Figure 2-27. Sludge drying bed with concrete walls.
d. Postchlorination. The ordinary combined primary and secondary
wastewater treatment processes remove or stabilize most of the organic matter present
and reduce the bacterial content by 50 percent or more. There is no assurance,
however, that the content of pathogenic organisms has been reduced to a satisfactory
level unless some disinfection process is applied. Chlorination is frequently applied to a
treatment plant effluent just prior to its disposal into a receiving stream. Chlorination at
this point is termed postchlorination. This chlorination is required when effluents are
discharged into bodies of water used for domestic water supply, shellfish culture,
training and recreational activities, or irrigation. Where water supplies are taken
relatively near the point of outfall, disinfection of effluent is vitally important. Bacterial
content of settled or secondary treated effluents is reduced approximately 99.5 percent
by chlorination to a residual of 0.3 to 0.5 ppm with a 15-minute contact period.
Complete sterilization is not obtained. The chlorine feed rate is proportioned with the
flow and the chlorine demand of the wastewater. Adequate mixing during the chlorine
contact period is ensured by the installation of adequate baffling or by either mechanical
or automatic means.
(1) Manual. When applied manually, the agent may be in either liquid or
powder form. Chloride of lime, calcium hypochlorite, and sodium hypochlorite are
agents that may be applied by hand and, if so, contact chambers are to be used to
effect the required contact time.
(2) Automatic. Commonly used gas-fed machines automatically chlorinate
wastewater effluent with chlorine gas. These machines have sensory devices that
control the quantity of gas dispensed according to the amount of effluent being