In some plants, sludge is removed from the sedimentation basin continuously using automated equipment.  In other plants, sludge must be removed manually.  If removed manually, the basin should be cleaned at least twice per year, or more often if excessive sludge buildup occurs.  It is best to clean the sedimentation basin when water demand is low, usually in April and October.  Many plants have at least two sedimentation basins so that water can continue to be treated while one basin is being cleaned, maintained, and inspected. 

If sludge is not removed from the sedimentation basin often enough, the effective (useable) volume of the tank will decrease, reducing the efficiency of sedimentation.  In addition, the sludge built up on the bottom of the tank may become septic, meaning that it has begun to decay anaerobically.  Septic sludge may result in taste and odor problems or may float to the top of the water and become scum.  Sludge may also become resuspended in the water and be carried over to the filters. 

The sludge which is found in the bottom of a sedimentation basin is primarily composed of water.  The solids in the sludge are mainly excess coagulant, such as alum.  Alum sludge has a solids concentration of only about 1% when automatically removed from the basin, or about 2% if manually removed.  (The greater solids concentration of manually removed sludge is due to a small amount of gravity thickening.)

Many options exist for disposal of sedimentation sludge.  Here we will discuss disposal in streams, sanitary sewers, lagoons, and landfills.

Disposal in Streams and Sewers

In the past, sludge and backwash water was typically released into streams and other bodies of water.  However, this practice is becoming much less common and is now well regulated.  Backwash water and sludge can only be released into streams if a discharge permit has been granted by the NPDES.  The NPDES further requires extensive daily monitoring of the water quality when the sludge is being discharged. 

Alternatively, sludge may be piped directly to the sanitary sewer.  However, this disposal option also has its disadvantages.  Sludge can cause sewer blockages.  In addition, fees charged by the wastewater treatment plant can be expensive.  Releasing sludge into the sewage lines in large batches makes treatment of the wastewater problematic, so sludge is typically released slowly over a long time period, which requires a large holding tank at the water treatment plant. 

Thickening Sludge

Most of the other alternatives require transporting sludge away from the treatment plant.  Sludge is typically dried before it is trucked away since the greater volume of wet sludge makes it much more expensive to transport.  This drying process is known as dewatering or thickening.  Alum sludge is difficult to thicken, but a variety of devices have been developed to thicken the sludge, some of which are explained below. 

In many cases, the sludge is treated by the addition of polymers to aid in the dewatering process.  Alternatively, the sludge can be heated or frozen and thawed to increase the solids concentration.  Treating the sludge to aid in thickening is known as conditioning the sludge.

Once the sludge has been conditioned, it may be thickened in a lagoon, drying bed, or one of several other devices. 

Sludge thickening in a lagoon.

Lagoons, which are small-volume storage ponds, are the simplest device used to thicken sludge.  These lagoons are filled with sludge and the solids are allowed to settled due to gravity to the bottom of the lagoon while the clear water is pumped off the top.  After a few months, gravity and evaporation will have reduced the sludge to a 30-50% solid state.  The sludge can then be covered with soil and left on site, or may be trucked to a landfill off-site.



Drying beds are often used to thicken sludge destined for a landfill more quickly than the sludge would be thickened in a lagoon.  A drying bed is similar in design to a sand filter, with a layer of sand underlain by a layer of gravel.  The sludge is applied to the top of the sand and the water percolates down through the sand and gravel and is drained away.  When the sludge is sufficiently dry, it is carefully removed from the top of the sand and is trucked to a landfill.  When sludge is preconditioned with chemicals, dewatering in a drying bed may take only a few days or weeks.

Drying bed.

Other processes used to thicken sludge include filter presses, belt filter presses, centrifuges, and vacuum filters.   These processes result in sludge with a solids content ranging from 30 to 50%.