§ 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.
(6) Distribution of effluent. Distribution of effluent to the buried sand filter shall meet the requirements of § § 73.44—73.46 (relating to pressurized distribution design; dosing tanks; and dosing pumps).
Authority 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).
Source The provisioins of this § 73.162 adopted November 7, 1997, effective November 8, 1997, 27 Pa.B. 5877.
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