Utah Administrative Code
Topic - Environmental Quality
Title R317 - Water Quality
Rule R317-3 - Design Requirements for Wastewater Collection, Treatment and Disposal Systems
Section R317-3-6 - Settling
Universal Citation: UT Admin Code R 317-3-6
Current through Bulletin 2024-06, March 15, 2024
6.1. General Considerations
A. Number of Units. Multiple units capable of
independent operation shall be provided in all plants where the design rate of
flow exceed 1 million gallons per day (3,785 cubic meters per day). Plants
where the design rate of flow is less than one (1) million gallons per day
(3,785 cubic meters per day), shall include other provisions to assure
continuity of treatment.
B.
Arrangement. Settling tanks shall be arranged for optimum site utilization, and
shall be consistent with the hydraulic head requirements for other ancillary
units.
C. Flow Distribution.
Effective flow measurement devices and control appurtenances (e.g. valves,
gates, splitter, boxes, etc.) should be provided to permit proper proportioning
of flow to each unit.
D. Tank
Configuration. The selection of tank size and shape, and inlet and outlet type
and location shall be based on the site and flow patterns.
6.2. Design Considerations
A. Dimensions.
1. The minimum length of flow from inlet to
outlet should not be less than be 10 feet (3 meters) unless special provisions
are made to prevent short circuiting. The sidewater depth for primary
clarifiers shall be not less than 8 feet (2.4 meters).
2. Clarifiers following an activated sludge
process shall have sidewater depths of at least 12 feet (3.7 meters) to provide
adequate separation zone between the sludge blanket and the overflow
weirs.
3. Clarifiers following
fixed film reactors shall have sidewater depth of at least 8 feet (2.4
meters).
B. Surface
Loading (Overflow) Rates
1. Primary Settling
Tanks
a. Surface loading or overflow rates at
the average design rate of flow for primary tanks shall not exceed:
(1) 600 gallons per day per square foot (24
cubic meters per square meter per day) for plants treating at the rate of flow
less than 1 million gallons per day (3,785 cubic meter per day), or
(2) 1,000 gallons per day per square foot (41
cubic meters per square meter per day) for plants treating at the rate of flow
more than 1 million gallons per day (3,785 cubic meter per day).
b. For primary settling, expected
influent BOD5 removal and surface loading is as shown by
the relationship: E = (41.5 - (0.01 x Surface loading at average design Q))
where, E = efficiency, percent, and surface loading less than or equal to 2,000
gallons per day per square foot (82 cubic meters per square meter per day).
However, anticipated higher BOD5 removal than the one
predicted using above relationship for sewage or sewage containing appreciable
quantities of industrial wastes (or chemical additions to be used), shall be
validated by plant performance data.
2. Intermediate Settling Tanks. Surface
loading or overflow rates for intermediate settling tanks following fixed film
reactor processes shall not exceed 1,000 gallons per day per square foot (41
cubic meters per square meter per day) at the average design rate of
flow.
3. Final Settling Tanks
a. Settling tests should be conducted
wherever a pilot study of biological treatment is warranted by unusual waste
characteristics or treatment requirements.
b. The applicant will conduct pilot testing
where proposed loadings go beyond the limits set forth in this
section.
c. Surface loading or
overflow rates for settling tanks following fixed film processes shall not
exceed 800 gallons per day per square foot (33 cubic meters per square meter
per day) at the average design rate of flow.
d. Settling tanks following activated sludge
processes must be designed to meet thickening as well as solids separation
requirements. Surface loading or overflow, and weir overflow rates must be
adjusted for the various processes to minimize the problems with sludge
loadings, density currents, inlet hydraulic turbulence, and occasional poor
sludge settleability. The high rate of recirculation of return sludge from the
final settling tanks to the aeration or reaeration tanks requires careful
consideration of above factors. The hydraulic design of intermediate and final
settling tanks following the activated sludge process shall be based upon the
average design rate of flow excluding activated sludge return flow as shown in
Table R317-3-6.2(B)(3)(d).
C. Inlet Structures. Inlets should be
designed to dissipate the inlet velocity and to distribute the flow equally
both horizontally and vertically and to prevent short circuiting. Channels
should be designed to maintain a velocity of at least one foot per second (0.3
meter per second) at the minimum design flow. Corner pockets and dead ends
should be eliminated and corner fillets or channeling used where necessary.
Provisions shall be made for elimination or removal of floating materials in
inlet structures.
D. Effluent
Overflow Weirs
1. General. Effluent overflow
weirs shall be adjustable for leveling.
2. Location. Effluent overflow weirs shall be
located to optimize actual hydraulic detention time, and minimize short
circuiting.
3. Design Rates. Weir
loadings shall not exceed 10,000 gallons per day per lineal foot (124 cubic
meters per meter per day) for plants treating the average design rate of flow
of one (1) million gallons per day (3,785 cubic meters per day) or less. Higher
weir loadings may be used for plants designed for larger average flows, but
shall not exceed 15,000 gallons per day per lineal foot (186 cubic meters per
meter per day). If pumping is required, weir loadings must be related to pump
delivery rates to avoid short circuiting.
4. Weir Troughs. Weir troughs shall be
designed to prevent submergence at the maximum design rate of flow (peak daily
flow), and to maintain a velocity of at least one foot per second (0.3 meter
per second) at one-half of the average design rate of flow. Submergence may be
permitted at the maximum design rate of flow (peak daily flow) with one unit
out of service.
E.
Submerged Surfaces. The tops of troughs, beams, and similar submerged
construction elements shall have a minimum slope of 1.4 vertical to 1
horizontal; the underside of such elements should have a slope of 1 to 1 to
prevent the accumulation of scum and solids.
F. Unit Dewatering. The bypass design shall
provide for redistribution of the plant flow to the remaining units in
operation.
G. Freeboard. Walls of
settling tanks shall extend at least 6 inches (15 centimeters) above the
surrounding ground surface and shall provide not less than 12 inches (30
centimeters) freeboard. Additional freeboard or the use of wind screens should
be provided where larger settling tanks are subject to high velocity wind
currents that would cause tank surface waves and inhibit effective scum
removal.
6.3. Sludge and Scum Removal
A. Scum Removal. Effective
scum collection and removal facilities, including baffling, shall be provided
for primary, intermediate and secondary settling tanks. The unusual
characteristics of scum which may adversely affect pumping, piping, sludge
handling and disposal, should be recognized in design. Provisions may be made
for the discharge of scum with the sludge; however, other special provisions
for disposal may be necessary.
B.
Sludge Removal. Sludge collection and withdrawal facilities shall be designed
to assure rapid removal of the sludge. Suction withdrawal of sludge from the
tank floor should be provided for activated sludge plants designed for
reduction of the nitrogenous oxygen demand.
1. Sludge Hopper. When scrapers are used to
move sludge into a discharge hopper, the minimum slope of the side walls shall
be 1.7 vertical to 1 horizontal. Hopper wall surfaces should be made smooth
with rounded corners to aid in sludge removal. Hopper bottoms shall have a
maximum dimension of two feet (0.6 meter). Deep sludge hoppers for sludge
thickening are not acceptable.
2.
Sludge Removal Piping. Each hopper shall have an individually valved sludge
withdrawal line at least six inches (15 centimeters) in diameter. The static
head available for withdrawal of sludge shall be 30 inches (76 centimeters) or
greater, as necessary to maintain a three foot per second (0.91 meter per
second) velocity in the withdrawal pipe. Clearance between the end of the
withdrawal line and the hopper walls shall be sufficient to prevent bridging of
the sludge. Adequate provisions shall be made for rodding or back-flushing
individual pipe runs for activated sludge secondary clarifiers except for
oxidation ditch clarifiers. Piping shall also be provided to return waste
sludge to primary clarifiers.
3.
Sludge Removal Control. Sludge wells shall be provided with telescoping valves
or other equipment for viewing, sampling and controlling the rate of sludge
withdrawal. The use of sight glass and sampling valves may be appropriate. A
means of measuring the sludge removal rate shall be provided. Air lift type of
sludge removal must not be used for removal of primary sludges. Sludge pump
motor control systems shall include time clocks and valve controls for
regulating the duration and sequencing of sludge removal.
6.4. Protective and Service Facilities
A. Operator Protection. All
settling tanks shall be equipped to provide safe working conditions for
operators. Such features shall include machinery covers, life lines, stairways,
walkways, handrails and slip resistant surfaces.
B. Mechanical Maintenance Access. The design
shall provide for convenient and safe access to routine maintenance items such
as gear boxes, scum removal mechanisms, baffles, weirs, inlet stilling baffle
area, sludge and scum pumps, and effluent channels.
C. Electrical Fixtures and Controls.
Electrical fixtures and controls in enclosed settling basins shall meet the
requirements of the National Electrical Code for Class 1, Group D, Division 1
locations. The fixtures and controls shall be located so as to provide
convenient and safe access for operation and maintenance. Walkways, bridge area
and area around settling tanks shall be illuminated with area lighting for
operating personnel safety.
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