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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.
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