Class 5 holding tanks, operation and maintenance (On-site Sewage Systems)

Last updated October 22, 2026

Values this chapter turns on

minimum working capacity of a holding tank: 9,000 L; minimum working capacity of a holding tank
of design flow a residential holding tank must hold: 7 days; of design flow a residential holding tank must hold
max temporary use that allows a holding tank: 12 months; max temporary use that allows a holding tank
of tank capacity in sludge and scum triggers pumping: 1/3; of tank capacity in sludge and scum triggers pumping

This chapter covers what happens when a leaching bed is not possible (Class 5 holding tanks) and what every owner and operator must do once a system is running (Section 8.9.). Both are favourite exam areas because the rules are short and full of exact numbers: 12 months, 9,000 L, 7 days, one-third, 36 months, 20 mg/L.

Class 5: when a holding tank is allowed

The default is no. A Class 5 system shall not be installed except as Article 8.8.1.2. allows (OBC Div. B 8.8.1.1.). The four permitted cases (OBC Div. B 8.8.1.2.(1)):

Case	Condition
Temporary operation	not over 12 months, and not seasonal recreational use
Remedy an unsafe system	where fixing it with a Class 4 system is impracticable
Upgrade for an existing building	where a Class 4 upgrade is not possible because of lot size, site slope or clearance limits
Interim until municipal sewers arrive	the municipality undertakes to keep an approved hauled sewage system operating until sewers are available

Whenever a Class 5 system is installed, the owner must have a written agreement with a hauled sewage system operator for disposal of the sewage (Sentence (2)).

Worked example: is a holding tank allowed?

Proposal	Allowed?
Site office for an 8 month road job	Yes, temporary operation of 12 months or less
New seasonal cottage, owner prefers a tank to a bed	No, seasonal recreational use is excluded and nothing else applies
Failing bed at an old house on a 0.1 ha lot where no Class 4 system fits	Yes, upgrade or remedy where Class 4 is not possible or impracticable
New subdivision lot where the town has committed to sewers in 3 years and to keep a hauler running	Yes, interim measure

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Holding tank construction and sizing

From Subsection 8.2.2. (OBC Div. B 8.2.2.4.): the tank must allow complete removal of settled solids and must have a working capacity of at least 9,000 L. Several connected tanks count as one, minus any volume that cannot drain.

From Section 8.8. (OBC Div. B 8.8.2.1.):

An audible and visual alarm warns that the system is nearing capacity. It must give enough warning considering Q, the location of the system and the hauler's response time.
A vent of at least 3 in. trade size, ending at least 300 mm above finished grade with a vent cap (600 mm in an area subject to flooding), and at least 3.5 m from any air inlet, window or door.
Alternatively, the vent may connect into the building's venting system if it is at least 3 in. and installed to Part 7.

Holding tanks in residential dwellings need at least 7 days of holding capacity based on Q (OBC Div. B 8.8.2.2.).

Worked example: holding tank size

A 3 bedroom house, Q = 1,600 L. 7 x 1,600 = 11,200 L. That exceeds 9,000 L, so the tank must hold at least 11,200 L.

A 1 bedroom cabin, Q = 750 L. 7 x 750 = 5,250 L, below the 9,000 L floor. The tank must hold at least 9,000 L.

Clearances for holding tanks are in Table 8.2.1.6.-C. For example, 1.5 m to a structure and 3 m to a property line.

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Operation of sewage systems

General duties for every system (OBC Div. B 8.9.1.2.): no sewage or effluent on the ground surface; no escape except where the system is designed to discharge; and no escape into a piped water supply, well, watercourse, ground water or surface water, except properly treated effluent discharged into soil by a system designed for it.

Every system is operated in line with the basis on which it was approved and the manufacturer's requirements (OBC Div. B 8.9.2.2.).

Class 4 operation

OBC Div. B 8.9.2.3.:

Operate per the manufacturer's literature required by 8.6.2.2.(6).
Nobody may operate a treatment unit other than a septic tank without an agreement for servicing by a person who has the literature and is authorized by the manufacturer.
That service provider must notify the chief building official if the agreement ends or the operator denies access.

Sampling for Level IV beds

For a treatment unit serving a shallow buried trench, Type A or Type B dispersal bed (OBC Div. B 8.9.2.4.):

Item	Rule
What	grab sample for CBOD5 and suspended solids, using the APHA/AWWA/WEF Standard Methods
First sample	once in the first 12 months of use
After that	once in every 12 month period, at least 10 and not more than 18 months after the last
Pass mark	each parameter not over 20 mg/L in the grab sample
If it fails	resample within 6 months and send results to the CBO
Reporting	results go promptly to the chief building official

Class 5 operation

Operate per the hauler agreement, and stop using the system once it is full until it is pumped (OBC Div. B 8.9.2.5.).

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Maintenance

Keep construction in line with the approval and the manufacturer, keep all parts working, and keep nearby land in a condition that will not harm the system (OBC Div. B 8.9.3.2.).
Grease interceptors serving restaurant kitchens are maintained to CSA B481.4 (OBC Div. B 8.9.3.3.).
Septic tanks and other treatment units are cleaned when sludge and scum take up one-third of the working capacity (OBC Div. B 8.9.3.4.).
The pressure head at the farthest point of pressurized distribution pipes is checked at least every 36 months (OBC Div. B 8.9.3.5.).

Worked example: when to pump

A 4,500 L septic tank is inspected. Sludge and scum together measure about 1,600 L. One-third of 4,500 L is 1,500 L. At 1,600 L the tank has passed the trigger and must be cleaned.

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Common traps

Seasonal recreational use never qualifies as "temporary".
9,000 L is the minimum for any holding tank; residential also needs 7 days of Q.
The vent is 300 mm above grade, 600 mm in flood areas, 3.5 m from openings.
Service agreements are required for treatment units other than septic tanks.
20 mg/L is the grab sample test; the Table 8.6.2.2. values are 30 day averages.
Pump at one-third full of sludge and scum; check pressure head every 36 months.

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
Class 5: when a holding tank is allowed	Whenever a Class 5 system is installed, the owner must have a written agreement with a hauled sewage system operator for disposal of the sewage (Sentence (2)).
Holding tank construction and sizing	From Subsection 8.2.2.: the tank must allow complete removal of settled solids and must have a working capacity of at least 9,000 L. Several connected tanks count as one, minus…
Operation of sewage systems	General duties for every system: no sewage or effluent on the ground surface; no escape except where the system is designed to discharge; and no escape into a piped water supply…
Maintenance	A 4,500 L septic tank is inspected.

Class 5: when a holding tank is allowed
Whenever a Class 5 system is installed, the owner must have a written agreement with a hauled sewage system operator for disposal of the sewage (Sentence (2)).
Holding tank construction and sizing
From Subsection 8.2.2.: the tank must allow complete removal of settled solids and must have a working capacity of at least 9,000 L. Several connected tanks count as one, minus…
Operation of sewage systems
General duties for every system: no sewage or effluent on the ground surface; no escape except where the system is designed to discharge; and no escape into a piped water supply…
Maintenance
A 4,500 L septic tank is inspected.

Usual exam traps

Seasonal recreational use never qualifies as "temporary".
9,000 L is the minimum for any holding tank; residential also needs 7 days of Q.
The vent is 300 mm above grade, 600 mm in flood areas, 3.5 m from openings.
Service agreements are required for treatment units other than septic tanks.
20 mg/L is the grab sample test; the Table 8.6.2.2. values are 30 day averages.
Pump at one-third full of sludge and scum; check pressure head every 36 months.

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Questions on Class 5 holding tanks, operation and maintenance, 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. B 8.8.1.1. Prohibited Installation
(1)Except as provided in Article 8.8.1.2., a Class 5 sewage system shall not be installed.
Open OBC Div. B 8.8.1.1. in the code viewer
OBC Div. B 8.8.1.2. Acceptable Installation
(1)A Class 5 sewage system may be installed in the following circumstances:
(a)where the proposed use of the sewage system is for a temporary operation, excluding seasonal recreational use, not exceeding 12 months in duration,
(b)to remedy an unsafe sewage system where the remediation of the unsafe condition by the installation of a Class 4 sewage system is impracticable,
(c)to upgrade a sewage system serving an existing building, where upgrading through the use of a Class 4 sewage system is not possible due to lot size, site slope or clearance limitations, or
(d)as an interim measure for a lot or parcel of land until municipal sewers are available, provided that the municipality undertakes to ensure the continued operation of an approved hauled sewage system until the municipal sewers are available.
(2)Where a Class 5 sewage system is installed, a written agreement for the disposal of sanitary sewage from the sewage system shall be entered into with a hauled sewage system operator.
Open OBC Div. B 8.8.1.2. in the code viewer
OBC Div. B 8.2.2.4. Holding Tanks
(1)All holding tanks shall be of such design and construction as will allow the complete removal of solid matter that can be expected to settle in the holding tank through an apparatus or device suitable for allowing the contents of the holding tank to be removed from the holding tank.
(2)A holding tank shall have a working capacity of not less than 9,000 L.
(3)Where two or more tanks are used to meet the requirement of Sentence (2), they shall be deemed to be one holding tank provided they are connected in such a manner as will allow the sanitary sewage contained in them to flow between the tanks.
(4)The working capacity of the tanks described in Sentence (3) shall not include any portion of any tank that cannot be completely drained due to the manner in which the connections are made.
Open OBC Div. B 8.2.2.4. in the code viewer
OBC Div. B 8.8.2.1. Construction Requirements
(1)All Class 5 sewage systems shall be equipped with a device that shall produce an audible and visual warning alarm so located to warn that the sewage system is nearing capacity.
(2)The device required in Sentence (1) shall be designed to provide suitable advance warning to the building occupants considering
(a)the total daily design sanitary sewage flow,
(b)the location of the Class 5 sewage system, and
(c)the response time of the hauled sewage system contractor.
(3)Except as provided in Sentence (4), all holding tanks shall be provided with a vent that
(a)is not less than 3 in. trade size,
(b)terminates at least,
(i)300 mm above finished grade with a vent cap, or
(ii)600 mm above finished grade with a vent cap when the holding tank is located in an area subject to flooding, and
(c)terminates at least 3.5 m away from any air inlet, window, or door.
(4)A vent from a holding tank may connect into the venting system of the building served by the holding tank provided that,
(a)the vent is not less than 3 in. trade size, and
(b)the installation of the vent shall conform to the requirements in Part 7.
Open OBC Div. B 8.8.2.1. in the code viewer
OBC Div. B 8.8.2.2. Sizing of Holding Tanks
(1)All holding tanks used in residential dwellings shall have a minimum 7 day holding capacity based on the total daily design sanitary sewage flow.
Open OBC Div. B 8.8.2.2. in the code viewer
OBC Div. B Table 8.2.1.6-C. Minimum Clearances for Holding Tanks Forming Part of Sentence 8.2.1.6.(3)
Table 8.2.1.6.-C — Minimum Clearances for Holding Tanks Forming Part of Sentence 8.2.1.6.(3) Object | Minimum Clearance, m Structure | 1.5 Well with a watertight casing to a depth of at least 6 m | 15 Any other well | 15 Spring | 15 Property Line | 3
Open OBC Div. B Table 8.2.1.6-C. in the code viewer
OBC Div. B 8.9.1.2. General Requirements for Operation and Maintenance
(1)Every sewage system shall be operated and maintained so that
(a)the sewage system or any part of it shall not emit, discharge or deposit sanitary sewage or effluent onto the surface of the ground,
(b)sewage system or effluent shall not emit, discharge, seep, leak or otherwise escape from the sewage system or any part of it other than from a place or part of the sewage system where the system is designed or intended to discharge the sanitary sewage or effluent, and
(c)except as provided in Sentence (2), sanitary sewage or effluent shall not emit, discharge, seep, leak or otherwise escape from the sewage system or any part of it into a piped water supply, well water supply, a watercourse, ground water or surface water.
(2)Clause (1)(c) does not apply to the use of a sewage system designed and operated such that properly treated effluent is discharged into soil.
Open OBC Div. B 8.9.1.2. in the code viewer
OBC Div. B 8.9.2.2. General
(1)Every sewage system shall be operated in accordance with
(a)the basis on which the construction and use of the sewage system was approved or required under the Act or predecessor legislation, as the case may be, and
(b)the requirements of the manufacturer of the sewage system.
Open OBC Div. B 8.9.2.2. in the code viewer

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OBC Div. B 8.9.2.3. Class 4 Sewage Systems
(1)Every Class 4 sewage system shall be operated in accordance with the literature required by Sentence 8.6.2.2.(6).
(2)No person shall operate a treatment unit other than a septic tank unless the person has entered into an agreement whereby servicing and maintenance of the treatment unit and its related components will be carried out by a person who
(a)possesses a copy of the literature required by Sentence 8.6.2.2.(6), and
(b)is authorized by the manufacturer to service and maintain that type of treatment unit.
(3)The person authorized by the manufacturer to service and maintain the treatment unit and who has entered into the agreement referred to in Sentence (2) with the person operating the treatment unit shall notify the chief building official if,
(a)the agreement is terminated, or
(b)access for service and maintenance of the treatment unit is denied by the person operating the treatment unit.
Open OBC Div. B 8.9.2.3. in the code viewer
OBC Div. B 8.9.2.4. Sampling of Treatment Units
(1)Every person operating 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
(a)take a grab sample of the effluent to determine the level of CBOD₅ and suspended solids in the effluent,
(b)carry out the sampling required by Clause (1)(a) in accordance with the methods described in the APHA/AWWA/WEF, "Standard Methods for the Examination of Water and Wastewater," and
(c)promptly submit the results of the sampling required by Clause (a) to the chief building official.
(2)Except as provided in Sentence (4), the sampling required by Sentence (1) shall be conducted
(a)initially, once during the first 12 months after the sewage system was put into use, and
(b)thereafter, once during every 12 month period, at least 10 months and not more than 18 months after the previous sampling has been completed.
(3)The concentration of CBOD₅ and suspended solids in the grab sample described in Sentences (1) and (4) is deemed to comply with the maximum concentration requirements set out in Table 8.6.2.2. when it does not exceed 20 mg/L for each of these parameters.
(4)If the results of the sampling required by Sentence (1) do not comply with Sentence (3), the person operating the treatment unit shall
(a)resample the effluent in accordance with Clauses (1)(a) and
(b)within 6 months after the previous sampling has been completed, and
(b) promptly submit the results of the resampling required by Clause (a) to the chief building official.
Open OBC Div. B 8.9.2.4. in the code viewer
OBC Div. B 8.9.2.5. Class 5 Sewage Systems
(1)Every Class 5 sewage system shall be operated in accordance with the agreement referred to in Sentence 8.8.1.2.(2).
(2)No Class 5 sewage system shall be operated once it is filled with sanitary sewage until such time as the sanitary sewage is removed from the sewage system.
Open OBC Div. B 8.9.2.5. in the code viewer
OBC Div. B 8.9.3.2. General
(1)Every sewage system shall be maintained so that
(a)the construction of the sewage system remains in accordance with
(i)the basis on which the construction and use of the sewage system was approved or required under the Act or predecessor legislation, as the case may be, and
(ii)the requirements of the manufacturer of the sewage system, and
(b)all components of the sewage system function in their intended manner.
(2)The land in the vicinity of a sewage system shall be maintained in a condition that will not cause damage to, or impair the functioning of, the sewage system.
Open OBC Div. B 8.9.3.2. in the code viewer
OBC Div. B 8.9.3.3. Interceptors
(1)Every grease interceptor referred to in Article 8.1.3.1. shall be maintained in accordance with CSA B481.4, "Maintenance of grease interceptors."
Open OBC Div. B 8.9.3.3. in the code viewer
OBC Div. B 8.9.3.4. Class 4 Sewage Systems
(1)Septic tanks and other treatment units shall be cleaned whenever sludge and scum occupy one-third of the working capacity of the tank.
Open OBC Div. B 8.9.3.4. in the code viewer
OBC Div. B 8.9.3.5. Pressurized Distribution Systems
(1)The pressure head at the furthest point from the pump in all distribution pipes shall be checked for compliance with Articles 8.7.6.1. and 8.7.8.2. and the design specification at least every 36 months.
Open OBC Div. B 8.9.3.5. in the code viewer