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Class 4 Back to all types of systems The most common type of sewage system, which is used to service single-family dwellings, multiple units, and commercial premises is Class 4. It is typically composed of a treatment unit (septic tank/tertiary treatment unit) and a leaching bed.
Sewage Systems  Typical Raised	The leaching bed is comprised of absorption trenches and distribution pipes which treats the sewage effluent (liquid portion) by a combination of physical, chemical and biological reactions within the soil mass.
Leaching Bed Typical Septic Tank (Class 4)	A septic tank is a watertight vault with two compartments which raw sewage accumulates. The main purpose of the septic tank is to collect the sewage and separate solid materials from the liquids. The heavier solids settle to the bottom of the tank (sludge layer) and the lighter oils and greases rise to the top forming the scum layer. The liquid in the septic tank then flows by displacement into the leaching bed.
Tile Trench Construction	The leaching bed is comprised of absorption trenches and distribution pipes which treats the sewage effluent (liquid portion) by a combination of physical, chemical and biological reactions within the soil mass.
Treatment of Effluent by the Soil	The sewage effluent (liquid portion) exits the septic tank and moves into the tile trench which treats the sewage effluent by a combination of physical, chemical and biological reactions within the soil mass.
Class 4 Filter Bed	Use of a Filter Bed The filter bed is an alternative type of leaching bed. It may be used on sites where there is insufficient area for a conventional bed, or where there are natural features, such as treed areas, that minimize the available leaching bed area. Unlike an absorption trench leaching bed, in which each line of the distribution pipe is in a separate trench, a filter bed allows the sewage effluent to enter a series of distribution pipes that are set in a common layer of stone. The stone layer covers a surface of filter sand overlying the native soils. Filter beds can be installed either in-ground, partially raised, partly excavated or fully raised as shown in Figures 5.9.3 to 5.9.6.