B Aerobic lagoons

Aerobic lagoons can be used if minimizing odors is critical (fig. 10-24). These lagoons operate within a depth range of 2 to 5 feet to allow for the oxygen entrainment that is necessary for the aerobic bacteria.

The design of aerobic lagoons is based on the amount of BOD5 added per day. If local data are not available, use the BOD5 values from the tables in chapter 4. Figure 10-25 shows the acceptable aerobic loading rates for the United States in lb-BOD5/acre/day. The lagoon surface area at the average operating depth is sized so that the acceptable loading rate is not exceeded.

Even though an aerobic lagoon is designed on the basis of surface area, it must have enough capacity to accommodate the waste volume (WV) and sludge volume (SV). In addition, depth must be provided to accommodate the normal precipitation less evapora tion on the lagoon surface, the 25-year, 24-hour storm precipitation on the lagoon surface, and freeboard. Should State regulations not permit an emergency outflow or for some other reason one is not used, the minimum freeboard is 1 foot above the top of the required volume. Figure 10-24 demonstrates these volume depth requirements.

Aerobic lagoons need to be managed similarly to anaerobic lagoons in that they should never be overloaded with oxygen demanding material. The lagoon should be filled to the minimum operating level, generally 2 feet, before being loaded with waste. The maximum liquid level should not exceed 5 feet. The water level must be maintained within the designed operating range. Sludge should be removed when it exceeds the designed sludge storage capacity. Aerobic lagoons should also be enclosed in fences and marked with warning signs.

Figure lO-24 Aerobic lagoon cross section rr

Required volume

Max. operating level

Freeboard (1.0 minimum)

Depth of 25-year, 24-hour storm event on lagoon surface

Depth of normal precipitation less evaporation on the lagoon surface accumulated during the treatment period

Volume of manure, wastewater, and clean water accumulated (WSV)

during the treatment period

Volume of accumulated sludge for period between sludge removal events (SV)

Crest of spillway or other outflow device (where permissible)

Max. drawdown

Crest of spillway or other outflow device (where permissible)

Max. drawdown

Note: An aerobic waste treatment lagoon has a required minimum surface area based on BOD

Figure 10-25 Aerobic lagoon loading rate

U.S. DEPARTMENT OF AGRICULTURE

NATURAL RESOURCES CONSERVATION SERVICE

Figure 10-25

Aerobic Lagoon Loading Rate (lb BOD5/ acre/day) (29)

Figure 10-25

NOTE:

CAUTION SHOULD BE USED IN INTERPOLATING THIS MAP IN MOUNTAINOUS AREAS. LINES HAVE BEEN SMOOTHED IN MOUNTAINOUS AREAS OF THE WESTERN STATES.

SOURCE:

NATIONAL WEATHER SERVICE PUBLICATIONS. MAP PREPARED USING AUTOMATED MAP CONSTRUCTION. NATIONAL CARTOGRAPHY AND GIS CENTER, FORT WORTH, TEXAS, 1995

REVISED JUNE 1995, 1005009

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