Questions and answers about wastewater

    Do you know what happens to your water when you pull the plug, flush the toilet or drain the washing machine? No? Well, it's worth knowing.

    This so-called 'wastewater' is not only a vital resource but, after treatment, will be released to our land, waterways or the ocean.

    Used water from toilets, showers, baths, kitchen sinks, laundries and industrial processes is known as wastewater. Domestic households produce an average of 200–300L of wastewater per person every day! Ninety-nine percent of this wastewater is water, the other one percent is the contaminating waste.

    Much of the wastewater we produce has been changed in a way that means it cannot be used again unless it is treated. Changes made to water that turn it into wastewater include:

    • warming or cooling it,
    • adding human wastes;
    • adding oil, grease or fat;
    • adding organic matter such as food wastes; and
    • adding poisons such as pesticides, some organic compounds, synthetic chemicals and heavy metals.

    Wastewater from the sewerage system is sent to the sewage treatment plant for treatment before it is released back into the environment.

    Where does wastewater come from?

    Sources of wastewater include homes, shops, offices and factories, farms, transport and fuel depots, vessels, quarries and mines.

    Water used in toilets, showers, baths, kitchen sinks and laundries in homes and offices is domestic wastewater.

    Wastewater from manufacturing and industrial operations such as food processing or metal refining is industrial or trade waste. This includes liquid waste from any process (e.g. water used to cool machinery or clean plant and equipment).

    Stormwater, a form of wastewater, is runoff that flows from agricultural and urban areas such as roofs, parks, gardens, roads, paths and gutters into stormwater drains, after rain. Stormwater flows untreated directly to local creeks or rivers, eventually reaching the ocean. For more information see the department's brochure Stormwater - An important natural resource to protect and Caring for our water.

    What is wastewater treatment?

    In Queensland, most wastewater is treated at sewage treatment plants. Wastewater is transported from domestic or industrial sites through a system of sewers and pump stations, known as sewerage reticulation, to a sewage treatment plant. Local governments build, maintain and operate most sewage treatment plants.

    Operators are licensed under the Environmental Protection Act 1994 to discharge treated wastewater at an acceptable environmental standard into waterways. The Act is administered by the department. The Department of Natural Resources advises local governments about managing, operating and maintaining sewerage systems and treatment plants.

    In unsewered areas, local governments may require householders to install individual or household sewage treatment systems to treat domestic wastewater from toilets, kitchens, bathrooms and laundries. The Department of Natural Resources authorises the use of household systems when they are proven to be effective. Local governments will approve their use within the local area.

    Most stormwater receives no treatment. In some new subdivisions, treatment of some stormwater to remove litter, sand and gravel has begun using gross pollutant traps.

    What happens in a sewage treatment plant?

    Wastewater treatment occurs in four stages:

    1—Primary treatment

    Removes solid matter. Larger solids, such as plastics and other objects wrongly discharged to sewers, are removed when wastewater is passed through screens. Smaller particles, such as sand, are removed in grit traps.

    Wastewater then flows into large tanks where solids settle and are removed as sludge. Grease and scum are skimmed from the surface.

    2—Secondary treatment

    Uses tiny living organisms knows as micro-organisms to break down and remove remaining dissolved wastes and fine particles. Micro-organisms and wastes are incorporated in the sludge.

    3—Nutrient removal

    Removes nitrogen and phosphorus nutrients that could cause algal blooms in our waterways and threaten aquatic life. Algal blooms can cause visual pollution; in some circumstances, use up dissolved oxygen essential for aquatic life; and some forms may be toxic.

    Nutrient removal is not available at all sewage treatment plants because it requires expensive specialised equipment. It is becoming more common in Queensland.

    Clear liquid effluent produced after treatment may still contain disease-causing micro-organisms. If this effluent is released into waterways such as rivers or the sea, the micro-organisms will eventually die out. Until this happens, these waterways could be a health risk. Where people use these waterways, effluent needs disinfection to make it safe for release.


    Removes disease-causing micro-organisms. Suitable and cost-effective disinfection methods for cities include adding chemicals to effluent and irradiation with ultraviolet light. In less populated areas, effluent may be held in lagoons or ponds for several weeks, allowing micro-organisms to die off before the effluent is released.

    How is waste water managed?

    Wastewater can only be disposed of as permitted by a licence under the Environmental Protection Act 1994, administered by the department. This usually means wastewater has to be treated or contaminants removed before it can be discharged to waterways. Most wastewater flows into the sewerage system.

    Under the Act, local governments administer approvals and licences for environmentally relevant activities (ERAs) involving wastewater releases that might have a local impact. The department administers approvals and licences to ERAs involving wastewater releases that might have a regional or statewide impact.

    The department also advises industries on how to manage wastewater effectively. For example, industry can:

    • avoid generating unnecessary wastewater,
    • minimise the amount of water used,
    • minimise the strength of contaminants,
    • treat and re-use wastewater,
    • use evaporation ponds,
    • dispose of wastewater to the sewerage system (with local government approval), and
    • treat wastewater before discharge to waterways or the ocean.

    The department's goals for water quality and strategies for achieving them are set out in the Environmental Protection (Water and Wetland Biodiversity) Policy 2019 (EPP). The EPP is an important tool for ensuring that the broad environmental protection measures included in the Environmental Protection Act 1994 are better defined when it comes to the specific issues of protecting our water.

    The EPP requires local governments to prepare and implement sewage management plans to ensure health and environmental standards are met and to minimise unnecessary discharges of pollutants from sewage treatment plants.

    Is wastewater treatment effective?

    Local governments, water utilities and industrial operators are required to monitor and report to the department wastewater discharges and the quality of waterways that receive wastewater. Tests can be carried out on the spot for water quality measures such as temperature, pH and dissolved oxygen. Otherwise, samples are taken for laboratory analysis. Often many tests are needed to establish the levels of each of the different pollutants such as oils, heavy metals and pesticides in water.

    Monitoring provides factual information about water quality and can confirm that licence conditions are being met. The information obtained through monitoring provides the basis for making water quality decisions.

    Industries must test their own discharges to ensure they meet their licence conditions. The department randomly checks industry to ensure that testing techniques and water quality standards are being maintained.

    If problems are found, the licence holder can be ordered to carry out works immediately to rectify problems. Should problems continue offenders can be prosecuted.

    You can help protect water quality by notifying the department or your local council of any noticeable change in water quality, such as fish kills or algal blooms, possibly caused by a discharge of wastewater.

    Future directions

    Scientists and engineers are searching for ways to reduce the quantity and improve the quality of wastewater treatment. Industries are seeking to comply with water quality criteria and are adopting best practice environmental management and cleaner production techniques.

    Local, State and Commonwealth Governments are encouraging individuals and companies to use water wisely through education programs, legislation and pricing.

    Industries are implementing management strategies to improve water quality by conserving water, eliminating use of contaminants and reducing degradation by treating wastewaters effectively before discharge.

    How can I help?

    Everyone can help to reduce the quantity and improve the quality of wastewater going to the sewage treatment plant.

    • Never connect stormwater drains to the sewerage system or sewerage drains to stormwater. This is illegal!
      • Stormwater drains take large amounts of water from roofs, buildings, land and paved areas after rain. Connecting toilets and other domestic waste drains to stormwater drains could result in wastewater flowing down open gutters. This is harmful to the environment and a serious heath hazard.
    • Compost your kitchen scraps
      • Disposing of kitchen scraps via an in-sink style garbage disposal units can place additional loads on sewage treatment systems and add nitrogen and phosphorus into our waterways. Try composting at home to convert your kitchen scraps and garden clippings into compost to fertilise your plants and enrich your soil. By composting kitchen scraps you can give nutrients to your garden, cut down on solid waste (household rubbish), conserve water and reduce wastewater disposal (water down the drain).
      • Because wastewater treatment is expensive, composting saves rate payers' money too!
    • Create less wastewater
      • Conserve water by turning the tap off when brushing your teeth, taking shorter showers, fixing dripping taps, using the washing machine only when you have a full load and installing a dual-flush toilet and water-saving shower nozzle all help reduce the amount of wastewater entering the sewerage system.
    • Never put harmful substances down sinks, toilets or stormwater drains
      • Substances including petrol, grease, oil, pesticides and herbicides, and solvents such as paint strippers should not be poured down sinks, toilets or stormwater drains. These substances are difficult to remove in the sewage treatment process and cause pollution problems in our local waterways.
      • Contact your local council for safe disposal options.
    • Use biodegradable and phosphate-free detergents or soap
      • Detergents that are phosphate free add fewer nutrients to the sewerage system. Read and compare detergent and washing powder labels at the supermarket, and select products that are more environmentally friendly. Alternatively, use pure soap or soap flakes for washing or try natural products such as borax, vinegar and old-fashioned elbow grease for cleaning.

    For more information about water pollution see Caring for our water.