| (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.
Cont'd... |
