Want to reach zero-emissions? The answer might lie in biosolids

Published 24 July 2017

Seedlings planted out on windrows of composted biosolids
Biosolids – that carbon-rich by-product of wastewater treatment – has gained attention as a potential solution to offset carbon emissions through sequestration. But is using it for carbon storage in soil the best application of this resource?

“We are already using biosolids in agriculture, but some in the water industry are curious if we can detect the carbon in soil, and if yes, can we generate carbon credits for that,” said Norman Goh, a PhD candidate studying wastewater biosolids at the University of South Australia. 

To know if this was a viable option, Goh first had to test if current methods are effective in both storing carbon in soil and measuring that carbon over long periods of time. A proper investigation of its efficacy would “take decades of monitoring”, he said, so much of his research relied on literature reviews and his own field tests. 

Biosolids are commonly used as fertilisers for agriculture, and Goh and his team focused their efforts on collecting soil cores from seven farms actively utilising biosolids. Each soil core had to be 30cm deep, and over the seven farms, this equated to approximately 150kg of samples.

From the start, he sensed that there would be barriers to this method. 

“Ideally, you would apply the biosolids to something like an empty pasture and then leave it undisturbed for several years. However, we investigated actively farmed fields because that is where the majority of the biosolids are applied,” Goh said. 

“There’s always something going on in an active farm field … there’s a lot of movement in the soil from operations like seeding and harvesting, which creates variability.

“We knew about these barriers going in. However, no one has tried to measure carbon stored in farm soil like this before, so we knew we had to try.”

Because this is an expensive and labour intensive process, Goh said a worthwhile result would be finding at least 6g of carbon within each core sample, or one and a half teaspoons, to prove the viability of carbon sequestration in soil. 

Currently, farms will apply about 30g of dried biosolids per A4-sized area of land, or between five and six teaspoons – “just a sprinkling”, Goh said. 

However, to reliably measure carbon, given the limitations in current methods, you need to apply at the very least about a stubby’s worth of dried biosolids per A4-sized area of land, or about 400g. 

“To put that in perspective, a medium-sized farm plot – about 40 hectares – would require just under 7 million stubbies-worth of dried biosolids. Trucking that around doesn’t make sense economically or environmentally, and from a nutrient point of view it’s overkill” he said.

His research found that as current methods stand, carbon sequestration in soil through biosolids is not a viable option.  

“It’s a bit of a bummer. On paper the idea looks good, but in practice carbon sequestration runs up against economic limitations, policy gaps, and lagging science and technology,” Goh said. 

“Effective carbon policy supported by science is critically important in this area and more work needs to be done with regard to gaining appropriate support for research in this area.”

However, biosolids still have a role to play in bringing utilities closer to a zero-emissions future. 

“If you want to generate carbon credits, a more feasible option is to look at the nutrients and see how much carbon-intensive chemical fertiliser we are offsetting with the use of biosolids,” Goh said. 

Part of this transition requires biosolids to go from being viewed as a by-product of wastewater treatment to a valuable product in and of itself. 

“A highly optimised wastewater treatment plant would produce fewer biosolids, and those it does produce would have a higher concentration of nutrients as a result of optimised biogas production,” Goh said. 

“This fits well with making wastewater treatment plants low-emissions because the carbon that would end up in biosolids is being consumed to produce more biogas, which in turn offsets the use of energy from fossil fuels. Additionally, if you apply nutrient-enriched biosolids to farmland as fertiliser, you’ll offset the use of chemical fertilisers, which are energy intensive to produce and can pollute waterways in the form of runoff.”

There is appetite for this in the water industry, and more can be done to catalyse the movement, said Goh.

“In the future carbon sequestration in biosolids might be possible, but right now you get more bang for your buck focusing on the nutrient aspect of it, which is easier to quantify than trying to measure carbon levels in soil,” he said. 

“In the short to mid term, nutrients are the way to go. However, in the long term, carbon should always be at the back of your mind.”
This research was supported by the CRC for Low Carbon Living Ltd. (project RP2008) whose activities are supported by the Cooperative Research Centres program, an Australian Government initiative. More information on biosolids CRC project RP2008 can be found here. For further information on this research, please contact Dr Michael Short or Professor Chris Saint.