(1) The treatment unit used in conjunction with a leaching bed constructed as a Type A dispersal bed shall provide an effluent quality that does not exceed the maximum concentrations set out opposite a Level IV treatment unit in Columns 2 and 3 of Table 8.6.2.2.
(2) A Type A dispersal bed shall be backfilled 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.
(3) The combined thickness of the sand layer and the stone layer, if utilized, of a Type A dispersal bed shall not be less than 500 mm.
(4) Except as provided in Sentence (5), the sand layer shall,
(a) be comprised of sand that has,
(i) a percolation time of at least 6 and not more than 10 min, and
(ii) not more than 5% fines passing through a 0.074 mm (No. 200) sieve,
(b) have a minimum thickness of 300 mm, and
(c) have an area that is not less than the lesser of,
(i) the area of the stone layer determined in accordance with Sentence (6) or, if leaching chambers are used, the area over which the leaching chambers are spaced determined in accordance with Sentence (6.1), and
(ii) the value determined by the formula,
A=QT/850
where,
A =the area of contact in square metres between the base of the sand 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.
(5) Where the underlying soil has a percolation time of more than 15 min, the sand layer referred to in Sentence (4) shall,
(a) extend to at least 15 m beyond the perimeter of the treatment unit, or the centrelines of the outer distribution pipes or leaching chambers if utilized, in any direction in which the effluent entering the soil or leaching bed fill will move horizontally, and
(b) have an area that is not less than the value determined by the formula,
A=QT/400
where,
A =the area of contact in square metres between the base of the sand and the underlying soil, or leaching bed fill if utilized,
Q =the total daily design sanitary sewage flow in litres, and
T =the lesser of 50 and the percolation time of the underlying soil.
(6) Where a stone layer is used, the stone layer shall,
(a) be rectangular in shape with the long dimension parallel to the site contours,
(b) have a minimum thickness of 200 mm,
(c) be protected in the manner described in Sentence 8.7.3.3. (2), and
(d) be constructed such that the bottom of the stone layer is at least 600 mm above the high ground water table, rock or soil with a percolation time of 1 min or less or greater than 50 min.
(e) have a minimum area not less than the value determined by the formula,
A = Q/B
where,
A =the area of the stone layer in square metres,
B =the following amount,
i. 50, if the total daily design sanitary sewage flow exceeds 3 000 litres, or
ii. 75, if the total daily design sanitary sewage flow does not exceed 3 000 litres, and
Q =the total daily design sanitary sewage flow in litres.
(6.1) Where leaching chambers are used,
(a) the Type A dispersal bed shall be rectangular in shape with the long dimension parallel to the site contours, and
(b) the leaching chambers shall,
(i) be evenly spaced over the area calculated in Subclause (iv), with a maximum distance of 200 mm between the exterior edges of the lines of leaching chamber,
(ii) be protected in the manner described in Clause 8.7.3.4. (1) (f),
(iii) be constructed such that the bottom of the leaching chambers is at least 600 mm above the high ground water table, rock or soil with a percolation time of 1 min or less or greater than 50 min, and
(iv) have a minimum area not less than the value determined by the formula,
A = Q/B
where,
A =the area over which the leaching chambers are spaced, in square metres,
B =the following amount,
i. 50, if the total daily design sanitary sewage flow exceeds 3 000 litres, or
ii. 75, if the total daily design sanitary sewage flow does not exceed 3 000 litres, and
Q =the total daily design sanitary sewage flow in litres.
(7) Leaching bed fill with a percolation time not exceeding 15 min may be used to satisfy the vertical separation requirements of Clause (6) (d) or Subclause (6.1)(b) (iii), provided that the leaching bed fill conforms to the requirements specified in Sentence (5) regardless of the percolation time of the underlying soil.
(8) Where a stone layer is used, the effluent shall be evenly distributed within the stone layer to within 600 mm of the perimeter of the stone layer.
(8.1) Where leaching chambers are used, the effluent shall be evenly distributed within the area over which the leaching chambers are spaced to within 600 mm of the perimeter of that area.
(9) The stone layer or area over which the leaching chambers are spaced shall not be located closer than the minimum horizontal distances set out in Table 8.2.1.6.B. and these distances shall be increased when required by Sentence 8.7.4.2. (11).