OBC On-site Sewage Systems cheat sheet

The Building Code Act, 1992 authorizes the building code, the chief building official (CBO), inspectors, and enforcement; Part 8 of Division B contains the sewage system rules.

A sewage system is defined as a system for the treatment and disposal of sewage serving one lot, including treatment units, distribution piping, and dispersal components.

Part 8 applies to systems with a design capacity of 10,000 L/day or less; larger systems fall outside Part 8 and require other approvals.

A building permit is required before constructing, extending, altering, or repairing a sewage system; the permit is issued by the principal authority (municipality or board of health).

The CBO may require plans, specifications, and a site evaluation report before issuing a permit; incomplete applications may be refused.

A sewage system shall not be used or covered until it has been inspected and the inspector has given authorization to cover or use it.

Where a system is found unsafe or non-compliant, the CBO may issue an order to remedy, which may require repair, replacement, or disconnection.

Owner obligations include maintaining the system, keeping records of maintenance, and ensuring the system is not overloaded beyond its design capacity.

A site evaluation identifies soil texture, depth to bedrock, depth to water table, slope, and lot size; these determine the feasible system type and size.

Soil permeability is assessed by percolation testing or by texture classification; percolation rate (T-time) is expressed in minutes per 25 mm of water drop.

Depth of unsaturated soil beneath the dispersal trench bottom must be adequate; shallow bedrock or high water table requires a raised or mound system.

Slope affects system selection: steep slopes require special design, and slopes over about 30 percent generally require an engineered design.

A minimum setback from wells, surface water, property lines, and buildings applies; these distances are set in Part 8 and must be verified on site.

Soil with very slow permeability (high T-time) or very fast permeability (sandy, coarse) may need a filter bed or mound to provide adequate treatment.

Seasonal high water table shall be determined from soil mottling, monitoring wells, or local records; it sets the elevation of the trench bottom.

Lot size and reserve area must allow a replacement system; a reserve area of comparable size to the primary area is typically required.

Design flow for a single-family dwelling is typically based on 1,000 to 1,500 L/day, or a per-bedroom or per-person allowance set by Part 8.

Daily flow estimates use fixture counts, occupancy, and water-use assumptions; non-residential flows are based on use type and occupant load.

Typical residential sewage strength is about 200 to 300 mg/L BOD5 and 200 to 300 mg/L suspended solids; grease and solids settle in the tank.

The design basis includes peak flow, organic loading, and hydraulic loading; the dispersal area is sized from the daily flow and soil loading rate.

Hydraulic loading rate is the volume applied per unit area per day and depends on soil texture and the treatment level provided.

A garbage grinder, high-flow fixtures, or a hot tub increases flow and organic load and must be included in the design basis.

Infiltration and inflow from footing drains or roof leaders shall not be connected to the sewage system; clear water connections are prohibited.

Design flow for a dwelling is based on the number of bedrooms or fixture units; the greater of the calculated flow and the minimum allowance applies.

A septic tank provides primary treatment by settling and flotation; typical liquid retention time is at least 24 hours at design flow.

Minimum tank capacity is set by Part 8 and is typically 3,600 L for a single-family dwelling, with additional capacity for extra bedrooms or fixtures.

Tanks must be watertight, have accessible inspection ports, and be constructed of durable materials such as concrete, fiberglass, or polyethylene.

Sludge and scum accumulate; the tank must be pumped when the combined sludge and scum depth reaches about one-third of the tank depth.

A two-compartment tank or a tank with an effluent filter improves solids retention and protects the dispersal field.

Effluent filters capture fine solids and must be cleaned regularly; a blocked filter can cause backup into the building.

Aerobic treatment units (ATUs) provide secondary treatment and require continuous power, regular maintenance, and effluent quality monitoring.

Grease interceptors are required for commercial kitchens and other high-grease sources before discharge to the sewage system.

A leaching bed consists of distribution pipe in gravel-filled trenches; trench bottom area is sized from the daily flow and the soil loading rate.

Distribution pipe shall be level or uniformly sloped, with perforations placed downward, and shall be surrounded by specified aggregate and geotextile.

A filter bed uses a sand or other filter medium to provide additional treatment where soil is marginal; effluent is applied to the filter surface or buried.

A mound system raises the dispersal area above natural grade using a sand fill; it is used where the water table or bedrock is close to the surface.

Trench spacing, sidewall overlap, and aggregate depth are specified in Part 8; trench width and spacing affect the effective absorption area.

Serial and distribution box layouts must provide equal flow to each trench; a distribution box must be level and accessible.

A reserve area of equal size must be identified and protected from construction traffic and future building expansion.

Dispersal systems shall not be located in areas subject to flooding or where surface water ponds over the trenches.

A pump chamber is sized to provide the required dose volume and to prevent pump short-cycling; the dose volume is typically the volume of the distribution pipe plus a fraction of the daily flow.

Dosing distributes effluent in controlled batches, improving treatment and resting the soil between doses.

Pump selection is based on total dynamic head (static lift plus friction losses) and the required dose rate.

Check valves and isolation valves are required on the discharge line to prevent backflow and to allow maintenance.

Alarms and high-level floats are required to warn of pump failure or high water in the chamber.

Controls shall be accessible, protected from water, and include a manual override for testing and emergency operation.

A pump chamber must be watertight and sized to hold the dose plus emergency storage; the alarm float is set below the inlet invert.

Gravity distribution is preferred where elevation allows; pumping is used where the site cannot achieve gravity flow.

Minimum setbacks from wells, surface water, property lines, and buildings are specified in Part 8 and must be measured horizontally from the system component.

A leaching bed shall not be located closer than the specified distance to a watercourse, lake, or pond; the setback protects surface water quality.

Setbacks to drinking-water wells are larger than to other features because of contamination risk; the required distance depends on well type.

Systems shall not be located under driveways, parking areas, or buildings, and must be accessible for maintenance and inspection.

Clearances from foundation drains, water lines, and buried utilities must be maintained to prevent cross-contamination and damage.

The system shall be located so that effluent does not surface or drain onto neighbouring property.

Setbacks from property lines apply to the entire system, including the reserve area, and must be shown on the site plan.

Where a variance is requested, the principal authority may require additional treatment or an engineered design to compensate for reduced setbacks.

Installation must follow the approved plans; any change requires approval before construction continues.

Inspections are required at specified stages: before covering the tank, before covering the distribution piping, and at final completion.

The installer must notify the inspector when the work is ready for inspection and must not cover work before authorization.

Maintenance includes regular tank pumping, filter cleaning, pump testing, and inspection of the dispersal area for ponding or odour.

A maintenance plan and record-keeping are required for systems with mechanical components such as pumps and ATUs.

The owner shall ensure the system is operated within its design capacity and shall not discharge prohibited wastes such as grease, solvents, or clear water.

Signs of failure include surfacing effluent, odour, slow drains, and soggy ground over the dispersal area; these require prompt repair.

As-built records and maintenance records should be retained and transferred to a new owner on sale of the property.

A permit application includes a site plan, design drawings, specifications, and the site evaluation report.

The site plan must show the lot boundaries, buildings, wells, watercourses, setbacks, the system layout, and the reserve area.

Design drawings must show tank size and location, distribution layout, pipe sizes and slopes, and pump chamber details where applicable.

The designer must be qualified; where required, drawings must be sealed by a professional engineer or designer.

A permit is valid for a specified period; work must commence within the time allowed and be completed within the permit term.

Changes to approved plans require an amendment to the permit before the work proceeds.

A final inspection and approval are required before the system is placed in service; the approval documents the as-built condition.

Records of permits, inspections, and maintenance must be kept and made available to the principal authority on request.