Section Env-Wq 711.04 - Secondary Settling Tanks: Design Criteria for Overflow Rates(a) Subject to (b), below, the design overflow rates shall be determined from solids flux analysis and solids loadings.(b) In lieu of (a), above, activated sludge settling tanks treating domestic wastes may be sized according to Table 711-1 below, wherein overflow rates are based on solids flux with a factor of safety of 1.3 against settling tank failure, overflow rates for systems with a selector are based on settling characteristics typical of mixed liquor with a sludge volume index (SVI) of 150 mL/g, and overflow rates for systems without selectors are based on settling characteristics typical of a mixed liquor with a SVI of 200 mL/g: Table 711-1: Secondary Settling Tank Overflow Rates Peak Hourly Flow
MLSS, mg/L | Surface Overflow Rates at Critical Loading with SVI = 150 mL/g, gpd/sf | Surface Overflow Rates at Critical Loading with SVI = 250 mL/g, gpd/sf |
1,500 | 2,100 | 1,850 |
2,000 | 1,710 | 1,450 |
2,500 | 1,400 | 1,140 |
3,000 | 1,140 | 900 |
3,500 | 930 | 690 |
4,000 | 760 | 550 |
4,500 | 610 | 440 |
5,000 | 510 | 340 |
(c) Side water depth for secondary settling tanks shall be as follows: (1) For rectangular units , 12 to 13 feet;(2) For circular units up to 40 feet in diameter, 12 feet;(3) For circular units 40 feet to 75 feet in diameter, 14 feet;(4) For circular units 75 feet to 125 feet in diameter, 16 feet; and(5) For circular units greater than 125 feet in diameter, 18 feet.(d) Circular settling tanks shall have a minimum bottom slope of 0.25 inches per foot.N.H. Code Admin. R. Env-Wq 711.04
Adopted byVolume XXXIV Number 46, Filed November 13, 2014, Proposed by #10693, Effective 10/15/2014, Expires10/15/2024.