In a continuous sand filter the water passes the
filter bed in upward direction. The flow of water through a porous medium
results in a hydraulic pressure drop. We call this pressure drop the filter bed
resistance. The actual value of this filter bed resistance is measured by
reading the difference in head in the feed pipeline to the filter and the
filtrate level in the filter, as per the figure. Either by using a pressure indicator
or by simply measuring the water level in the de-aeration pipe at the top of
the filter.


Filter bed resistance

The filter bed resistance,  Hfilter bed, can be divided into two components:

                                                Hfilter bed = H0 + Hdeposition                                                                                                 (1)

H0 is referred to as the clean bed resistance; this is the hydraulic resistance due to the water flow of a clean filter bed, without any deposits in the pore space.

The second component, Hdeposition, represents the additional filter bed resistance due to deposits in the pore space of the filter medium.

In blog #5 we showed how the clean bed resistance is calculated. In this blog we are going to describe the relevancy of the second component Hdeposition in the equation of filter bed resistance on the filter performance.


Head loss due to deposition of solids within
the filter bed

present in the water will be captured in the pore space of the filter bed. Due
to these deposits the pores become narrower, leading to additional filter bed

An important
factor that influences filtration efficiency is the filling degree of the
pores. A higher filling degree of the pores within the filter bed will act as
an additional barrier for solids entering the filter. This will generally lead
to a better filtrate quality. Also known as “dirt catching dirt”. 
So, to improve filtration efficiency, a certain amount of deposits are necessary, which can be achieved by decreasing the sand circulation rate. The lower the applied sand circulation rate, the higher the amount of deposits, the higher the additional filter bed head loss, the higher the filtration efficiency. This holds true until the maximum applicable head loss is achieved. As indicated in one of our earlier blogs this maximum head loss is defined as 2/3 times bed height. Changing sand circulation rates in continuous filtration is executed by adjusting the air flow to the airlift. The Sand-Cycle monitoring and control of continuous filters has been developed to obtain precisely these favorable conditions. Sand

circulation rates which allow the filter system to operate within a head loss
range to meet the best performance conditions.