Lab 3: Build a Rapid Sand Filter

Objectives

Build a rapid sand filter including necessary valves and pumps so it can be backwashed

Background

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.

Experimental Setup

Filter Specifications

parameter	specification
Filter diameter	2.5 cm
Sand density	2.65 g/cm³
Sand depth	30 cm
porosity*	0.4
filtration velocity	5 m/hr
backwash velocity	50 m/hr

* (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)

Apparatus schematic

Procedure

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 controller.
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 then repeat.
Carefully drain the apparatus without causing any leaks or spills
Place the assembled apparatus on one of the carts and label with your group name
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.