On-site sewage calculations: BCIN Calculation Lab

Last updated October 22, 2026

When this shows up on the exam

The On-site Sewage Systems exam is built around one chain of numbers. You find the total daily design sanitary sewage flow (Q), then use Q to size the septic tank, the length of distribution pipe, the filter bed area and the contact area. A typical question gives a house (bedrooms, finished area, fixture units) or a business (seats, employees, floor area) and asks for one link in that chain. Clearance questions add a twist: a raised bed pushes the distances out.

The method

Residential flow. Use Table 8.2.1.3.-A (OBC Div. B 8.2.1.3.(1)). For a dwelling, start with the bedroom row (1 to 5 bedrooms). Then work out each "additional flow" item separately: 500 L for each bedroom over 5, the floor area steps over 200 m², and 50 L for each fixture unit over 20. Note 2 of the table says that where more than one calculation is allowed, the highest one governs. So add the largest additional amount, not all of them.
Floor area steps. Count each 10 m² or part of it: 100 L per step from 200 to 400 m², 75 L from 400 to 600 m², 50 L over 600 m². Note 3 says to use total finished area and leave out a finished basement. Always round a part step up.
Other occupancies. Use Table 8.2.1.3.-B. Items joined by "and" are added; items joined by "or" are alternatives, and the highest governs (Note 3). Two establishments in one building are added together (8.2.1.3.(3)).
Septic tank. Working capacity is the greater of 3,600 L and 2 × Q (residential) or 3 × Q (non-residential). The first compartment needs at least 1.3 × Q and never less than 2,400 L. Each later compartment is at least half the first. With tanks in series, the first tank needs at least the first compartment volume and can never be under 3,600 L, and the tanks together still need the full working capacity (8.2.2.3.).
Design percolation time (T). With percolation tests, use the highest result of at least 3 locations (8.2.1.2.(3)).
Absorption trenches. Pipe length L = QT/200 with a septic tank, or QT/300 after a Level II, III or IV treatment unit. Never less than 40 m (30 m for a shallow buried trench) (8.7.3.1.). Leaching chambers: Type I uses /200 and Type II uses /300 (8.7.3.2.). Each trench is at most 30 m long (8.7.3.3.). At 150 m or more you need a pump or siphon (8.6.1.3.).
Shallow buried trench. Use Table 8.7.3.1.: Q is divided by 75, 50 or 30 depending on the T range. Watch the boundaries: the middle row includes T = 50.
Fill based beds. The fill mantle area must keep the loading rate at or under Table 8.7.4.1. (8.7.4.1.), so area = Q ÷ loading rate.
Filter beds. Effective area = Q ÷ 75 when Q is 3,000 L or less, Q ÷ 50 when Q is over 3,000 L (and then more than one bed), or Q ÷ 100 after a Level II to IV unit. Each bed is 10 to 50 m² (8.7.5.2.). Contact area with the soil below is A = QT/850, with T capped at 50 (8.7.5.3.(6)). Pipe lines are evenly spaced, at most 1,200 mm apart (Table 8.7.5.3.), and the outer line is within 600 mm of the edge (8.7.5.3.(2)).
Raised bed clearances. Take the distribution pipe clearance from Table 8.2.1.6.-B and add twice the height the bed is raised above original grade (8.7.4.2.(11)).

Worked example 1: a large house and its tank

A 4 bedroom house has 340 m² of finished area above grade, plus a finished basement, and 28 fixture units.

Step	Working	Result
Bedroom row	Table 8.2.1.3.-A, 4 bedroom dwelling	2,000 L
Area addition	340 − 200 = 140 m², 14 steps × 100 L (basement left out)	1,400 L
Fixture unit addition	28 − 20 = 8, 8 × 50 L	400 L
Largest addition	1,400 L beats 400 L	1,400 L
Q	2,000 + 1,400	3,400 L
Tank	greater of 3,600 L and 2 × 3,400	6,800 L
First compartment	greater of 1.3 × 3,400 and 2,400	4,420 L
Second compartment	half of 4,420	2,210 L

Answer: Q = 3,400 L and a 6,800 L tank (OBC Div. B 8.2.2.3.(1)).

Worked example 2: trench bed for the same house

Percolation tests at three spots give 9, 11 and 12 min, so T = 12 min.

Step	Working	Result
Length	3,400 × 12 ÷ 200	204 m
Minimum check	204 m is over 40 m	204 m
Trenches	204 ÷ 30 = 6.8, round up	7 trenches
Dosing	204 m is 150 m or more	pump or siphon

If the bed is built in fill instead, the fill area for T = 12 min comes from the first row of Table 8.7.4.1. (10 (L/m²)/day): 3,400 ÷ 10 = 340 m² (OBC Div. B 8.7.4.1.).

Worked example 3: filter bed

A house with a septic tank has Q = 2,100 L. The soil under the bed has T = 40 min.

Step	Working	Result
Loading limit	Q is not over 3,000 L	75 L/m² per day
Effective area	2,100 ÷ 75	28 m²
Contact area T	lesser of 50 and 40	40
Contact area	2,100 × 40 ÷ 850	98.8 m²

The filter medium surface needs at least 28 m² (8.7.5.2.(3)). Its base must spread at least 250 mm thick over about 98.8 m² of soil (OBC Div. B 8.7.5.3.(6)).

Worked example 4: business flow and a raised bed

A bowling alley with 8 lanes has no food service. Table 8.2.1.3.-B gives 400 L per lane: 8 × 400 = 3,200 L. It is non-residential, so the tank is the greater of 3,600 L and 3 × 3,200 = 9,600 L, so 9,600 L.

Its fill based bed is raised 0.6 m above original grade. Table 8.2.1.6.-B gives 3 m to a property line. Add 2 × 0.6 = 1.2 m, so the outer pipe centre line must be at least 4.2 m from the lot line (OBC Div. B 8.7.4.2.(11)).

Common mistakes

Adding every "additional flow" item together instead of using the largest one (Note 2).
Counting a finished basement in the floor area (Note 3).
Rounding a part 10 m² step down instead of up, or charging all area over 200 m² at one rate.
Using 2 × Q for a business, or forgetting the 3,600 L floor.
Treating "or" items in Table 8.2.1.3.-B as additive, or "and" items as alternatives.
Averaging percolation tests instead of taking the highest.
Using /200 after a Level II to IV unit, or /300 with a plain septic tank.
Picking the wrong T row at a boundary (T = 20, 35 or 50).
Using T over 50 in A = QT/850.
Adding the raised height once instead of twice, or applying it to the wrong table.

Find it fast in the code PDF

Bookmark Division B > Part 8. Section 8.2 holds flows, clearances and tanks; Section 8.7 holds leaching beds. Useful Ctrl+F terms:

"Total finished area" (Note 3 of Table 8.2.1.3.-A)
"the calculation resulting in the highest flow"
"three times the daily design sanitary sewage flow" (septic tank)
"L = QT/200" and "Q/50" (trench length)
"Loading Rates for Fill Based Absorption Trenches"
"A = QT/850" (filter bed contact area)
"twice the height that the leaching bed is raised"

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Drill On-site Sewage Systems

Code cited in this method

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

OBC Div. B Table 8.2.1.3-A. Residential Occupancy — Forming Part of Sentence 8.2.1.3.(1)
Table 8.2.1.3.-A Residential Occupancy — Forming Part of Sentence 8.2.1.3.(1) Residential Occupancy | Volume, litres Apartments, Condominiums, Other Multi-family Dwellings - per person[1] | 275 Boarding Houses (a) Per person, (i) with meals and laundry facilities, or, | 200 Boarding Houses (a) Per person, (ii) without meal or laundry facilities, and | 150 Boarding Houses (b) Per non-resident staff per 8 hour shift | 40 Boarding School - per person | 300 Dwellings (a) 1 bedroom dwelling | 750 Dwellings (b) 2 bedroom dwelling | 1,100 Dwellings (c) 3 bedroom dwelling | 1,600 Dwellings (d) 4 bedroom dwelling | 2,000 Dwellings (e) 5 bedroom dwelling | 2,500 Dwellings (f) Additional flow for[2] (i) each bedroom over 5, | 500 Dwellings (f) Additional flow for[2] (f) (ii) (A) each 10 m² (or part of it) over 200 m² up to 400 m²[3], | 100 Dwellings (f) Additional flow for[2] (f) (ii) (B) each 10 m² (or part of it) over 400 m² up to 600 m²[3], and | 75 Dwellings (f) Additional flow for[2] (f) (ii) (C) each 10 m² (or part of it) over 600 m²[3], or | 50 Dwellings (f) Additional flow for[2] (iii) each fixture unit over 20 fixture units | 50 Hotels and Motels (excluding bars and restaurants) (a) Regular, per room | 250 Hotels and Motels (excluding bars and restaurants) (b) Resort hotel, cottage, per person | 500 Hotels and Motels (excluding bars and restaurants) (c) Self service laundry, add per machine | 2,500 Work Camp/Construction Camp, semi-permanent per worker | 250 Notes to Table 8.2.1.3.-A: (1) The occupant load shall be calculated using Subsection 3.1.17. (2) Where multiple calculations of sanitary sewage volume is permitted, the calculation resulting in the highest flow shall be used in determining the design daily sanitary sewage flow. (3) Total finished area, excluding the area of the finished basement.
Open OBC Div. B Table 8.2.1.3-A. in the code viewer
OBC Div. B 8.2.1.3. Sewage System Design Flows
(1)For residential occupancies, the total daily design sanitary sewage flow shall be at least the value determined in accordance with Table 8.2.1.3.-A. (See Note A-8.2.1.3.(1) and (2))
(2)For all other occupancies, the total daily design sanitary sewage flow shall be at least the value determined in accordance with Table 8.2.1.3.-B. (See Note A-8.2.1.3.(1) and (2))
(3)Where a building contains more than one establishment, the total daily design sanitary sewage flow shall be the sum of the total daily design sanitary sewage flow for each establishment.
(4)Where an occupancy is not listed in Table 8.2.1.3.-B, the highest of metered flow data from at least 3 similar establishments shall be acceptable for determining the total daily design sanitary sewage flow.
Open OBC Div. B 8.2.1.3. in the code viewer
OBC Div. B Table 8.2.1.3-B. Airports, Bus Terminals, Train Stations, Dock/Port Facilities (Food Services excluded)
Airports, Bus Terminals, Train Stations, Dock/Port Facilities (Food Services excluded) Establishments[1] | Volume, litres (a) Per passenger, and | 20 (b) Per employee per 8 hour shift | 40 Assembly Hall — per seat Establishments[1] | Volume, litres (a) No food service, or | 8 (b) Food service provided | 36 Barber Shop/Beauty Salon Establishments[1] | Volume, litres per service chair | 650 Bowling Alleys (Food Service not included) Establishments[1] | Volume, litres per lane | 400 Churches and Similar Places of Worship — per seat Establishments[1] | Volume, litres (a) No kitchen facilities, or | 8 (b) Kitchen facilities provided | 36 Country Club (excluding Food Service) Establishments[1] | Volume, litres (a) Per resident, | 375 (b) Per employee per 8 hour shift, and | 50 (c) Per member or patron | 40 Day Care Facility Establishments[1] | Volume, litres per person (staff and children) | 75 Dentist Office Establishments[1] | Volume, litres (a) Per wet service chair, and | 275 (b) Per dry service chair | 190 Doctors Office Establishments[1] | Volume, litres (a) Per practitioner, and | 275 (b) Per employee per 8 hour shift | 75 Factory (excluding process or cleaning waters) — per employee per 8 hour shift Establishments[1] | Volume, litres (a) No showers, or | 75 (b) Including showers | 125 Flea Markets[2] (open not more than 3 days per week) Establishments[1] | Volume, litres (a) Per non-food service vendor space, | 60 (b) Per food service establishment / 9.25 m² of floor space, and | 190 (c) Per limited food service outlet | 95 Food Service Operations Establishments[1] | Volume, litres (a) Restaurant (not 24 hour), per seat | 125 (b) Restaurant (24 hour), per seat | 200 (c) Restaurant on controlled-access highway, per seat | 400 (d) Paper service restaurant, per seat | 60 (e) Donut shop, per seat | 400 (f) Bar and cocktail lounge, per seat | 125 (g) Drive-in restaurant per parking space | 60 (i) Cafeteria — per meal | 12 Take-out restaurant (no seating area) Establishments[1] | Volume, litres (h) (i) per 9.25 m² of floor area, and | 190 (h) (ii) per employee per 8 hour shift | 75 Food outlet Establishments[1] | Volume, litres (j) (i) excluding delicatessen, bakery and meat department, per 9.25 m² of floor space, | 40 (j) (ii) per 9.25 m² of delicatessen floor space, | 190 (j) (iii) per 9.25 m² of bakery floor space, | 190 (j) (iv) per 9.25 m² of meat department floor space, and | 380 (j) (v) per water closet | 950 Hospitals — per bed Establishments[1] | Volume, litres (a) Including laundry facilities, or | 750 (b) Excluding laundry facilities | 550 Long-Term Care Homes, etc. Establishments[1] | Volume, litres per bed | 450 Office Building[3] Establishments[1] | Volume, litres (a) Per employee per 8 hour shift, or | 75 (b) Per each 9.3 m² of floor space | 75 Public Parks Establishments[1] | Volume, litres (a) With toilets only per person, or | 20 (b) With bathhouse, showers, and toilets per person | 50 Recreational Vehicle or Campground Park Establishments[1] | Volume, litres (a) Per site without water or sewer hook-up, or | 275 (b) Per site with water and sewer hook-up | 425 Schools — per student Establishments[1] | Volume, litres (a) Day school, | 30 (b) With showers, | 30 (c) With cafeteria, and | 30 (d) Per non-teaching employee per 8 hour shift | 50 Service Stations (no vehicle washing)[3] Establishments[1] | Volume, litres (a) Per water closet, and | 950 (a) Per water closet, and (i) per fuel outlet[4], or | 560 (a) Per water closet, and (ii) per vehicle served | 20 Shopping Centre (excluding food and laundry) Establishments[1] | Volume, litres per 1.0 m² of floor space | 5 Stadiums, Race Tracks, Ball Parks Establishments[1] | Volume, litres per seat | 20 Stores[3] Establishments[1] | Volume, litres (a) Per 1.0 m² of floor area, or | 5 (b) Per water closet | 1,230 Swimming and Bathing Facilities (Public) Establishments[1] | Volume, litres per person | 40 Theatres Establishments[1] | Volume, litres (a) Indoor, auditoriums per seat, | 20 (b) Outdoor, drive-ins per space, or | 40 (c) Movie theatres per seat | 15 Veterinary Clinics Establishments[1] | Volume, litres (a) Per practitioner, | 275 (b) Per employee per 8 hour shift, and | 75 (c) Per stall, kennel or cage if floor drain connected | 75 Warehouse Establishments[1] | Volume, litres (a) Per water closet, and | 950 (b) Per loading bay | 150 Notes to Table 8.2.1.3.-B: (1) The occupant load shall be calculated using Subsection 3.1.17. (2) Flea markets open more than 3 days per week shall be assessed using the volumes stated under the heading "Stores". (3) Where multiple calculations of sanitary sewage volume is permitted, the calculation resulting in the highest flow shall be used in determining the design daily sanitary sewage flow. (4) The number of fuel outlets is considered the maximum number of fuel nozzles that could be in use at the same time
Open OBC Div. B Table 8.2.1.3-B. in the code viewer
OBC Div. B 8.2.2.3. Septic Tanks
(1)The minimum working capacity of a septic tank shall be the greater of 3,600 L and
(a)in residential occupancies, twice the daily design sanitary sewage flow, or
(b)in non-residential occupancies, three times the daily design sanitary sewage flow.
(2)Every septic tank shall be constructed in such a manner that any sanitary sewage flowing through the tank will pass through at least 2 compartments.
(3)The working capacity of the compartments required in Sentence (2) shall be sized such that
(a)the first compartment is at least 1.3 times the daily design sanitary sewage flow but in no case less than 2,400 L, and
(b)each subsequent compartment shall be at least 50% of the first compartment.
(4)Where multiple tanks are to be used to meet the requirements of Sentences (2) and (3), the tanks shall be connected in series such that
(a)the first tank in the series shall have at least a capacity as calculated in Clause (3)(a), however at no time shall a tank having a working capacity of less than 3,600 L be used,
(b)all additional tanks after the first tank, excluding pump or dosing tanks shall have at least a working capacity equal to the volume required by Clause (3)(b),
(c)the pipe between the outlet of one tank and the inlet of the next tank in the series shall have a minimum slope of 2 percent,
(d)there shall be no partitions in the tank except where a partition is required to maintain the structural integrity of the tank, in which case openings within the partition shall be provided to allow the free movement of sanitary sewage throughout the tank, and
(e)all piping between tanks shall be continuous and shall be connected to the tank through the use of flexible watertight seals that will permit differential movement between the tanks.
(5)Partitions separating the septic tank into compartments shall extend at least 150 mm above the liquid level at the outlet, and there shall be one or more openings through or above the partition.
(6)The openings required between compartments referred to in Sentence (2) shall have a total cross-sectional area of at least three times the area of the inlet pipe and be located between the top and a level 150 mm above the liquid level at the outlet to provide for the free flow of air between compartments.
(7)Sanitary sewage shall pass from one compartment to another of the septic tank as follows:
(a)by means of a device similar to that described in CSA B66, "Design, Material, and Manufacturing Requirements for Prefabricated Septic Tanks and Sewage Holding Tanks" for outlet devices, or
(b)through two or more openings through the partition located in a horizontal line, and evenly spaced across the width of the partition, centred at approximately 40% of the liquid depth below the surface of the liquid, and having a total area of between three and five times that of the cross-sectional area of the inlet pipe.
(8)A septic tank shall be of such design and construction as will permit the collection and holding of sanitary sewage in it to a depth of not less than 1,000 mm, except that a depth of not less than 900 mm is permitted where the excavation is in rock, or to avoid rupture or displacement of the tank due to ground water pressure.
(9)Except as provided in Sentences (10) and (11), every septic tank shall be installed in such a manner that the access openings are located not more than 300 mm below the ground surface.
(10)Where the top of the septic tank is located more than 300 mm below the ground surface, it shall be equipped with risers that extend from the access opening of the septic tank to within 300 mm of the ground surface.
(11)Where risers are used they shall conform to the requirements of CSA B66, "Design, material, and manufacturing requirements for prefabricated septic tanks and sewage holding tanks," and shall have adequate access openings to allow for regular maintenance of the septic tank.
Open OBC Div. B 8.2.2.3. 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 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 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

Show all 17 articles

OBC Div. B 8.6.1.3. Pumps and Siphons
(1)Where the total length of distribution pipe or leaching chamber required is 150 m or more, the sewage system shall have at least one pump or a siphon contained in a dosing tank that may be a separate compartment within the tank structure, for distribution of the effluent.
(2)Where the total length of leaching chamber is 150 m or more, a distribution pipe shall be installed at the centre line of the leaching chamber and extend the total length of leaching chamber to allow for dosing of the effluent.
(3)Alternating siphons shall not be installed in a sewage system.
(4)Where 2 or more pumps are employed within a dosing tank, the pumps shall be designed such that the pumps alternate dosing, and dosing shall continue in the event that one pump fails.
(5)Where a pump or siphon is required, the pump or siphon shall be designed to discharge a dose of at least 75% of the internal volume of the distribution pipe within a time period not exceeding 15 min.
(6)Where a pump or siphon is required, the pump or siphon shall be equipped with a device that shall produce an audible and visual alarm signal that indicates a high water level in the pump or siphon chamber.
Open OBC Div. B 8.6.1.3. 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 8.7.4.1. 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
Open OBC Div. B Table 8.7.4.1. in the code viewer
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.
Open OBC Div. B 8.7.4.1. in the code viewer
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.
Open OBC Div. B 8.7.5.2. in the code viewer
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))
Open OBC Div. B 8.7.5.3. in the code viewer
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
Open OBC Div. B Table 8.7.5.3. in the code viewer
OBC Div. B Table 8.2.1.6-B. Minimum Clearances for Distribution Piping and Leaching Chambers Forming Part of Sentence 8.2.1.6.(2)
Table 8.2.1.6.-B — Minimum Clearances for Distribution Piping and Leaching Chambers Forming Part of Sentence 8.2.1.6.(2) Object | Minimum Clearance, m Structure | 5 Well with a watertight casing to a depth of at least 6 m | 15 Any other well | 30 Lake | 15 Pond | 15 Reservoir | 15 River | 15 Spring not used as a source of potable water | 15 Stream | 15 Property Line | 3
Open OBC Div. B Table 8.2.1.6-B. in the code viewer
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.
Open OBC Div. B 8.7.4.2. in the code viewer