Definition

More sophisticated unit with equipment in multi-chambered tanks to treat sewage and produce clean water.

A covered, porous-walled chamber that allows water to slowly soak into the ground.

Multi-chambered tank with an outlet.

A cesspool is a sewage holding tank with no outlet.

How does it work?

Package sewage treatment plants create an environment, which facilitates the growth of bacteria which break down sewage into non- polluting end products. Not only does primary treatment take place but also secondary treatment. This requires an electricity supply, which is used to artificially introduce air to the treatment plant; it is this oxygen transfer through the sewage which enables the growth of aerobic bacteria which are more effective in the breakdown of sewage than the bacteria present in a septic tank. This results in a higher quality effluent being produced, which can (subject to Environment Agency Consent to Discharge) be discharged directly to a watercourse.

Pre-settled effluent from a collection and storage/treatment is discharged to the underground chamber from which it infiltrates into the surrounding soil.

Primary tanks facilitate primary treatment to take place (the separation of liquids and solids by gravity). Sewage flows into the tank and the heavy solids ‘sludge’ sink to the bottom, lighter solids, grease and oils or ‘scum’ float to the surface. Some of the sludge is degraded by naturally occurring anaerobic (without oxygen) bacteria. The liquid effluent flows via gravity out of the tank and discharges to land by soakaway.

Sewage flows in and is stored, when the tank is full the waste is tankered away.

Site Suitability

From single domestic house up to thousands of occupants

• Single domestic house or small developments
• Where there is sufficient porosity in the ground to allow for soakaway.

Best suited for soil with good absorptive properties; clay, hard packed or rocky soil is not appropriate

• Sites where the ground is unsuitable for the waste to soak away to ground.
• Sensitive sites e.g. sites close to drinking water supplies

Advantages

• Sewage treated to higher standard.
• Treated effluent can be reused for irrigation, landscaping, dust suppression etc.
• No bad odors in compound.
• Regular exhauster costs eliminated.
• Reduced water costs as recycled water is used for secondary purposes instead of paying for fresh water.
• Suitable for Large scale developments.
• Simple installation.
• Minimal basic Maintenance requirements.
• Simple installation.
• Elimination of pollution for fresh water and underground sources.
• Increased property value.
• No need for separation of greywater and black water.

• Can be built and repaired with locally available materials.
• Small land area required.
• Low capital and operating costs

• Relatively low installation cost .
• Some minimal treatment.
• Do not require electricity.

• Low installation cost.
• Do not require electricity.

Disadvantages

High initial installation cost. – Requires minimal electricity to operate.

• Primary treatment is required to prevent clogging
• Soak pits swamp get flooded, especially in soils with poor drainage.
• Soak pits need separation of greywater and black water.
• Recycled water can’t be reused.
• not suitable for large developments.
• Decreased property value, especially if the soak pit water floods the ground surface.

• Incomplete treatment of sewage possibility of pollution.
• Regular exhauster costs.
• Bad Oduors.
• Flooding, overflow, backflow risks where porosity is a problem.
• Effluent cannot be re-used.

• No treatment of sewage possibility of pollution.
• Regular exhauster costs.
• Bad Oduors.
• Flooding, overflow, backflow risks.