25 Pa. Code § 73.162 - Intermittent sand filters
(a) There
are two types of intermittent sand filters available for use with individual
residential spray irrigation systems. The standards for free access sand
filters and buried sand filters are included in this section.
(b) Free access sand filters shall meet the
following standards:
(1)
Filter. The filter shall be constructed in a tank meeting the
following specifications:
(i) The surface area
of a filter tank shall be a minimum of 40 square feet for systems using an
aerobic treatment tank and serving a single family residence of three bedrooms
or less. The filter area shall be increased by 10 square feet for each
additional bedroom over three.
(ii)
Systems proposing the use of a septic tank to serve a single family dwelling of
three bedrooms or less shall be designed using two filter tanks or a single
tank with two chambers. Each tank or chamber shall have a surface area of 40
square feet. The filter area of each filter shall be increased by 10 square
feet for each additional bedroom over three.
(iii) Tanks shall be watertight and made of a
sound, durable material which is not subject to excessive corrosion or
decay.
(iv) Concrete tanks shall
have a minimum wall thickness of 2 1/2 inches and be adequately
reinforced.
(v) If precast slabs
are used as tank tops to support the access covers, the slabs shall have a
thickness of at least 3 inches and be adequately reinforced.
(vi) Tanks shall be designed and constructed
so that the depth from the cover to the top of the sand layer provides
sufficient freeboard to allow for maintenance of the sand surface.
(vii) Access shall be provided by a minimum
of two access openings. These access openings shall be a minimum of 36 inches
by 36 inches and provide access to the entire surface of the filter.
(viii) The tank wall shall be extended a
minimum of 6 inches above final grade.
(ix) Access covers shall be insulated against
severe weather, secured by bolts or locking mechanisms, prevent water
infiltration and the entrance of debris, and be lightweight to facilitate
routine maintenance.
(2)
Media. Sand suppliers shall provide certification, in writing
to the sewage enforcement officer and permittee, with the first delivery to the
job site, that the sand to be supplied meets the following specifications:
(i) The fine aggregate shall have an
effective size of between 0.3 to 0.6 mm, a uniformity coefficient of less than
3.5 and less than 4% of the coarse aggregate passing the #100 sieve. The sieve
analysis shall be conducted in accordance with Department of Transportation PTM
#616 and the uniformity coefficient shall be determined by using Department of
Transportation PTM #149.
(ii) The
sand may not contain more than 15% by weight deleterious material as determined
by Department of Transportation PTM #510.
(3)
Contents of
certification. The written certification shall include the name of the
supplier, the testing results, the testing date, the amount of material
purchased under this certification and the delivery date.
(4)
Construction. The sand
filter shall be constructed according to the following standards:
(i) A 4-inch diameter perforated underdrain
pipe with a minimum 2,500 pound crush test specification shall be placed on the
bottom of the tank.
(ii) Two rows
of perforations between 1/2 to 3/4 inch in diameter shall be drilled in the
underdrain pipe at 6 inch intervals and the pipe shall be placed so the
perforations face downward and the rows are approximately 45° from each
other.
(iii) Aggregate shall be
placed around the underdrain to a total depth of 5 inches from the bottom of
the tank. Coarse aggregate used in the underdrains and distribution system
shall meet the Type B requirements posted in the Department of Transportation
specifications Publication #408, section 703, Table B and uniform size and
grading of the aggregate shall meet AASHTO No. 57 requirements, as described in
Form 408, section
703.2, Table C from a Department
of Transportation certified stockpile.
(iv) A minimum depth of 4 inches of aggregate
shall be placed over the aggregate underdrain material. Coarse aggregate used
in the transition layer shall meet the Type B requirements posted in the
Department of Transportation specifications Publication #408, section 703,
Table B. The size and grading shall meet AASHTO No. 8 requirements, as
described in Form 408, section
703.2, Table C from a Department
of Transportation certified stockpile.
(v) Sand shall be placed over the aggregate
to a depth of at least 24 inches.
(vi) The sand in the filter may not be
greater than 36 inches deep.
(vii)
The central distribution system shall be designed and installed to convey a
minimum 2 inch flood dose of effluent to the surface of the sand filter. A high
water alarm shall be installed in the filter tank which produces an audible and
visual alarm when effluent backs up on the filter surface to 12 inches above
the surface of the sand.
(viii)
When two filters or chambers are required to treat septic tank effluent, the
duplicate units shall, at the discretion of the designer, be flooded
alternately, periodically by using valves, or simultaneously.
(ix) The central distribution piping may not
be more than 2 inches in diameter.
(x) The height of the central distribution
system's effluent outlet above the sand surface shall allow for the
installation of a splash plate and the maximum flooding depth of the sand
filter.
(xi) A concrete splash
plate or other suitable material shall be located under each effluent outlet to
prevent scouring of the sand surface. Movement of the splash plate during the
flooding operation shall be prevented.
(c) Buried sand filters shall meet the
following standards:
(1)
Location.
(i) When buried
sand filters are proposed to be installed in areas where bedrock is encountered
above the proposed depth of the sand filter, or where the seasonal high
groundwater table rises above the proposed depth of the sand filter, the
designer should consider measures to prevent filter and liner damage and
groundwater infiltration.
(ii) A
buried sand filter may not be constructed in unstabilized fill.
(2)
Size.
(i) The size of the sand filter shall be
determined on the basis of the appropriate application rate and the estimated
daily sewage flow in accordance with §
73.16(a)
(relating to absorption area requirements) but the sand filter area shall be at
least 300 square feet for use with either an aerobic treatment tank or septic
tank with solids retainers units.
(ii) For a single family residence, the
minimum sand filter area shall be based on a maximum hydraulic loading of 1.15
square feet per gallon per day.
(iii) Where aerobic treatment precedes the
sand filter, a 1/3 reduction to the filter area may be used to size the
filter.
(3)
Media.
(i) At least 2 inches
of clean aggregate meeting subsection (b)(4)(iii) shall surround underdrains
and distribution pipes. A minimum of 4 inches of aggregate meeting subsection
(b)(4)(iv) shall be placed over the underdrain. A layer of porous geotextile
material may be placed on top of both layers of aggregate to prevent migration
of soil or sand into the aggregate.
(ii) At least 24 inches of clean sand shall
be placed over the underdrain aggregate. The sand shall meet the specifications
in §
73.55(c)
(relating to elevated sand mounds).
(iii) The minimum depth of earth cover over
the coarse aggregate in all installations shall be 12 inches. When the top of
the aggregate is less than 12 inches from the undisturbed soil surface, the
soil cover shall extend beyond the filter area by at least 3 feet on all sides.
The soil over the sand filter shall be so graded that surface water will run
off, consist of soil suitable for the growth of vegetation and be seeded to
control erosion.
(4)
Underdrain piping.
(i)
Underdrain piping shall be laid on a grade of 3 to 6 inches per 100 feet sloped
to the outfall pipe.
(ii)
Underdrain piping shall be positioned between the distribution laterals to
maximize effluent travel through the filter sand.
(iii) Underdrain piping holes shall be equal
or greater in number and size to the distribution piping holes.
(iv) Underdrain piping shall have two rows of
holes placed at approximately 45° angle from each other along the bottom
half of the pipe.
(v) The outfall
pipe from the underdrain header shall have an antiseep collar and bentonite
clay plug or a leak proof boot sealed as per manufacturer's instructions to the
subsurface sand filter liner.
(5)
Filter base and liner.
The base of the filter shall be sloped to the underdrain pipe a maximum of 1%.
An impervious liner of hyplon, polyvinyl chloride or polyethylene sheeting of
20 millimeter thickness or equal shall be installed on a tamped earth base to
prevent seepage to the groundwater. A concrete bottom and sides may also be
used at the discretion of the designer. A 2-inch layer of sand or a layer of 10
ounce porous geotextile material shall be provided on each side of the liner to
prevent punctures and tears. Seams shall be made according to manufacturer's
specifications.
Notes
The provisions of this § 73.162 issued under section 9 of the Pennsylvania Sewage Facilities Act (35 P. S. § 750.9); The Clean Streams Law (35 P. S. §§ 691.1-691.1001); and section 1920-A of The Administrative Code of 1929 (71 P. S. § 510-20).
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