HPF is also an option for high temperature, high pressure applications operating at a maximum temperature of 200°F and a maximum of 250 psig. These filters include a microprocessor controller, dual 3-way motorized valve, an inline rotameter to monitor flow rates, and inlet and outlet gauges and sample ports.

Super Performance Filters (SPF)

Super Performance Filters (SPF) can remove suspended solids down to 0.5 microns. The SPF uses 0.2 mm sand and filters along the media surface, operating at higher flow rates. The SPF is designed for closed loop systems operating at less than 120°F and a maximum of 100 psig.

Operating an SPF in closed loops where particles are greater than 5 micron will require an increase in backwashes and an increased loss of treated system water. The SPF also uses fresh makeup water for backwash, which minimizes the amount of treated water lost from the closed loop system to basically the volume within the filter vessel itself. A mounted digital counter displays the number of backwashes completed.

HPF/SPF Filter Media vs Conventional Filter Media


Nominal Rating

Loading Capacity*


20 µm

1.0 X

Sand and Anthracite

15 µm

1.6 X

Typical Multimedia

10 µm

1.8 X

HPF Media

5 µm

2.8 X

SPF Media

0.5 µm

1.0 X

*Measure of the amount of suspended solids a given volume of media can hold before the bed needs to be backwashed. Higher capacities require fewer backwashes, lower capacities require more frequent backwashes.



Filtration is important for the success of a closed loop water treatment program. The benefits of filters are very clear.

  • Reduce system component damage
  • Increase equipment efficiency
  • Allow the chemical treatment to properly work
  • Prevent unexpected system shutdown
  • Reduce labor and maintenance costs
  • Reduce energy and water usage

Chem-Aqua’s HPF and SPF filtration options offer the advantages of

  • High solids loading capacity reducing backwash frequency
  • Pre-assembled and pre-programmed for easy installation
  • Require minimal plant involvement

A Chem-Aqua Representative can perform a simple onsite test to help determine the best filter model to use.

Contact us today to learn more about our water treatment programs and to have a free survey and filtration test performed by a trained Chem-Aqua Representative.

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