As urbanisation increases throughout New Zealand local bodies and councils have been seeking innovative ways to safely dispose of biosolids in an environmentally sustainable way.
Biosolids are sewage sludges (the semi liquid end products of sewage treatment) that sewage companies often treat and stabilise to make them safe to spray on pastures where they help to fertilise and condition the soil as they contain beneficial nutrients (e.g., nitrogen and phosphorus). Unfortunately, biosolids may also contain other less desirable compounds (e.g. heavy metals such as cadmium, zinc and lead), often produced from industrial activity, that may be toxic to humans and plant life when they are used as fertilisers.
“In New Zealand there are 250 public wastewater treatment plants from which 77 000 tonnes of dry solids could be beneficially used.”
PhD student Zahra (Giti) Talebi Gheshlaghi from the Soils and Physical Sciences Group at Lincoln University is undertaking research into the pre-treatment of biosolids- amended soils with different compounds to see which one best binds up (immobilizes) these undesirable chemicals and prevents them from causing harm.
“At present, the application of biosolids to land is strictly controlled to restrict the build up of metals to potentially harmful levels,” said Giti. “My work involves finding a way to treat the soils that have received biosolids to render the metals unavailable for plant uptake (the process is called remediation).”
Giti’s results showed that after treatment of the biolsolids-amended soils with different waste products, some of these products did reduce the levels of bioavailable heavy metals - bioavailable means able to be taken up by living things.
On the practical level, her research involved taking soil samples from medium and highly contaminated sites where biosolids had been applied since the 1960s, then growing sunflowers in each soil after treating then with inexpensive products such as iron waste, treated zeolite and lime (at high and low levels of application).
Giti then analysed the levels of the contaminant chemicals in the soil and the plants to find out what treatment worked best; as shown by the reduced levels of contaminant metals in the plants in comparison with the soil.
Her results showed that the addition of different rates and combinations of the three materials significantly decreased the uptake of nickel and zinc by the plant shoots. There was no reduction in the copper uptake by the plants
Professor of Environmental Soil Science, Ron McLaren, who is supervising Giti’s research, says that Giti’s work has the potential to improve the management of biosolids treated soils and increase the use of industrial waste products such as iron waste for the remediation of metal-contaminated soils.
The research is on-going.
Giti has most recently presented her work at the New Zealand Soil Science conference in Rotorua.
FOR FURTHER INFORMATION CONTACT
Janette Busch, Technical Writer
Lincoln University, Canterbury
Tel: 64 3 325 2811 ext 8114
Email: Janette Busch