Section Env-Wq 711.01 - Primary Settling Tanks(a) Inlets to primary settling tanks shall be designed to: (1) Dissipate the inlet velocity in each tank to prevent short circuiting; and(2) Distribute the flow equally to multiple tanks.(b) Channels in settling tanks shall be designed to prevent sedimentation. Corner pockets and dead ends are not permitted and shall be eliminated if they occur.(c) Provisions shall be made for elimination or removal of floating materials in inlet structures having submerged ports.(d) The minimum length of flow from inlet to outlet in rectangular settling tanks shall be 20 feet.(e) Primary settling tanks shall have a minimum side water depth of 12 feet.(f) Effective scum collection and removal facilities shall be provided ahead of the outlet weirs on all settling tanks.(g) Overflow weirs shall be adjustable for leveling.(h) The tops of troughs, beams, and similar construction features that are submerged shall have a minimum slope of 1.4 vertical to one horizontal.(i) If primary settling tanks are part of the WWTP design, a minimum of 2 primary settling tanks shall be provided.(j) All primary settling tanks shall provide safe and easy access for maintenance and protection of operators. Access stairways and elevated walkways shall be equipped with handrails. Walls of primary settling tanks shall extend a minimum of 6 inches above the surrounding ground surface, be provided with safety railings, and have not less than 12 inches freeboard.(k) For sludge removal from primary settling tanks, provision shall be made for: (1) Sampling and measuring flow of the sludge; and(2) For sludge hoppers in rectangular settling tanks, a 1.7 horizontal to one vertical minimum slope of the side walls.(l) Settling tanks designed for use without mechanical equipment for sludge collection and removal shall be prohibited.(m) Air lift type of sludge removal shall be prohibited. Primary sludge shall be removed from the sludge hoppers by positive displacement pumps with timers for control of pumping periods.(n) Average surface overflow rates for primary settling tanks shall not exceed 600 gpd per square foot (gpd/sf) for WWTPs having an average design flow of 1 million gallons per day (MGD) or less.(o) Average surface overflow rates for primary settling tanks shall not exceed 1,200 gpd/sf for WWTPs having an average design flow greater than 1 mgd, unless reduced primary removal rates are provided in the design loadings for subsequent secondary treatment units.(p) Surface overflow rates for peak hourly flow shall not exceed 3,000 gpd/sf.(q) If activated sludge is wasted to the primary tanks, average overflow rates shall not exceed 800 gpd/sf and peak hourly overflow rate shall not exceed 1,200 gpd/sf.N.H. Code Admin. R. Env-Wq 711.01
(See Revision Note at chapter heading for Env-Wq 700) #8590, eff 3-25-06
Amended byVolume XXXIV Number 46, Filed November 13, 2014, Proposed by #10693, Effective 10/15/2014, Expires10/15/2024.