| (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.
Cont'd... |
