Primary vs secondary wastewater treatment: what is the difference?

Think about everything that goes down your household drains: toilet waste, soap, food particles, grease and water from showers and washing.

In a conventional septic tank, this mixture is held long enough for heavier solids to settle at the bottom. Fats and lighter material float to the top, forming a layer known as scum.

The liquid between those layers flows out to the disposal field. Bacteria in the tank also break down some of the waste, but plenty of dissolved waste and fine particles remain in the liquid.

That is primary treatment. It removes the bulk of the settleable material, while leaving the soil to do much of the remaining treatment.

Some waste will not separate out just by letting it sit. It stays dissolved in the water or floats through as tiny particles.

Secondary treatment uses bacteria and other organisms to consume more of that material. Depending on the system, wastewater may pass through a living filter bed or enter a chamber where air is added to support the bacteria.

Further filtering or settling then helps separate remaining particles from the treated water.

The result is less organic waste and fewer suspended solids leaving the system. In everyday terms, more of the cleaning happens inside the treatment system, before the water reaches your land.

The soil beneath a disposal field does an important job. It helps filter and treat wastewater as the water moves through it.

However, the soil has limits. Sending it wastewater with less organic material and fewer solids reduces the treatment work it needs to do and helps protect the disposal system from clogging.

Cleaner wastewater also allows methods such as pressure compensating drip irrigation, often shortened to PCDI. This distributes wastewater through small outlets along irrigation pipes. Those outlets need well-treated wastewater to help keep them clear.

That can give the designer more options for fitting the disposal area into your property.

No. Secondary-treated wastewater is not drinking water.

It may look much clearer, but appearance does not tell you whether harmful microorganisms are still present. Secondary treatment is one part of managing wastewater safely; the disposal method and separation from people and water sources still matter.

Where additional disinfection is needed, a system may include another treatment step, such as UV. That is separate from the basic primary-versus-secondary comparison.

A conventional septic tank can still be suitable where the assessed soil and disposal design can safely manage primary-treated wastewater, and local requirements allow it.

For example, a rural property with suitable soil, adequate separation from groundwater and enough usable disposal land may be able to use a straightforward septic system.

It is generally a simpler, lower-cost treatment option. WaterFlow supplies both concrete and plastic septic tanks because there are properties where that approach makes sense.

But “we have plenty of land” is not the whole answer. A large paddock that stays waterlogged may offer fewer options than a smaller area of suitable soil.

Secondary treatment may be needed when the proposed disposal method requires cleaner wastewater, or when the site assessment identifies a need for more treatment before discharge.

For instance, a design using fine drip irrigation needs secondary-treated wastewater. Ground conditions and nearby sensitive features can also influence the required treatment level and layout.

Heavy clay is a useful example of why treatment and drainage need separate attention. Cleaner wastewater can help protect the field, but it does not make clay absorb water faster. The designer still needs to spread the water at a rate the soil can handle.

You may see these terms when comparing systems:

WaterFlow’s range includes conventional septic tanks, NaturalFlow configurations with different treatment levels, BioPod secondary treatment and EconoTreat advanced secondary treatment. The specific model and configuration need to match the design requirements.

Advanced secondary treatment does not automatically mean every contaminant has been removed. If nitrogen reduction or disinfection is needed, ask specifically how the proposed system provides it.

No. Secondary describes the treatment achieved, rather than one particular technology.

Aerated systems use added air to support bacteria. Worm-based systems use organisms living within filter beds. Both approaches can provide secondary treatment when designed and configured to do so.

That is why “worm-based”, “aerated” and “secondary” are not interchangeable terms. One describes how a system works; the other describes the treatment it delivers.

The most useful question is:

“What treatment level does my property need, and what is the reason for it?”

Your designer should be able to explain whether the answer comes from the soil, the disposal method, the surrounding environment or applicable requirements.

Then ask which systems can meet that need, what the complete installation will cost and what maintenance each involves.

At WaterFlow, we can explain those differences in plain language and help you choose a system that provides the treatment your property needs.