Engineering Insight: What do you do with all that waste?
A sample of dried biosolids. Photo: Mike Bourke.
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,” says Garry Macdonald of Beca. “What do you do with all that waste?”
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.
“They say ‘it will cost me $6 million to buy a dryer. What’s the cost of that annually versus spending $100 per tonne to go to landfill?’”
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. “A lot of them have sustainability policies and don’t want to send their biosolids to landfill, but financial justification kicks in”, he says. “They say ‘it will cost me $6 million to buy a dryer. What’s the cost of that annually versus spending $100 per tonne to go to landfill?’ But effectively you are taking up space that has to be replaced at some stage. The issue applies particularly to councils that own landfills, as they don’t face the full commercial cost. Many of these local landfills are nearing the end of their operating life and the costs of a replacement landfill haven’t been factored in.’”
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.”
“People think sewage is sewage and that it never changes.”
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 (see sidebar story). 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. Garry says biosolids have great potential as an agricultural fertiliser, but there’s no great willingness to explore it. “The issue is ‘faecal aversion’,” he says. “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].”
“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.”
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.”
“Some state governments such as Queensland now legally recognise biosolids as a resource rather than a waste product.”
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.
“Australia is quite progressive when it comes to biosolids, with nearly 70 per cent being applied to land as a fertiliser and/or soil improver,” says Jonathan McKeown, Chief Executive of the Australian Water Association. “In terms of legislation, biosolids fall under waste management or minimisation acts in Australia, and most have some sort of intent to restrict or prevent disposal of recyclable organic material to landfill.”
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.
“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.”
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,” says Garry. “There are [landfills] such as Hampton Downs in the Waikato who think that biosolids actually catalyse the breakdown of municipal waste and produce an increase in landfill gas.”
Nevertheless, it strikes him 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,” says Garry. “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 loo, they’re exacerbating the issue.”
Waste not, want not
Since the 1970s, when liquid slurry was applied to sandy-soiled council-owned farmland, the Christchurch City Council has taken the view that the end product of its Bromley wastewater treatment plant ought to be put to some useful purpose.
“We’ve always reused it as a soil conditioner and fertiliser,” says Mike Bourke MIPENZ, Senior Planner of Assets and Network Planning. “From 1997 onwards we began to de-water the material and that gave us some more options. We did some forestry application, but mostly we used it for the rehabilitation of the Burwood landfill.”
The council upped the ante in 2011, when it commissioned a drying plant. The dryer produces 3,500 tonnes of dried biosolids annually, the vast bulk of which is trucked to Westport to rehabilitate the Stockton mine site.
Christchurch City Council Biosolids Drying Plant at the Bromley Wastewater Treatment Plant site. Photo: Joseph Wildy.
Before it gets anywhere near Stockton, however, there’s a lengthy process involved. Mike explains: “The primary treatment takes out all the organic solids that are suspended in the effluent, and a secondary treatment process produces solids from what is dissolved in the effluent. We recover those produced solids through a biological process. We then settle those solids out, put them through an anaerobic digestion process, which breaks down organic matter and produces methane and carbon dioxide. We use that biogas onsite for power generation, then dry the biosolids using a heat source either from the Burwood landfill gas or a wood-waste boiler. That drives the moisture out of the de-watered cake, following dewatering in belt presses, and that becomes the feed stock for the dryer.”
At 98 per cent dry, the end product looks like charcoal coloured wood pellets. This is applied liberally at Stockton, with a couple of hundred tonnes per hectare.
“Biosolids also provide a huge benefit in terms of the ability of the soil to grow plants.”
Why are biosolids so useful for remediating a mine site? “Mine sites traditionally produce what they call acid rock drainage following rehabilitation, where the water flowing through the land becomes very acidic”, says Mike. “The aim is to reduce that. By applying biosolids you reduce the amount of oxygen available in the soil, and so reduce the amount of acid that’s being produced. Biosolids also provide a huge benefit in terms of the ability of the soil to grow plants. With carbon nitrogen, a bit of phosphorous and a bit of potassium it makes a good low grade fertiliser.”
Mike says the council has also rehabilitated a couple of parks on the eastern side of the city. “The beauty of the drying process is you produce an absolutely pathogen-free material. I like to tell people ‘You can put it on your cereal.’”