New solutions to deal with biosolids are being implemented globally. But do the costs outweigh the benefits? Call it the price of progress. As we lift our game on wastewater treatment, we are inevitably creating growing amounts of treated sewage sludge, or biosolids.

Thank you to the Institute of Professional Engineers New Zealand (IPENZ), for letting us republish a version of the article, first published in the IPENZ Engineering Insight Magazine and written by Matt Philp. You can view the full article on the IPENZ website here.

New solutions to deal with biosolids are being implemented globally. But do the costs outweigh the benefits? Call it the price of progress. As we lift our game on wastewater treatment, we are inevitably creating growing amounts of treated sewage sludge, or biosolids.

“It’s an increasingly problematic area in New Zealand. What do you do with all that waste?” Garry Macdonald, Beca Business Director

Unsurprisingly, the historical answer in New Zealand has been to de-water biosolids and dispose of them in landfill. But that solution is looking increasingly unsatisfactory, not least because these biosolids take up space that otherwise could be used for household refuse.

Garry, a Fellow of IPENZ whose 39-year engineering career has been dedicated to finding more environmentally sustainable ways to manage both waste and wastewater, says that while some of the larger city councils around the country are thinking creatively, most are swayed by the economics.

Untapped energy sources
There’s another compelling argument for looking beyond the landfill: biosolids are potentially rich sources of both energy and nutrients that in New Zealand are going largely untapped. A lot of the energy we take from our food abides through the wastewater treatment process and is harboured in this treated sludge, along with nitrogen, phosphorus and various other useful elements.

“Other countries are starting to extract energy and nutrients from biosolids,” says Garry. “Australia, for example, is a huge user of biosolids to bring poor soil up to productive use. In Japan, biosolids are incinerated and the ash is used in bricks. America is rapidly getting into the resource recovery market from sewage sludges, as well as using biosolids for agriculture. In Europe, where land is limited, they’re using high technology to reduce the volume of biosolids, and the ash is sometimes used as an additive to concrete. So everyone’s doing something a bit different.”

New Zealand is starting to get in on the act. For many years, treated biosolids have been sprayed on forestry land at Nelson’s Rabbit Island. The Christchurch City Council uses its biosolids to remediate the Stockton mine site on the West Coast. After some to-ing and fro-ing in the Environment Court, Auckland’s Watercare Services was given permission to truck its solids byproduct from the Mangere treatment plant to nearby Puketutu Island, where it’s being used to rebuild the volcanic landform that in the 20th Century was quarried to near sea level. In New Plymouth, meanwhile, biosolids are dried and pelletised into a commercial fertiliser product called BioBoost. But these are outliers.

“Biosolids have great potential as an agricultural fertiliser”

Garry adds “But there’s no great willingness to explore it. The issue is ‘faecal aversion’. People think sewage is sewage and that it never changes.”

The benefits outweigh the risks
In fact, wastewater treatment markedly reduces the threat of pathogens, while subsequent drying processes are capable of producing a largely pathogen-free material. There is, however, an additional risk from contaminants – heavy metals and organic compounds are the biggest worry – which in combination with the pathogen issue means biosolids can’t be treated like regular fertilisers. Instead, both the process of manufacture of biosolids and their application to land are subject to government guidelines, with the particular risk posed by contaminants mitigated by established permissible limits in soils.

“Copper cylinders and zinc water pipes can leach and the metals get into the wastewater system, where they are very difficult to extract,” says Garry. “Instead, you tackle it at the application end. The New Zealand guidelines clearly state what the limits are. There’s one grading system for acceptability of various heavy metals, and another for [pathogens].”

He argues that the risks of using biosolids on the land are minimal, and far outweighed by the benefits.

“These are organically-rich substances, with lots of nitrogen phosphorous for plant growth, along with other beneficial trace elements. It would be great to see some public understanding that this material could be useful for parks and reserves and suchlike. It could produce some real cost savings for councils. Conversely, by not using biosolids, there’s a double cost, because you still have to find a place to put the sludge.”

The Australian story is somewhat different. Across the Ditch, the vast majority of biosolids are applied directly to the land as fertiliser, with only a small amount sent to landfill. What’s more, some state governments such as Queensland now legally recognise biosolids as a resource rather than a waste product.

Garry would like to see New Zealand take a leaf out of the Australian book, but concedes it’s not a straightforward matter. Quite apart from the public perception issue, there’s the question of cost. Treating biosolids to the standard required for use as fertiliser, for example, is not a cheap business, and smaller New Zealand councils in particular might struggle to justify the outlay.

There’s also an argument that sending biosolids to landfill is not a complete waste of potential energy.

“As long as you have properly designed landfill gas recovery, then you are effectively still recovering that energy, it’s just over a longer period of time.”

Nevertheless, it strikes Garry that in the age of sustainable thinking we ought to be doing far more to exploit this particular waste product.

“The New Zealand economy is largely driven by agriculture. Biosolids reuse can actually help, rather than hinder, this economic activity. The science is supportive, but we need to work with our communities to demonstrate there is another, more sustainable pathway. Because, really, what’s the point of cleaning up all our waterways and being good on liquid waste if we have a brown stream that’s just going into a hole in the ground? We’re big on recycling in New Zealand, but apparently not in this area. And every time someone flushes the toilet, they’re exacerbating the issue.”

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Garry Macdonald

Business Director - Water

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