(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.
(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
(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.
(4) Screenings and grit disposal. All screenings and
grit shall be disposed of in an approved manner. Suitable containers
with lids shall be provided for holding screenings. Runoff control
must be provided around the containers where applicable. Fine screen
tailings are considered as infectious waste; therefore, containers
must provide vector control if wastes are not disposed of daily at
a Type 1 landfill.
(5) Preaeration. Because preaeration may be proposed
when a particular problem is anticipated, evaluation of these units
will be on a case-by-case basis. Diffuser equipment shall be arranged
for greatest efficiency, with consideration given to maintenance and
(6) Flow equalization. Equalization should be considered
to minimize random or cyclic peaking of organic or hydraulic loadings.
Equalization units should be provided after screening and grit removal.
(A) Aeration. Aeration may be required for odor control.
When required, air supply must be sufficient to maintain 1.0 milligrams
per liter (mg/liter) of dissolved oxygen in the wastewater.
(B) Volume. A diurnal flow graph with supporting calculations
used for sizing the equalization facility must be provided in the
engineering report. Generally, an equalization facility requires a
volume equivalent to 10% to 20% of the anticipated dry weather 30-day
average flow. Tankage should be divided into separate compartments
to allow for operational flexibility, repair, and cleaning.
(c) Flow measuring devices and sampling points. A means
for measuring effluent flow shall be provided at all plants. Consideration
should be given to providing a means to monitor influent flow. Where
average influent and effluent flows are significantly different, e.g.,
plants with large water surfaces located in areas of high rainfall
or evaporation or plants using a portion of effluent for irrigation,
both influent and effluent must be measured. Consideration should
be given to internal flow monitoring devices to measure returned activated
sludge and/or to facilitate splitting flows between units with special
attention being given when units are of unequal size. All plants shall
be provided with a readily accessible area for sampling effluent.
(1) Inlets. Clarifier inlets shall be designed to provide
uniform flow and stilling. Vertical flow velocity through the inlet
stilling well shall not exceed 0.15 feet per second at peak flow.
Inlet distribution channels shall not have deadened corners and shall
be designed to prevent the settling of solids in the channels. Inlet
structures should be designed to allow floating material to enter
(2) Scum removal. Scum baffles and a means for the
collection and disposal of scum shall be provided for primary and
final clarifiers. Scum collected from final clarifiers in plants utilizing
the activated sludge process, or any modification thereof, and aerated
lagoons may be discharged to aeration basin(s) and/or digester or
disposed of by other approved methods. Scum from all other final clarifiers
and from primary clarifiers shall be discharged to the sludge digester
or other approved method of disposal. Discharge of scum to any open
drying area is not acceptable. Mechanical skimmers shall be used in
units with a design flow greater than 25,000 gallons per day. Smaller
systems may use hydraulic differential skimming provided that the
scum pickup is capable of removing scum from the entire operating
surface of the clarifier. Scum pumps shall be specifically designed
for this purpose.
(3) Effluent weirs. Effluent weirs shall be designed
to prevent turbulence or localized high vertical flow velocity in
the clarifiers. Weirs shall be located to prevent short circuiting
flow through the clarifier and shall be adjustable for leveling. Weir
loadings shall not exceed 20,000 gallons per day peak design flow
per linear foot of weir length for plants with a design flow of 1.0
mgd or less. Special consideration will be given to weir loadings
for plants with a design flow in excess of 1.0 million gallons per
day (mgd), but such loadings shall not exceed 30,000 gallons per day
peak flow per linear foot of weir.
(4) Sludge lines. Means for transfer of sludge from
primary, intermediate, or final clarifiers for subsequent processing
shall be provided so that treatment efficiency will not be adversely
affected. Gravity sludge transfer lines shall not be less than eight
inches in diameter.
(5) Basin sizing. Overflow rates are based on the surface
area of clarifiers. The surface areas required shall be computed using
the following criteria. The actual clarifier size shall be based on
whichever is the larger size from the two surface area calculations
(peak flow and design flow surface loading rates). The final clarifier
solids loading for all activated sludge treatment processes shall
not exceed 50 pounds of solids per day per square foot of surface
area at peak flow rate. The following design criteria for clarifiers