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2/1/2017 | 2 MINUTE READ

Filtration: A Simple Solution for a Big Problem

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Cloth blinding can be a major issue. Precoating might be the answer.


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To filter oily or slimy solids from wastewater, sometimes all that’s needed is a simple solution. Pollution control and filtration manufacturer Met-Chem offers filtration and wastewater analysis services and expertise to handle those challenges.

With less than a gallon of typical wastewater, the company can run tests to determine if a filter press, chemical pretreatment, clarifier or other option will provide the desired result.

When conducting a filter press feasibility study, lab technicians begin by analyzing customer needs. A sample is then introduced to a pressurized testing chamber, replicating the conditions of a filter press. If a good filter cake is not formed, due to cloth blinding or another issue, additional testing is conducted.  
Cloth blinding happens when the filter cloth is clogged with small, oily or slimy solids. Many solutions can easily be filtered without blinding, but some contain fine particles that quickly obstruct the filter cloth. Although this can destroy the whole system, it doesn’t necessarily mean the system or the cloths are the problem. 

 “We often get calls from clients looking for a new filter press or a new filtration solution because their current setup is not performing correctly,” says Met-Chem President Wally Senney. “However, after testing their wastewater, we find that their filter cloths are blinding, often extremely early in the filtration process.”

Even after the right cloth is selected, solids may still cause the filtrate to blind. With a precoat, the filter aid coats the cloth, enabling solids to effectively build on the media. Particles from the influent create a filter cake that becomes trapped in the multi-layers of the precoat, keeping the filter cloth clear and enabling filtration.

Additional equipment is needed to perform a precoat, such as a mixing tank, mixer, plumbing and possibly a feed pump, all offered by Met-Chem.

There are two major methods for using filter aids with a filter press: 

  • Precoat before filtration 
  • Body feed during filtration


If blinding is happening early in the process, the best solution is to precoat before filtration begins. Precoat materials can be mixed with water to form a slurry, which is then circulated through a filter until the water is clear and the filter cloths have been coated.

If the blinding is more gradual, a body feed would be more effective. In this process, the filter aid material is mixed directly into the solution being filtered. This can be accomplished either by injecting it into the feed line or mixing it directly in the tank feeding the filter press. However, this will also result in a mixed cake with both particulates from the incoming stream and filter aid, whereas using a precoat before filtration will create more defined layers.

Before moving on from existing filtration and wastewater equipment, a comprehensive analysis can often point to a simple, inexpensive solution.

For more information, visit




Originally published in the February 2017 issue. 





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