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Primary and Tertiary wastewater systems – what’s the difference?

Primary on-site wastewater systems.

Commonly known as a septic tank and soakage trench or bed the traditional septic tank and soakage trench systems worked well for large lots (1/4 acre or more), but had limitations in clay soils (poor soakage conditions) or gravelly soils (too rapid soakage resulting in pollution of wells). For small lot subdivisions, septic tank effluent resulted in groundwater contamination in some locations. Un-maintained septic tanks exacerbated these problems. A primary systems do not treat wastewater in any way, they simply work to reduce solids before discharging the liquid to the soakage trench.

Secondary on-site wastewater systems.

On-site wastewater treatment has made rapid progress in the last 20 to 30 years due to advances in design methods, manufacturing techniques, and materials for enhancing and supporting small-scale wastewater treatment processes. A secondary systems does pre-treat wastewater before discharging in a controlled manner to a land treatment area on-site. Secondary systems are able to be used in less than ideal soil conditions and work to reduce harmful groundwater contamination.

There is also a greater understanding of the role of soils and soil bacteria in adsorbing and treating waste nutrient material in on-site land-application areas. This has resulted in much wider choices in providing pre-treatment of domestic wastewaters and then subsequent discharge to a land treatment area on-site.

Primary Treatment versus Secondary Treatment

Traditional septic tank systems provide only ‘primary’ treatment and require ample topsoil and organic matter to enable bacteria to break down septic tank effluent and allow the water in the effluent to soak away effectively. The soil provides the majority of the treatment in a septic tank and soakage trench system. Poor soils can be a contributing factor to system failure. The ‘secondary’ treatment process provided by modern treatment systems (aerated treatment or modern septic tank and packed bed filter systems) breaks down most of the waste matter and creates nutrients for plant growth. The soil is a less important part of the treatment process, but is still required to capture bacteria and viruses and enable them to die off with time.

So which is the best method?

Usually the most environmentally effective on-site wastewater system comprises either an aerobic treatment plant, or a modern septic tank and packed bed (sand or textile filter) treatment plant, both of which produce high quality effluent for use in supporting plant growth in landscaped areas via dripline irrigation.