Composting Toilet

Parts of this article appeared first in IChemE’s Wet News Issue 81 and parts in the Institute of Water Magazine – Issue 206.

Our COVID-19 Experience

At three o’clock on a sunny Friday afternoon my family and I were walking through the woods – getting our daily permitted exercise. Laura, my wife, had a dull ache in her chest and had to sit down. By nine o’clock that night, it had developed into a sharp pain. I started to worry that it might be a heart attack. We didn’t want to waste NHS time if it wasn’t serious, so I called 111. We got through after 15 minutes and, within an hour, a doctor called us back and put Laura into COVID-19 isolation.

I quickly moved my stuff out of our room & set up camp in my office. Being a process engineer, I made Laura a bottle of 70% isopropanol spray, before leaving a flask of tea outside her prison door.

The WHO and UK government guidance is that people with COVID-19 symptoms should “…use a separate bathroom from the rest of the household”. If you can’t do that then they recommend that you “clean a shared bathroom each time you use it”.

For environmental reasons, we had been considering installing a composting toilet for some time. I was also concerned that the spray disinfectant Laura had to use each time she went to the bathroom wasn’t helping her lungs.

So, we rapidly made a decision to purchase an install a composting toilet.

Choosing a Composting Toilet

You can categorise composting toilets into two types: separating and non-separating. Separating models aim to keep solid and liquids separate, which can reduce odours. Non-separating units, also called “internal composting” units mix the solids and liquids in order to allow composting internally.

We considering non-separating units like Sun Mar, MullToa Biolet, Envirolet, Ekolet and separating units like Natures Head (a popular model for marine/campervan use), Airhead (similar, but a number of design features mean it tends to gets better online reviews), Simploo and the Separett Villa 9000.

A friend who was in my Atkins team in Dubai often spoke at conferences on the benefits of segregating urine… That was good news as separating toilets are generally half the price of internally composting toilets and I’m a stereotypically tight-fisted Scot.

You can then subdivide the separating toilets a few other ways. One is whether they have mechanical solids mixing. Another is around how the liquids are collected.

Natures Head, Airhead, and similar units have cranks to mix the solids. Separett has an automatic facility to rotate its solids collection bucket, but that is just stop things piling up. As far as I can tell, Simploo’s solids collection is just a stationary bucket.

I wasn’t convinced by the sales agents who suggest that the organics added to the units with cranks starts off the composting. I imagine they basically help dehydrate the faecal matter and absorb odour producing compounds.

It is my understand that none of the separating units should do any significant composting internally as, from what I read, 40-60% moisture content is required for biological composting. The separating units all deliberately dehydrate the solids by use of constant vapour extraction (an extractor fan). I would be interested to know whether mixing units with their ‘starter’ material significantly help that process?

However we decided that we would prefer to collect the liquids in a tank in the garden rather than having to walk through to empty a tank every couple of days. That left us with a choice of one unit that we could find which was available for next day delivery.

The Installation

My dad has a tool hire account, so he rented a diamond core cutting drill, and I proceeded to make my first mistake. We had our house roughcast a couple of years ago and I was overly concerned that we might knock off a sheet of render. So, I donned my PPE and drilled a couple of pilot holes out through our attic space at the correct location, so that my dad could core drill from the outside-in. The excuse I’m sticking to is that I couldn’t drill outside, as Laura couldn’t leave the house, so I had nobody to hold the bottom of the ladder. My dad drilled back more horizontally than I had… The key learning point is to core drill out rather than in, in order to ensure the exit holes are exactly where you need them.

We have videoed all this, but haven’t yet had time to edit the hours of footage. Please let us know in the comments, if you would like me to – and if so, how long a video you’d like to watch.

COVID-19 Toilet Aerosol Risk

Reading the WHO interim guidance, I got the impression that the main risk of COVID-19 contagion through wastewater is due to aerosols when infected individuals flush the toilet. It is therefore recommended that, as well as washing our hands, we all close the lid when we flush. One advantage of composting-toilets in these times is that you don’t need to flush.

Our Experience of Composting Toilets

Our experience so far is that:

They can be installed in a day and some don’t require any plumbing – so can make excellent emergency toilets for those who need to isolate.
They don’t normally smell – in fact ours smells much less than a regular flush toilet as the constant fan immediately extracts any gaseous emissions. There are occasionally times (as with all bathrooms) when things smell a little. We also had one major issue with our first emptying – email me if you really want the details.
They use tonnes less water than even the most efficient low-flush toilet.
While they do extract air from your house, I believe they remove less heat, since the specific heat of water is >4x that of air.
The extractor fan has quite a cooling effect on one’s undercarriage – an experience that requires some getting used to. I’m now quite used to it, although it could just be that cooling is pleasant in summer & wasn’t so nice in cold weather.
Ours lets us apply our liquid nutrients direct to the garden – at least in theory: we still need to check with the local council, but the liquid collection tank has an educator that mixes irrigation water 8:1 with yellow water.
No matter how hard I try, the model we chose won’t work standing up – I consider this to be a design-flaw to be remedied, as I often find myself in the hall wondering whether I should waste water using our regular toilet or waste time with the composting one. Perhaps I should just install a waterless urinal…
You never forget to flush – or have to flush a stubborn floater multiple times. The model we chose opens a hatch and rotates the drum automatically when you sit on the seat. When you have finished there’s nothing more to do – you don’t even need to flush.

I don’t believe our “composting toilet” actually does much composting. It came with three buckets, one of which we have filled. We are now determining what to do when the third bucket is full. In particular how safe it would be for us to add this material directly into our vegetable-waste garden composter, or whether it requires disinfection first? We are considering something like a hot bin composter for this purpose.

I am told by their salespeople that “internal composting” toilets (which mix solid and liquids) actually do the composting in the unit, and produce useable compost. I don’t know whether I believe that, but we may replace our main toilet with an “internal composting” unit for a long-term trial. Let us know in the comments if you would value a head to head like this…

Carbon Benefits of Composting Toilets

The Energy Saving Trust have said that “Flushing the toilet is an unavoidable fact of life”. However, if we could change the facts of life by enabling mass deployment of composting toilets, what sort of impact could that have on the water industry’s carbon footprint?

The Water Industry is already committed to achieving net zero emissions by 2030, which will require lots of renewable energy for us to pump water around the country. If we can reduce our total energy demands, that frees this renewable energy to be used elsewhere.

The carbon cost of potable water treatment is much higher than it need be, because we treat much more water to potable standards than we need to. The economic and embodied carbon costs of a national twin pipe (potable and non-potable) network make it a non-starter. Decentralised grey water treatment and reuse on-site is much more feasible. But again, twinning up sewers into black and grey at a national level is unfeasible.

If an area could move entirely to composting toilets, as well as saving the 20-30% of potable water that is used for toilet flushing, there would be no black water. That would allow for local treatment and reuse of grey water. This could save at least a further 50% of potable water demand, reducing average potable water requirements from 143 to under 40 litres per person per day.

Our excellent system of flushing human waste doesn’t make for the most efficient means of nutrient recovery. The second law of thermodynamics advises against dissolving your salt in water if you subsequently want to put that salt on your chips.

We could save huge amounts of energy for both nutrient and water recovery, if we stopped using water as our means of human waste disposal.

Composting toilets and decentralised grey water treatment may require a larger water industry workforce and could have an impact on existing infrastructure. Conversely, potable water treatment and distribution energy & resource requirements could drop by over 70%, to say nothing of the wastewater collection and treatment savings.

This may be going too far, but each composting toilet in use reduces potable water demand and wastewater production, as well as the load on wastewater treatment works & the risk of pollution events caused by inappropriate flushing. The first few million installed shouldn’t require any changes to our existing national infrastructure. They even produce local fertiliser. Why don’t we talk about them more?