On-site Wastewater Systems

Wastewater is really wasted water. It is a resource we cannot afford to literally throw away. There are three main ways in which our daily household wastewater is treated. Many urbanised and modern cities have a reticulated sewer system, whereby household wastewater is piped to a central treatment plant and then pumped to discharge – either to the ocean or on land.

This is an expensive way to treat wastes, simply because of the huge investment in infrastructure, operation and maintenance that is undertaken for these large-scale projects.

Did you know?

The aim of sewage treatment is to remove or reduce the following pollutants:

• organic matter (monitored as BOD – Biochemical Oxygen Demand)
• suspended solids
• nutrients such as nitrogen and phosphorus, too much of which negatively impact on our waterways
• pathogens (organisms that cause disease).

If household wastewater is not piped off the property then some type of on-site treatment plant is required. The first, and cheapest, option is the septic tank. This can be two tanks (or one tank with a baffle to separate two chambers or compartments) that are essentially an anaerobic digestion system (hence the term ‘septic’).

The first tank or chamber is the primary sedimentation tank. It holds all of the solids from the household, and much of this is broken down by bacteria and other micro-organisms. Treated effluent is drawn from the middle of the first tank and enters the second tank where further digestion of the wastes occurs.

Septic tanks are designed to hold all of the daily wastes for a few days to enable break down and separation. The retention time that wastes are held is crucial to the success of waste digestion, and if too much wastewater enters the system then the bacteria don’t have enough time to break it all down.

Partially-treated effluent is then passed into some type of soil absorption system. This may be leach drains, stone-filled beds or trenches, or soak wells. The size of the leach field is proportional to the daily volume of wastewater and inversely proportional to the porosity of the soil. For example, if the soil is clayey in nature and water only percolates slowly into it, then the drains need to be larger or longer.

All wastewater treatment systems require maintenance. Unfortunately, most people ignore the septic tank until it floods or starts to smell or the toilets back up and a plumber is called. Once the tanks and drains are buried most people have an ‘out of sight, out of mind’ attitude.

In non-sewered areas, septic tank systems are still the norm. However, sometimes the soil is not suitable for continuous discharge and there may be sensitive waterways nearby or a high water table. As more councils require stringent effluent discharge quality, the installation of secondary treatment systems is steadily increasing. These whole-of-house systems treat all wastewater to a much higher standard and the discharge water can then be used to irrigate gardens.

An Aerated Wastewater Treatment System (AWTS) is also called an Aerobic Treatment Unit (ATU) and they provide both primary and secondary treatment of domestic wastewater. Typically, they consist of several chambers (or even separate tanks) where some combination of biological, physical and chemical processes is employed to remove the pollutants.

The first chamber is the primary sedimentation chamber and it operates in a similar manner to a septic tank. Anaerobic bacteria digest much of the wastes and produce various gases such as carbon dioxide, methane and nitrogen, which are vented.

The second stage is the aeration stage. A blower pumps air, either continuously or on a cycle, through a diffuser, which forces air bubbles into and throughout the wastewater effluent. Different types of bacteria exist when air is plentiful and the chemical processes that occur are different also.

Once all of the effluent has been stirred up it needs time to allow the floc (minute undigested or insoluble particles) to settle to the bottom of the clarification chamber as sludge.

Every septic tank and AWTS will accumulate scum (floating on top) and sludge (muck that sinks to the bottom). This is because the bacteria that digest human wastes are not that good at breaking down oil and fats (scum) and some solids (and this includes toothpicks, cigarette butts, plastic, condoms, cotton buds and lumps of toothpaste, which unfortunately end up in the system).

Much of the sludge is also dead bacteria. Eventually every tank needs to be pumped out and desludged, most often between 3 to 10 years, depending on the system and what enters the system.

Some of the sludge in the clarification chamber is returned, often by an air-driven lift pump, back to the primary chamber. Monitoring the volume of sludge in the aerated mix is also a good indication of when the tanks need pumping out by a liquid waste contractor.

The clear, settled liquid is then passed through some type of disinfection process, using chlorine tablets, UV light or ozone, and then into the pump chamber. As the treated water rises in this chamber, the float switch activates and water is pumped to the irrigation area.

All of these very complex processes are designed to treat sewage to a level suitable for surface irrigation. This includes dripline or special sprinklers, which only permit a short width and height spray so that the treated effluent is not showered on buildings, paths, animals and people.

Every state or country has rules and regulations about how much irrigation is required, plume (spray) height and the setback distances from buildings and boundaries.

Aerated treatment systems are biological systems and require servicing and maintenance by experienced, registered service technicians. They are more expensive than septic tank systems to install and maintain.

Furthermore, pumps and blowers use electricity and occasionally need replacement. All of this adds on-going costs to the overall operation of the system, but about 200,000 L each year can be used to irrigate lawns and gardens.

Did you know?

Most municipal wastewater treatment plants are designed for tertiary treatment. This is a further stage of wastewater processing before any effluent is released to the receiving environment. Here, more nutrients are removed (so less nitrogen and phosphorus), greater filtration and settling may occur to produce higher-quality effluent and greater levels of disinfection ensure complete pathogen kill.