Image: Tina Tiller

Why industrial anaerobic digestion is not the answer to food waste

New Zealand’s first commercial bioenergy facility has been lauded as a sustainable way to deal with the nation’s food waste. It’s anything but, says community composter Kate Walmsley.

A couple of weeks ago, it was announced that construction was starting on Aotearoa’s first commercial bioenergy facility, owned by Ecogas. 

Many people might see this as a win for the zero-waste movement, and a step towards a more sustainable economy – but I and many others see it as quite the opposite.

I have a lot of bones to pick with the claims made in this press release, which seemed to go unchallenged by media. The cherry-picked data and misleading use of terms like “carbon-neutral”, “circular economy solution”, “sustainable”, and “renewable energy” to describe this infrastructure project amount to greenwashing and cannot be left unchallenged. Three claims made in the article I found particularly dubious: the carbon sequestration claim, the biofertiliser claim and the sustainable, renewable energy claim.

Industrial-scale anaerobic digestion really isn’t as good as it sounds at face value. It produces high amounts of CO2, and has negative implications for soil health and food security. It is costly infrastructure that can only be considered a good investment with blinkers on. There are far better and cheaper ways to cut food waste to landfill, create jobs and deliver environmental outcomes.

Converting food waste to energy by anaerobic digestion (AD) is a form of “recovery”, a last-ditch attempt to glean some value before disposal. At the best of times it is only marginally better than landfilling, and where the digester is remote from the source of the food “waste”, as in the present case, it’s a pointless exercise. In contrast, composting cycles the carbon and other nutrients embedded in food “waste” back into the soil, building soil carbon structure and fertility to grow the next harvest, and the next.

The local living compost hub at OMG market garden on Symonds St in Auckland (Photo: Supplied)

Investing in composting must be prioritised over AD if we are to shift to a sustainable, circular economy. AD works against the beneficial outcomes of composting – it’s a reversal of the natural order of photosynthesis, biomass growth and decomposition that results in a net flow of carbon dioxide from the atmosphere into the soil. This tech results in the systemic depletion of soil carbon and fertility and emits carbon dioxide on an industrial scale, the inevitable effect of which will be to hasten warming and food insecurity.   

Good waste policy follows the waste hierarchy, which guides how we should prioritise activities to deal with waste. Actions higher up the hierarchy are preferred to those lower down. Informed waste hierarchies place “recycling” and “composting” on the same level. Unfortunately, NZ’s Waste Minimisation Act 2008 defines composting as a form of “recovery”, placing it a tier further down the hierarchy, alongside AD. This makes no sense, and composters and waste activists around the country are calling for the legislation to be updated.

This is a better example of a waste hierarchy – read more here.

So, why has $30 million been invested in a commercial anaerobic digestion facility, instead of composting? And why does Biogas claim AD is a sustainable solution for food waste? To explain, we need to backtrack a bit.

A big underlying problem is that we think of food waste as rubbish, not a resource. Food waste is rich in organic carbon and nutrients that must be returned to the soil to create a truly sustainable circular system. To create a circular economy, we must switch from the mindset of “how much of this waste can we make ‘disappear’?” to “how much of this resource can we recycle and use again?”. Nature doesn’t “waste” anything – to create true circular solutions, we should follow nature’s lead.

Food waste is organic matter – “organic” here meaning anything that was once living. All organic matter is mainly made up of carbon and water, with a dash of nitrogen and a few other ingredients. The balance of carbon and nitrogen is important, and governs how nature recycles organic matter.

Composting is how humans speed up nature’s recycling process, and return carbon and nutrients to the soil to be grown into fresh food. As a really simple overview:

When done well, composting turns about half of the carbon in the food waste into compost. The other half is released as CO2, as part of the natural carbon cycle, to be absorbed by plants and once again built into organic matter. Good compost is a balanced, living mixture of carbon and nitrogen, teeming with good bacteria, fungi, protozoa, worms, and all kinds of other critters. This “soil food web” is the main reason why compost is so important and supports healthy and productive plant growth.

Anaerobic digestion to turn organic waste into energy is a very different process. It’s driven by different kinds of microorganisms under controlled conditions, whereby these microorganisms chew up as much as possible of the carbon in the food waste into biogas, mainly methane (CH4). This methane is then burnt to produce energy, with byproducts of CO2 (lots of it) and heat. Simplified:

#1 The carbon sequestration claim

So, where does all the CO2 go? The press release claims it will be piped to glasshouses to grow tomatoes, but the process produces huge amounts of CO2, and plants can only absorb so much before it escapes from the glasshouse. So, without using incredibly expensive CO2 scrubbers throughout the facility and glasshouses, most of the CO2 byproduct will head straight into our atmosphere – the last place we need more carbon.

This is one reason I am baffled by outrageous claim #1: that this facility is “expected to remove up to 10,000 tonnes of carbon dioxide – the equivalent of planting 218,400 trees every year”. I would love to know the working behind this figure. Does it also factor in carbon impacts such as the trucking of food waste and biofertiliser to and from the facility and around the North Island? Seems like a vague environmental claim to me.

An artist’s impression of the new Ecogas anaerobic digestion facility at Reporoa (Photo: Supplied)

#2 The biofertiliser claim

Putting this issue aside for the moment, let’s look at the other main product of anaerobic digestion. When carbon is stripped from the food waste to produce methane for bioenergy, a small amount of nitrogen-heavy sludge remains. This is being touted as a “clean biofertiliser”, but if applied to land in a nitrogen-heavy state, it would have similar negative effects to synthetic nitrogen fertilisers like urea – ie damaging the soil system, leading to soil degradation and erosion, and running off into waterways, polluting them. The idea of then composting the digestate to rebalance the carbon and nitrogen is ludicrous – um, replacing the carbon that was just ripped out and burnt? Should have just used composting in the first place…

Thus my beef with outrageous claim #2: that this facility will “enrich local farmland” and “produce clean biofertiliser for approximately 2000 hectares of local farmland”. This “biofertiliser” may well be harmful, not helpful; and definitely nowhere near as good as compost. Has market demand been validated for such a product? Some in the know are doubtful – so it could end up in landfill anyway.

If this 75,000 tonnes of organic waste were instead channelled into best-practice composting, it could yield 37,500 tonnes of compost. Assuming a typical application rate of 2.5 tonnes of compost per hectare, that would genuinely enrich 15,000 hectares of local farmland – improving, not degrading, the soil. Destroying these nutrients to produce bioenergy increases dependence on synthetic fertilisers for food production, which carry a high carbon price tag, from their manufacture to their effect on the land – and is probably higher than we realise

#3 The sustainable, renewable energy claim

Onto dubious claim #3 (perhaps the most far-fetched of all): that the anaerobic digestion facility will turn the organic waste “into sustainable renewable clean energy”. The organic waste being used as fuel is certainly not being renewed in the process – it’s being converted into gas and burnt. I think the use of the term “bioenergy”, which includes renewable in its definition, is deeply misleading.

When rubbish incineration has been “widely reported as the most expensive and dirtiest form of energy production on the planet” and dismissed by associate environment minister Eugenie Sage as “using the atmosphere as a tip” (in a recent webinar at 11.45), why has AD enjoyed such a lot of favourable attention from the same minister, our government, and industry? AD may happen at a lower temperature than incineration, but is still a form of waste-to-energy, so should be subjected to the same kinds of evaluation by decision-makers.

In Aotearoa NZ, we are well-endowed with genuinely renewable sources of energy. Last week I drove past the Hau Nui Wind Farm near Martinborough – its 15 turbines power around 4,200 households in the region, with no harmful byproducts. The Ecogas AD facility expects to provide only around 2,500 households with power, burning up 75,000 tonnes of food waste to do so. Doesn’t sound very efficient to me. 

While we have no shortage of renewable energy sources in this country, we do have a shortage of compost, with prices rising quickly and suppliers selling out, unable to meet demand from growers who want to transition to more sustainable practices, such as replacing synthetic nitrogen fertiliser with compost.

The AD process releases far more carbon than the natural carbon cycle can soak up. As a double whammy, using the nitrogen-loaded residue as a fertiliser strips yet more carbon out of the soil, as it unbalances and eventually collapses the soil biology that keeps natural carbon and nitrogen cycles going. (Soil degradation is a huge source of carbon emissions – further degrading the credibility of dubious claim #1.) We are losing soil every year due to unsustainable farming practices, with very concerning implications for food security. If our food production is inherently unsustainable, how on earth could food waste be considered a sustainable source of energy?

In Aotearoa, we are well-endowed with genuinely renewable sources of energy (Photo: PETER PARKS/AFP/Getty Images)

Investing in our future

To feed our country going forward into an uncertain future, let alone meet MPI’s new food export targets, we need to start seriously investing in the health of our soils. Composting is the best way to do this.

It’s widely acknowledged that we can no longer carry on with “business as usual”. And yet, waste policy and infrastructural investment continue to favour expensive, fancy tech “solutions” to our problems. Investing $30 million in the Ecogas facility ($7 million being taxpayer dollars) will create path dependency or the “lock-in” effect, as Ecogas will require a constant high flow of food waste to run its operations and recoup its initial investment. To make matters worse, Auckland Council has locked itself into a 20-year contract to provide Ecogas with kerbside-collected food waste for anaerobic digestion, despite concerns voiced by the composting community. Here’s hoping that other councils do not follow suit. These decisions will undermine efforts to reduce food waste in the first place, while chewing up funds that could have gone towards world-class composting services and waste avoidance work instead.

As to anaerobic digestion technology being “well proven overseas”, it’s true that the UK (as an example) has 579 AD facilities in operation, and over 300 more in development, owing to a strategy launched in 2011. It’s also true that their soil health and productivity is in grave danger – alarm bells are now really ringing, with a warning that parts of the country are 30 to 40 years away from the fundamental eradication of soil fertility. Let’s heed the warning and not follow in these destructive footsteps.

In contrast, a “not business as usual” solution would involve investing this $7 million of taxpayer dollars in an innovative decentralised composting system, such as those being piloted and advocated for by members of the Urban Farmers’ Alliance. Such investment would form the foundation of a true circular economy solution that is regenerative, producing far more and far better fertiliser for local growers (thus better food), employing more people in highly skilled work, and minimising carbon emissions. Combining localised composting with regenerative farming practices has a broad spectrum of positive outcomes beyond just reducing waste to landfill – including for food security, soil health, carbon sequestration, water quality, biodiversity, employment and upskilling, public health and wellbeing, and the overall resilience of our society.

There are many excellent examples of community-led composting solutions around the country. Just one is Kaicycle in Wellington, which is working hard on a scalable model of decentralised food waste collection and composting. Kaicycle Composting currently employs four part-timers paid a living wage, and diverts 40 tonnes of food waste per year from landfill. Small, yes, but there is plenty of demand and potential to scale. Scaling this model to cater for 75,000 tonnes of food waste could employ over 1500 FTE in long-term jobs. Whereas the Ecogas facility will “generate close to 60 jobs through the construction process”, probably dropping to a handful once up and running. Localised, smaller-scale composting runs on local people power – just the kind of industry we should be looking to for the post-Covid recovery. Furthermore, all the compost created would support more jobs in regenerative farming to turn the nutrients in the food waste back into good quality, fresh, local kai. 

Public investment in the waste sector and Covid recovery must build, rather than hinder, a low-waste, low-carbon, sustainable and regenerative future. The waste sector is enjoying a big injection of cash from MfE, which will continue thanks to the raising of the waste levy. But will this money be allocated wisely and according to the waste hierarchy? Do the decision makers have a good understanding of the impacts of different options for organic waste? Or, are they trying to shovel money out the door as quickly as possible, as a knee-jerk reaction to the post-lockdown economic situation, and as it’s an election year?

I am deeply concerned that a lot of the “education” about what good waste, energy and infrastructural solutions comes from profit-driven private business with their own interests in first place. Those with the purse strings must urgently make space to resource and then listen to independent, thorough research that applies a systems-thinking lens, as well as the voices of the community and other stakeholders who will be impacted by these investments for decades to come.

I’m also disgusted by the concept of food waste resources being removed from our communities, trucked across the country and burnt to generate big profits for private business, at the expense of our long-term social and environmental wellbeing.

This anaerobic digestion facility, and potentially another 21 that will follow if our government doesn’t take action, has dire implications for our food security and ability to shift to a sustainable circular economy. Now is not the time to fritter away taxpayer dollars on second-rate solutions. We need truly sustainable and regenerative solutions to guide our recovery and pathway to a low-carbon, food-secure, equitable future. Food waste is a resource to recycle, not to burn for pseudo-renewable energy. Our harvests are numbered otherwise.




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