TM 5-814-3/AFM 88-11, Volume III
(1) Step-aeration. This type of upgrading generally involves only minimum capital investment. To
implement this type of upgrading, it is necessary to modify influent piping, renovate the air system, and
expand the sludge recycle capacity. Primary and secondary clarifier capacity must be checked and increased
to handle the higher loadings.
(2) Contact stabilization. An overloaded, conventional activated sludge plant can be upgraded by
converting the existing aeration tank to two separate tanks: one for stabilization of return sludge and the
second as the contact tank for the raw wastewater. In addition to the physical conversion of the single
aeration tank to two tanks, this type of modification requires revamping the influent piping, expanding the
sludge recycle capacity, and installing new aerators. Expanding the secondary clarifier capacity can be
achieved, in many cases, by introducing the raw wastewater directly to the stabilization tank and operating
in parallel with the secondary clarifier.
(3) Completely mixed activated sludge. The flow in a conventional aeration tank can be modified
to provide for uniform distribution by changing the influent wastewater piping and the recycle piping.
Complete mixing in the tank can be achieved through installation of an agitator sparger air system.
(4) Oxygen aeration. An overloaded, conventional activated sludge plant can be upgraded by using
oxygen instead of air for aeration. The upgrading modifications will include oxygen generation facilities,
installation of aeration tank cover and baffles, and new piping arrangements. The structural integrity of the
existing aeration tank and foundation must be checked before modification. The design will also include
provisions for precautions against explosion and protection against potential accelerated corrosion.
(5) Use of activated sludge to treat partially treated wastewater This modification is considered
to be the simplest of all upgrading procedures since the activated sludge process will be built as an addition
to an existing facility. If nitrification is required, aeration to provide at least 4.5 pounds oxygen per pound
of ammonia nitrogen will be provided in addition to the air requirements for carbonaceous biochemical
oxygen demand removal. No modifications will be required for the existing facilities, and the additional
facilities will be designed as add-on units.
b. Upgrading to increase organic removal efficiency. The increasingly stringent requirements on
effluent quality may dictate a need for upgrading even though the existing facilities are not hydraulically or
organically overloaded. Table 8-2 contains the incremental biochemical oxygen demand removal percentages
that can be achieved at an activated sludge plant by various upgrading modifications.
8-5. Techniques for upgrading waste treatment ponds.
In many cases, ponds have been designed without physical design factors being considered. Therefore,
improvements in pond performance can be obtained through physical changes such as installing diversion
curtains to prevent "short circuiting:' Also, improvements in the inlet and outlet configurations can be
effective. In some cases, installation of additional aeration equipment at critical locations in the pond will
improve its efficiency. Paragraph 14-3 provides design criteria for aeration requirements. Finally, adequate
retention time is essential. Some existing ponds will have a gradual loss of retention time due to solids
deposition in quiescent zones. This problem can be corrected through periodic removal of solids from existing