Virginia Administrative Code › Title 9 - ENVIRONMENT › Agency 25 - STATE WATER CONTROL BOARD › Chapter 790 - VIRGINIA POLLUTANT DISCHARGE ELIMINATION SYSTEM (VPDES) PERMIT REGULATION › Part III - Manual of Practice for Sewerage Systems and Treatment Works › Article 4 - Preliminary Processes › Section 9VAC25-790-530 - Clarifiers

Current through Register Vol. 40, No. 24, July 15, 2024
A. Conventional
					 solids settling basin design information shall apply to clarifiers not preceded
					 by chemical flash mix and flocculation. Where clarifiers are preceded by
					 chemical flash mix and flocculation, chemical clarification requirements shall
					 apply.
B. Design. Conventional
					 clarifiers shall be designed to dissipate the inlet velocity, to distribute the
					 flow uniformly across the basin, and to prevent short-circuiting hydraulic
					 currents.
1. Inlet channels should be
					 designed to maintain a velocity of at least one foot per second at 1/2 design
					 flow. Corner pockets and dead ends shall be eliminated, and corner fillets or
					 channeling used where necessary.
2.
					 Provisions shall be made for elimination or removal of floating materials in
					 inlet structures having submerged ports.
3. The minimum length of flow from inlet to
					 outlet of a clarifier should be 10 feet unless special provisions are made to
					 prevent short circuiting.
4. The
					 liquid depth of mechanically cleaned clarifiers shall be as shallow as
					 practicable but not less than 10 feet for an overflow rate of 300 gpd per
					 square foot. For each three-foot increase or decrease in depth, the overflow
					 rate shall be increased or decreased by 200 gpd per square foot respectively.
					 Final clarifiers receiving flow from biological reactors should not be less
					 than 12 feet in depth.
5. A minimum
					 of two hours of average design flow detention volume should be provided within
					 the settling zone of conventional clarifiers, at the design loading.
6. Multiple clarifiers capable of independent
					 operation shall be provided at treatment works having a capacity of more than
					 40,000 gallons per day; however, single clarifiers may be allowed at
					 Reliability Class II and Class III treatment works having a capacity up to
					 100,000 gpd when appropriate reliability and continuous operability
					 requirements are satisfied.
7.
					 Where multiple clarifiers are utilized in suspended growth processes,
					 provisions for combining the effluent from the reactors (aeration basins) and
					 proportionally distributing the reactor effluent to each clarifier shall be
					 included, for the purpose of evenly distributing the biomass to the
					 clarifiers.
8. Overflow weir plates
					 shall be adjustable. In cases in which clarifier designs have a potential for
					 short circuiting hydraulic inefficiencies, weir loadings rates should not
					 exceed 10,000 gallons per day per linear foot for treatment works designed for
					 average flows of 1.0 mgd or less. Special consideration will be given to weir
					 loading rates for treatment works designed for flows in excess of 1.0 mgd, but
					 such loading rates should not exceed 15,000 gallons per day per linear foot if
					 short circuiting problems may affect performance. If pumping is required, pump
					 capacity shall be related to clarifier design to avoid excessive weir
					 loading.
9. The tops of beams and
					 similar construction features which are submerged shall have a minimum slope of
					 1.4 vertical to 1 horizontal. The underside of such features should have a
					 slope of one to one to prevent the accumulation of scum and solids. Effective
					 scum collection and removal facilities, including baffling, shall be provided
					 ahead of the outlet weirs on all clarifiers. Provisions may be made for
					 discharge of scum with the sludge; other provisions may be necessary to dispose
					 of floating materials which may adversely affect sludge handling and
					 management.
10. Clarifier design
					 should include provisions for reasonable access for maintenance and protection
					 of operators. Such features may include slip resistant stairways and walkways,
					 protective handrails, etc., in accordance with VOSH requirements. If side walls
					 are extended some distance above the liquid level to provide flood protection,
					 or for other purposes, stairs and walkways with handrails should be provided to
					 facilitate housekeeping and maintenance. Access for cleaning and maintenance of
					 weirs should also provide proper safety features in accordance with VOSH
					 requirements.
11. Where primary
					 clarifiers are used, provisions for emergency bypassing, or discharging sewage
					 which has received preliminary treatment directly to the biological treatment
					 unit operation, may be desirable.
12. Shallow depth sedimentation will be
					 considered on a case-by-case basis.