How do you calculate the appropriate flow rate (particularly when designing a non-standard biosand filter)?

Flow Rate = Filtration Rate*Surface Area

          We recommend that the filtration rate through the sand should not be more than 0.4 m3/m2/hr (or 400 liters per square meter of sand surface area per hour).  

         The V10 concrete biosand filter has a surface area of 0.06m2.

Flow Rate = Filtration Rate*Surface Area =  400 L/m2/hr* 0.06 m2 = 24 L/hr

Now, we are interested in the peak rate. As the filration rate declines over the course of the filter's run, knowing the what the flow rate would be per hour if the filtration rate had stayed the same isn't very useful. Because of this, we convert the flow rate to a per minute value.

24 L/hr  / 60 min/hr = 0.4 L/min

If the surface area is smaller or larger, then the maximum flow rate must be calculated using the same formula as above with the actual surface area.

Flow rates should be measured when the filter is just installed and filled with water to the very top as this is when the maximum rate occurs.  In practice, it is difficult to get the exact flow rate however it is acceptable to come within about 25% of these values.  If it is outside this range, the filtration sand should be changed until the flow rate is within range.