Biosolids
Sustainable resource management - biosolids
At Melbourne Water, we aim to undertake sustainable activities that can continue into the future, reduce impacts on the environment over time and balance environmental, social and economic factors.
We treat about 92% of Melbourne's sewage, and since our treatment plants in Werribee and Bangholme were commissioned it has been standard practice to store the dried and stored sewage 'sludge' onsite. Despite the vast amount of land at our sewage treatment plants, stockpiling this treated sludge - known as biosolids - is not sustainable.
We are seeking new, beneficial uses for biosolids.
Melbourne Water is working to beneficially reuse 100% of biosolids produced at our sewage treatment plants by 2010. To meet this target, we are considering a range of options that vary in cost, commercial viability and social acceptance.
Biosolids have been used successfully in Australia for soil conditioning and potting mixes, composting, land rehabilitation, landscaping, forestry, brick manufacture, agriculture and silviculture.
Moving forward
Biosolids are the final outcome of the sewage treatment process. The sludge produced during sewage treatment is dried and stored for three years, creating biosolids. This storage time ensures that public health and environmental guidelines are met.
Some of the biosolids produced and stored at the Eastern Treatment Plant at Bangholme (right) have been used in landscaping, site rehabilitation and blended organic soil products. Biosolids stored at the Western Treatment Plant at Werribee are yet to be used offsite.
The stockpiles of biosolids at the Eastern Treatment Plant, which opened in 1975, are different from those at the Western Treatment Plant, which has been in operation since 1897. Sewage has been treated at the Western Treatment Plant for more than 100 years and the biosolids stored there contain different types and levels of contaminants to those at the Eastern Treatment Plant.
These physical differences are the reason we have set different targets for use of the biosolids generated at each of our treatment plants.
At the Eastern Treatment Plant, we aim to:
- Use biosolids generated each year beneficially by 2005
- Reduce the stockpile of biosolids to sustainable levels by 2010.
At the Western Treatment Plant, we aim to:
- Use biosolids generated each year beneficially by 2010
- Reduce the stockpile of biosolids to sustainable levels over the long-term.
Achieving our targets
Melbourne Water is examining a range of options for using biosolids based on EPA Victoria guidelines that aim to protect the environment and public health.
Eastern Treatment Plant
Biosolids from the Eastern Treatment Plant have been added to compost and sand to produce blended organic soils for gardens, lawns, landscaping, and for growing plants, flowers and vegetables. We are expanding this soil improvement scheme and investigating other similar opportunities.
We are also examining the use of biosolids in large-scale projects such as freeway construction, landfill and site rehabilitation.
One such project at Woodlands Estate in Braeside (right) has used biosolids from the plant to rehabilitate a quarry site and create a wetland to treat stormwater run-off. Some 110,000 tonnes - or four years of annual biosolids generation - has been used. The wetland lakes have been built on a foundation of biosolids.
An onsite landscaping project at the plant has also used about 75,000 tonnes of biosolids, or about three years of the Eastern Treatment Plant stockpile.
Western Treatment Plant
Biosolids produced at the Western Treatment Plant (right) are not currently used offsite, although Melbourne Water and Blue Circle Southern Cement are investigating the use of the plant's biosolids as fuel in the company's Waurn Ponds cement kiln.
Research into the use of dried sludge as a component of brick manufacture is also currently underway, and could be a sustainable use for sludge that has high levels of contaminants, as these are permanently contained within the brick. A pilot project by Re-Brick Pty Ltd is being undertaken as part of the Smart Water Fund program, a grants scheme managed by Melbourne Water and three retail water businesses.
New facilities are being provided to manage the increased amount of biosolids being generated as a result of upgrading sewage treatment lagoons. We are also studying treatment options for contaminated stockpiles over the long-term, including phytoremediation, in which pollutants are removed from soils by uptake into plants.
Investigations are also underway into the use of dried sludge as a fuel in power generation.
The treatment process
Eastern Treatment Plant
The Eastern Treatment Plant treats about 40% of Melbourne's sewage. Activated sludge technology is currently being trialled in the sewage treatment process at the plant. This provides conditions that encourage nitrogen-removing bacteria to multiply.
The treatment process at the plant includes screening to remove debris, sedimentation and biological processes to remove solids. Sewage sludge is discharged from digesters to drying pans, where it is mechanically turned and dried to a consistency resembling dry soil.
The sludge is then harvested and stored for three years to remove pathogens.
Western Treatment Plant
The Western Treatment Plant treats about 52% of Melbourne's sewage. Sewage is mostly treated in vast lagoon systems, which have been complemented by activated sludge technology as part of a major $124 million environmental upgrade to the plant. Sludge, which settles to the bottom of the lagoons, is pumped into a contained area where it dries to create biosolids.
More sludge will be generated as part of the upgrade, but it is likely to be of higher quality, providing greater scope for using the biosolids.
Developing new opportunities
The quality of biosolids at the Eastern and Western treatment plants varies markedly.
For many years, sewage flows to the Western Treatment Plant included industrial waste with contaminants that concentrated in the biosolids.
This has limited the options for using the resulting biosolids, particularly the older stockpiles. In this case, the most prudent management option is long-term storage onsite with a view to future remediation. Trade waste restrictions in recent years have improved the level of contaminants in the sewage flowing to the plant.
Trade waste restrictions and significantly less trade waste in the sewage flows entering the Eastern Treatment Plant have allowed for more uses of biosolids from the plant in land rehabilitation projects. Biosolids have also been used to create products such as blended organic soils after being diluted with sand, organic waste and other materials.
Storage space at our treatment plants is also a factor. The Western Treatment Plant is one of the largest sewage treatment plants in the world, occupying more than 11,000 hectares (about the size of Phillip Island). In contrast, the Eastern Treatment Plant covers 1000 hectares, and there is a more immediate need to develop a sustainable alternative to storing biosolids there.
In densely populated cities such as Paris and London, sewage sludge is burned to generate power. This addresses the limited land for storage and the prohibitive amount of truck movements required to move large amounts of sludge offsite from the treatment plants.
Melbourne Water is also investigating the use of biosolids from our treatment plants as an alternative source of power.
Because of a lack of space, many water authorities have their biosolids transported offsite. For example, Melbourne Water provides land for Barwon Water to store and treat biosolids at the Western Treatment Plant. Barwon Water's own biosolids storage facility at its Black Rock Treatment Plant between Barwon Heads and Torquay has been full for some time. We are also currently working with Barwon Water to investigate options for establishing a joint program for the beneficial use of biosolids.
In the past, Melbourne Water has not used the biosolids produced because of high costs and issues in moving biosolids offsite, and because of extensive space at our sewage treatment plants for storage.
But much has changed in recent years. Sustainability is a key business priority, and it is no longer appropriate for the stockpiles to grow.
EPA Victoria has signalled its intention to review the environmental risks associated with biosolids management practices. EPA Victoria licences for sewage treatment plants have also been amended to include a requirement for sustainable biosolids management.
International technologies are being reviewed to determine the potential uses for biosolids generated at our sewage treatment plants.
Research and development
Melbourne Water has undertaken studies to determine the quantity and quality of sludge stored at our Eastern and Western treatment plants.
This has enabled the biosolids to be classified under EPA Victoria guidelines, according to levels of pathogen and chemical contaminants, and will allow us to identify further options for the beneficial use of biosolids.
Melbourne Water is well positioned to take advantage of high technology processes that are being developed, and that are expected to become more reliable and cost-competitive over time.
We maintain an awareness of new technologies through literature review, industry contacts and study tours, and working closely with local suppliers to analyse the most appropriate schemes.
Managing trade waste
Chemical contaminants (heavy metals such as zinc, mercury, cadmium and copper) in biosolids limit options for beneficial use.
We have a long-term aim of reducing the levels of such metals in biosolids through changes to trade waste inputs to the sewerage system.
We are working with the Trade Waste Acceptance Advisory Committee, an independent group of scientific professionals, our retail customers and EPA Victoria to improve standards of trade waste, and to reduce the amount and impact of problem metals entering the sewerage system.
Emerging technologies
There is potential to use a range of technologies to extract the energy value of biosolids, which has been compared with brown coal.
Biosolids can be converted to methane, oil and electricity in high technology processes. However, these technologies are generally costly and require further investigation.
Thermal processes have been developed that extract energy and generate power from sludges. For example, incinerators handle all biosolids produced in the London metropolitan area, generating more than 11 megawatts a year of electricity, but the system still requires some 17,000 tonnes of ash to be disposed of each year.
An alternative is to use biosolids in cement kilns, where ash is locked into cement use.
Energy recovery is a capital-intensive process, in contrast to our air-drying system, which is cost-efficient but requires a large area of land.
Energy recovery is likely to become more attractive for large treatment plants driven by:
- Value of land and proximity of residences to treatment plants
- Odour management
- Technology advances
- Regulation and public attitudes to the application of biosolids to agricultural land.
These processes are becoming more feasible with the introduction of the Federal Government's Renewable Energy Certificates program, in which organisations that produce renewable energy receive certificates for each megawatt-hour of electrical energy generated, enabling them to sell surplus certificates. Melbourne Water is registered in this program as a generator of renewable energy through our hydro-electricity and power generation plants that use biogas produced during the sewage treatment process.
Other sources and useful links
EPA Victoria - www.epa.vic.gov.au
Department of Sustainability and Environment - www.dse.vic.gov.au
Downloads
- Biosolids (PDF, 149kb)