8.7.

(1) This Section is applicable to the construction of leaching beds.

(1) A leaching bed shall not be located,

(a) in an area that has an average slope that exceeds one unit vertically to four units horizontally,

(b) in soil or leaching bed fill having a percolation time of,

(i) less than one minute, or greater than 125 minutes if constructed as a shallow buried trench, or

(ii) less than one minute, or greater than 50 minutes for all other leaching beds, or

(c) in or on an area that is subject to flooding that may be expected to cause damage to the leaching bed or impair the operation of the leaching bed.

(2) A leaching bed shall not be covered with any material having a hydraulic conductivity less than 0.01 m/day.

(3) The surface of the leaching bed shall be shaped to shed water and together with the side slopes of any raised portion, shall be protected against erosion in such a manner as to not inhibit the evaporation and transpiration of waters from the soil or leaching bed fill, and to not cause plugging of the distribution pipe.

(4) No part of a leaching bed shall be sloped steeper than 1 unit vertically to 4 units horizontally.

(5) A leaching bed shall be designed to be protected from compaction or any stress or pressure that may result in,

(a) the impairment or destruction of any pipe in the leaching bed, or

(b) the soil or leaching bed fill.

(1) The total length of distribution piping shall,

(a) not be less than 30 m when constructed as a shallow buried trench, or

(b) not be less than 40 m for any other absorption trench.

(2) Except as provided in Sentences (1), (3), and (4) every leaching bed constructed by means of absorption trenches shall have a total length of distribution pipe not less than the value determined by the formula,

where,

L = total length of distribution pipe in metres

Q = the total daily design sanitary sewage flow in litres

T = the design percolation time

(3) Except as provided in Sentence (1), where the treatment unit is described in Article 8.6.2.2.v1, the leaching bed may have a total length of distribution pipe not less than the value determined by the formula,

where,

L = total length of distribution pipe in metres

Q = the total daily design sanitary sewage flow in litres

T = the design percolation time

(4) Except as provided in Sentence (1), where the leaching bed is constructed as a shallow buried trench, the total length of the distribution pipe shall not be less than the value determined by Table 8.7.3.1.

where,

Q = the total daily design sanitary sewage flow in litres, and

T = the design percolation time.

(1) Except as provided in Sentence (2), absorption trenches shall be,

(a) approximately the same length and not more than 30 m in length,

(b) not less than 500 mm and not more than 1 000 mm in width,

(c) not less than 300 mm and not more than 900 mm in depth,

(d) centred not less than 1 600 mm apart,

(e) located so that the bottom of the trench is not less than 900 mm above the high ground water table, rock or soil with a percolation time more than 50 minutes, and

(f) backfilled, after the installation of the distribution pipe with leaching bed fill, so as to ensure that after the leaching bed fill settles, the surface of the leaching bed will not form any depressions.

(2) Absorption trenches constructed as shallow buried trenches shall be,

(a) approximately the same length and not more than 30 m in length,

(b) not less than 300 mm and not more than 600 mm in width,

(c) not less than 300 mm and not more than 600 mm in depth,

(d) centred not less than 2 000 mm apart,

(e) not less than 900 mm at all points on the bottom of the absorption trench above the high ground water table or rock, and

(f) backfilled, after the installation of the distribution pipe with leaching bed fill, so as to ensure that after the leaching bed fill settles, the surface of the leaching bed will not form any depressions.

(1) Except for shallow buried trenches, the distribution pipe used in the construction of a leaching bed shall be,

(a) of not less than 3 in. trade size for gravity flow systems, or 1 in. trade size for pressurized systems,

(b) installed with a uniform downward slope from the inlet with a drop of not less than 30 mm and not more than 50 mm for each 10 m of distribution pipe, and

(c) installed within a layer of stone conforming to Sentence (5).

(2) Prior to backfilling, the stone layer required in Clause (1)(c) shall be protected in such a manner so as to prevent soil, or leaching bed fill from entering the stone by completely covering with,

(a) untreated building paper, or

(b) a permeable geo-textile fabric.

(3) Every pressurized distribution pipe shall be self-draining so as to prevent freezing of its contents.

(4) Every pressurized distribution pipe shall have orifices of at least 3 mm in diameter, spaced equally along the length of the pipe.

(5) The layer of stone required by Clause (1)(c) shall,

(a) be comprised of washed septic stone, free of fine material, with gradation conforming to Table 8.7.3.3.A.,

(b) be not less than 500 mm in width,

(c) extend not less than 150 mm below the distribution pipe, and

(d) extend not less than 50 mm above the distribution pipe.

(1) The area described in Sentence 8.7.4.2.(1) shall be designed such that the loading rate does not exceed, for soil having a percolation time set out in Column 1 of Table 8.7.4.1.A., the maximum value set out opposite it in Column 2 of Table 8.7.4.1.A.

(1) A leaching bed comprised of absorption trenches may be constructed in leaching bed fill if unsaturated soil or leaching bed fill complying with Clause 8.7.2.1.(1)(b) extends,

(a) to a depth of at least 250 mm over the area covered by the leaching bed fill, and

(b) for at least 15 m beyond the outer distribution pipes in any direction in which the effluent entering the soil or leaching bed fill will move horizontally.

(2) If the unsaturated soil or leaching bed fill described in Sentence (1) has a percolation time greater than 15 minutes, any leaching bed fill added to form the leaching bed shall have a percolation time not less than 75% of the percolation time of the unsaturated soil or leaching bed fill.

(3) Leaching bed fill that does not meet the requirements of Sentence (2) may be used to form the leaching bed if,

(a) the distance from the bottom of the absorption trench to native soil is not less than 900 mm, or

(b) where the distance from the bottom of the absorption trench to native soil is less than 900 mm, the percolation time of the least permeable soil or leaching bed fill within 900 mm from the bottom of the absorption trench is used to calculate the length of the distribution pipe under Article 8.7.3.1.

(4) Sentence (2) does not apply to any leaching bed fill added as backfill above the stone layer in which the distribution pipe is located.

(5) All leaching bed fill added shall be stabilized against erosion.

(6) The site to which the leaching bed fill is added shall be generally clear of vegetation.

(7) The leaching bed fill that is added shall be compacted in layers in such a manner as to avoid uneven settlement of the distribution pipes.

(8) Any distribution boxes, header lines, absorption trenches, or distribution pipes shall be installed only after the leaching bed fill has been compacted in accordance with Sentence (7).

(9) Except as provided in Sentence (10), the sides of the added leaching bed fill shall be sloped to ensure stability, but shall not be steeper than one unit vertically to four units horizontally.

(10) The side slope of the leaching bed fill may be increased up to one unit vertically to three units horizontally if measures are taken to prevent erosion and ensure stability of the leaching bed fill.

(11) The distances as set out in Column 2 of Table 8.2.1.6.B.v1 shall be increased by twice the height that the leaching bed is raised above the original grade.

(1) The total daily design sanitary sewage flow shall not exceed 5 000 L where the treatment unit is a septic tank, or 10 000 L where the treatment unit is described in Article 8.6.2.2.v1

(1) The effective area of the surface of the filter medium in each filter bed shall be at least 10 m² and not more than 50 m².

(2) The area described in Sentence 8.7.4.2.(1) shall be designed such that the loading rate does not exceed, for soil having a percolation time set out in Column 1 of Table 8.7.4.1.A., the maximum value set out opposite thereto in Column 2 of Table 8.7.4.1.A.

(3) Except as provided in Sentence (5), where the total daily design sanitary sewage flow does not exceed 3 000 L, the effective area shall be such that the loading on the surface of the filter medium does not exceed 75 L/m² per day.

(4) Except as provided in Sentence (5), where the total daily design sanitary sewage flow exceeds 3 000 L,

(a) the effective area shall be such that the loading on the surface of the filter medium does not exceed 50 L/m² per day, and

(b) the leaching bed shall be comprised of more than one filter bed, each of similar size and adjacent to each other.

(5) Where a treatment unit designed to produce effluent not exceeding the maximum concentrations stipulated in Column 2 of Table 8.6.2.2.A.v1 is used in conjunction with a filter bed, the effective area shall be such that the loading on the surface of the filter medium does not exceed 100 L/m² per day.

(1) Sentences 8.7.4.2.(1), (2) and (4) to (11) apply to the construction of a filter bed.

(2) The lines of distribution pipe shall be evenly spaced over the surface of the filter medium to which the sanitary sewage is applied.

(3) The filter medium shall have a minimum depth of 750 mm below the stone layer and shall be clean sand comprised of particles ranging in size between the limits of,

(a) an effective size of 0.25 mm with a uniformity coefficient not less than 3.5,

(b) an effective size of 2.5 mm with a uniformity coefficient not greater than 1.5, and

(c) having a uniformity coefficient not greater than 4.5.

(4) The filter medium shall be unsaturated for its entire depth.

(5) Where there is more than one filter bed in a leaching bed, the filter beds shall be separated by at least 5 m between the distribution pipes of the filter beds.

(6) The base of the filter medium shall extend to a thickness of at least 250 mm over an area meeting the requirements of the following formula:

where,

A = the area of contact in square metres between the base of the filter medium and the underlying soil,

Q = the total daily design sanitary sewage flow in litres, and

T = the lesser of 50 and the percolation time of the underlying soil.

(7) The stone layer required by Clause 8.7.3.3.(1)(b) shall be not less than 900 mm above the high ground water table, rock or soil with a percolation time more than 50 minutes,

(1) The design and installation of a shallow buried trench shall be carried out by a person competent in this field of work.

(1) The treatment unit shall provide an effluent quality as required in Sentence 8.6.2.2.(2)v1.

(2) The effluent shall be distributed through a pressurized distribution system having a pressure head of not less than 600 mm when measured to the most distant point from the pump.

(3) The pump chamber shall be sized to provide sufficient storage volume so that the effluent is evenly dosed on an hourly basis over a 24-hour period.

(4) A shallow buried trench shall not be constructed unless the soil or leaching bed fill is sufficiently dry to resist the compaction and smearing during excavation.

(5) Every chamber shall be as wide as the shallow buried trench in which it is contained, and the cross-sectional height of the chamber at its centre point shall not be less than half the width of the trench.

(6) Every chamber shall contain only one pressurized distribution pipe.