Biosolids
What are biosolids?
Biosolids are the organic wastewater solids treated to a standard suitable for reuse in the environment. Biosolids are normally a damp soil-like material that contain nutrients – particularly nitrogen, trace metals at low concentrations, and bacteria. With a high organic content they improve soil moisture retention and plant growth. When applied to the soil in a controlled manner, biosolids are a beneficial soil conditioner and fertiliser. Biosolids can be fully dried to a flowable granule using heat and can then be used as a fuel source in boilers or as an easily spread fertiliser.
If we value the future or our natural resources, care and attention must be given to what we produce and how we use it. Each year approximately 6,000 dry tonnes of biosolids are produced from the Christchurch Wastewater Treatment Plant. This is a contribution of 1 kilogram of nitrogen per person per year. The biosolids are the result of treating the wastewater to a standard suitable for discharge to the coastal environment. Christchurch City Council is about to embark on a new strategy of drying biosolids using landfill gas with the primary intention of using the dried material as a fuel source to replace coal.
What has been done in the past?
Since 1900, wastewater solids in Christchurch have been applied to land. In the early years of sewage treatment, both the liquid and the solids were applied to land near the plant at Bromley.
From the commissioning of the new plant in 1962, the treated wastewater solids have been applied to council farmland near the Estuary. The biosolids have also been used on old landfill areas in Bexley. These lands are still being used to fatten cattle.
In addition, biosolids have also been used to re-soil and rejuvenate both Cuthberts Green and Rangers Park in Christchurch. Since 1997 the biosolids have been used primarily in the restoration of the now closed Burwood Landfill. The restoration is expected to be complete in 2008. Biosolids in form of damp cake have also been applied to forests as a fertiliser in the City Council area and further south near Dunsandel.
Why the new strategy?
With the final capping and restoration of the Burwood Landfill nearing completion, and the reduction in forest areas available for biosolids application other opportunities for the use of biosolids need to be found. In 2005 the Council asked the University of Canterbury Waste to Resources Group to lead a consultation process involving all stakeholder groups to recommend to the Council the preferred future direction for the use of biosolids. A consultation process that was new to New Zealand was used and the process finally recommended to the Council that use of the biosolids as a fuel source was the preferred option, with application to non-food producing land as the other option. The “backstop” option is to dump the biosolids into the Kate Valley landfill. The consultation process and recommendations are documented in Invitation to Design the Future Christchurch Scenario Workshop on Biosolids Management [PDF 615KB].
The advantages of drying the biosolids mean that the product can be used as a fuel source, has lower transportation costs than the current dewatered cake, achieves a grade “A” pathogen standard (pathogen free) and when using landfill gas as the fuel for drying has a lower greenhouse gas profile than other options.
Proposed Biosolids Drying Facility
The Christchurch City Council proposes building a biosolids drying facility at the Christchurch Wastewater Treatment Plant and has applied for Resource Consent from Environment Canterbury (ECan) to do so.
The drying facility would reduce the water content of the biosolids from 80% to less than 10%. The drier biosolids could be used as fuel or fertiliser, and the volume of biosolids sent to landfill would greatly reduce.
More information can be found here:
 These documents are PDFs. You will require the free Adobe Reader to view them.
Reports on air discharges and environmental effects have been prepared on the proposed biosolids drying facility and can be read below:
What are the legal requirements?
Use of the dried biosolids as a fuel will require a consent in the same way as a consent is required for a coal fired boiler. The dried biosolids has approximately half the heating value of coal and would be used to replace a small proportion of the coal fed to a boiler. Obtaining such a consent within Christchurch, would have its challenges. Land application of the biosolids either as dewatered cake or as a dried product requires consent which includes the assessment of a wide range of possible environmental effects. Conditions on these consents would also include monitoring requirements.
What do biosolids look and smell like?
The fully dried product has no smell and can be produced in a range of granule sizes. The dewatered cake is a damp soil like material that does have some odour when applied to land.
What do other countries do with their biosolids?
In Europe and the UK biosolids are applied to land, sometimes as a composted product, however in some countries there is a strong trend away from land application, and away from landfill dumping towards burning with coal. In the USA there is a wide variety of end uses for biosolids, including land application as a soil conditioner and fertiliser, landfill dumping, and composting. Drying is also used to produce a more readily usable product. In New Zealand most major centres dispose of their biosolids to landfill although some compost, and some dry their product.
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