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Flushing money down the drain

11 January 2023

Can the sewage network play a role in reducing greenhouse gas emissions?

This is a blog about food waste, biogas and the Victorian sewage network in the UK, so if you have inadvertently started reading this whilst eating your lunch, you may want to stop now and come back later. Let’s start with the big picture: globally, humanity grows and then throws away about 1.3 billion tons of food per annum. This means that one third of all agricultural land in the world is used to grow food that is never eaten and this waste is mostly sent to landfill where it then cheerfully rots away and emits 70 million tons of methane per annum (which is 85 times more harmful than CO2). If food waste was a country it would be the world’s 3rd highest emitter of greenhouse gasses after China and the US.

Most of this food waste (61%) is produced by households and each household throws away about 700 kg per annum. As well as producing methane in landfill sites, food waste in domestic bins contaminates other materials like paper and cardboard making them uneconomic to recycle. This in part is why UK domestic recycling rates have plateaued at 40% for years now.

So buying a lot of food but then not eating it, scraping it into your kitchen bin to contaminate your recycling, and then having it all ending up in landfill producing methane is a series of bad ideas, all things considered. Some UK councils have tried to address the contamination issue by collecting household food waste separately, but without much success. Collecting lots of little green bins is costly for the councils, and the sudden accessible presence of food on the kerbside has contributed to the recent growth of the rat population in urban areas (as much as 25% in some cases). Understandably, people are becoming less keen on kerbside collection as a viable solution although presumably the rodents think it’s a great idea.  

Here’s the big lightbulb moment: almost every household in the western world is connected to an underground sewage pipe. Taking the UK as an example, there is a network of 215,000 miles of pipes which connect nearly every house to 9,000 sewage plants. 66% of the sewage waste that flows through these pipes is already processed in existing Anaerobic Digestion (AD) plants. As you will know, AD is a proven technology which uses bacteria to convert the long chain hydrocarbons in the ‘stuff’ that is in the sewage pipes into methane (CH4), CO2 and fertilizer.

Could we not use that same sewage network to transport our food waste to the same AD plants where it can be used to feed the bacteria to make more biogas and more fertilizer? This is not a new concept and most households in the US have a waste disposal unit in the kitchen sink, which turns their waste into a sort of murky soup and flushes it down the drain. These units cost about GBP 500 and can be easily retrofitted to most kitchen sinks.

Putting liquidized food waste into the sewer instead of the bin would solve 4 problems at once:

  1. It would reduce the amount of food waste going to landfill and hence it would reduce methane emissions
  2. It would stop you having to store your rancid food waste on hot days and reduce the contamination of the other recyclable materials in the household bin, such as paper and card
  3. It would turn the food waste into fertilizer for farming
  4. And finally, it would enhance the energy content of the other ‘stuff’ that goes into the same pipe and turns up at the sewage plant. This means more biogas and fertilizer generated from the AD plant

The last point is obvious, if you think about it. Food going into your body has more energy content than what subsequently comes out and this is why you eat it in the first place. That said, the potential biogas content of food waste varies enormously in line with how fattening/tasty it is. Pork pies are both very tasty and contain huge amounts of energy, and hence potential biogas. Lettuce less so on all three counts.

That said, improving the biogas yield of the AD process by adding food waste to sewage is not without its technical challenges. AD plants are unique amongst their renewable energy peers in that they are at heart a living breathing process that relies on bacteria to break down the organic feedstock. However, they are notoriously fussy eaters and much prefer a bland predictable diet rather than a smorgasbord of different types of food from thousands of different households. Evolution plays a part here and different types of bacterium which prefer different types of feedstock eventually become dominant within the plant. If you keep feeding the plant a lot of expired cream cakes (high in fat and sugars), the alpha bacteria which have thrived on cake won’t take kindly to a sudden switch to green vegetables and gas production will plummet. However, economies of scale work in our favour here. The more people who discharge food waste into the sewer, the more the variation in chemical composition evens out to a pretty predictable mix of proteins, carbs, fats, starches and sugars. That said, all bacterium particularly dislike metal, glass and plastic so households would need to depackage their waste before putting it through the blender.

Another big advantage of this idea is that part of the food waste is transformed into fertilizer. Producing fertilizer at commercial scale is an astonishingly environmentally harmful process. Producing 1 ton of agricultural grade fertilizer requires 1 ton of oil and a 108 tons of clean water and the process emits 6 tons of CO2. The mass loss to gas from an AD process is very small and most of the waste that goes into an AD plant comes out as a fertilizer-type product, so adding food to the mix on the way in will directly increase the volume of fertilizer that comes out.

Apart from the potential negative impact to the bacteria as a result of co-mingling food waste and the other ‘stuff ‘in the sewage, the other main objection seems to be regulatory overlap. As an example, different government departments in the UK regulate different industries. The civil servants who regulate food and sanitation don’t care much about methane but are understandably obsessed with the spread of viruses such as Foot and Mouth. Typically, the products of AD plants gets spread to land. If this contains live viral particles, diseases can spread. Pasteurizing the feedstock on the way in to kill the viruses is the solution but this requires energy and would be an additional step in the process that would require both regulation and additional pasteurizing equipment at the sewage plant. Not an insurmountable legal or technical challenge but a cost to the government alongside retrofitting waste disposal units to household sinks.

There are approximately 28 million households in the UK. Fitting every home with a waste disposal unit will be a once-off cost to the government of approximately GBP 20 billion. That’s a huge number, but as we saw with solar panels, costs should go down with a mass roll-out and could be amortized over a number of years. To put this into context, turning all the UK’s food waste into biogas would reduce greenhouse gas emissions by 5% and potentially produce about 3 million MWh of energy in the form of biogas. This retrofit is slightly smaller and cheaper than Hinckley Point C nuclear power station which will produce only slightly more power and will cost at least GBP 25 billion.    

In conclusion, we think putting liquidized food waste into the existing sewer network makes a lot of sense. Capturing the methane from this waste in a controlled manner and making electricity with it is clearly preferable to letting it freely escape from landfill sites, and there is the obvious collateral benefit in reducing contamination of recyclable materials such as paper and cardboard, and in the production of more fertilizer. In the UK, we gratefully inherited a whole network of sewage pipes from our Victorian forefathers so we have a ready-made and free underground route for moving the waste for free from our kitchens to our local AD plants. Technically, with a big retrofit of waste disposal units and some smart chemical engineers at the AD plants, this solution is feasible. We just need some government department collaboration and good decision-making. As any parent of a fussy toddler will ruefully testify, humanity is never going to stop wasting food, so doing something useful with it is the next best step.