Leaching beds: BCIN On-site Sewage Systems Study Guide

Last updated October 22, 2026

Values this chapter turns on

trench pipe length for septic tank effluent: QT/200; trench pipe length for septic tank effluent
minimum distribution pipe for absorption trenches: 40 m; minimum distribution pipe for absorption trenches
max percolation time for most leaching beds: 50 min; max percolation time for most leaching beds
max daily flow to a filter bed with a septic tank: 5,000 L; max daily flow to a filter bed with a septic tank

Section 8.7. is the biggest part of the exam and the one with the most arithmetic. It covers six kinds of leaching bed: conventional absorption trenches, fill based absorption trenches, filter beds, shallow buried trenches, and Type A and Type B dispersal beds. Every one of them is sized from two numbers: the daily design flow Q (Chapter 6) and the percolation time T. Learn which formula goes with which bed and which "T" each formula uses.

Percolation time and SB-6

Percolation time is the average time in minutes for water to drop one centimetre (def.). If you do not run percolation tests, you can estimate T from the soil type using Supplementary Standard SB-6 (OBC Div. B 8.2.1.2.(2)).

SB-6 Table 2 (coarse grained soils) and Table 3 (fine grained soils) give each Unified Soil Classification group a range of permeability (K) and a range of T, with a comment (SB-6 Table 2, SB-6 Table 3). Read the row for the soil symbol. For example, SM (silty sands) shows T of 8 to 20 min, and the clean gravels GW and GP show T under 1 and are marked unacceptable. Pick within a range from the soil's characteristics: more clay, density, plasticity or organics push T up (SB-6 note, SB-6 note).

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General requirements for every leaching bed

A leaching bed may not be located (OBC Div. B 8.7.2.1.):

on an average slope steeper than 1 vertical to 4 horizontal,
in soil or fill with T under 1 min, or over 50 min (over 125 min for a shallow buried trench),
where flooding may damage it or impair its operation.

Also: no cover with hydraulic conductivity under 0.01 m/day, a surface that sheds water, and protection from compaction.

Pipes must be detectable, for example with 14 gauge TW solid copper tracer wire (OBC Div. B 8.7.2.2.). Leaching chambers may be used in absorption trenches, shallow buried trenches, filter beds and Type A beds (not Type B), must meet IAPMO PS 63, and come as Type I or the wider Type II (OBC Div. B 8.7.2.3., Table 8.7.2.3.).

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Absorption trenches: length

Bed	Pipe or chamber length
Conventional trench, septic tank effluent	L = QT/200, minimum 40 m (8.7.3.1.)
Conventional trench, Level II, III or IV effluent	L = QT/300, minimum 40 m
Type I leaching chamber	L = QT/200; Type II L = QT/300 (8.7.3.2.)
Leaching chamber with an 8.6.2.2. treatment unit	L = QT/300
Shallow buried trench	from Table 8.7.3.1. or 8.7.3.2.: Q divided by a number that depends on the T band; minimum 30 m

Worked example: conventional bed

Q = 1,600 L, T = 15 min, septic tank. L = 1,600 x 15 / 200 = 120 m. That is under 150 m, so no pump is required (Chapter 8). With trenches limited to 30 m each, use at least 4 trenches of 30 m.

With a Level III unit instead: L = 1,600 x 15 / 300 = 80 m.

Small numbers: Q = 1,100 L, T = 6 min gives 33 m, which is below the minimum, so use 40 m.

Worked example: shallow buried trench

Q = 1,600 L, T = 30 min. T falls in the band over 20 up to 50, where the table divides Q by 50: 1,600 / 50 = 32 m, above the 30 m minimum.

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Absorption trench construction

OBC Div. B 8.7.3.3. sets dimensions:

Item	Conventional trench	Shallow buried trench
Length	about equal, not over 30 m	about equal, not over 30 m
Width	500 to 1,000 mm	300 to 600 mm
Depth	600 to 900 mm	300 to 600 mm
Centre spacing	at least 1,600 mm (2,400 mm for Type II chambers not in fill)	at least 2,000 mm
Separation below	900 mm to high ground water table, rock or soil with T over 50 min	900 mm to high ground water table or rock

Gravity distribution pipe is at least 3 in., sloped down 30 to 50 mm per 10 m, in washed septic stone (Table 8.7.3.4.) at least 500 mm wide, 150 mm below and 50 mm above the pipe, covered with untreated building paper or geotextile (OBC Div. B 8.7.3.4.). Leaching chambers go in level, uncut and full length, with end caps and an inlet splash plate (OBC Div. B 8.7.3.5.).

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Fill based absorption trenches

Trenches may be built in leaching bed fill if unsaturated soil or fill (T from 1 to 50 min) extends at least 250 mm deep over the fill area and at least 15 m beyond the outer pipe centre lines in the direction effluent moves (OBC Div. B 8.7.4.2.). That area is limited by the loading rates in Table 8.7.4.1., read by the T of the underlying soil (OBC Div. B 8.7.4.1.). As a reading example, soil with T over 20 up to 35 min allows 8 L/m² per day.

Other fill rules: where the native soil has T over 15 min, added fill must have T at least 75% of the native soil's T; fill is compacted in layers before pipes go in; side slopes no steeper than 1 in 4 (1 in 3 with erosion and stability measures); and clearances increase by twice the raise height.

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Filter beds

Filter beds are limited to Q of 5,000 L with a septic tank, or 10,000 L with a Level II to IV unit (OBC Div. B 8.7.5.1.). Each filter bed's effective area is 10 to 50 m² (OBC Div. B 8.7.5.2.).

Situation	Max loading on filter medium
Q up to 3,000 L	75 L/m² per day
Q over 3,000 L	50 L/m² per day, and more than one filter bed
Level II, III or IV effluent	100 L/m² per day

Construction (OBC Div. B 8.7.5.3.): clean sand filter medium at least 750 mm deep, unsaturated; pipe spacing per Table 8.7.5.3.; beds 5 m apart. The base extends at least 250 mm thick over an area A = QT/850, where T is the lesser of 50 and the underlying soil's T.

Worked example: filter bed

Q = 2,400 L, septic tank, underlying T = 30 min. Filter medium area = 2,400 / 75 = 32 m² (one bed, within 10 to 50 m²). Base contact area = 2,400 x 30 / 850 = 84.7 m². If T were 70 min, use 50: 2,400 x 50 / 850 = 141.2 m².

Q = 4,000 L: loading drops to 50, so 4,000 / 50 = 80 m², split into at least two similar beds, each 50 m² or less.

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Shallow buried trenches

Shallow buried trenches and Type A and B beds must be designed and installed by a competent person (OBC Div. B 8.7.1.2.). Shallow buried trenches need Level IV effluent, pressurized distribution with at least 600 mm head at the farthest point, hourly dosing over 24 hours, dry soil when dug, and one pressurized pipe per chamber (OBC Div. B 8.7.6.1.).

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Type A dispersal beds

Level IV effluent over an unsaturated sand layer (OBC Div. B 8.7.7.1.). Key numbers: sand plus stone at least 500 mm; sand T of 6 to 10 min with not more than 5% fines, at least 300 mm thick; stone layer at least 200 mm thick, rectangular along the contours, bottom at least 600 mm above high ground water, rock, or soil with T of 1 or less or over 50. Stone area A = Q/B, with B = 75 where Q is 3,000 L or less and 50 where Q exceeds 3,000 L. Where the underlying soil's T exceeds 15 min, extend the sand area with fill of T 15 or less, 15 m downslope and over an area A = QT/400 (T is the lesser of 50 and the soil's T).

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Type B dispersal beds

Level IV effluent through pressurized pipes in septic stone (OBC Div. B 8.7.8.2.), on an average slope not over 1 in 7 (OBC Div. B 8.7.8.1.). Stone bottom 600 mm above high ground water, rock or soil with T over 50; head at least 600 mm; pipes at most 1.2 m apart. Area A = QT/400 (or the BC manual method); linear loading at most 40 L/m where T is 24 min or more, 50 L/m where under 24; width not over 4 m (OBC Div. B 8.7.8.3.).

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Common traps

Septic tank effluent: QT/200. Treated (Level II to IV): QT/300. Shallow buried trench: table, Q only.
Minimums: 40 m for absorption trenches, 30 m for shallow buried trenches.
Shallow buried trenches allow T up to 125 min; other beds stop at 50 min.
QT/850 and QT/400 for Type A cap T at 50; Type B's QT/400 uses the soil's actual T.
Leaching beds: average slope 1 in 4; Type B: 1 in 7.

Chapter review

Chapter at a glance

One line per section: the idea to carry into the exam. Then the answers that most often catch candidates out.

Chapter at a glance: the key idea in each section
Section	Key idea
Percolation time and SB-6	Percolation time is the average time in minutes for water to drop one centimetre.
General requirements for every leaching bed	Also: no cover with hydraulic conductivity under 0.01 m/day, a surface that sheds water, and protection from compaction.
Absorption trenches: length	Q = 1,600 L, T = 15 min, septic tank.
Absorption trench construction	Gravity distribution pipe is at least 3 in., sloped down 30 to 50 mm per 10 m, in washed septic stone at least 500 mm wide, 150 mm below and 50 mm above the pipe, covered with…
Fill based absorption trenches	Trenches may be built in leaching bed fill if unsaturated soil or fill (T from 1 to 50 min) extends at least 250 mm deep over the fill area and at least 15 m beyond the outer pipe…
Filter beds	Filter beds are limited to Q of 5,000 L with a septic tank, or 10,000 L with a Level II to IV unit.
Shallow buried trenches	Shallow buried trenches and Type A and B beds must be designed and installed by a competent person.
Type A dispersal beds	Level IV effluent over an unsaturated sand layer.
Type B dispersal beds	Level IV effluent through pressurized pipes in septic stone, on an average slope not over 1 in 7. Stone bottom 600 mm above high ground water, rock or soil with T over 50; head at…

Percolation time and SB-6
Percolation time is the average time in minutes for water to drop one centimetre.
General requirements for every leaching bed
Also: no cover with hydraulic conductivity under 0.01 m/day, a surface that sheds water, and protection from compaction.
Absorption trenches: length
Q = 1,600 L, T = 15 min, septic tank.
Absorption trench construction
Gravity distribution pipe is at least 3 in., sloped down 30 to 50 mm per 10 m, in washed septic stone at least 500 mm wide, 150 mm below and 50 mm above the pipe, covered with…
Fill based absorption trenches
Trenches may be built in leaching bed fill if unsaturated soil or fill (T from 1 to 50 min) extends at least 250 mm deep over the fill area and at least 15 m beyond the outer pipe…
Filter beds
Filter beds are limited to Q of 5,000 L with a septic tank, or 10,000 L with a Level II to IV unit.
Shallow buried trenches
Shallow buried trenches and Type A and B beds must be designed and installed by a competent person.
Type A dispersal beds
Level IV effluent over an unsaturated sand layer.
Type B dispersal beds
Level IV effluent through pressurized pipes in septic stone, on an average slope not over 1 in 7. Stone bottom 600 mm above high ground water, rock or soil with T over 50; head at…

Usual exam traps

Septic tank effluent: QT/200. Treated (Level II to IV): QT/300. Shallow buried trench: table, Q only.
Minimums: 40 m for absorption trenches, 30 m for shallow buried trenches.
Shallow buried trenches allow T up to 125 min; other beds stop at 50 min.
QT/850 and QT/400 for Type A cap T at 50; Type B's QT/400 uses the soil's actual T.
Leaching beds: average slope 1 in 4; Type B: 1 in 7.

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Questions on Leaching beds (8.7), in the exam format, every answer explained and cited to its article. Sign in to start; a free account gets the free questions.

Practice this chapter On-site Sewage Systems calculations

Code cited in this chapter

Based on the official 2024 Building Code Compendium on ontario.ca.

OBC Div. A 1.4.1.2.(1) Percolation time
Percolation time means the average time in minutes that is required for water to drop one centimetre during a percolation test or as determined by a soil evaluation or analysis.
Open OBC Div. A 1.4.1.2.(1) in the code viewer
OBC Div. B 8.2.1.2. Site Evaluation
(1)A site evaluation shall be conducted on every site where a new or replacement sewage system is to be installed. (See Note A-8.2.1.2.(1))
(2)The percolation time shall be determined by
(a)conducting percolation tests, or
(b)classifying the soil according to one of the following methods,
(i)the Unified Soil Classification System as described in MMAH Supplementary Standard SB-6, "Percolation Time and Soil Descriptions," or
(ii)the Soil Texture Classification as described in Chapter 3 of USDA, "Soil Survey Manual." (See Note A-8.2.1.2.(2))
(3)Where the percolation time is determined by a percolation test, there shall be a minimum of 3 locations selected, suitably spaced to accurately evaluate the leaching bed area, with the highest percolation time of the tests being used. (See Note A-8.2.1.2.(3))
Open OBC Div. B 8.2.1.2. in the code viewer
OBC SB-6 Table 2. Approximate Relationship of Coarse Grained Soil Types to Permeability and Percolation Time
Soil Type (Unified Soil Classification): Coarse Grained, More than 50% Larger than #200 | Coefficient of Permeability, K — cm/sec | Percolation Time, T — mins/cm | Comment G.W. — Well graded gravels, gravel-sand mixtures, little or no fines. | 10⁻¹ | <1 | very permeable unacceptable G.P. — Poorly graded gravels, gravel-sand mixtures, little or no fines. | 10⁻¹ | <1 | very permeable unacceptable G.M. — Silty gravels, gravel-sand-silt mixtures. | 10⁻²–10⁻⁴ | 4–12 | Permeable to medium permeable depending on amount of silt. G.C. — Clayey gravels, gravel-sand-clay mixtures. | 10⁻⁴–10⁻⁶ | 12–50 | Important to estimate amount of silt and clay S.W. — Well graded sands, gravelly sands, little or no fines. | 10⁻¹–10⁻⁴ | 2–12 | medium permeability S.P. — Poorly graded sands, gravelly sand, little or no fines. | 10⁻¹–10⁻³ | 2–8 | medium permeability S.M. — Silty sands, sand-silt mixtures. | 10⁻³–10⁻⁵ | 8–20 | medium to low permeability S.C. — Clayey sands, sand-clay mixtures. | 10⁻⁴–10⁻⁶ | 12–50 | medium to low permeability depending on amount of clay Column 1 | 2 | 3 | 4
Open OBC SB-6 Table 2. in the code viewer
OBC SB-6 Table 3. Approximate Relationship of Fine Grained Soil Types to Permeability and Percolation Time
Soil Type (Unified Soil Classification): Fine Grained, More than 50% Passing #200 | Coefficient of Permeability, K — cm/sec | Percolation Time, T — mins/cm | Comment M.L. — Inorganic silts and very fine sands, rock flour, silty or clayey fine sands, clayey silts with slight plasticity | 10⁻⁵–10⁻⁶ | 20–50 | medium to low permeability C.L. — Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays | 10⁻⁶ and less | over 50 | unacceptable O.L. — Organic silts, organic silty clays of low plasticity; liquid limit less than 50 | 10⁻⁵ and less | 20–over 50 | acceptable depends on clay content M.H. — Inorganic silts, micareaous or diatomageous fine sandy or silty soils, elastic silts | 10⁻⁶ and less | over 50 | unacceptable C.H. — Inorganic clays of medium to high plasticity, organic silts | 10⁻⁷ and less | over 50 | unacceptable O.H. — Organic clays of medium to high plasticity, organic silt; liquid limit over 50 | 10⁻⁶ and less | over 50 | unacceptable Column 1 | 2 | 3 | 4
Open OBC SB-6 Table 3. in the code viewer
OBC SB-6 Selection of "T" Time From The Above Tabulation
A range of “T” times for each soil type is shown above. Select from within this range by determining if the soil is within the low, middle or high part of the range considering the soil identifiers and soil characteristics. Consider structure, density, colour, prevalence or organics, the clay content and mineralogy, the plasticity index and liquid limit and the functioning of existing systems in similar soils in the area.
Notes:
The following Ministry of the Environment Reports provide further information on the relationship between grain size, coefficient of permeability and percolation time.
(1)Study on the Feasibility of Correlating Percolation Time with Laboratory Permeability - 1975 - Research Report No. S56 by H. T. Chan, PhD., P.Eng.
(2)Study of Conventional Tile Fields in Fine-Grained Soils - 1979 Research Report 74 by H. T. Chan, PhD., P.Eng.
Open OBC SB-6 in the code viewer
OBC SB-6 Estimation of Percolation Time
(a)The purpose of this Section and the associated Tables and Charts is to provide assistance to those who must decide on the percolation time(s) to be used in design. Suggested relationships between percolation time, coefficient of permeability and soils of various types are given. it must be emphasized that, particularly for fine grained soils, there is no consistent relationship due to the many factors involved. The following guidance is presented for the soil types outlined in the Unified Soil Classification System (Table 1). In order to assess a particular soil.
(i)Table 2 and Table 3 - Approximate relationship of soil types to permeability and percolation time.
(ii)Charts 1 to 14 - Typical grain size distribution curves for soil types in the Unified Soil Classification System.
(b)In Table 2 and Table 3, a range of values of “K” and of “T” are given for various soil descriptions. The principal modifiers which will influence selection of a “T” value within the range given are:
(i)The structure - “massive” fine-grained soils have high values of “T”.
(ii)The density - For a given soil higher density produces a higher value of “T”.
(iii)The percentage of clay - the higher the percentage the higher the value of “T”.
(iv)The mineralogy of the clay portion - The more it “swells” the higher the value of “T”.
(v)The plasticity of the soil - The higher the plasticity index the higher the value of “T”.
(vi)Liquid Limit - the higher the liquid limit the higher the value of “T”.
(vii)Organic content - The presence of fine organic particles, detectable by colouration and odour, can significantly reduce the permeability and raise the value of “T”.
Open OBC SB-6 in the code viewer
OBC Div. B 8.7.2.1. General Requirements
(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 1 min, or greater than 125 min if constructed as a shallow buried trench, or
(ii)less than 1 min, or greater than 50 min 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 smearing of the soil or leaching bed fill.
Open OBC Div. B 8.7.2.1. in the code viewer
OBC Div. B 8.7.2.2. Distribution Pipes within Leaching Beds
(1)Sentence (2) applies to the design and construction of a leaching bed with distribution pipes used within the leaching bed.
(2)The header line and distribution pipes within a leaching bed shall be designed and constructed so that they can be detected by
(a)magnetic means,
(b)means of a 14 gauge TW solid copper light coloured plastic coated tracer wire, or
(c)other means of subsurface detection.
Open OBC Div. B 8.7.2.2. in the code viewer

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OBC Div. B 8.7.2.3. Leaching Chambers within Leaching Beds
(1)Leaching chambers are permitted for use in conjunction with an absorption trench, shallow buried trench, filter bed or Type A dispersal bed.
(2)Leaching chambers shall comply with the dimension requirements for either a Type I or Type II leaching chamber listed in Table 8.7.2.3.
(3)Leaching chambers shall conform to the requirements of IAPMO PS 63, "Plastic Leaching Chambers".
(4)The header line and leaching chambers within a Leaching bed shall be designed and constructed so that they can be detected by
(a)magnetic means,
(b)means of a 14 gauge TW solid copper light coloured plastic coated tracer wire,
(c)means of a 12 gauge copper clad steel light coloured plastic coated tracer wire, or
(d)other means of subsurface detection.
Open OBC Div. B 8.7.2.3. in the code viewer
OBC Div. B Table 8.7.2.3. Leaching Chamber Dimensions Forming Part of Sentence 8.7.2.3.(2)
Table 8.7.2.3. Leaching Chamber Dimensions Forming Part of Sentence 8.7.2.3.(2) Type of Leaching Chamber | Width, mm | Height, mm Type I | 380 to 410 | 280 to 305 Type II | 555 to 575 | 300 to 320
Open OBC Div. B Table 8.7.2.3. in the code viewer
OBC Div. B 8.7.3.1. Length of Distribution Pipe
(1)The total length of distribution pipe 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,
L = QT/200
where,
L = total length of distribution pipe in metres,
Q = the total daily design sanitary sewage flow in litres, and
T = the design percolation time.
(3)Except as provided in Sentence (1), where a leaching bed receives effluent from a Level II, Level III or Level IV treatment unit as described in Table 8.6.2.2., the leaching bed may have a total length of distribution pipe not less than the value determined by the formula,
L = QT/300
where,
L = total length of distribution pipe in metres,
Q = the total daily design sanitary sewage flow in litres, and
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.
Open OBC Div. B 8.7.3.1. in the code viewer
OBC Div. B 8.7.3.2. Length of Leaching Chamber
(1)The total length of leaching chamber 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) and (3), the total length of leaching chamber shall not be less than the value determined by the formula,
L = QT/200, for a Type I leaching chamber, or
L = QT/300, for a Type II leaching chamber,
where,
L = total length of leaching chamber in metres,
Q = the total daily design sanitary sewage flow in litres, and
T = the design percolation time.
(3)When a treatment unit described in Article 8.6.2.2. is used in conjunction with a leaching chamber, the total length of leaching chamber shall not be less than the value determined by the formula,
L = QT/300
where,
L = total length of leaching chamber in metres,
Q = the total daily design sanitary sewage flow in litres, and
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 leaching chamber shall not be less than the value determined by Table 8.7.3.2.
Open OBC Div. B 8.7.3.2. in the code viewer
OBC Div. B Table 8.7.3.1. Length of Distribution Pipe in Shallow Buried Trench Forming Part of Sentence 8.7.3.1.(4)
Table 8.7.3.1. Length of Distribution Pipe in Shallow Buried Trench Forming Part of Sentence 8.7.3.1.(4) Percolation Time (T) of Soil, min | Length of Distribution Pipe, m 1 < T ≤ 20 | Q/75 20 < T ≤ 50 | Q/50 50 < T < 125 | Q/30 where, Q = the total daily design sanitary sewage flow in litres, and T = the design percolation time.
Open OBC Div. B Table 8.7.3.1. in the code viewer
OBC Div. B Table 8.7.3.2. Length of Leaching Chamber in Shallow Buried Trench Forming Part of Sentence 8.7.3.2.(4)
Table 8.7.3.2. Length of Leaching Chamber in Shallow Buried Trench Forming Part of Sentence 8.7.3.2.(4) Percolation Time, (T) of Soil, min | Length of Leaching Chamber, m 1 < T ≤ 20 | Q/75 20 < T ≤ 50 | Q/50 50 < T < 125 | Q/30 where, Q = the total daily design sanitary sewage flow in litres, and T = the design percolation time.
Open OBC Div. B Table 8.7.3.2. in the code viewer
OBC Div. B 8.7.3.3. Absorption Trenches
(1)Except as provided in Sentence (2), absoprtion 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 600 mm and not more than 900 mm in depth,
(d)centred not less than,
(i)1,600 mm apart where used in conjunction with
(a) a distribution pipe,
(b) a Type I leaching chamber, or
(c) a Type II leaching chamber constructed in leaching bed fill, or
(ii)2,400 mm apart where used in conjunction with a Type II leaching chamber, other than a Type II leaching chamber constructed in leaching bed fill,
(e)located so that the bottom of the absorption trench is not less than 900 mm above the high ground water table, rock or soil with a percolation time of more than 50 min, and
(f)backfilled, after the installation of the distribution pipe or leaching chamber 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 a shallow buried trench 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.
Open OBC Div. B 8.7.3.3. in the code viewer
OBC Div. B Table 8.7.3.4. Gradation of Septic Stone — Forming Part of Sentences 8.7.3.4.(5) and 8.7.8.2.(6)
Table 8.7.3.4. Gradation of Septic Stone — Forming Part of Sentences 8.7.3.4.(5) and 8.7.8.2.(6) Particle Size | Percent Passing 53 mm | 100 19 mm | 0 - 5 75 µm | 0 - 1
Open OBC Div. B Table 8.7.3.4. in the code viewer
OBC Div. B 8.7.3.4. Distribution Pipe
(1)Except for a shallow buried trench, the distribution pipe used in the construction of a leaching bed shall be
(a)not less than 3 in. trade size for gravity flow 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 for gravity flow systems, and
(c)installed within a layer of stone conforming to Sentence (5).
(2)Prior to backfilling, the stone layer required by 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 it 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,
(a)be not less than 1 in. trade size, and
(b)have orifices of at least 3 mm in diameter, spaced equally along the length of the pipe to provide even distribution of the effluent.
(5)The stone layer required by Clause (1)(c) shall
(a)be comprised of washed septic stone, free of fine material, with gradation conforming to Table 8.7.3.4.,
(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.
(6)This Article does not apply to a distribution pipe within a leaching chamber.
Open OBC Div. B 8.7.3.4. in the code viewer
OBC Div. B 8.7.3.5. Leaching Chamber
(1)Prior to backfilling, the leaching chamber shall be
(a)installed level over the length of the absorption trench,
(b)securely connected together, section to section,
(c)free of structural damage, uncut and used full length,
(d)equipped with end caps installed on both ends,
(e)equipped with an integrated splash plate at the inlet end of each line of leaching chamber, to prevent soil scouring, and
(f)protected in such a manner so as to prevent soil or leaching bed fill from entering the leaching chamber.
(2)Except for a shallow buried trench, the distribution pipe within a leaching chamber shall be not less than 3 in. trade size for dosed systems.
(3)Except for a shallow buried trench, every pressurized distribution pipe within a leaching chamber shall
(a)extend over the entire length of the leaching chamber,
(b)be not less than 1½ in. trade size,
(c)have orifices of at least 6 mm in diameter, spaced equally along the length of the pipe to provide even distribution of the effluent,
(d)be supported,
(e)be self-draining so as to prevent freezing of its contents, and
(f)have cleanouts installed at the downstream end of each line of leaching chamber to allow for servicing of the system.
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OBC Div. B 8.7.4.2. Construction Requirements
(1)Except for a shallow buried trench, a leaching bed comprised of absorption trenches may be constructed in leaching bed fill, if unsaturated soil or leaching bed fill complying with Subclause 8.7.2.1.(1)(b)(ii) 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 centre lines of the outer distribution pipes or leaching chambers 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 min, any additional leaching bed fill added to it 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 to which it is added.
(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 the underlying soil is not less than 900 mm, or
(b)where the distance from the bottom of the absorption trench to the underlying 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. or the length of the leaching chamber under Article 8.7.3.2.
(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 or leaching chambers.
(8)Any distribution boxes, header lines, absorption trenches, distribution pipes or leaching chambers 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 set out in Column 2 of Table 8.2.1.6.B. shall be increased by twice the height that the leaching bed is raised above the original grade.
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OBC Div. B Table 8.7.4.1. Loading Rates for Fill Based Absorption Trenches and Filter Beds Forming Part of Sentences 8.7.4.1.(1) and 8.7.5.2.(2)
Table 8.7.4.1. — Loading Rates for Fill Based Absorption Trenches and Filter Beds Forming Part of Sentences 8.7.4.1.(1) and 8.7.5.2.(2) Percolation Time (T) of Soil, min | Loading Rates, (L/m²)/day 1 < T ≤ 20 | 10 20 < T ≤ 35 | 8 35 < T ≤ 50 | 6 T > 50 | 4 Column 1 | 2 Column 1 | 2
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OBC Div. B 8.7.4.1. Loading Requirements
(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., the maximum value set out opposite it in Column 2 of Table 8.7.4.1.
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OBC Div. B 8.7.5.1. Application
(1)The total daily design sanitary sewage flow shall not exceed
(a)5,000 L where the treatment unit is a septic tank, or
(b)10,000 L where the treatment unit is a Level II, Level III or Level IV treatment unit as described in Table 8.6.2.2.
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OBC Div. B 8.7.5.2. Loading Requirements
(1)The effective area of the surface of the filter medium in each filter bed shall be at least 10 m^2 and not more than 50 m^2 .
(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., the maximum value set out opposite it in Column 2 of Table 8.7.4.1.
(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 Level II, Level III or Level IV treatment unit as described in Table 8.6.2.2. 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/m^2 per day.
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OBC Div. B 8.7.5.3. Construction Requirements
(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 pipes or leaching chambers shall be evenly spaced over the surface of the filter medium to which the sanitary sewage is applied, with the outer most distribution pipe or leaching chamber not more than 600 mm from the perimeter of that area with a maximum spacing between the centre lines of the distribution pipes or leaching chambers in accordance with Table 8.7.5.3.
(3)The filter medium shall have a minimum depth of 750 mm below the stone layer or bottom of the leaching chambers 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 or leaching chambers 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:
A = QT/850
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.
(See Note A-8.7.5.3.(6) and (7))
(7)The stone layer or bottom of the leaching chambers shall be not less than 900 mm above the high ground water table, rock or soil with a percolation time of more than 50 min. (See Note A-8.7.5.3.(6) and (7))
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OBC Div. B Table 8.7.5.3. Maximum Spacing Between Centre Lines of Distribution Pipes or Leaching Chambers Forming Part of Sentence 8.7.5.3.(2)
Table 8.7.5.3. Maximum Spacing Between Centre Lines of Distribution Pipes or Leaching Chambers Forming Part of Sentence 8.7.5.3.(2) Distribution Method | Maximum Spacing Between Centre Lines, mm Distribution pipes | 1,200 Type I leaching chambers | 900 Type II leaching chambers | 1,000
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OBC Div. B 8.7.1.2. Limitation on Installation
(1)The design and installation of a shallow buried trench, Type A dispersal bed or Type B dispersal bed shall be carried out by a person competent in this field of work.
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OBC Div. B 8.7.6.1. Construction Requirements
(1)The treatment unit used in conjunction with a leaching bed constructed as a shallow buried trench 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)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 compaction and smearing during excavation.
(5)Every chamber or leaching chamber shall be as wide as the shallow buried trench in which it is contained, and the cross-sectional height of the chamber or leaching chamber at its centre point shall not be less than half the width of the trench.
(6)Every chamber or leaching chambers shall contain only one pressurized distribution pipe.
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OBC Div. B 8.7.7.1. Construction Requirements
(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 (7), 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 be extended using unsaturated soil or leaching bed fill having a percolation time of not more than 15 min and a depth of at least 300 mm
(a)to at least 15 m beyond the perimeter of the treatment unit, or the centre lines 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)over 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 combined sand layer and extension 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.
(See Note A-8.7.7.1.(5))
(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 8.7.3.4.(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.
(7)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 8.7.3.5.(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.
(8)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 (7)(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.
(9)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. (See Note A-8.7.7.1.(9))
(10)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.
(11)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).
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OBC Div. B 8.7.8.2. Construction Requirements
(1)The treatment unit used in conjunction with a leaching bed constructed as a Type B 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 B dispersal bed shall be
(a)rectangular in shape with the long dimension parallel to the site contours,
(b)not more than 1,000 mm in depth measured from the bottom of the stone layer to the finished grade when installed in soil with a percolation time that exceeds 15 min, and
(c)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 bottom of the stone layer shall be at least 600 mm above the high ground water table, rock or soil with a percolation time greater than 50 min.
(4)The effluent shall be distributed over the Type B dispersal bed 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.
(5)The distribution pipes shall
(a)be self-draining so as to prevent freezing of their contents, and
(b)have orifices of at least 3 mm in diameter, spaced equally along the length of the pipes.
(6)The stone layer containing the distribution pipes shall
(a)be comprised of washed septic stone, free of fine material, with gradation conforming to Table 8.7.3.4.,
(b)extend not less than 250 mm below the distribution pipe, and
(c)extend not less than 50 mm above the distribution pipe.
(7)The distribution pipes shall be spaced not more than 1.2 m apart with the outermost pipe spaced not more than 600 mm from the edge of the bed.
(8)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.
(9)When there is more than one Type B dispersal bed in a leaching bed, the Type B dispersal beds shall be separated by at least 5 m measured from the edge of the stone layers.
(10)A Type B dispersal bed 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).
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OBC Div. B 8.7.8.1. General Requirements
(1)Except as provided in Sentence (2) and Sentence 8.7.8.2.(2), a Type B dispersal bed shall conform to the requirements of Article 8.7.2.1.
(2)A Type B dispersal bed shall not be located in an area that has an average slope that exceeds one unit vertically to seven units horizontally.
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OBC Div. B 8.7.8.3. Design Requirements
(1)The area of a Type B dispersal bed shall not be less than the minimum area determined in accordance with Clause (2)(a) or (b).
(2)For the purposes of Sentence (1), the minimum area is either of the following:
(a)the area calculated based on the loading rates for Type 2 effluent set out in the Column headed "Type 2" found in Table 2-8 of the BCMOH, "Sewerage System Standard Practice Manual," or
(b)the value determined by the formula,
A = QT/400
where,
A = the area of contact in square metres between the stone layer and the underlying soil,
Q = the total daily design sanitary sewage flow in litres, and
T = the percolation time of the underlying soil.
(3)The linear loading rates of the underlying soil shall not be greater than
(a)the linear loading rates set out in Table 2-11 of BCMOH, "Sewerage System Standard Practice Manual," where the area of the Type B dispersal bed is determined in accordance with Clause (2)(a), or
(b)the following linear loading rate, where the area of the Type B dispersal bed is determined in accordance with Clause (2)(b)
(i)40 L/m, for soil having a percolation time equal to or greater than 24 min, or
(ii)50 L/m, for soil having a percolation time less than 24 min.
(4)The width of a Type B dispersal bed shall not exceed 4 m.
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