Current environmental laws do not allow this because such discharge harms the
receiving body of water (cloudy water, toxicity to aquatic life, formation of sludge
banks). Following are alternative methods of ultimate sludge disposal, which, in some
cases, may be economical and environmenally sound solutions.
Discharge to Sanitary Sewer. In general, water treatment plant residues
can be disposed of by discharge to a sanitary sewer without upsetting the wastewater
treatment processes. However, problems can result if the amount of sludge is too
The sewer can be overloaded hydraulically by large batch dumps
of sludge. This problem can be handled by storing the sludge in a holding tank, then
bleeding the sludge slowly into the sewer during periods of low wastewater flow (such
as after midnight). However, sewer flow needs to be sufficient to prevent sludge solids
from accumulating in the sewer, since the solids may then clog the sewer.
The water treatment sludge solids increase the amount of sludge to
be disposed of at the sewage treatment plant. Therefore, the dewatering and disposal
problems are not eliminated, but simply shifted elsewhere. Water treatment plant
sludge is not affected by sludge digestion processes at the sewage treatment plant but
does take up digester volume. In some cases, water plant sludges have been reported
to clog digesters.
Landfill. Modern sanitary landfills are designed and operated to keep the
amount of water leaching from the filled material to a minimum. For this reason, landfill
regulations often require that sludges contain at least 20 percent dry solids, and
sometimes require as high as 50 to 60 percent. Wet sludges are not acceptable
because they are difficult to mix well with other solid wastes before covering, and
because the large amount of water could percolate through the soil and pollute water
Lagoons. Disposal lagoons are simply dewatering lagoons that are never
cleaned out, thus eliminating the main operating problem of drying lagoons. The main
disadvantage is that large land areas are permanently committed for use as lagoons.
For plants with small sludge quantities and plentiful land, lagoons can be practical for
Land Spreading of Lime Sludge. In many agricultural areas, particularly
in the Midwest, farming practices require that lime or limestone be added to the fields
periodically to control soil pH. Sludge from lime water softening processes can be used
for this purpose if it is sufficiently dewatered to allow easy handling.