Tech Corner – Conventional Septic Tank Systems

Tech corner will be a regular feature of the project newsletter aimed at providing basic 101 information on aspects of onsite wastewater systems. For the inaugural tech corner we thought we’d start with an overview of the components of conventional septic tank systems.

A conventional septic tank system comprises three major components: a septic tank, a distribution device and an absorption/infiltration field or trench (Figure 1).

The septic tank is a watertight single or multi-chambered tank that wastewater flows through (Figure 2a). The tank is primarily a settling tank where the organic matter contained in the wastewater settles and is decomposed (digested) by anaerobic bacteria (Crites and Tchobanoglous 1998).

Solids that are not digested either settle to the bottom as sludge or float to the top and form a scum layer, both of which must be periodically removed to ensure correct system operation. 

Wastewater from within the clear zone of the septic tank (i.e., between the scum and sludge layers (Figure 2a & b)) flows to a distribution device and then into the adsorption field where it is discharged to ground. The distribution device aims to split the flow equally to each pipe in the absorption field.

The absorption field or trench is a sub-surface system within the soil (typically a network of perforated pipes buried in free-draining material) which allows effluent to seep into the surrounding soil (both through the base and side walls of the trenches). Effluent from the septic tank typically flows via gravity through to the absorption field or trench. The effluent receives further treatment in the soil through filtering, and further bacterial action. The absorption field must be designed based on the properties of the soil at the site otherwise the system will not function properly.

An appropriately designed and installed septic tank system is capable of adequately managing wastewater, however inappropriately designed, sited and/or managed systems can fail.

Reasons for failure can include excessive water entering the system, lack of maintenance or improper system design (sizing and siting). Septic tank failure may be obvious (e.g., effluent emerging on the ground surface, drainage system backing up and overflows) or less conspicuous (e.g., inadequate treatment resulting in ground/surface water contamination). The cause of failure may be obvious, you may require a professional to help determine the cause.

One of the most common causes of failure of septic tank systems is the carryover of solids and oils/grease from the septic tank to the absorption field. These can be a result of overloading the system or from lack of maintenance. Use of appropriate grease traps and effluent filters help to protect the disposal field from solids and oils carry over from the septic tank. While grease traps and filters require periodic cleaning, this is far more cost effective than having to replace or refurbish the infiltration field or trench. Filters can also be retrofitted to existing septic tank outlets with relative ease.

Cleaning/desludging a septic tank is normally recommended every 3–5 years, but this can vary depending on the size of the septic tank relative to the wastewater load. Ideally sludge levels should be measured at least once a year and desludging done when the depth between the sludge and scum reduces to half or less of the water depth. If your septic tank requires very frequent desludging to stop back-ups (e.g., more than once per year), then it may be undersized and/or there are problems in the infiltration trenches.

For more detailed information on onsite wastewater management systems refer to Auckland Regional Council’s Technical Publication 58 “On-site Wastewater Systems: Design and Management Manual,” Third Edition ARC Technical Publication 2004, prepared by A. W. Ormiston (ARC Consultant) and R. E. Floyd.

References and links

Auckland Regional Council Technical Publication 58 (2004) On-site Wastewater Systems: Design and Management Manual, Third Edition ARC Tech. Publication 2004.

Crites, R. & Tchobanoglous, G. (1998) Small and decentralised wastewater management systems. Boston, WCB/McGraw-Hill.

Schultheis, R. A. (2010) Septic Tank/
Absorption Field Systems: A Homeowners Guide to Installation and Maintenance. University of Missouri-Columbia MU Extension, EQ401. MU Extension, University of Missouri- Columbia.

Conventional septic tank system [Source:].
Single chambered septic tank [Source:].
Dual chambered septic tank. [Source:].