On-site Wastewater Systems
The horizontal flow reedbed
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.
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).
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.
Double septic tank system
A singLe septic tank may have a baffle to separate chambers
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 alternating leach field
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.
Wetland plants getting established in the vertical flow reedbed
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.
A cross-section of an AWTS
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.
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.
Water is a resource that we all depend on and equally so, it's a resource that we're all responsible to use in a sustainable way. From simple water conservation measures through to complete household wastewater treatment and reuse systems, there are ways that any household can play their role in conserving this precious resource.
Eco Wastewater Solutions
In non-sewered areas, septic tanks systems were once the norm. As more Councils require stringent effluent discharge quality, the installation of aerobic treatment units (ATU) is steadily increasing. These whole-house systems treat all wastes to a secondary standard and the discharge water can then be used to irrigate gardens.
Dr Ross Mars