(a) General requirements. Whenever possible, existing
data of flows and raw waste strength from the same plant or nearby
plants with similar service areas should be used in design of treatment
facilities. When using such data for design purposes, the variability
of data should be considered and the design based on the highest flows
and strengths encountered during normal operating periods taking into
consideration possible infiltration/inflow. In the absence of existing
data, the following are generally acceptable parameters to which must
be added appropriate allowances for inflow and infiltration into the
collection system to obtain plant influent characteristics.
Attached Graphic
(1) Effluent quality. Wastewater treatment plants shall
be designed to consistently meet the effluent concentration and loading
requirements of the applicable waste disposal permit.
(2) Effluent quantity. The design flow of a treatment
plant is defined as the wet weather, maximum 30-day average flow.
The design basis shall include industrial wastewaters which will enter
the sewerage system. The engineering report shall state the flow and
strength of wastewaters from industries which individually contribute
5.0% or more of plant flow or loading and discuss the aspect of hazardous
or toxic wastes. It is the intent of these design criteria that the
permit conditions not be violated. The engineering report shall list
the design influent flow and concentration of five-day biochemical
oxygen demand (BOD5 ), total suspended
solids (TSS), or other parameters for the following:
(A) dry weather 30-day average (QD W);
(B) wet weather maximum 30-day average (QD W); and
(C) two-hour peak flow (Qp W).
(3) Piping. The piping within all plants shall be arranged
so that when one unit is out of service for repairs, plant operation
will continue and emergency treatment can be accomplished. Valves
and piping shall be provided and sized to allow dewatering of any
unit, in order that repairs of the unit can be completed in as short
a period of time as possible. Portable pumping units may be used for
dewatering small treatment plants (design flow of less than 100,000
gallons per day) or interim facilities. Removed wastes must be stored
for retreatment or delivered to another treatment facility for processing.
Consideration shall be given in design for means to clean piping,
especially piping carrying raw wastewater, sludges, scum, and grit.
(4) Peak flow. For treatment unit design purposes,
peak flow is defined as the highest two-hour average flow rate expected
to be delivered to the treatment units under any operational condition,
including periods of high rainfall (generally the two-year, 24-hour
storm is assumed) and prolonged periods of wet weather. With pumped
inflow, clarifiers shall have the capacity of all pumps operating
at maximum wet well level unless a control system is provided that
will limit the pumping rate to the firm capacity. This flow rate may
also include skimmer flow, thickener overflow, filter backwash, etc.
All treatment plants must be designed to hydraulically accommodate
peak flows without adversely affecting the treatment processes. The
engineer shall determine, by methods acceptable to the commission,
the appropriate peak flow rate, including the possibility of utilizing
standby pumps. The proposed two-hour peak flow rate, together with
a discussion of rationale, calculations, and all supporting flow rate
data shall be, unless presented in the preliminary engineering report,
included in the final engineering design report. Special storm flow
holding basins or flow equalization facilities can be specified to
partially satisfy the requirements of this section where all treatment
units within a plant are not sized for peak flow. See §317.9
of this title (relating to Appendix A) for referencing a two-year
24-hour rainfall event.
(5) Auxiliary power. The need for auxiliary power facilities
shall be evaluated for each plant and discussed in the preliminary
and final engineering reports. Auxiliary power facilities are required
for all plants, unless dual power supply arrangements can be made
or unless it can be demonstrated that the plant is located in an area
where electric power reliability is such that power failure for a
period to cause deterioration of effluent quality is unlikely. Acceptable
alternatives to auxiliary power include the ability to store influent
flow or partially treated wastewater during power outage. Auxiliary
power may be required by the commission for plants discharging near
drinking water reservoirs, shellfish waters, or areas used for contact
recreation, and for plants discharging into waters that could be unacceptably
damaged by untreated or partially treated effluent. For more information
on power reliability determination and emergency power alternatives,
refer to §317.3(e) of this title (relating to Lift Stations).
(6) Component reliability. Multiple units may be required
based upon the uses of the receiving waters and the significance of
the treatment units to the treatment processes.
(7) Stairways, walkways, and guard rails. Basins having
vertical walls terminating four or more feet above or below ground
level shall provide a stairway to the walkway. Guard rails on walkways
shall have adequate clearance space for maintenance operations (see §317.7
of this title (relating to Safety)).
(8) Public drinking water supply connections. There
shall be no water connection from any public drinking water supply
system to a wastewater treatment plant facility unless made through
an air gap or a backflow prevention device, in accordance with American
Water Works Association (AWWA) Standard C506 (latest revision) and
AWWA Manual M14. All backflow prevention devices shall be tested annually
with their test and maintenance report forms retained for a minimum
of three years. All washdown hoses using potable water must be equipped
with atmospheric vacuum breakers located above the overflow level
of the washdown area.
(9) Ground movement protection. The structural design
of treatment plants shall be sufficient to accommodate anticipated
ground movement including any active geologic faults and allow for
independent dewatering of all treatment units. Plants should not be
located within 50 feet of geologic faults.
(10) Odor control facilities. The need for odor control
facilities shall be evaluated for each plant. Factors to be considered
are the dissolved oxygen level of the incoming sewage and the type
of treatment process proposed.
(b) Preliminary treatment units. Bar screens, screens,
or shredders through which all wastewater will pass should be provided
at all plants with the exception of plants in which septic tanks,
Imhoff tanks, facultative, aerated, or partially mixed lagoons represent
the initial treatment unit. In the event bar screens, screens, or
shredders are located four or more feet below ground level, appropriate
equipment shall be provided to lift the screenings to ground elevation.
Where mechanically cleaned bar screens or shredders are utilized,
a backup unit or manually cleaned bar screen shall be provided. A
means of diverting flow to the backup screen shall be included in
the design.
(1) Bar screens. Manually cleaned bar screens shall
be constructed having a 30-degree to 60-degree slope to a horizontal
platform which will provide for drainage of the screenings. Bar screen
openings shall not be less than 3/4 inch for manually cleaned bar
screens and 1/2 inch for mechanically cleaned bar screens. The channel
in which the screen is placed shall allow a velocity of two feet per
second or more at design flow. Velocity through the screen opening
should be less than three feet per second at design flow.
(2) Grit removal. Grit removal facilities should be
considered for all wastewater treatment plants. Grit washing facilities
shall be provided unless a burial area for the grit is provided within
the plant grounds, or the grit is handled otherwise in such a manner
as to prevent odors or fly breeding. Grit removal units shall have
mechanical means of grit removal or other acceptable methods for grit
removal. Plants which have a single grit collecting chamber shall
have a bypass around the chamber. All grit collecting chambers shall
be designed with the capability to be dewatered. The method of velocity
control used to accomplish grit removal in gravity settling chambers
shall be detailed in the final engineering report.
(3) Fine screens. Fine screens, if used, shall be preceded
by a bar screen. Fine screens shall not be substituted for primary
sedimentation or grit removal; however, they may be used in lieu of
primary treatment if fully justified by the design engineer. A minimum
of two fine screens shall be provided, each capable of independent
operation at peak flow. A steam cleaner or high pressure water hose
shall be provided for daily maintenance of fine screens.
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