d. Disposal of Liquid Sludge.
(1) Fertilizer. In some instances, particularly where drying-bed facilities are
deficient, well-digested liquid sludge may be drawn into tank trucks and applied directly
as fertilizer to areas being used to grow nonsubsistence crops.
(2) Lagooning. Digested sludge may be discharged from the digesters
directly to lagoons instead of to drying beds. The lagoons must be located in isolated
areas, preferably where the sludge will flow by gravity. Lagooning of sludge or digester
supernatant is generally done only as an expedient when other facilities are inadequate.
Lagoon storage can be a continuous operation or can be confined to peak load
situations. Land requirements and possible ground water pollution are the major
Section V. FINAL EFFLUENT AND STABILIZATION PONDS
2-16. FINAL EFFLUENT
The final effluent from a wastewater treatment plant is normally discharged into a
watercourse or receiving stream. This final effluent must be of such quality that it will
not create a nuisance, will not have a harmful effect on the receiving waters, and will be
in compliance with all local, state, and Federal standards. Although the effluent from an
efficient treatment plant may be relatively free of nonfiltrable residue, BOD, and
microorganisms, it is usually quite low in dissolved oxygen content. Therefore, the
effluent is provided some type of mechanical treatment to increase the oxygen assets of
the receiving stream. One method of adding oxygen to the effluent is by permitting it to
flow in open channels, thus absorbing air as it flows. Another method used when there
is a sufficient drop in elevation between the plant outflow and the receiving stream is to
construct successive steps (cascades) along the waterway. The effluent thus creates a
miniature waterfall, thereby absorbing oxygen from the air.
Dechlorination (partial or complete reduction of residual chlorine) of the effluent
may be necessary to comply with applicable Federal, state, and local discharge
standards and thus prevent chronic effects on the waste-receiving stream.
Dechlorination may be accomplished by sulfur dioxide and its derivatives. Nonchemical
means may also be used to accomplish dechlorination (activated carbon, for example).
2-18. STABILIZATION PONDS
a. General. A stabilization pond, or oxidation pond, is an artificial shallow pond
into which wastewater is discharged for natural purification by biological processes
under the influence of air and sunlight. The aerobic bacteria present in the wastewater
convert organic matter into carbon dioxide, water, and various nutrients. Algae use
these nutrients in their metabolic processes and produce oxygen needed by the aerobic