Begin developing a water balance table starting with an empty reservoir. All calculations will be in acre-inches.
| Month | Wastewater volume | Water applied | Change in storage | Accumulated in storage |
|---|---|---|---|---|
| Nov | 27.6 | 27.8 | 0 | 0 |
| Dec | 28.5 | 0 | 28.5 | 28.5 |
| Jan | 28.5 | 0 | 28.5 | 57.0 |
| Feb | 25.8 | 0 | 25.8 | 82.8 |
| Mar | 28.5 | 0 | 28.5 | 111.3 |
| Apr | 27.6 | 36.5 | -8.9 | 102.4 |
| May | 28.5 | 42.6 | -14.1 | 88.3 |
| Jun | 27.6 | 48.7 | -21.1 | 67.2 |
| Jul | 28.5 | 48.7 | -20.2 | 47.0 |
| Aug | 28.5 | 48.7 | -20.2 | 26.8 |
| Sep | 27.6 | 43.5 | -15.9 | 10.9 |
| Oct | 28.5 | 40.0 | -11.5 | 0 |
Identify the required minimum storage volume using the maximum accumulated volume. Remember, this is the actual wastewater that must be stored. If an open storage system is selected, additional storage must be provided for average annual rainfall minus average annual evaporation, which in Ohio ranges from 6 to 12 inches (Figure 4). Finally, a freeboard of 3 feet is recommended for reservoirs of from 15 to 50 acre/feet. A freeboard of 2 feet is recommended for smaller reservoirs.
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| Figure 4. Recommended volumes for wastewater storage basins. |