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

Design flow:
Net evaporation per year:
4 persons x 45 gpcd = 180 gpd
46.7 in. - 15.3 in. = 31.4 in.
(31.4 in.)(l44 in.2) = 4,522 in.3 of water/ft2 water surface
(4,522 in.3)(ft3/1,728 in.3)(7.48 gal/ft3) = 19.6 gal of water/ft2
water surface
Lagoon area required:
(180 gpd)(365 days)/(19.6 gal/ft') = 3,352 ft2
This can be provided by a round lagoon, 65.3-ft diameter.
At this point, we need to ensure that the lagoon will have adequate
storage capacity to allow accumulation of water to a depth of no more
than 4 or 5 ft in low-evaporation months (usually winter), and to allow
sufficient surface area for evaporation of the accumulated water plus
new influent flows during the months when evaporation rates exceed the
monthly wastewater flow (usually summer). This is done by comparing the
wastewater flow against the evaporation rate for each months, and by
performing a water balance (i.e., calculating the gain or loss in gal-
lons for each months). Table 7-16 shows such a balance.
From October.through April, the lagoon will gain 35,443 gal of volume.
This is equivalent to a gain of 1.4 ft:
(35,443 gal) (
Beginning with a 2-ft minimum depth, the depth of the lagoon varies from
2 ft to 3.4 ft.
Some sources indicate that BOD loadings should also be considered in
lagoon sizing for odor control. Loadings in the range of 0.25 to 0.8
#BOD/day/l,OOO ft2 (1.2 to 3.9 kg/day/1000 m2) have been recommended,
but supporting data for onsite systems are not available (43)(45)(46).
If infiltration is permitted and feasible considering local soils, the
size of the lagoon can be reduced by the amount of water lost through
percolation.
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