As part of its waste and resources strategy, the UK government proposes that – subject to certain conditions – every household in England should have access to a separate food waste collection from 2023, when at present barely half do.
At present, households in England produce around 386,000 tonnes of separately collected food waste. WRAP estimated that, in 2015, 7.1 million tonnes of domestic household food waste were produced across the UK, which would equate to 5kg per household per week. Of this, some 4.9 million tonnes was collected by local authorities (whether separately or in the residual waste stream), with the remainder home composted or washed down the drain. I doubt that the actual numbers are quite that large today.
Reviewing recent data on food waste arisings from some 140,000 households in Buckinghamshire, who have been on a weekly collection system for over 3 years, it shows an average capture of just under 1.5kg per household per week. Even if the scheme is capturing barely half of all food waste in the area, the figures would suggest that, if applied across the UK, the total amount likely to be separately collected is far less than 4.9 million tonnes. This observation is backed up by a number of as yet unpublished studies indicating that the amount of food households actually throw out may somewhat less than had previously been estimated.
However, a doubling of the number of authorities collecting food waste might reasonably be expected to bring about at least a doubling of the amount of food waste that is captured. The question then arises – what shall we do with it?
The default option in the UK has been to send it for wet anaerobic digestion (wAD), the name given to anaerobic digestion (AD) processes where the proportion of dry matter in the material that is processed is less than 15%. The outputs of this process are:
- biogas, which is often burned onsite to generate electricity but which can also be valorised and injected into the natural gas grid or compressed for use in transport fuel; and
- digestate, which can be spread on land as a fertiliser.
England has had an excess of wAD capacity for some time, as plants were built to take advantage of renewable energy subsidies, which allowed early adopters of wAD to effectively offer very low gate fees. The business model for wAD depends heavily on income from gas generation, which operators seek to maximise. Without the subsidies, which are greatly reduced and coming to an end, many plants would have had to charge far higher gate fees, perhaps in excess of £80 per tonne to generate their expected revenue and profit margin.
However, too little thought was given to how the supply of food waste could be secured, resulting in some food waste travelling more than 100 miles to reach an AD plant, sometimes bypassing more local facilities.
Still less was given to the land-market for digestate. The issue of what to do with digestate is critical. Farmers don’t want to add material to their land if it is full of plastic, so AD needs front-end depackaging and significant screening at the back end. Further, the digestate from wAD is a slurry with a high ammonia content; it can only be applied to land for a few months of the year, and often needs to be stored for some months before application. When applied correctly at the appropriate time of year based on crop need, it is a good replacement for artificial fertiliser, but does little or nothing to add carbon to the soil. For digestate in slurried form, there are no meaningful alternative markets to the agricultural one.
With the impending prospect of needing to treat twice as much food waste, is more wAD the right way to go, or should we be considering alternatives?
One such alternative is dry AD (dAD), which is capable of processing a feedstock with a dry matter content of 15-45%, to provide both biogas and compost.
There is a common belief that wAD generates more gas than dAD. To the extent that this is true, it simply reflects the fact that food waste, the typical feedstock for wAD, has a higher gas-yield potential than does the comingled food and green waste normally processed in dAD. Indicatively, based on sources including the biogas calculator, fresh leafy garden waste may have a biogas potential of about c.110 m3 of biogas per tonne while the potential of food waste is more like 160m3 per tonne.
The amount of the potential gas yield that is harvested depends on system efficiency and biological process efficiency. This is a complex subject, but most wAD uses a mixer-agitated technology where all biochemical processes occur concurrently in a single tank. Efficiency can be as little as 60% and rarely exceeds 80%. In other words, even if the input material has a high biogas potential, the system may simply not realise it.
By contrast, dAD systems are typically designed for digestion to occur in steps: material transits a plugflow along a digestion vessel, or by batch control. Input material can receive an inoculant of recirculated digestate at the infeed point to ‘kick-start’ the AD process, resulting in rapid production of biogas and short residence time in the digester. This results in a typical efficiency of around 90%, harvesting as much of the available biogas as is realistically possible.
Putting the gas potential and efficiency figures together:
- wAD of material with a biogas potential of 160Nm3 per tonne, operating at 60% efficiency will harvest 96Nm3 of biogas per tonne; and
- dAD of material with a biogas potential of 110Nm3 per tonne, operating at 90% efficiency will harvest 99Nm3 of biogas per tonne.
A Normal Cubic Meter of a gas (Nm3) is the volume measured under the standard conditions of 0⁰C and 1 atmosphere of pressure.
So, the greater gas potential of wAD’s feedstock tends to lead it to be perceived as higher-yielding than dAD, when the latter’s greater efficiency can mean that the opposite is the case. Its performance was clearly demonstrated in detailed UK research carried out to establish criteria for PAS110 thresholds, including blind-tests of residual biogas potential of digestate from both dAD and wAD. . If all 6 million tonnes of UK green waste could be treated at the efficiencies indicated above, it would generate at least 594 million Nm3 of gas. In reality, the gas yield would be much higher due to the mix of food and green waste in the feedstock.
Betting the farm
In addition to the efficiency benefit, dAD systems typically incorporate an aerobic compost phase after the anaerobic phase. As a result, they produce a PAS100QP compost that helps build soil carbon, along with some liquid fertiliser. These are much more readily usable in agriculture than the digestate from wAD, and the potential exists for screening the output into products of varying quality, helping to access a wider range of markets. It follows that there is far less difficulty in making beneficial use of the outputs from the process. If all UK garden waste was treated with dAD, then according to research for Defra’s New Technology Demonstrator Programme, it would produce some 3.6 million tonnes of PAS100QP compost.
A processing technology that can manage both food and garden waste, whilst extracting gas energy as well as carbon-rich organic material, has considerable advantages, not least because opens up a great deal of additional material for biogas production – at a rough estimate. However, it might also allow more flexibility about how food and garden waste are collected. While Defra is eager to promote separate food waste collections, dAD could be used to treat material where mixed food and garden appears to be the most cost-effective collection option – which I suspect might often by the case.
WRAP and Defra are of the view that yields of food waste are considerably higher where it is collected separately from garden waste. I’m not sure that this makes sense psychologically – from the householder’s point of view, once I fill my kitchen caddy, it doesn’t really make any difference if they put this waste into the typical small weekly bin or into a larger wheelie bin with garden waste. The larger bin may even be more convenient. I suspect that part of the issue could be that, by the time mixed food and garden waste is sampled, some moisture from the food has been lost, while a proportion may be difficult to meaningfully separate from the garden waste.
This is not an argument for the abandonment of our existing wAD facilities. They will continue to be need to process separately collected food waste from commercial sources, and from households in areas where separate collection makes sense. The usability of digestate could be improved by adding composting at the end of the process to treat garden waste alongside digestion outputs. However, a shift from composting to dAD, fed by mixed food and green (whether collected together, or blended after collection) would be the most efficient way to extract economic and environmental value from the hundreds of thousands of additional tonnes of food waste that will need to be collected in order to meet municipal recycling targets.
While the opinions expressed in this article are those of the author, he would like to acknowledge technical contributions from Dr Robin Szmidt of Target Renewables.
Featured image: Peter O’Connor (CC BY-SA 2.0), via Flickr.