Filtering

The purpose of filtration is to remove suspended particles from water by passing the water through a medium such as sand.  As the water passes through the filter, floc and impurities get stuck in the sand and the clean water goes through.  The filtered water collects in the clearwell, where it is disinfected and then sent to the customers. 

Filtration is usually the final step in the solids removal process which began with coagulation and advanced through flocculation and sedimentation.  In the filter, up to 99.5% of the suspended solids in the water can be removed, including minerals, floc, and microorganisms. 



Requirements

Filtration is now required for most water treatment systems.  Filters must reduce turbidity to less than 0.5 NTU in 95% of each month's measurements and the finished water turbidity must never exceed 5 NTU in any sample. 

As you will recall, turbidity alone does not have health implications.  So, why the strict regulations?  Although turbidity is not harmful on its own, turbid water is difficult to disinfect for a variety of reasons.  Microorganisms growing on the suspended particles may be hard to kill using disinfection while the particles themselves may chemically react with chlorine, making it difficult to maintain a chlorine residual in the distribution system.  Turbidity can also cause deposits in the distribution system that create tastes, odors, and bacterial growths. 

However, turbid drinking water has other troublesome implications as well.  Sand filtration removes some cyst-forming microorganisms, such as Giardia which cannot be killed by traditional chlorination.  Cysts are resistant covers which protect the microorganism while it goes into an inactive state.  

Regulations require that at least 99.9% of Giardia cysts and 99.99% of viruses be removed from drinking water.  Since it is difficult to test directly for these microorganisms, turbidity in water can be used as an indicator for their presence.  By requiring a low turbidity in the finished water, treatment plants are ensuring that few or no Giardia are present in finished drinking water.

In a few locations, surface waters are used for domestic purposes without filtration.  In these situations, the water is obtained from a watershed which includes only undeveloped areas.  The watershed is patrolled and carefully managed to prevent contamination.



Location in the Treatment Process

In the typical treatment process, filtration follows sedimentation (if present) and precedes disinfection.  Depending on the presence of flocculation and sedimentation, treatment processes are divided into three groups - conventional filtration, direct filtration, and in-line filtration.

The most common method of filtration is conventional filtration, where filtration follows coagulation/flocculation and sedimentation.  This type of filtration results in flexible and reliable performance, especially when treating variable or very turbid source water.

Some treatment plants operate without some or all of the sediment removal processes which precede filtration.  If filtration follows coagulation and flocculation, without sedimentation, it is known as direct filtration.  This method can be used when raw water has low turbidity.

Another type of filtration, known as in-line filtration, involves operating the filters without flocculation or sedimentation.  A coagulant chemical is added to the water just before filtration and coagulation occurs in the filter.  In-line filtration is often used with pressure filters, but is not as efficient with variable turbidity and bacteria levels as conventional filtration is. 



Polymer Aids

Although filtration does not require the addition of any chemicals, polymer aids may sometimes be added to the influent water.  These chemicals improve the quality of the effluent water by helping the floc get caught in the filter. 

Polymer aids come in two main types.  Moderate molecular weight cationic polymers (DADMA) are added ahead of flocculation to strengthen the floc while relatively high molecular weight nonionic polymers (polyacrylamides) are added just before filtration to aid in floc removal.

Polymer aids can be troublesome in some respects.  The powdered form of the polymer is very slippery, so spills should be cleaned up quickly.  In addition, extended use of polymer aids may gum up the filters.  As a result, polymer aids are often used like coagulant aids - in extreme situations to improve the water quality for a short time.