On-Site Wastewater Treatment and Disposal Systems - Forced ...

humidity, and the resulting net evaporation potential. Other features
that affect performance include:
1. Soil permeability (evaporation/infiltration only)--lagoon size
and soil permeability are inversely proportional
2. Salt accumulation (evaporation only)--results in decreased
evaporation rate
3. Hydraulic loading--size must accommodate peak flows
4. Inlet configuration-- center inlet tends to improve mixing and
minimize odors
5. Construction techniques
7.3.3.4 Design
Lagoons can be circular or rectangular. The maximum wastewater depth is
normally 3 to 5 ft (0.9 to 1.5 m) with a freeboard of 2 or 3 ft, (0.6 to
0.9 m), although depths greater than 8 ft (2.4 m) have also been used
(42)(43)(44)(45)(46)(47). The minimum wastewater depth is generally 2
ft (0.6 m). This may necessitate the addition of fresh water during
high-evaporation summer months. Figure 7-37 shows the dimensional re-
qujrements for a typical fnsite lagoon. The size ranges from 3 to 24
ft /gpcd (0.07 to 0.57 m /lpcd), depending primarily on the type of
lagoon (evaporation or evaporation/infiltration), soil permeability,
climate, and local regulations.
Lagoon design is usually based on locally available evaporation and pre-
cipitation data, soil percolation rates (evaporation/infiltration only),
and an assumed wastewater flow. Since runoff is excluded by the con-
tainment berms, evaporation lagoons need only provide adequate surface
area to evaporate the incident precipitation and the influent waste-
water. Calculations may be made initially on an annual basis, but must
then be checked to insure that adequate volume is provided for storage
during periods when liquid inputs exceed evaporation. A brief design
example is outlined below.
Assumptions:
1. Four occupants of home
2. 45-gpcd wastewater flow
3. Annual precipitation: 15.3 in.
4. Annual evaporation: 46.7 in.
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