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
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
the operator's skill and knowledge at maximizing the efficiency of
coagulation/flocculation and sedimentation. Physical features of
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
run time, should be recorded.
Factors Influencing Efficiency
The efficiency of a filter is influenced by a variety of factors.
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
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
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
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
caused by improper operation of the filter.
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.
on the surface of filters when adhesive materials cause particles out
of the water and media grains to stick together. If the filter is
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
loss of filter capacity, since water will not pass through the mudballs
and must flow around them.
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
effluent water. The untreated water may contain microorganisms
such as Giardia and is thus
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
(negative head) or from filtering very cold, supersaturated
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,
in the filter can damage the filter media.