| are based upon a side water depth of ten feet and shall be considered
(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).
(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
(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
(ii) Crushed rock, slag, and similar media shall be
washed and screened or forked to remove clays, organic material, and
(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
(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
(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).
(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
(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.