It’s not a binary choice of one or the other, writes Brian Cox, who says the two technologies can work together for the good of the environment.

In an opinion piece titled “Why industrial anaerobic digestion is not the answer to food waste”, Kate Walmsley argued that composting provides better environmental and societal outcomes than anaerobic digestion. Globally, both composting and anaerobic digestion (AD) of organic waste are recognised as two well-proven and often complementary technologies. The choice of processing pathway depends on the composition of the waste and the desired products produced.

In general, high-energy feedstocks such as food waste are better suited for AD, whereas high-fibre feedstocks such as green waste are more suited to composting. Most importantly, they both keep waste out of landfills and hence substantially reduce greenhouse gas emissions and other environmental risks and impacts.

The waste hierarchy

In both cases, the processing is of residual organic wastes after minimisation, recycling or reuse has been applied. The Reporoa processing facility is a good example of how this works in action. One of the parties sourcing waste for processing is Ecostock which, for a number of years, has been collecting Auckland food wastes and extracting material that can be processed into stock food. They will now provide the food wastes that cannot be fed to cattle to the Reporoa processing plant for processing into energy and biofertiliser that is derived from the solid residues of AD.

Both composting and anaerobic digestion require effective source segregation of organic waste from other types of waste and contaminants. Improved separation of waste at source has been internationally proven to increase the level of recycling and reuse of organic waste that would otherwise have gone to landfill, hence underpinning the goal of the waste hierarchy.

With the focus on resource recovery, most food waste AD plants produce biogas for heat, electricity and/or use as a vehicle fuel, and biofertiliser from the residual solids.

Greenhouse gas emissions

Biogas from anaerobic digestion reduces harmful methane emissions otherwise coming from landfill, carbon dioxide from natural gas used in boilers and generators, or natural gas currently used to make synthetic fertilisers.

The Reporoa AD plant will convert the unavoidable fraction of food waste and other commercial or industrial organic wastes unsuitable for reuse in its raw form into biogas composed of 65% biomethane and 35% biogenic CO2. Up to 3,000 tonnes per year of the biogenic CO2 will be made available for glasshouse fertilisation, with the potential for improving horticultural growth by up to 30%. More than 9,500 tonnes of fossil fuel CO2 emissions will also be avoided by replacing the natural gas currently being used for heating of the glasshouses and by generation of renewable electricity.

Walmsley’s article failed to acknowledge the inherent methane and nitrous oxide emissions produced during composting. With methane and nitrous oxide having 21 and 289 times more greenhouse effect than the CO2 equivalent respectively, the total CO2 equivalent emitted per tonne of waste processed are similar for both compost and AD.

A significant amount of meat-sourced food waste to be processed at the Reporoa plant could not be composted. The type of food waste typically treated using AD would require the addition of at least twice as much again of bulking material to allow good compost to be formed. This would result in three times the transport cost and consequently higher transport emissions. There is also substantial energy cost and greenhouse gas emissions associated with composting operation, including energy for aeration, odour capture and processing. This demonstrates that even composting is not completely free of emissions.

Biofertiliser

Both composting and AD preserve nutrients contained in the original feedstock, but they differ in degree of organic matter breakdown, with AD achieving higher (60-90%) conversion of organic matter to biogas. Consequently, the biofertilisers from composting and AD processing are different fertilisers. Compost offers better soil conditioning properties due to the higher residual carbon content, while biofertiliser from AD contains nutrients in a balanced slow-release form, with some soluble nutrients to give a little boost on application. AD biofertiliser has a significant humic content that improves the soil in the same way that compost does. In general terms, compost is an organic soil improver, and the solid residue from AD is an organic fertiliser.

The nutrient concentration of AD biofertiliser is much higher than compost, but is still lower than that of mineral fertilisers. As for all fertilisers, different ones are used for different plant growth outcomes. AD-sourced biofertiliser can also be mixed with composts to provide an enhanced compost. Farmers have a choice of using either or both products and will select what suits their specific needs.

The Reporoa AD plant will produce around 85,000 tonnes per annum of biofertiliser product that will be spread onto more than 2,500 hectares of pasture or cropping land within a 50km radius of the Reporoa plant. Biofertiliser will displace nitrogen, phosphorous, potassium and other macro- and micro- nutrients that would otherwise be supplied from synthetic fertilisers transported around the world or produced using natural gas here in New Zealand.

Biofertiliser from AD plants is an established product overseas, while New Zealand lags far behind. For example, Italy produces up to 30 million tonnes of AD biofertiliser annually, equal to about €400 million in fossil-derived fertiliser savings.

Sustainable, renewable energy

Compost cannot be used to offset natural gas or coal emissions from boilers or from electricity generators, whereas AD biogas can be used to replace fossil fuel emissions. Renewable energy supplied by the Reporoa processing facility from AD biogas will replace more than 10,000 tonnes CO2 per annum of emissions from fossil fuel alternatives.

Both processing pathways use living organisms to break down the organic matter. As a result they both have to be carefully and professionally managed. The difference between the technologies is that composting is undertaken with air and AD is undertaken in the absence of oxygen. Each technology needs to be installed and managed to best practice.

Comparing anaerobic digestion to waste incineration is not appropriate. Waste incineration is typically applied to mixed municipal waste, rather than source-segregated food waste due to the low energy content of organic wastes.

Unlike in other markets, AD facilities in New Zealand are not driven by the aim to produce electricity. While electricity production is an added benefit, it is the avoidance of greenhouse gas emissions from landfilling of organic waste and from the use of natural gas as energy source that are the key drivers.

In sum, anaerobic digestion and composting are complementary technologies. They both provide sustainable alternative to landfill disposal; reduce greenhouse gas emissions generated at landfills and assist New Zealand in achieving its emission and sustainable development targets; preserve nutrients contained in food waste, enabling their reuse on land; reduce New Zealand’s dependence on largely imported synthetic fertilisers produced using mainly fossil fuel-driven processes; require adequate management of the biofertiliser product application to avoid adverse effects on the environment; and need to be designed and operated to high standard.

In addition to the above, anaerobic digestion also provides a sustainable fuel source to replace fossil fuels and reduce greenhouse gas emissions and a renewable source of CO2 for glasshouse enrichment.

The choice between using anaerobic digestion or composting for processing of organic waste needs to be based on lifecycle analysis, market demand for products and types and amounts of waste available.

Brian Cox is executive officer of the Bioenergy Association