In this article we cover:

  • The stages of treating wastewater
  • Quality performance criteria
  • Most common system types
  • Graphic comparing the types

Here’s an overview of the types of domestic sewage systems available today.

Generally, the stages of treating domestic sewage will involve settlement to begin with, where solids are separated out from the liquid. This is followed by secondary treatment, in which biological processes get to work to break down the organic matter. A final process will use a filtration system, or in some cases, UV treatment to disinfect the water before it is discharged.

 The quality of effluent is chiefly based on the degree of reduction of biological oxygen demand (BOD). In Northern Ireland, discharge to a waterway will usually require effluent quality of 97.5 per cent BOD reduction or greater and discharge to a sub-surface irrigation system will usually require effluent quality of 95 per cent BOD reduction or greater.

 Other quality performance criteria includes the levels of suspended solids (SS) or total suspended solids (TSS), chemical oxygen demand (CoD), ammonia, phosphate and nitrate remaining in the treated effluent.

In Ireland, the system must comply to SR66 and the Environmental Protection Agency (EPA) Code of Practice for Domestic Waste Water Treatment Systems – Population Equivalent of less than 10 (published in 2021)

System types

Package wastewater treatment plants (PWWTPs) are nowadays the most commonly used solution for treating effluent before it is discharged. These systems introduce air into the effluent during the secondary stage, using electrical air pumps or motors to drive a mechanical system, circulating air through the sewage, which promotes the growth of aerobic bacteria to break down the sewage, producing a higher quality effluent.

 The older standard block-built septic tanks with percolating filter beds performed a similar function as a PWWTP but without the added aeration function and do not produce as high a quality of effluent. They would not meet the requirements for discharge to a waterway nowadays. The percolating filter bed (usually filled with gravel) needs to be properly maintained. Other designed systems may incorporate the use of reed beds, constructed wetland systems, soakaway mounds and other methods that further improve the quality of effluent before final discharge. Other options include tertiary treatment, e.g. drip systems.

 Prefabricated septic tank systems are usually constructed from plastic, glass reinforced plastic (GRP) or concrete. They were intended to be connected to a sub-surface irrigation or other filtration system and not directly to a waterway. Many are still found serving rural dwellings today in the form of the widespread ‘bottle’ tank and similar systems.

 Cess pits are less often encountered nowadays, but they still exist on some older properties. As they do not process the sewage in any way, they must not be permitted to leak or overflow, so must be regularly emptied (at a cost) by the local council or a licenced waste disposal specialist.

Other designed systems may incorporate the use of reed beds, constructed wetland systems, soakaway mounds and other methods that further improve the quality of effluent before final discharge. Other options include tertiary treatment, e.g. drip systems.

Note that the word ‘soakaway’ should not be used to describe a sub-surface irrigation system for effluent discharge. A soakaway is usually what you would use to receive surface water running off roofs and other hard surfaces. The two perform different jobs and should neither be physically linked in any way nor should one be confused with the other.

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