e. Maintenance. StabiIization ponds are simple in operation, but they do
require a minimum of maintenance. Weeds and mosquitoes must be controlled. One
way to control mosquitoes is to stock the pond with surface-feeding fish, such as
gambusia. Scum must sometimes be removed. Though much less frequently, sludge
must sometimes be removed also. Therefore, provisions should be made in
construction for draining ponds.
Section VI. PACKAGE WASTEWATER TREATMENT PLANTS
Small installations have several options available for wastewater treatment.
They may utilize the wastewater systems of neighboring communities, when possible.
If permitted, they may install soil absorption systems. If sufficient land area is available,
they may construct stabilization ponds. If none of these options are available, the
package wastewater treatment plant should meet their needs. The term "package" as
applied to wastewater treatment plants usually means a compact, relatively simple to
operate unit designed to give complete treatment to the wastewater from a small
community. Figure 2-29 illustrates the package unit for such a treatment plant.
Package plants are generally installed where a high degree of biochemical oxygen
demand removal is required but other factors prohibit the use of conventional types of
secondary treatment. Units of this type are often used also for treatment of industrial
2-20. PRINCIPLES OF OPERATION
Most of the plants currently being used employ a modification of the activated-
sludge process. The conventional activated-sludge plant aerates settled wastewater
and return activated sludge at a mixed-liquor suspended solids concentration of about
2000 milligrams per liter for about 6 hours. The activated sludge is then settled in a
secondary settling tank. Return sludge amounting to about 25 percent of the influent
flow is recycled ahead of the aeration tank. The remainder of the activated sludge is
wasted. The modifications to the activated sludge process that are used for package
plants are the extended aeration process and the contact stabilization process.
a. Extended Aeration. Most extended aeration plants by-pass primary settling
and aerate the mixed-liquor suspended-solids for about 24 hours at a concentration of
about 5000 milligrams per liter. Return sludge for these plants is usually about 100 to
150 percent of the influent flow. Because of the long aeration times and low food to
microorganism ratio (low organic loadings of less than 20 pounds BOD per 1,000 cubic
feet of aeration tank volume), synthesis rates are low and a low buildup of residue
results. Because of the low residue buildup, a digestion unit is not needed. This
aeration is considered an option in larger plants.