Last updated
Values this chapter turns on
- pipe or chamber length that triggers dosing
- 150 m
- pipe or chamber length that triggers dosing
- of distribution pipe volume per dose, in 15 min
- 75%
- of distribution pipe volume per dose, in 15 min
- minimum area of a septic tank effluent filter
- 550 cm²
- minimum area of a septic tank effluent filter
- Level IV limit for suspended solids and CBOD5
- 10 mg/L
- Level IV limit for suspended solids and CBOD5
A Class 4 system is a treatment unit connected to a leaching bed. The treatment unit is either a septic tank or an other treatment unit (often called an advanced or tertiary unit) rated Level II, III or IV. This chapter covers Section 8.6.: the general rules, dosing with pumps and siphons, septic tank effluent filters, and the effluent standards for other treatment units. Leaching beds are in Chapter 9.
Class 4 general requirements
Section 8.6. applies to the construction of Class 4 systems (OBC Div. B 8.6.1.1.OBC Div. B 8.6.1.1. Scope(1) This Section applies to the construction of a Class 4 sewage system.). The treatment unit must be connected to a leaching bed built to Section 8.7. (OBC Div. B 8.6.1.2.OBC Div. B 8.6.1.2. General Requirements(1) The treatment unit shall be connected to a leaching bed constructed in accordance with the requirements of Section 8.7.).
Pumps and siphons
OBC Div. B 8.6.1.3.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… sets the dosing rules:
| Rule | Detail |
|---|---|
| When dosing is required | total length of distribution pipe or leaching chamber of 150 m or more |
| What is required | at least one pump, or a siphon in a dosing tank (which may be a compartment in the tank structure) |
| Leaching chambers 150 m or more | a distribution pipe at the centre line, running the full length, to allow dosing |
| Alternating siphons | not allowed |
| Two or more pumps | must alternate, and dosing must continue if one fails |
| Dose size | at least 75% of the internal volume of the distribution pipe, delivered in 15 min or less |
| Alarm | audible and visual high water alarm in the pump or siphon chamber |
Worked example: is a pump needed?
A house has Q = 2,000 L and T = 18 min, with a conventional absorption trench bed and a septic tank. Pipe length L = QT/200 = 2,000 x 18 / 200 = 180 m (Chapter 9). Because 180 m is 150 m or more, the system needs a pump or a siphon in a dosing tank, with a high water alarm.
If T were 12 min: L = 2,000 x 12 / 200 = 120 m. Below 150 m, so gravity distribution is allowed.
Septic tank systems
The septic tank itself is sized and built under Article 8.2.2.3. (OBC Div. B 8.2.2.3.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…), covered in Chapter 6. Quick recap for a house: working capacity is the greater of 3,600 L and twice Q; at least 2 compartments; the first at least 1.3 x Q and not less than 2,400 L.
Section 8.6. adds the effluent filter (OBC Div. B 8.6.2.1.OBC Div. B 8.6.2.1. Septic Tank Systems(1) An effluent filter shall be installed in the outlet flow path of every septic tank that discharges effluent to a leaching bed. (2) The septic tank effluent filter required by Sentence (1) shall, (a) conform to the requirements of NSF/ANSI 46, "Evaluation of Components and Devices Used in Wastewater Treatment…):
- Required in the outlet flow path of every septic tank that discharges to a leaching bed.
- Must conform to NSF/ANSI 46, filter particles of 1.6 mm, have a minimum area of 550 cm², and be installed to the manufacturer's recommendations.
- A secured access opening at the ground surface must allow regular maintenance of the filter.
Worked example: septic tank for a 3 bedroom house
Q = 1,600 L (3 bedrooms, under 200 m², 20 fixture units or fewer).
| Item | Working | Result |
|---|---|---|
| Working capacity | greater of 3,600 and 2 x 1,600 = 3,200 | 3,600 L |
| First compartment | greater of 2,400 and 1.3 x 1,600 = 2,080 | 2,400 L |
| Second compartment | at least 50% of 2,400 | 1,200 L |
| Effluent filter | 1.6 mm, at least 550 cm², access at grade | required |
Other treatment units
A treatment unit other than a septic tank must produce effluent that does not exceed the limits for its level in Table 8.6.2.2. (OBC Div. B 8.6.2.2.(1)OBC Div. B 8.6.2.2. Other Treatment Units(1) Except as provided in Sentence (2), a treatment unit, other than a septic tank, shall be designed such that the effluent does not exceed, for the level of the treatment unit set out in Column 1 of Table 8.6.2.2., the maximum concentrations set out opposite it in Columns 2 and 3 of Table 8.6.2.2. (2) A treatment…). The table (Table 8.6.2.2.OBC Div. B Table 8.6.2.2. Other Treatment Unit Effluent Quality Criteria Forming Part of Sentences 8.6.2.2.(1) and (2)Table 8.6.2.2. — Other Treatment Unit Effluent Quality Criteria Forming Part of Sentences 8.6.2.2.(1) and (2) Classification of Treatment Unit[1] | Suspended Solids[2] | CBOD₅[2] Level II | 30 | 25 Level III | 15 | 15 Level IV | 10 | 10 Column 1 | 2 | 3 Notes to Table 8.6.2.2.: (1) The classifications of treatment…) has three rows (Level II, III and IV) and two limits in each: suspended solids and CBOD5, in mg/L as a 30 day average (Note (2)). The levels match the classes of treatment in CAN/BNQ 3680-600 (Note (1)).
Level IV is the cleanest. As a reading example, a Level IV unit's limits are 10 mg/L for each parameter. Higher level numbers mean lower allowed concentrations.
Other rules in 8.6.2.2.:
- Where the leaching bed is a shallow buried trench, Type A dispersal bed or Type B dispersal bed, the unit must meet Level IV limits (Sentence (2)).
- Units with mechanical components need an audible and visual alarm to warn occupants or the operator of a malfunction (Sentence (3)).
- All such units must allow sampling of the effluent (Sentence (4)).
- A unit certified to CAN/BNQ 3680-600 using the listed temperature options is deemed to comply (Sentence (5)). You do not need to know the test details.
- The operator must get literature from the manufacturer or distributor that describes the unit and gives full operation, servicing and maintenance instructions (Sentence (6)).
Why the level matters downstream
Better effluent earns smaller beds. Under Section 8.7., a Level II, III or IV unit allows the pipe length formula QT/300 instead of QT/200, a filter bed loading of 100 L/m² per day, and a filter bed flow cap of 10,000 L instead of 5,000 L. Only Level IV effluent can go to shallow buried trenches and Type A or B dispersal beds.
Worked example: matching unit to bed
A designer wants a Type A dispersal bed on a tight lot. The treatment unit must meet Level IV (10 mg/L suspended solids and 10 mg/L CBOD5, 30 day average). A Level III unit would be fine for a conventional bed with QT/300, but not for a Type A bed.
Operation of these units (service agreements, sampling at 20 mg/L grab sample limits) is in Chapter 10.
Common traps
- Dosing is triggered at 150 m or more of pipe or chamber, not "more than 150 m".
- Alternating siphons are banned outright.
- Dose: 75% of the pipe's internal volume, within 15 minutes.
- The effluent filter is required on every septic tank feeding a leaching bed, and needs access at the ground surface.
- Table 8.6.2.2. limits are 30 day averages. The 20 mg/L figure is the grab sample test in 8.9.2.4., a different rule.
- Shallow buried trenches and Type A and B beds need Level IV, not just "a treatment unit".
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.
- Class 4 general requirements
Section 8.6. applies to the construction of Class 4 systems.
- Septic tank systems
The septic tank itself is sized and built under Article 8.2.2.3., covered in Chapter 6. Quick recap for a house: working capacity is the greater of 3,600 L and twice Q; at least 2…
- Other treatment units
A treatment unit other than a septic tank must produce effluent that does not exceed the limits for its level in Table 8.6.2.2.. The table has three rows (Level II, III and IV)…
Usual exam traps
- Dosing is triggered at 150 m or more of pipe or chamber, not "more than 150 m".
- Alternating siphons are banned outright.
- Dose: 75% of the pipe's internal volume, within 15 minutes.
- The effluent filter is required on every septic tank feeding a leaching bed, and needs access at the ground surface.
- Table 8.6.2.2. limits are 30 day averages. The 20 mg/L figure is the grab sample test in 8.9.2.4., a different rule.
- Shallow buried trenches and Type A and B beds need Level IV, not just "a treatment unit".
Practice this chapter
Questions on Class 4 systems: septic tanks and treatment units, in the exam format, every answer explained and cited to its article. Sign in to start; a free account gets the free questions.
Code cited in this chapter
Based on the official 2024 Building Code Compendium on ontario.ca.
© King's Printer for Ontario, 2024. Reproduced with permission.
OBC Div. B 8.6.1.1. Scope
Open OBC Div. B 8.6.1.1. in the code viewer(1)This Section applies to the construction of a Class 4 sewage system.
OBC Div. B 8.6.1.2. General Requirements
Open OBC Div. B 8.6.1.2. in the code viewer(1)The treatment unit shall be connected to a leaching bed constructed in accordance with the requirements of Section 8.7.
OBC Div. B 8.6.1.3. Pumps and Siphons
Open OBC Div. B 8.6.1.3. in the code viewer(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.
OBC Div. B 8.2.2.3. Septic Tanks
Open OBC Div. B 8.2.2.3. in the code viewer(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.
OBC Div. B 8.6.2.1. Septic Tank Systems
Open OBC Div. B 8.6.2.1. in the code viewer(1)An effluent filter shall be installed in the outlet flow path of every septic tank that discharges effluent to a leaching bed.
(2)The septic tank effluent filter required by Sentence (1) shall,
(a)conform to the requirements of NSF/ANSI 46, "Evaluation of Components and Devices Used in Wastewater Treatment Systems,"
(b)be sized to filter particles of 1.6 mm,
(c)have a minimum area of 550 cm², and
(d)be installed in accordance with the manufacturer's recommendations.
(3)A secured access opening to allow for regular maintenance of the effluent filter shall be provided at the ground surface.
OBC Div. B 8.6.2.2. Other Treatment Units
Open OBC Div. B 8.6.2.2. in the code viewer(1)Except as provided in Sentence (2), a treatment unit, other than a septic tank, shall be designed such that the effluent does not exceed, for the level of the treatment unit set out in Column 1 of Table 8.6.2.2., the maximum concentrations set out opposite it in Columns 2 and 3 of Table 8.6.2.2.
(2)A treatment unit that is used in conjunction with a leaching bed constructed as a shallow buried trench, Type A dispersal bed or Type B dispersal bed shall be designed such that the effluent 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.
(3)All treatment units referred to in Sentences (1) and (2) that contain mechanical components shall be equipped with an audible and visual warning alarm so located to warn the occupants of the building served or the operator of the treatment unit of a malfunction in the operation of the treatment unit.
(4)All treatment units referred to in Sentences (1) and (2) shall permit the sampling of the effluent.
(5)A treatment unit is deemed to comply with Sentences (1) and (2) if it has been certified to CAN/BNQ 3680-600, "Onsite Residential Wastewater Treatment Technologies" using a temperature condition listed under option a) or b) of Annex A.4.2. of that standard. (See Note A-8.6.2.2.(5))
(6)Every operator of a treatment unit shall obtain, from the manufacturer or distributor of the treatment unit, literature that describes the unit in detail and provides complete instructions regarding the operation, servicing, and maintenance requirements of the unit and its related components necessary to ensure the continued proper operation in accordance with the original design and specifications.
OBC Div. B Table 8.6.2.2. Other Treatment Unit Effluent Quality Criteria Forming Part of Sentences 8.6.2.2.(1) and (2)
Open OBC Div. B Table 8.6.2.2. in the code viewerTable 8.6.2.2. — Other Treatment Unit Effluent Quality Criteria Forming Part of Sentences 8.6.2.2.(1) and (2) Classification of Treatment Unit[1] | Suspended Solids[2] | CBOD₅[2] Level II | 30 | 25 Level III | 15 | 15 Level IV | 10 | 10 Column 1 | 2 | 3 Notes to Table 8.6.2.2.: (1) The classifications of treatment units specified in Column 1 correspond to the levels of treatment described in CAN/BNQ 3680-600, "Onsite Residential Wastewater Treatment Technologies." (2) Maximum concentration in mg/L based on a 30 day average.