Lab 3: Build a Rapid Sand Filter
- Build a rapid sand filter including necessary valves and pumps so it can
As a third step in building a water filtration plant you will
build a rapid sand filter that can be backwashed.
Rapid sand filters are a polishing step in water purification.
Most of the suspended solids are removed by the flocculation and sedimentation
steps that normally precede rapid sand filtration. Rapid sand filters are
cleaned by reversing the flow of water and suspending the bed of sand. The
agitation during the backwash is sufficient to remove most of the accumulated
solids. This makes it possible for a rapid sand filter to treat large quantities
of water for many years while only requiring regular backwashing.
* (void volume/total volume of sand plus voids)
The rapid sand filtration apparatus must have the following capabilities
- control flow rate through filter in both forward and backwash modes
- pump filtered water up through filter to backwash (clean) the sand
- control the flow direction using shutoff valves (and by turning the pump
on and off)
- Calculate the desired forward and backwash flow rates
- Calculate the mass of sand required for your filter
- Design your apparatus. Note that it is better to have the valve modules
resting on the lab bench rather than directly attached to the filter column.
- Assemble the apparatus making sure that all plumbing connections are tight
- Fill the clear well reservoir with tap water
- Verify that the centrifugal pump is connected in such a way that it fills
with water before turning it on!
- Open the flow control valve and turn on the solenoid valve on the effluent
side of the centrifugal pump. You can create a special "prime pump" state to
open up the S2 and S5 (or S3) valves.
- Turn on the centrifugal pump. If water begins to flow you can adjust the
flow rate with F2. If water doesn't begin to flow immediately turn
off the pump and investigate why water wasn't filling able to enter the pump.
Running the pump without water will damage the pump!
- Fill the filter with water in the backwash mode using the centrifugal pump
- Check for leaks
- Regulate the backwash flow using a flow control valve so that the bed
expands approximately 50%
- Change the filter to forward flow and set the flow rate using the process
- Filter several liters of "raw" water and then backwash the
filter again to remove the entrapped solids
- Experiment with the
Process Controller software to see if you can set up an automated
sequence of filter, backwash, wait for sand to settle, filter to waste and
- Carefully drain the apparatus without causing any leaks or spills
- Place the assembled apparatus on one of the carts and label with your
- Clean up your lab bench
The raw water for this lab contains 100 mg/L kaolin and 50 mg/L alum to make
it easier to filter.