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Sewage System Types

Class 4 Sewage System

A Class 4 sewage system is the most common type of sewage disposal system currently in use. The system consists of a septic tank or treatment unit and a subsurface disposal area for the sanitary sewage. Class 4 sewage systems are sized based on the total daily design sanitary sewage flow, (Q), and the percolation rate, (T), of the receiving soil. For residential dwellings, there are three determining factors that contribute to the total daily design sanitary sewage flow which are the total finished area (excluding the area of the finished basement), the number of bedrooms and the number of fixture units.


A septic tank is a primary treatment unit consisting of two compartments that receives the wastewater from the household plumbing. The capacity is based on the septic tank being sized to a minimum of twice the total daily design sanitary sewage flow but not less than 3600 L. The main function of the septic tank is to remove the solids and start the treatment process (anaerobic) on the sewage effluent.

Subsurface Sewage Disposal

The leaching bed or tile bed is the most common method used for the final disposal of the liquid portion of the wastewater stream. The leaching bed evenly distributes the effluent into the soil to continue the treatment process in an aerobic environment. The leaching bed is manifested in different forms such as conventional leaching bed, filter bed, shallow buried trench and area distribution bed.

There are other approved systems that fall under the Class 4 category but are too numerous and complicated to list. Most are tertiary treatment systems that replace the septic tank with an aerobic treatment plant. The final effluent disposal is either via a leaching bed or a uniform stone layer overlying an unsaturated sand layer.

Further information can be accessed by visiting the Ministry of Municipal Affairs and Housing Building and Development Branch Website.