Current through Reg. 49, No. 52; December 27, 2024
(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 (QDW);
(B) wet weather maximum 30-day average
(QDW); and
(C) two-hour peak flow
(QpW).
(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.
(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
inspection.
(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.
(d) Clarifiers.
(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 the clarifier.
(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 are based upon a
side water depth of ten feet and shall be considered acceptable.
Attached
Graphic
(6)
Sidewater depth (SWD). The minimum SWD for conventional primary and
intermediate clarifiers is seven feet. All final clarifiers shall have a
minimum SWD of eight feet. Final clarifiers having a surface area equal to or
greater than 1,250 square feet (diameter equal to or greater than 40 feet) must
be provided with a minimum SWD of 10 feet.
(7) Hopper bottom clarifiers. Hopper bottom
clarifiers without mechanical sludge collecting equipment will only be approved
for those facilities with a permitted design flow of less than 25,000 gallons
per day. The required SWD for hopper bottom clarifiers may be computed using
the following equation: SWD = 160 QD + 4, where SWD equals required SWD in feet
and QD equals design flow in million gallons per day. Furthermore, SWD as
computed previously for any flow may be reduced by crediting the upper
one-third of the hopper as effective SWD if the following conditions are met:
(A) clarifier surface loading rate is reduced
by at least 15% from maximum loading rate as per paragraph (5) of this
subsection;
(B) influent stilling
baffle and effluent weir are designed to prevent short circuiting;
(C) detention time at peak flow is at least
1.8 hours for secondary treatment and 2.4 hours for advanced treatment;
and
(D) an appropriate form of flow
equalization is used.
(8) Sludge collection equipment. All
conventional clarifier units that treat flow from a treatment plant facility
with a design flow of 25,000 gallons per day or greater shall be provided with
mechanical sludge collecting equipment. Hopper bottom clarifiers must have a
smooth wall finish and a hopper slope of not less than 60 degrees.
(9) BOD5 removal. It
shall be assumed that the BOD5 removal in a primary
clarifier is 35%, unless satisfactory evidence is presented to indicate that
the efficiency will be otherwise. In plant efficiency calculations, it shall be
assumed that the BOD5 removal in intermediate and final
clarifiers is included in the calculation for the efficiency of the treatment
unit preceding the intermediate or final clarifier.
(e) Trickling filters.
(1) General. Trickling filters are secondary
aerobic biological processes which are used for treatment of sewage.
(2) Basic design parameters. Trickling
filters are classified according to applied hydraulic loading in million
gallons per day per acre (mgd/acre) of filter media surface area, and organic
loadings in pounds of biochemical oxygen demand (BOD) per day per 1,000 cubic
feet of filter media (lb BOD/day-1,000 cu ft). The following factors should be
considered in the selection of the design hydraulic and organic loadings:
strength of the influent sewage, effectiveness of pretreatment, type of filter
media, and treatment efficiency required. Typical ranges of applied hydraulic
and organic loadings for the different classes of trickling filters are
presented in the following table for illustrative purposes. The design engineer
shall submit sufficient operating data from existing trickling filters of
similar construction and operation to justify his efficiency calculations for
the filters, and a filter efficiency formula from a reliable source acceptable
to the commission. The formula of the National Research Council may be used
when rock media is used in the trickling filter(s).
Attached
Graphic
(3)
Pretreatment. The trickling filter treatment facility shall be preceded by
primary clarifiers equipped with scum and grease removal devices. Design
engineers may submit operating data as justification of other alternative
pretreatment devices which provide for effective removal of grit, debris,
suspended solids, and excess oil and grease. Preaeration shall be provided
where influent wastewater contains harmful levels of hydrogen sulfide
concentrations.
(4) Filter media.
(A) Material specifications for rock media.
The following are minimum requirements.
(i)
Crushed rock, slag, or similar media should not contain more than 5.0% by
weight of pieces whose longest dimension is greater than three times its least
dimension. The rock media should be free from thin, elongated, and flat pieces
and should be free from dust, clay, sand, or fine material. Rock media should
conform to the following size distribution and grading when mechanically graded
over a vibrating screen with square openings:
(I) passing five-inch sieve--100% by
weight;
(II) retained on three-inch
sieve--95% to 100% by weight;
(III)
passing two-inch sieve--0.2% by weight;
(IV) passing one-inch sieve--0.1% by
weight;
(V) the loss of weight by a
20-cycle sodium sulphate test, as described in the American Society of Civil
Engineers Manual of Engineering and Engineering Practice Number 13, shall be
less than 10%.
(ii) Rock
media shall not be less than four feet in depth (at the shallowest point) nor
deeper than eight feet (at the deepest point of the filter).
(B) Synthetic (manufactured or
prefabricated) media.
(i) Application of
synthetic media shall be evaluated on a case-by-case basis. Suitability should
be evaluated on the basis of experience with installations treating similar
strength wastewater under similar hydraulic and organic loading conditions. The
manufacturer's recommendations shall be included, as well as case histories
involving the use of the media.
(ii) Media shall be relatively insoluble in
sewage and resistant to flaking or spalling, ultraviolet degradation,
disintegration, erosion, aging, all common acids and alkalies, organic
compounds, biological attack, and shall support the weight of a person when the
media is in operation.
(iii) Media
depths should be consistent with the recommendations of the
manufacturer.
(C)
Placing of media.
(i) The dumping of media
directly on the filter is unacceptable. Instructions for placing media shall be
included in the specifications.
(ii) Crushed rock, slag, and similar media
shall be washed and screened or forked to remove clays, organic material, and
fines.
(iii) Such materials should
be placed by hand to a depth of 12 inches above the underdrains and all
material should be carefully placed in a manner which will not damage the
underdrains. The remainder of the material may be placed by means of belt
conveyors or equally effective methods approved by the engineers. Trucks,
tractors, or other heavy equipment should not be driven over the filter media
during or after construction.
(iv)
Prefabricated filter media shall be placed in accordance with recommendations
provided by the manufacturer.
(5) Filter hydraulics.
(A) Dosing. Wastewater may be applied to the
filters by siphons, pumps, or by gravity discharge from preceding treatment
units when suitable flow characteristics have been developed.
(B) Distribution equipment. Settled
wastewater may be distributed over the filter media by rotary, horizontal, or
travelling distributors, provided the equipment proposed is capable of
producing the required continuity and uniformity of distribution over the
entire surface of the filter. Deviation from a calculated uniformly distributed
volume per unit surface area shall not exceed 10% at any portion of the filter.
Filter distributors shall be designed to operate properly at all flow rates.
Excessive head in the center column of rotary distributors shall be avoided,
and all center columns shall have adequately sized overflow ports to prevent
the head from building up sufficiently for the water to reach the bearings in
the center column. Distributors shall include cleanout gates on the ends of the
arms and shall also include an end nozzle to spray water on the wall of the
filter to keep the edge of the media continuously wet. The filter walls shall
extend at least 12 inches above the top of the ends of the distributor
arms.
(C) Seals. The use of mercury
seals is prohibited in the distributors of newly constructed trickling filters.
If an existing treatment facility is to be modified, any mercury seals in the
trickling filters shall be replaced with oil or mechanical seals.
(D) Distributor clearance. A minimum
clearance of six inches shall be provided between the top of the filter media
and the distributing nozzles.
(E)
Recirculation. In order to insure that the biological growth on the filter
media remains active at all times, provisions shall be included in all designs
for minimum recirculation during periods of low flow. This minimum
recirculation shall not be considered in the evaluation of the efficiency of
the filter unless it is part of the proposed specified continuous recirculation
rate. Minimum flow to the filters shall not be less than 1.0 mgd/acre of filter
surface. In addition, the minimum flow rate must be great enough to keep rotary
distributors turning and the distribution nozzles operating properly. For
facilities with a design capacity greater than or equal to 0.5 mgd and in which
recirculation is included in design computations for
BOD5 removal, recirculation shall be provided by
variable speed pumps and a method of conveniently measuring the recycle flow
rate shall be provided.
(F) Surface
loading. The engineering report shall include calculations of the maximum,
design, and minimum surface loadings on the filter(s) in terms of mgd/acre of
filter area per day (for the initial year and design year). Hydraulic loadings
of filters with crushed rock, slag, or similar media shall not exceed 40
mgd/acre based on design flow. The minimum surface loading shall not be less
than 1.0 mgd/acre. Loadings on synthetic (manufactured or prefabricated) filter
media shall be within the ranges specified by the manufacturer.
(6) Underdrain system.
(A) Underdrains. Underdrains with
semicircular inverts or equivalent shall be provided and the underdrainage
system shall cover the entire floor of the trickling filter. Inlet openings
into the underdrains shall provide an unsubmerged gross combined area of at
least 15% of the surface area of the filter.
(B) Hydraulics. Underdrains and the filter
effluent channel floor shall have a minimum slope of 1.0%. Effluent channels
shall be designed to produce a minimum velocity of two feet per second at
average daily flow rate of application to the trickling filter.
(C) Drain tile. Underdrains for rock media
trickling filters shall be either vitrified clay or precast reinforced
concrete. The use of half tile for underdrain systems is
unacceptable.
(D) Corrosion.
Underdrain systems for synthetic media trickling filters shall be resistant to
corrosion.
(E) Ventilation. The
underdrain system, effluent channels, and effluent pipe shall be designed to
permit free passage of air. Drains, channels, and effluent pipes shall have a
cross-sectional area such that not more than 50% of the cross-sectional area
will be submerged at peak flow plus recirculation. Provision shall be made in
the design of the effluent channels to allow the possibility of increased
hydraulic loading. The underdrain system shall provide at least one square foot
of ventilating area (vent stacks, ventilating holes, ventilating ports) for
every 250 square feet of rock media filter plan area. Ventilating area for
synthetic media underdrains will be provided as recommended by the
manufacturer, but shall be at least one square foot for every 175 square feet
of synthetic media trickling filter plan area.
(F) Maintenance. All flow distribution
devices, underdrains, channels, and pipes shall be designed so they may be
maintained, flushed, and properly drained. The units shall be designed to
facilitate cleaning of the distributor arms. A gate shall be provided in the
wall to facilitate rodding of the distributor arms.
(G) Flooding. Provisions shall be made to
enable flooding of the trickling filter for filter fly control; however,
consideration will be given by the commission to alternate methods of filter
fly control provided that the effectiveness of the alternate method is verified
at a full scale installation. This information shall be submitted with the
plans and specifications.
(H) Flow
measurements. Means shall be provided to measure flow to the filter and
recirculation flows.
(f) Rotating biological contactors (RBC).
(1) General.
(A) RBC units shall be covered and ample
ventilation provided. Working clearance of approximately 30 inches should be
provided within the cover unless the covers are removable, utilizing equipment
normally available on site. Enclosures shall be constructed of a suitable
corrosion-resistant material.
(B)
The design of the RBC media shall provide for self-cleaning action due to the
flow of water and air through the media. Careful selection of media that will
not entrap solids should be made.
(C) The RBC tank should be designed to
minimize zones in which solids will settle out.
(D) RBC media should be selected which is
compatible with the wastewater. Selection of media can be critical where the
wastewater has an industrial waste portion which either significantly increases
the wastewater temperature or contains a chemical constituent which may
decrease the life of the RBC media.
(2) Design.
(A) Pretreatment. RBC units shall be preceded
by pretreatment to remove any grit, debris, and excess oil and grease which may
hinder the treatment process or damage the RBC units. The design engineer
should consider primary clarifiers with scum and grease collecting devices,
fine screens, and oil separators. For wastes with high hydrogen sulfide
concentrations, preaeration shall be provided.
(B) Organic loading. The organic loading for
the design of RBC units shall be based on total BOD5 in
the waste going to the RBC, including any side streams. The design engineer
should consider a maximum loading rate of five pounds
BOD5 per day per 1,000 square feet of media in any
stage, depending on the character of the influent wastewater. The maximum
loading rate shall not exceed eight pounds BOD5 per day
per 1,000 square feet of media in any stage. The design engineer should also
consider the ratio of soluble BOD5 to total
BOD5 and its possible effect on required RBC media area.
Allowable organic loading for the entire RBC system shall not exceed the
following criteria.
Attached
Graphic
(C)
Stages of treatment. The number of RBC units in series (stages) for BOD removal
only shall be a minimum of three stages. For BOD removal and nitrification,
there shall be a minimum of four stages. If the plant is designed with less
stages than noted in the previous sentences of this subparagraph, the engineer
must provide justification based on either full-scale operating facilities or
pilot unit operational data. Any pilot unit data used in the justification must
take into consideration an appropriate scale-up factor.
(D) Drive system. The drive system for each
RBC unit shall be selected for the maximum anticipated media load. A variable
speed system should be considered to provide additional operator flexibility.
The RBC units may be mechanically driven or air driven.
(i) Mechanical drives.
(I) Each RBC unit shall have a positively
connected mechanical drive with motor and speed reduction unit to maintain the
required rpm.
(II) A fully
assembled spare mechanical drive unit for each size shall be provided
on-site.
(III) Supplemental
diffused air should be considered for mechanical drive systems to help remove
excess biomass from the media and to help maintain the minimum dissolved oxygen
concentration.
(ii) Air
drives.
(I) Each RBC unit shall have air
diffusers mounted below the media and off-center from the vertical axis of the
RBC unit. Air cups mounted on the outside of the media shall collect the air to
provide the driving force and maintain the required rpm.
(II) Blowers shall provide enough air flow
for each RBC unit plus additional capacity to double the air flow rate to any
one unit while the others are running normally.
(III) The blowers shall be capable of
providing the required air flow with the largest unit out of service.
(IV) The air diffuser line to each unit shall
be mounted such that it can be removed without draining the tank or removing
the RBC media.
(V) An air control
valve shall be installed on the air diffuser line to each RBC unit.
(E) Dissolved oxygen.
The RBC plant shall be designed to maintain a minimum dissolved oxygen
concentration of one milligram per liter at all stages during the peak organic
flow rate. Supplemental aeration may be required.
(F) Nitrification. The design of an RBC plant
to achieve nitrification is dependent upon a number of factors, including the
concentration of ammonia in the influent, effluent ammonia concentration
required, BOD5 removal required, minimum operational
temperatures, and ratio of peak to design hydraulic flow. Each of these factors
will impact the number of stages of treatment required and the allowable
ammonia nitrogen loading (lb NH3/day/1,000
ft2 media) required to achieve the desired levels of
nitrification for a given facility. The engineer shall submit appropriate data
supporting the design.
(G) Design
flexibility. The designer of an RBC plant should consider provisions to provide
additional operational flexibility such as controlled flow to multiple first
stages, alternate flow and staging arrangements, removable baffles between
stages, and provision for step feed and supplemental aeration.
(g) Activated sludge
facilities.
(1) Organic loading rates.
Aeration tank volumes should be based upon full scale experience, pilot scale
studies, or rational calculations based upon commonly accepted design
parameters such as food to microorganism ratio, mixed liquor suspended solids,
and the solids retention time. Other factors to be considered include size of
the treatment plant, diurnal load variations, return flows and soluble organic
loads from digesters, or sludge dewatering operations and degree of treatment
required. Temperature, pH, and dissolved oxygen concentration are particularly
important to consider when designing for nitrification. As a general rate,
minimum aeration tank volumes shall be as set forth in the following table.
Calculations must be submitted to fully justify the basis of design for any
aeration basins not conforming to these minimum recommendations.
Attached
Graphic
(A) The
conventional activated sludge process is characterized by having a plug flow
hydraulic regime wherein particles are discharged in the same sequence in which
they enter the aeration basin. Plug flow may be approximated in long tanks with
a high length-to-width ratio.
(B)
The contact stabilization process divides the aeration tank volume between the
reaeration zone and the contact zone. The ratio of reaeration volume to contact
volume ranges from 1:1 to 2:1. The hydraulic detention time in the contact zone
shall be sufficient to provide removals of soluble substrates to the required
levels. For domestic flows normally two hours is sufficient in the contact
zone. Contact zone volume shall be based upon acceptable removal kinetics for
soluble BOD5 and ammonia nitrogen.
(C) Oxidation ditches (which are organically
loaded consistent with this paragraph) shall have a minimum hydraulic retention
time of 20 hours based on design flow. These oxidation ditch systems shall
provide final clarification and return sludge capability equal to that required
for the extended aeration process. There shall be a minimum of two rotors per
ditch, each capable of supplying the required oxygenation capacity and
maintaining a minimum channel velocity of 1.0 foot per second with one rotor
out of service. The ditch shall be lined with reinforced concrete or other
acceptable erosion-resistant liner material. Provision shall be made to easily
vary the liquid level in the ditch to control the immersion depth of the rotor
for flexibility of operation. A motor of sufficient size to maintain the proper
rotor speed for continuous operation shall be provided. Rotor bearings should
have grease fittings that are readily accessible to maintenance personnel. Gear
housing and outboard bearings should be shielded from rotor splash.
(2) Aeration basin general design
considerations. Aeration tank geometry shall be arranged to provide optimum
oxygen transfer and mixing for the type aeration device proposed. Aeration
tanks must be constructed of reinforced concrete, steel with
corrosion-resistant linings or coatings, or lined earthen basins. Liquid depths
shall not be less than 8.0 feet when diffused air is used. All aeration tanks
shall have a freeboard of not less than 18 inches at peak flow. Access walkways
with properly designed safety handrails shall be provided to all areas that
require routine maintenance. Where operators would be required to climb heights
greater than four feet, properly designed stairways with safety handrails
should be provided. The shape of the tank and the installation of aeration
equipment should provide a means to control short circuiting through the tank.
For plants designed for design flows greater than 2.0 mgd the total aeration
basin volume shall be divided among two or more basins. Each treatment facility
shall be designed to hydraulically pass the design two-hour peak flow with one
basin out of service.
(3) Sludge
pumps, piping, and return sludge flow measurement. The pumps and piping for
return activated sludge shall be designed to provide variable underflow rates
of 200 to 400 gallons per day per square foot for each clarifier. If mechanical
pumps are used, sufficient pumping units shall be provided to maintain design
pumping rates with the largest single unit out of service. Sludge piping and/or
channels shall be so arranged that flushing can be accomplished. A minimum pipe
line velocity of three feet per second should be provided at an underflow rate
of 200 gallons per day per square foot. Some method shall be provided to
measure the return sludge flow from each clarifier.
(4) Aeration system design.
(A) General design consideration. Aeration
systems shall be designed to maintain a minimum dissolved oxygen concentration
of 2.0 mg/liter throughout the basin at the maximum diurnal organic loading
rate and to provide thorough mixing of the mixed liquor. The design oxygen
requirements for activated sludge facilities are presented in the following
table. The minimum air volume requirements may be reduced with appropriate
supporting performance evaluations from the manufacturer.
Attached
Graphic
(i) Minimum air
volume requirements are based upon a transfer efficiency of 4.0% in wastewater
for all activated sludge processes except extended aeration, for which a
wastewater transfer efficiency of 4.5% is assumed.
(ii) Value in parentheses represents the
minimum oxygen requirement for ditch type systems which will achieve
nitrification.
(B)
Diffused air systems.
(i) Volumetric aeration
requirements. Volumetric aeration requirements shall be as determined from the
preceding table unless certified diffuser performance data is presented which
demonstrates transfer efficiencies greater than those used in the preparation
of the table. Wastewater transfer efficiencies may be estimated for:
(I) coarse bubble diffusers by multiplying
the clean water transfer efficiency by 0.65%;
(II) fine bubble diffusers by multiplying the
clean water transfer efficiency by 0.45%. The maximum allowable wastewater
transfer efficiency shall be 12%. Plants treating greater than 10% industrial
wastes shall provide data to justify actual wastewater transfer efficiencies.
Wastewater oxygen transfer efficiencies greater than 12% are considered
innovative technology. See § 317.1(a)(2)(C) of this title (relating to General
Provisions) for performance bond requirements. Clean water transfer
efficiencies obtained at 20 degrees Celsius shall be adjusted to reflect field
conditions (i.e., wastewater transfer efficiencies) by use of the following
equation.
Attached
Graphic
(ii) Mixing requirement. Air requirements for
mixing should be considered along with those required for the design organic
loading. The designer is referred to Table 14-V, aerator mixing requirements in
Wastewater Treatment Plant Design, a joint publication of the American Society
of Civil Engineers and the Water Pollution Control Federation.
(iii) Blowers and compressors. Blowers and
compressors shall be of such capacity to provide the required aeration rate as
well as the requirements of all supplemental units such as airlift pumps.
Multiple compressor units shall be provided and shall be arranged so the
capacity of the total air supply may be adjusted to meet the variable organic
load to be placed on the treatment facility. The compressors shall be designed
so that the maximum design air requirements can be met with the largest single
unit out of service. The blower/compressor units shall automatically restart
after a period of power outage or the operator or owner shall be notified by
some method such as telemetry or an auto-dialer. The specified capacity of the
blowers or air compressors, particularly centrifugal blowers, should take into
account that the air intake temperature may reach 104 degrees Fahrenheit (40
degrees Celsius) or higher and the pressure may be less than standard (14.7
pounds per square inch absolute). The capacity of the motor drive should also
take into account that the intake air may be 10 degrees Fahrenheit (-12 degrees
Celsius) or less and may require oversizing of the motor or a means of reducing
the rate of air delivery to prevent overheating or damage to the
motor.
(iv) Diffusers and piping.
Each diffuser header shall include a control valve. These valves are basically
for open/close operation but should be of the throttling type. The depth of
each diffuser shall be adjustable. The air diffuser system, including piping,
shall be capable of delivering 150% of design air requirements. The aeration
system piping should be designed to minimize headlosses. Typical air velocities
in air delivery piping systems are presented in the following table.
Attached
Graphic
(5) Mechanical aeration systems. Mechanical
aeration devices shall be of such capacity to provide oxygen transfer to and
mixing of the tank contents equivalent to that provided by compressed air. A
minimum of two mechanical aeration devices shall be provided. Two speed or
variable speed drive units should be considered. The oxygen transfer capability
of mechanical surface aerators shall be calculated by the use of a generally
accepted formula and the calculations presented in the engineering report.
Proposed clean water transfer rates in excess of 2.0 pounds per horsepower-hour
shall be justified by performance data. In addition to providing sufficient
oxygen transfer capability for oxygen transfer, the mechanical aeration devices
shall also be required to provide sufficient mixing to prevent deposition of
mixed liquor suspended solids under any flow condition. A minimum of 100
horsepower per million gallons of aeration basin volume shall be
furnished.
(h) Nutrient
removal.
(1) Nitrogen removal. Biological
systems designed for nitrification and denitrification may be utilized for the
conversion/removal of nitrogen. Various physical/chemical processes may be
considered on a case-by-case basis.
(2) Phosphorus removal.
(A) Chemical treatment. Addition of lime or
the salts of aluminum, or iron may be used for the chemical removal of soluble
phosphorus. The phosphorus reacts with the calcium, aluminum, or iron ions to
form insoluble compounds. These insoluble compounds may be flocculated with or
without the addition of a coagulant aid such as a polyelectrolyte to facilitate
separation by sedimentation. When adding salts of aluminum or iron, the
designer should evaluate the wastewater to ensure sufficient alkalinity is
available to prevent excessive depression of the wastewater or effluent pH.
This is of particular importance when the system will also be required to
achieve nitrification. The designer is referred to Nutrient Control, Manual of
Practice FD-7 Facilities Design, published by the Water Pollution Control
Federation and the Process Design Manual for Phosphorus Removal, published by
the Environmental Protection Agency, for additional information.
(B) Biological phosphorus removal. Biological
phosphorus removal systems will be considered on a case-by-case basis for
systems which can produce operating data which demonstrate the capability to
remove phosphorus to the required levels. All biological systems which are
required to meet a 1.0 mg/liter effluent phosphorus concentration shall make
provision for standby chemical treatment to ensure the 1.0 mg/liter is
achieved.
(i)
Aerated lagoon.
(1) Horsepower. Mechanical
aeration units in aerated lagoons shall have sufficient power to provide a
minimum of 1.6 pounds of oxygen per pound of BOD5
applied with the largest unit out of service. If oxygen requirements control
the amount of horsepower needed, proposed oxygen transfer rates in excess of
two pounds per horsepower-hour must be justified by actual performance data.
The amount of oxygen supplied or the pounds of BOD5 per
hour that may be applied per horsepower-hour may be calculated by the use of
any acceptable formula. The combined horsepower rating of the aeration units
shall not be less than 30 horsepower per million gallons of aerated lagoon
volume.
(2) Construction. Earthen
ponds shall have large sections of concrete slabs or equivalent protection
under each aeration unit to prevent scouring of the earth. Concrete scour pads
shall be used in all areas where the velocity exceeds one foot per second.
Earthen ponds shall have protection on the slopes of the embankment at the
water line to prevent erosion of the slopes from the turbulence in the lagoon.
Where the horsepower level is more than 200 horsepower per million gallons of
lagoon volume, the pond embankment at the water line shall be protected from
erosion with riprap which may be concrete, gunite, a six-inch thick layer of
asphalt-saturated or cement-stabilized earth rolled and compacted into place,
or suitable rock riprap. The crest and dry slopes of embankments shall be
protected from erosion by planting of grass.
(3) Subsequent treatment, discharge systems.
Aerated lagoon effluent will normally be routed to additional ponds for
secondary treatment and to provide sufficient detention time for disinfection.
The secondary ponds system shall consist of two or more ponds. Secondary pond
sizing shall not exceed 35 pounds of BOD5 per acre per
day. Hydraulic detention time in a combined aerated lagoon and secondary pond
system shall be a minimum of 21 days (based on design flow) in order to provide
adequate disinfection. In designing the secondary ponds,
BOD5 removal efficiency in the aerated lagoon(s) may be
calculated using the following formula.
Attached
Graphic
(j) Wastewater stabilization ponds (secondary
treatment ponds).
(1) Pretreatment.
Wastewater stabilization ponds shall be preceded by facilities for primary
sedimentation of the raw sewage. Aerated lagoons or facultative lagoons may be
utilized in place of conventional primary treatment facilities.
(2) Imperviousness. All earthen structures
proposed for use in domestic wastewater treatment or storage shall be
constructed to protect groundwater resources. Where linings are necessary, the
following methods are acceptable:
(A) in-situ
or placed clay soils having the following qualities may be utilized for pond
lining:
(i) more than 30% passing a 200-mesh
sieve;
(ii) liquid limit greater
than 30%;
(iii) plasticity index
greater than 15; and
(iv) a minimum
thickness of two feet;
(B) membrane lining with a minimum thickness
of 20 mils, and an underdrain leak detection system;
(C) other methods with commission
approval.
(3)
Distribution of flow. Stabilization ponds shall be of such shape and size to
insure even distribution of the wastewater flow throughout the entire pond.
While the shapes of ponds may be dictated to some extent by the topography of
the location, long narrow ponds are preferable and they should be oriented in
the direction of the prevailing wind such that debris is blown toward the
inlet. Ponds with narrow inlets or sloughs should be avoided.
(4) Access area. Storm water drainage shall
be excluded from all ponds. All vegetation shall be removed from within the
pond area during construction. Access areas shall be cleared and maintained for
a distance of at least 20 feet from the outside toes of the pond embankment
walls.
(5) Multiple ponds. The use
of multiple ponds in pond systems is required. The operation of the ponds shall
be flexible, enabling one or more ponds to be taken out of service without
affecting the operation of the remaining ponds. The ponds shall be operated in
series during routine operation periods.
(6) Organic loading. The organic loading on
the stabilization ponds, based on the total surface area of the ponds, shall
not exceed 35 pounds of BOD5 per acre per day. The
loading on the initial stabilization pond shall not exceed 75 pounds of
BOD5 per acre per day.
(7) Depth. The stabilization ponds or cells
shall have a normal water depth of three to five feet.
(8) Inlets and outlets. Multiple inlets and
multiple outlets are required. The inlets and outlets shall be arranged to
prevent short circuiting within the pond so that the flow of wastewater is
distributed evenly throughout the pond. Multiple inlets and outlets shall be
spaced evenly. All outlets shall be baffled with removable baffles to prevent
floating material from being discharged, and shall be constructed so that the
level of the pond surface may be varied under normal operating conditions.
Submerged outlets shall be used to prevent the discharge of algae.
(9) Embankment walls. The embankment walls
should be compacted thoroughly and compaction details shall be covered in the
specifications. Soil used in the embankment shall be free of foreign material
such as paper, brush, and fallen trees. The embankment walls shall have a top
width of at least 10 feet. Interior and exterior slope of the embankment wall
should be one foot vertical to three feet horizontal. There shall be a
freeboard of not less than two feet nor more than three feet based on the
normal operating depth. All embankment walls shall be protected by planting
grass or riprapping. Where embankment walls are subject to wave action,
riprapping should be installed. Erosion stops and water seals shall be
installed on all piping penetrating the embankments. Provisions should be made
to change the operating level of the pond so the pond surface can be raised or
lowered at least six inches.
(10)
Partially mixed aerated lagoons.
(A)
Horsepower. With partially mixed aerated lagoons, no attempt is made to keep
all pond solids in suspension. Mechanical or diffused aeration equipment should
be sized to provide a minimum of 1.6 pounds of oxygen per pound of
BOD5 applied with the largest unit out of service. Where
multiple ponds are used in series, the power input may be reduced as the
influent BOD5 to each pond decreases. Proposed oxygen
transfer rates in excess of two pounds per horsepower-hour must be justified by
actual performance data.
(B) Pond
sizing. Partially mixed aerated lagoons should be sized in accordance with the
formula in subsection (i)(3) of this section using K-0.28. Pond length to width
ratios should be three to one or four to one.
(C) Imperviousness. Requirements for
imperviousness, multiple cells, embankment walls, and inlets and outlets shall
be the same as for other secondary treatment ponds.
(k) Facultative lagoon (raw
wastewater stabilization pond).
(1)
Configuration. The length to width ratio of the lagoon should be three to one,
with flow along the length from inlets near one end to outlets at the opposite
end (other configurations may be approved if adequate means of prevention of
short circuiting are provided). The length should be oriented in the direction
of the prevailing winds with the inlet side located such that debris will be
blown toward the inlet (generally, the north-northwest side). Inlet baffles
shall be provided to collect flotable material. The outlets shall be
constructed so that the water level of the lagoon may be varied under normal
operating conditions. Storm water drainage shall be prevented from entering the
lagoon. The design engineer may wish to locate the facultative lagoon in a
central location with regard to the surrounding secondary ponds to facilitate
compliance with the buffer zone requirement specified in Chapter 309 of this
title (relating to Domestic Wastewater Effluent Limitations and Plant
Siting).
(2) Imperviousness.
Requirements for imperviousness shall be the same as those for secondary
treatment ponds.
(3) Depth. The
portion of the lagoon near the inlets shall have a 10 to 12 foot depth to
provide sludge storage and anaerobic treatment. This deeper portion should be
approximately 25% of the area of the lagoon bottom. The remainder of the pond
should have a depth of five to eight feet.
(4) Organic loading. The organic loading,
based on the surface area of the facultative lagoon, shall not exceed 150
pounds of BOD5 per acre per day.
(5) Odor control. The facultative lagoon
shall have multiple inlets and the inlets should be submerged approximately 24
inches below the water surface to minimize odor but not disturb the anaerobic
zone. Capabilities for recirculation at 50% to 100% of the design flow should
be provided. Care should be taken to avoid situations where siphoning of lagoon
contents through submerged inlets can occur.
(6) Embankment walls. Refer to subsection
(j)(9) of this section.
(7)
Subsequent treatment. The facultative lagoon effluent will normally be routed
to a wastewater stabilization pond system for secondary treatment. In designing
the stabilization pond system, it may be assumed that BOD removal in the
facultative lagoon is 50%. The stabilization pond system shall contain two or
more ponds.
(l)
Filtration. Filtration must be employed as a unit operation to supplement
suspended solids removal for those treatment facilities with tertiary effluent
limitations (suspended solids effluent quality equal to or less than 10
mg/liter). Filtration may be employed as a unit operation for those treatment
facilities with secondary or advanced secondary effluent limitations. The
utilization of filtration in the design of the treatment facility normally
provides effective removal of suspended biological floc and neutral density
trash material which may remain in secondary clarifier effluent. Intermittent
filter operation is acceptable where on line controls monitor plant performance
or filters are not necessary to meet a specific discharge limitation.
(1) General requirements.
(A) Filter units shall be preceded by final
clarifiers designed in accordance with subsection (d) of this section for
secondary treatment criteria.
(B)
Filtered effluent, and not potable water, shall be utilized as the source of
backwash water.
(2) Deep
bed, intermittently backwashed granular media filters.
(A) Single media (sand filters), dual media
(anthracite-sand filters), or mixed media filter types (nonstratified
anthracite, sand, garnet, or other media) are acceptable for application;
however, single media filters shall be designed for maximum filtration runs of
six hours between backwash periods.
(B) Design filtration rates shall not exceed
three gpm/square foot for single media filters, four gpm/square foot for dual
media filters, and five gpm/square foot for mixed media filters. The filter
area required shall be calculated utilizing the previously listed specified
rates at the design flow of the facility. A minimum of two filter units shall
be provided with the required filter area calculated with one unit out of
service.
(C) Facilities to provide
periodic treatment utilizing chlorine or other suitable agents, introduced to
the influent stream of the filter units, shall be provided as an operational
technique to control slime growth on the filter surface and the backwash
storage basin.
(D) A graded gravel
layer of a minimum of 15 inches or variable thickness of other filter media
support material shall be provided over the filter underdrain system. Filter
media support material other than gravel will be reviewed on a case-by-case
basis. Normal media depths for the various filter types are as specified below.
Media depths significantly different than these must be justified to the
commission. The justification must include an analysis of the backwash rates.
The uniformity coefficient shall be 1.7 or less. The particle size distribution
for dual and mixed media filters shall result in a hydraulic grading of
material during backwash which will result in a filter bed with a pore space
graded progressively coarse to fine from the top of the media to the supporting
layer.
Attached
Graphic
(E) The
unit piping for the filter units shall be designed to return backwash waste to
upstream treatment units. In order to minimize a hydraulic surge, a backwash
tank must be included into the design for those plants that do not have some
means of flow equalization or surge control. A backwash tank shall be designed
to provide storage for filter backwash based upon the number of design daily
backwash cycles and the volume required for each backwash. Calculations must be
provided to the commission demonstrating that the performance of the plant will
not diminish with the discharging of the backwash water into the treatment
process. Enclosed backwash tanks shall be vented to maintain atmospheric
pressure. Surge control shall be provided to the backwash system to limit flow
rate variations to no more than 15% of the design flow of the treatment units
that will receive the backwash water. For these calculations, an influent lift
station is not considered as a treatment unit and, therefore, is not bound by
the 15% design flow requirement.
(F) Pumps for backwashing filter units shall
be designed to deliver the required rate with the largest pump out of service.
The backup pump unit may be uninstalled provided that the commission is
satisfied that the spare unit can be quickly installed and placed into
operation. Valve arrangement for isolating a filter unit for backwashing shall
provide ready access for the operator. Provision for manual override shall be
provided for any backwash system employing automatic control.
(G) Head loss indicators shall be provided
for all filter units.
(H) Backwash
for dual or mixed media filters shall provide a minimum bed expansion of 20%. A
surface scour shall be provided prior to or during the backwash cycle. Backwash
flow rates at 15 to 20 gpm/square foot and at a cycle time of 10 to 15 minutes
should be provided. The backwash cycle shall provide media fluidization at the
end of the cycle to restratify the media. Backwash for single media filters
should be provided by a surface air scour or combination air-water scour and
washwater at recommended rates as follows.
Attached
Graphic
(I) The
filter underdrain system shall be of a design adaptable to wastewater
treatment, providing a uniform distribution of filter backwash and freedom from
excessive orifice plugging. Wash water collection trough bottoms shall be
located a minimum of six inches above the maximum elevations of the expanded
media. A minimum freeboard of three inches shall be provided in addition to the
design upstream depth of the wash water media. A minimum freeboard of three
inches shall be provided in addition to the design upstream depth of the wash
water trough to prevent submerged trough conditions during filter
backwashing.
(3)
Multi-compartmented low head filters with continuous operation (automatic
backwash). This paragraph contains the design criteria for multi-compartmented
low head filters where the applicable criteria are different than those
contained in paragraphs (1) and (2) of this subsection. All other criteria
included in paragraphs (1) and (2) of this subsection will apply to
multi-compartmented low head filters with continuous operation.
(A) Filtration rates. Filtration rates shall
not exceed three gpm/square foot for single media filters and four gpm/square
foot for dual media filters based on the design flow rate applied to the
filters. The total filter area should be provided in two or more units and the
filtration rate shall be calculated on the total available filter area with one
cell of each unit out of service. Manufacturer's recommended rates should be
utilized if substantiated by test data.
(B) Backwash. The backwash rate shall be
adequate to fluidize and expand each media layer a minimum of 20%. Provision
should be made for an approximate rate of 10 gpm/square foot over a 30 to 60
second interval. Manufacturer's recommended rates should be utilized if
substantiated by test data. Pumps for backwashing filter units shall be
adequate to provide the required rate with the largest pump out of service. It
is permissible for the backup unit to be an uninstalled unit, provided that the
installed unit can be easily removed and replaced. Waste filter backwash water
shall be returned to upstream units, preferably the final clarifiers, for
treatment.
(C) Backwash surge
control. The rate of return of waste filter backwash water to treatment units
shall be controlled such that the rate does not exceed 15% of the design flow
of the treatment units. The hydraulic and organic load from waste backwash
water shall be considered in the overall design of the treatment plant. Where
waste backwash water is returned for treatment by pumping, adequate pumping
capacity shall be provided with the largest unit out of service. It is
permissible for the backup unit to be an uninstalled unit, provided that the
installed unit can be easily removed and replaced.
(4) Alternative design for effluent
polishing. Where filters are proposed to remove remaining visible particles,
other criteria will be considered on a case-by-case basis.