Australasian Science: Australia's authority on science since 1938

Carbon Emissions Can be Offset by Sewage

By Stephen Luntz

A small but significant portion of Australia’s carbon emissions could be offset by using solid waste from sewage treatment to improve soils, according to the CRC for Contamination Assessment and Remediation of the Environment (CRC CARE).

The majority of the biosolids produced by sewage treatment farms in Australia are already applied to soils, says Prof Nanthi Bolan. However, he adds that “currently Australia stockpiles about 70,000 tonnes of biosolids a year from its major urban sewage works”.

In other countries sewage waste is often contaminated by heavy metals from industrial sources, but this is not generally a problem in Australia. Instead the obstacle here has been the transportation costs to shift biosolids from treatment plants to locations where they can be of use. “Our research indicates this can be offset not only by the boost to fertility and soil organic structure, but also by its ability to increase carbon retention in the soil,” Bolan says.

Biosolids are around 30% carbon, and some of this is in forms that tend to stay in the soil for long periods of time. Bolan says that if the water content is brought below 50% during processing, and iron oxide and clays are added, this stable carbon can be maximised. Meanwhile, the nutrients in the biosolids promote plant growth, leading to higher carbon levels in the soil.

“Once we tried to encourage decomposition of carbon from waste in order to increase soil fertility. Now there is a lot of emphasis on biochar (AS, November 2010, p.7), so the pendulum has gone from encouraging decomposition to restricting decomposition,” Bolan says.

“However, the problem is that you release some carbon in making biochar. We think there is a middle way where you keep some carbon in non-degradable form.”

The amount of carbon that biosolids can lock into soil depends on factors such as the type of crop grown, how the soil is cultivated and the way the biosolid is produced, but Bolan told the CleanUp 2013 conference that under the right circumstances it was possible to store more than enough carbon to balance the costs of transportation and application.