Filter Efficiency

Monitoring

The efficiency of a filter can be measured in a variety of ways.  Effluent turbidity, which should be monitored continuously, gives an indication of the efficacy of the filtration process.  Particle counters can be used to count the number of particles in the effluent which are within the size range of Giardia and Cryptosporidium to determine how efficiently the filter has removed these microorganisms. 

The length of the run time between backwashing can also be used as a measure of filter efficiency.  Filter run time depends largely on the clarity of the water passing through the filter since clearer water will contain less material to be filtered out and clog the filter.  This clarity, in turn, usually reflects the operator's skill and knowledge at maximizing the efficiency of coagulation/flocculation and sedimentation.  Physical features of the plant can also have considerable influence on the run time.

The operator should test the influent and effluent turbidity, the effluent color, and head loss.  These factors, as well as the filter run time, should be recorded. 



Factors Influencing Efficiency

The efficiency of a filter is influenced by a variety of factors.  To a large extent, the efficiency is determined by the characteristics of the water being treated and by the efficiency of previous stages in the treatment process. 

The chemical characteristics of the water being treated can influence both the preceding coagulation/flocculation and the filtration process.  In addition, the characteristics of the particles in the water are especially important to the filtration process.  Size, shape, and chemical characteristics of the particles will all influence filtration.  For example, floc which is too large will clog the filter rapidly, requiring frequent backwashing, or can break up and pass through the filter, decreasing water quality. 

The types and degree of previous treatment processes greatly influence filtration as well.  Conventional, direct, and in-line filtration will all have different levels of efficiency. 

Finally, the type of filter used and the operation of the filter will influence filter efficiency.  The next section will discuss problems caused by improper operation of the filter. 



Filter Problems

Large mudball.
Photo Credit: Know Your Filters

Mudballs are approximately round conglomerations of filter material, ranging in size from pea-sized to two inches or more in diameter.  The picture above shows a very large mudball.  Mudballs form on the surface of filters when adhesive materials cause particles out of the water and media grains to stick together.  If the filter is not properly backwashed and surface washed, mudballs will continue accumulating material and will grow larger, eventually sinking down into the filter media.  Mudballs in the media result in shortened filter runs and in loss of filter capacity, since water will not pass through the mudballs and must flow around them. 


Breakthrough.
Another problem associated with filters is breakthroughs, cracking of the filter media and/or separation of the media from the filter wall.  Breakthroughs are caused by running the filter at an excessive filtration rate or by extending filter runs too long between backwashing.  Breakthroughs can result in untreated water flowing through the filter, which in turn results in a sudden high turbidity in the effluent water.  The untreated water may contain microorganisms such as Giardia and is thus not safe to drink. 

Air binding is the release of dissolved gases from the water into the filter or underdrain.  Air binding may result from low pressure in the filter (negative head) or from filtering very cold, supersaturated water.  The air in the filter and underdrain prevents water from passing through the filter, which in turn results in abnormally high head loss even when the filter has recently been backwashed.  During backwash, the air in the filter can damage the filter media.