Lesson 4: Water Supplies

°Introduction to Groundwater
°Aeration
°Iron and Manganese Removal
°Corrective Treatment
°Hydrogen Sulfide
°Turbidity Removal
°Filters
°Summary

Filters

In this section we will answer the following questions:

• How do the filters used in groundwater systems work?

 

Filtration

Filtration removes impurities and floc from the water being treated.  In general, filtration consists of passing the water through sand and gravel or some other filter.  The floc and impurities get stuck in the sand while the water passes through.  Filtration is usually one of the last steps in the water treatment process. 

There are three types of filters associated with water treatment: rapid sand, pressure, and slow sand. 

Rapid sand filters are mainly used in connection with surface water treatment.

Pressure filters are commonly used when iron and manganese must be removed from well water, especially in smaller water systems. 

Slow sand filters have been used for the treatment of relatively clear water when there is a possibility of the water becoming turbid.  This filtration method can be used to treat spring or well water that is relatively safe from contamination, but should not be used to treat surface water (lakes, ponds, etc.)

 

Pressure Filters

Pressure filters and rapid sand filters have the same requirements for sand and gravel for filtration.  Both use the same types of manifolds and laterals (types of pipes) and have the same velocities in the pipes.  Here, we will consider pressure filters, since they are used in groundwater treatment.  But many aspects of the two systems are similar. 

A pressure filter is an upright, closed cylinder containing a filter bed of layered sand and gravel on top of a collection system.  Water under pressure passes through the filter and then continues on through the water treatment system. 

As the water passes through the filter, oxidized minerals (such as iron and manganese) and foreign matter collect in the top portion of the sand.  Continued build-up of these particles tends to clog the filter.  The clogged filter requires more force or pressure to pass water through and filter at the same rate as an unclogged filter. 

A head loss gauge measures the pressure of the water leaving the filter.  By comparing the pressure of the water leaving the filter to the pressure of the water entering the filter, you can determine how much pressure was lost due to the water passing through a clogged filter - a measurement also known as head loss.

When the head loss gauge shows that there is excessive build-up on the filter, then the filter must be washed.  Excessive build-up can cause clogging, restricted flow, pressure build-ups, and possible breakthroughs.  A breakthrough is a crack or break in a filter bed which allows the water to pass through without contacting the filter and being cleaned.  In addition to damaging the filter bed and piping, a clogged filter can allow poor quality water to go into the system, causing problems and complaints. 

Filters are cleaned by backwashing.  The influent valve is closed and a waste line is opened.  Treated water from the system is pumped upward through the filter bed.  The water pumped upward has the velocity and volume to agitate the sand and carry away the foreign matter that has collected there. 

Backwashing normally takes about 10 minutes, though the time varies depending on the length of the filter run and the quantity of material to be removed.  Filters should be backwashed until the backwash water is clean. 

The filter rate can be increased to 3 gpm (gallons per minute) per square foot of filter and the backwash rate may be reduced to 12 gpm per square foot of filter if this provides proper washing.

 

Slow Sand Filters

The slow sand filter is similar in design to a rapid sand filter and a pressure filter, except that the sand is usually 36 inches in depth and the filter cannot be backwashed.  Instead, the filter is cleaned by removing the top two inches of sand from the filter.  Once 6 to 12 inches of sand have been removed, additional sand must be added to bring the filter back to the original depth.

Unlike pressure filtration in which the water is forced through the filter medium, water is drawn through a slow sand filter by gravity.  The raw water is pumped onto the filter bed and is filtered at a rate of 3 to 6 mgad (million gallons per acre per day) or 3 to 6 gallons per hour per square foot of sand area.  The filter rate is controlled by a set valve on the effluent line (the pipe containing the water flowing out of the filter system.)

Water is usually chlorinated before slow sand filtration, but may also be chlorinated as the filtered water goes to the sump pump for pumping to the system.  The slow sand filters are usually covered as well to prevent the growth of algae which will clog the filters. 

Although the operation and maintenance cost of slow sand filters is low, they cover large areas of land.  So the initial cost of land and construction makes this type of unit uneconomical in most instances. 

 

Part 8: Summary