TM 5-814-3/AFM 88-11, Volume III
15-2. Polishing ponds.
Wastewater treatment ponds may be used as a practical and economical method for upgrading existing
secondary treatment facilities to obtain improved organic and suspended solids removal. Both aerobic and
aerobic-anaerobic ponds can be used for this purpose. Ponds used for polishing purposes are subject to the
same operating characteristics as those used for primary or secondary treatment, and the same precautionary
design considerations must be applied. The design information and criteria presented in chapter 14 are
applicable to design of polishing ponds. (See also Culp and Culp, 1971.)
This can be accomplished by either diffused, cascade, U-tube or mechanical aeration. Diffused aeration is
carried out in tanks 9 to 15 feet deep and 10 to 50 feet wide (depth-to-width ratio is maintained at less than
2), with detention time of about 20 minutes. The maximum air requirement is approximately 0.15 cubic feet
per gallon of wastewater treated. Mechanically aerated basins are 8 feet deep and require 15 to 50 square feet
per aerator. Surface aeration is the most efficient mechanical aeration in terms of required horsepower (0.1
horsepower per 1,000 gallons of effluent). The drop required for cascade aeration in a stepped-weir structure
or in a rapidly sloping channel filled with large rocks or concrete blocks will depend on the desired oxygen
uptake: 2 feet of drop will be provided for each milligram per liter of dissolved oxygen increase required.
a. Description of process. A microstrainer consists of a rotating drum supporting a very fine, stainless
steel or plastic screen. Wastewater is fed into the inside of the drum and filters radially outward through the
screen, with the mat of solids accumulating on the screen inside the drum. The solids are flushed into a
removal trough at the top of the drum by a pressurized backwash system. From this trough, the solids are
returned to the head of the system. Process effluent wastewater can be used for the backwash. Table 15-2
provides performance data for several microscreen installations.