Biosolids - Frequently Asked Questions
What are biosolids?
Is there a difference between biosolids and sewage sludge?
How are biosolids produced?
What do biosolids look like?
Do biosolids have an odour?
Can biosolids be recycled?
Why are biosolids considered beneficial?
How are biosolids used?
How are biosolids transported?
Are biosolids used in other countries?
How long have biosolids been in use?
How are biosolids regulated?
How does the Water Corporation determine if it is safe to apply biosolids?
How are biosolids classified?
How safe are biosolids?
Are there health risks if wildlife comes into contact with biosolids?
Do biosolids contain toxic pollutants?
Do biosolids breed flies?
How often are biosolids tested to ensure safety?
Is it safe to eat food that has been grown using biosolids?
Can biosolids pollute or contaminate surface and groundwater?
Future trends in Western Australia
Who can I contact for more information?
I want to know about...
Use the following email address to submit a question to Water Corporation and we will try to respond to you within two weeks.
wwt.businessadmin@watercorporation.com.au
What are biosolids?
Biosolids produced from wastewater sludge once it has undergone additional treatment to significantly reduce disease-causing pathogens and volatile organic matter. Essentially, biosolids are the stabilised, nutrient rich, organic solid residues from the wastewater treatment process.
Wastewater consists of wastes primarily from household areas such as the kitchen (dishwasher), laundry and the bathroom.
Strict state and federal regulations ensure biosolids are an excellent source of safe inorganic nutrients.
Only biosolids that meet strict quality standards for pollutants and pathogens can be land-applied for beneficial purposes. All other biosolids must be disposed of to landfill, or further processed until they meet the quality standard.
Substances that are not included under the term biosolids are animal manures, untreated septage (septic tanks), municipal solid waste and untreated wastewater sludges, hazardous wastes, industrial sludges (oil refinery and waste) and grit and screenings removed during the initial wastewater treatment process.
Is there a difference between biosolids and sewage sludge?
Sewage sludge is not biosolids. Sewage sludge is the solids that are collected from the wastewater process without any further treatment. Biosolids is the sludge that has undergone further treatment to significantly reduce disease-causing pathogens and volatile organic matter, producing a stabilised product suitable for beneficial use.
How are biosolids produced?
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The production of biosolids from Water Corporation’s metropolitan operations involves the following stages:
The first stage in the production process is mechanical screening, followed by settling and skimming for the removal of large floating objects, sand, grit and settleable solids.
The second stage is a biological breakdown and stabilisation process. Microorganisms in the wastewater feed on the sewage converting them into the natural, mostly organic solids, or sludge
At the third stage, the collected organic solids, or sludge, undergo extensive stabilisation by liming or anaerobic (without oxygen) digestion. This process stabilises the organic matter while reducing the quantity of solids and the level of pathogens (disease-causing microorganisms). In this process, a large amount of organic matter is biologically converted into methane gas that can be used by the treatment facility to generate power or heat for the process. The stabilised solids resulting from the digestion process are called biosolids.
Finally a polymer is added to assist in the mechanical dewatering of the biosolids. This results in a black, soft-textured substance with an earthy odour and easy handling properties.
Types of biosolids
Water Corporation produces two types of biosolids suitable for direct land application. They are:
- Biosolids cake: produced at both the Beenyup and Woodman Point wastewater treatment plants (WWTP). Raw sludge is stabilised by anaerobic digestion, producing biosolids. The liquid biosolids are dewatered, resulting in biosolids cake. The ‘cake’ has an average of 80% water, with the balance being solids.
- Lime-amended Biosolids: produced at the Subiaco WWTP. The raw sludge is first dewatered followed by stabilisation with the addition of lime.
Another more processed form of biosolids is produced by blending biosolids with green waste and other organic materials. This form of biosolids is suitable for use in domestic gardens, as well as public parks and gardens. Several private companies undertake this activity using biosolids produced by Water Corporation.
What do biosolids look like?
Biosolids cake produced by Water Corporation have a plasticine-like texture and are dark brown/black in colour. Once these biosolids are further dried they resemble soil.
Lime-amended biosolids have a similar texture to biosolids, but are light brown in colour.
Do biosolids have an odour?
Biosolids may have their own distinctive odour depending on the type of treatment it has been through. Some biosolids have a stronger odour that may be offensive to some people, while most biosolids have a slight musty, ammonia odour. Sulphur and ammonia compounds (both plant nutrients) in biosolids are normally the cause of these odours. Typically odours are reduced after the biosolids have been applied and incorporating into the topsoil.
Can biosolids be recycled?
Just as glass, aluminium cans and old newspapers can be recycled, so can biosolids. Various forms of biosolids have been recycled in many countries around the world for more than 50 years. Biosolids recycling varies and is dependant on quality. Biosolids recycling has included uses such as a renewable alternative fuel, as an additive to bitumen, concrete and brick manufacturing and as a mineral fertiliser replacement to promote plant growth.
Why are biosolids considered beneficial?
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Biosolids are rich in nutrients and organic matter so they are a good natural fertiliser and soil improver. The nutrients include nitrogen and phosphorus (elements essential for crop growth) and micronutrients such as copper, zinc and iron, which release slowly during plant growth.
Among their benefits, biosolids can:
- Improve crop production
Land application of biosolids that meet strict quality criteria help improve, replenish and maintain healthy soil and add important slow-release nutrients. Biosolids boost soil water-holding capacity and reduce topsoil runoff, which helps to increase crop yields. The largest component of biosolids is organic matter. It works as a soil conditioner to promote natural microbial activity, loosening clay and improving the consistency of soils. The improved texture of these treated soils promotes dense, healthy root growth, allowing better nutrient uptake by plants. - Enrich tree plantations
Biosolids can promote tree growth and increase forest productivity for certain tree species. Biosolids used on pine stands it can shorten pulpwood and lumber production cycles, particularly in marginally productive soils. As biosolids enhance vegetation growth, wildlife also benefits from more abundant understorey vegetation. - Reduce landfill
The use of biosolids through land application and other methods offers an environmentally sound alternative to disposal as it conserves landfill space for non-recyclable materials. - Improve economic returns
The productivity of farming in areas with marginal soils is improved by the addition of biosolids. This generates an income for the farmers by the beneficial use of a product that otherwise may have been a cost to dispose. - Provide topsoil for land used for recreational uses
In Australia's dry climate, composted biosolids provide much need organic material to increase soil moisture retention and promote plant growth for both domestic and landscape use.
How are biosolids used?
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Farmers and gardeners have been beneficially using biosolids for years because they help promote crop and plant growth. Unlike mineral fertilisers that have the potential for excess nutrients to leach into waterways, biosolids bind quickly to soil particles with nutrients slowly mineralising for plant growth over a longer period of time. Biosolids also assist with the water-holding capacity of the soil. This is particularly beneficial for Western Australia’s poorly structured, sandy soils.
In Western Australia biosolids can only be:
- applied directly to land at agronomical rates: the rate at which plants can up take the nutrients (most of the biosolids produced in Western Australia are used in this way);
- composted at a licensed facility where the end-product is suitable for use on home gardens or municipal parks and gardens, (about 20% of the overall biosolids cake production from the metropolitan wastewater treatment plants is used in this way); or
- disposed at a suitable landfill site.
Only biosolids that meet specific quality criteria are suitable for land application. In Western Australia, biosolids are used to grow broadacre crops such as wheat, oat, canola and lupins, as well as forestry applications.
Specifically designed spreaders apply accurate rates of biosolids across paddocks, after which it is incorporated into the soil.
Water Corporation has also applied biosolids as a nutrient source for tree growth in forests. Similar application methods to that used in broadacre agriculture are used for this.
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How are biosolids transported?
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Biosolids cake from Water Corporation's metropolitan wastewater treatment plants are transported in trailers, which are fitted with rubber-sealed solid lids that completely encase the load. Lime-amended biosolids are transporrted in enclosed metal silo containers, which have a sealed, solid door at the rear (see photograph opposite).
Are biosolids used in other countries?
Biosolids are used widely in the United States of America (USA), Europe and several other countries. For the past 40 years, field trials and intensive investigations have been undertaken in these countries to determine safe and environmentally sound biosolids management practices. Australia's national and state guidelines for the use of biosolids are founded on this experience.
How long have biosolids been in use?
Recycling sewage wastes is not a new concept. For thousands of years, Chinese society returned sewage to farmland to maintain soil structure and encourage plant growth. In parts of Europe and elsewhere, biosolids have been applied to agricultural land for more than a century.
In the USA, biosolids recycling is as old as farm reclamation. Today thousands of municipalities across the USA successfully recycle biosolids. Similarly, biosolids use has been practised in Australia for more than 20 years.
How are biosolids regulated?
In Western Australia, biosolids applied to agricultural land must comply with both the Environmental Protection Act and the Health Act, which are administered by the Department of Environment and Conservation; and the Department of Health respectively. Both of these bodies require that biosolids application comply with the Western Australian Guidelines for Direct Application of Biosolids (Guidelines), unless approved otherwise by these bodies.
In some cases instances, for biosolids applications greater than 1,000 tonnes, the Department of Environment and Conservation issues a licence under the Environmental Protection Act for the activity. These Licences are issued for the property concerned (Prescribed Premises) to the Owner/Occupier of that property.
In all cases, Licensed or otherwise, the Owner/Occupier of the property who is applying the biosolids is responsible for applying them in accordance with the Guidelines (and any specific additional approvals/commitments).
For any biosolids application the regulation of environmental requirements to ensure compliance, whether under the guidelines, a licence or general provisions of the Environmental Protection Act, is the responsibility of the Department of Environment and Conservation, which can be contacted on (08) 9222 7000.
Regulation of biosolids health requirements is the responsibility of the Department of Health, which can be contacted on (08) 9388 4999.
Water Corporation has no regulatory role in the application of biosolids. However, it maintains a key interest in their proper application.
The following guidelines are designed to ensure the application of biosolids is undertaken in a responsible manner that is safe for the environment and does not in any way endanger public health or livestock:
- Guidelines for Sewerage Systems Biosolids Management (the national guidelines) and Western Australian Guidelines for Direct Land Application of biosolids and biosolids products.
The state guidelines are based upon the National Water Quality Management Strategy and international experiences but have been adapted specifically to Western Australian conditions.
How does the Water Corporation determine if it is safe to apply biosolids?
Water Corporation evaluates a site's suitability to determine whether biosolids can be applied. The evaluation examines water supplies, soil characteristics, slopes, vegetation, crop needs, the distances to surface and groundwater and the level of access the public have to the site. This evaluation is then passed to the Department of Environment and Conservation; and Department of Health for approval prior to the application proceeding.
How are biosolids classified?
Biosolids must be ‘classified’ to be suitable for use. Primarily there are two parameters examined when determining the classification of biosolids: the pathogen grading (P) and the contaminate grading (C).
State and national guidelines require biosolids to be adequately treated to significantly reduce pathogens. Biosolids must also contain very low concentrations of heavy metals, organo-chlorides and pesticides to permit use.
Three classes of biosolids (P1C1, P2C2 & P3C2) are safe for use in direct land application such as agriculture and forestry where routine public contact does not occur.
Other biosolids classes and uses are:
- Class P1C1 – these biosolids can be safely used in home gardens and on lawns, parkland and golf courses.
- Class P2C1 and P2C2 - These two levels of biosolids are suitable for use in horticulture and low-level public contact areas, such as landscaping.
Water Corporation's Perth metropolitan wastewater treatment plants produce P3C2 biosolids, which easily meet the requirements for restricted use for direct land application.
How safe are biosolids?
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Biosolids have been used for centuries, with no demonstrated adverse effects on human health. One of the most important considerations in the production of biosolids is the treatment to reduce pathogens. The treatment processes include digestion, drying or temperature elevation such as the addition of lime or composting. When biosolids are applied to the land, conditions in the soil quickly kill any remaining pathogenic organisms.
In Western Australia, biosolids are typically applied to agricultural land for the production of broadacre crops such as canola, wheat, and oats. These crops are in turn processed after harvest, which further reduce the potential health risk.
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Are there health risks if wildlife comes into contact with biosolids?
Typically the pathogens found in sewage are sourced from human waste and the vast majority are removed during processing of the biosolids. The majority of these pathogens, in particular viruses, do not infect other animals as they are specific for humans. However, a few pathogens, such as some strains of the protozoan parasite Cryptosporidium, can infect certain domestic animals, including cattle and the bacterium Salmonella is known to infect young chickens. This is why the biosolids guidelines recommend a withholding period of 30 days before allowing stock to graze on, or domestic fowl access to, land following a biosolids application.
This withholding period is not required for domestic animals such as dogs and native wildlife as they can not become infected by human pathogens.
Do biosolids contain toxic pollutants?
Biosolids are routinely tested for metal concentrations to ensure compliance with state and national guidelines. These tests (for nine metals and nine pesticides and organo-chlorides) are based on extensive research and recommendations from the United States Environmental Protection Authority.
As municipal wastewater treatment facilities and systems have improved during the past 20 years, metal concentrations in biosolids have decreased dramatically and typically are well below national and state guidelines. Many of the trace metals in biosolids, including cadmium, lead, copper and zinc, enter wastewater from industrial drains and metal pipes in homes and businesses. Some of these metals, known as micronutrients, are essential in small amounts for plant survival. Others such as cadmium, (which is a common ingredient of toothpaste), are monitored very closely in biosolids to ensure they remain well below the guideline accepted values.
Organic compounds, including pesticides, solvents and polychlorinated biphenyls (PCBs) can be present in biosolids in concentrations near the lowest detectable limits. Studies have found the risks associated with these to be negligible.
Water authorities in Australia are constantly investigating other potential contaminants to ensure the beneficial use of biosolids does not compromise public health or the environment.
Do biosolids breed flies?
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Flies have been associated with man wherever settlement has occurred, with the first housefly having been recorded in Australia in 1849. There are several fly species that prefer to breed in moist organic matter such as animal manure, kitchen refuse, lawn clippings and rotten vegetables. Biosolids, being a moist organic product, have at times supported fly breeding, but this has only ever been found in stockpiled (not spread) biosolids. For most fly species, breeding occurs during the warmer, drier months of the year, which for south western part of Western Australia typically occurs from October through to May.
To reduce the potential of fly emergence in biosolids, Water Corporation commissioned an independent research consultancy in March 2000. The entomologists continue to investigate and make recommendations to Water Corporation to assist with ongoing improvements. If fly emergence is detected Water Corporation will initiate the contingency best suited for that particular incident, which may include chemical treatment of the biosolids or immediate spreading of the biosolids.
Of all the recommendations to date, minimising the duration that biosolids are held on site before being applied to the land and not applying biosolids in identified fly-prone areas, such as the Wanneroo locality, have been the most effective in reducing fly emergence.
Water Corporation considers the most effective long-term management in the prevention of fly emergence is to temporarily contain the biosolids cake in a storage facility prior to it be applied to the land.
In 2009, an above-ground biosolids storage facility was built and trialled by Water Corporation near the town of Moora, about 200kn from Perth in the State's Mid West. The main purpose of the facility was to gauge the effectiveness of containing flies and to prevent fly emergence to the environment.
How often are biosolids tested to ensure safety?
As a minimum, Water Corporation biosolids are regularly analysed to ensure they meet the quality standard for direct land application. Nutrients, metals, pesticides and pathogens are analysed by an external, quality-assured laboratory.
Perth’s three main wastewater treatment plants have established a monitoring program, whic is the equivalent to monitoring every 100 dry tonnes of biosolids produced. Biosolids can be monitored as frequently as weekly. In addition, the wastewater treatment process is assessed to establish stabilisation; and for treated sludge to be classified as biosolids.
Is it safe to eat food that has been grown using biosolids?
Yes. The USA has conducted more than 30 years of scientific research. This includes a United States' Environmental Protection Authority risk assessment of biosolids use. These studies included the independent evaluation of data on organic chemicals, viruses and parasites that might be found in biosolids. The research examined how biosolids could directly or indirectly affect human health and the environment and established minimum safe concentrations for biosolids applications to land. It concluded that if correctly managed, biosolids application to land has no adverse environmental or health impacts.
Similar research has been conducted throughout Australia in the past 15 years.
Can biosolids pollute or contaminate surface and groundwater?
Biosolids pose little risk to ground or surface water quality if applied and managed correctly. As with any fertiliser the main water quality concern is the use of nitrogen and phosphorus, the application of which is regulated. Unlike commercial mineral fertilisers about 85% of the nitrogen in biosolids is present in a slow-release organic form, making biosolids less likely to cause water pollution from the release of nitrates (the mobile form of nitrogen) or phosphates.
Regulations limit the use of biosolids on agricultural land to strict agronomic loading rates, the rate that is necessary for crop growth but which minimises passage beyond the root zone. In addition, when biosolids are applied to land with appropriate soil pH the solubility of metals is controlled, preventing them from moving into groundwater. State regulations require that the soil pH be maintained at 5.0 or higher to ensure that metals do not contaminate soil and groundwater
Future trends in Western Australia
The beneficial use of all forms of biosolids from Perth metropolitan wastewater plants is close to 100%.
Biosolids production from the Perth metropolitan area is expected to increase due to:
+ new wastewater treatment process enhancements, and;
+ increases in wastewater inflow to metropolitan WWTPs.
Due to the unique soil and climate conditions in Western Australia, the demand for biosolids products outweighs supply. Ths trend is expected to continue, particularly as research consistently shows biosolids as a safe source of fertiliser, which meets plant nutrient needs and has no detrimental impact on the environment.
Who can I contact for more information?
| Department of Environment and Conservation | Enquiries related to biosolids land application licences and approvals. | Licensing Office | (08) 6250 8000 |
Enquiries related the Western Australian Guidelines for Direct Land Application of Biosolids and Biosolids Products. | Licensing and Policy Office | (08) 9222 7000 | |
| Department of Health | Enquiries related to the Western Australian Guidelines for Direct Land Application of Biosolids and Biosolids Products. | Department of Health Waste Management Branch | (08) 9388 4999 |
| Water Corporation | All other enquiries. | Faults, Emergencies and Security | 13 13 75 |
