Self-Contained Composting Toilets for the Chemically Sensitive
I have the SunMar Spacesaver in my tiny house because it's the smallest indoor self-contained composting toilet I could find in Canada, and the only one that will fit in my tiny bathroom. It was also the best priced unit. I absolutely do not recommend this toilet as it completely fails at handling liquids. Here are a few of the other challenges followed by some recommended brands:
Toxic Additives in Composting Toilets?
I had such an acute reaction to the microbe mix. It smells somewhat like a urinal cake. Everything online said it was non-toxic and natural... hmm. I called them to ask what is in it and they said citronella.
Citronella contains methyl eugenol which repels bugs and is considered toxic by Canadian governmental standards. There are a few essential oils I consider to be harmful for MCSers and citronella is definitely one of them. I had to figure out what kinds of microbes were needed for the toilet and I found out that EBokashi MCSers and citronella is definitely one of them. I had to figure out what kinds of microbes were needed for the toilet and I found out that EBokashi will work just as well and doesn't have fragrance added.
After using the toilet for a few months, I can say that the enzyme spray is not necessary. You could just use the Bokashi or Bokashi and a bulking material. However, I am not happy with this model at all. The upkeep is very smelly work and it often overflows with only one person using it. Dealing with the overflow is horrible. Even before it overflows the system is such that liquid accumulates below the tray where it becomes very smelly, and mouldy. One person using this toilet fulltime requires that it overflows into a septic system or blackwater system. Not very self-contained at all.
Offgassing the Unit
Another chemical issue is with the unit itself. It needed some offgassing outside to get rid of the plastic and glue smells. I left it outside for a week. A month would have been ideal.
There is some installation necessary that ideally would be contemplated before building the bathroom of a new house! A vent has to go through the wall, outside, and above the roof line. An emergency overflow valve needs to go through the floor and out to.... somewhere (a bucket)... or to the septic or sewage drain if you are on the grid. You do not need to have water as an input for this type of unit or bolt it down in any way. There are electric and non-electric self-contained units. The Spacesaver is electric. There is a need for a special outdoor compost for self-contained indoor composting toilets; they say the humus the toilets produce is totally benign but there is no way it is in there long enough to be benign.
Benefits to Having a Composting Toilet
There is a huge benefit to being off the septic system and that is the freedom to put your tiny house/yurt/dome on any piece of land with some extra amps to spare & a fresh water hose (and of course you could get totally off the grid with solar panels and rainwater collection).
If you are comparing the initial cost and cost of (ongoing) inputs to just hooking up to a city sewage line, then the self-contained composting toilets are going to seem expensive. If you are comparing the cost to installing a rural septic tank and the maintenance of that septic tank, then it starts to seem like a really good deal.
If you are extremely sensitive you're going to want to make sure you can source some tolerable bulking material before you start. A lot of the maintenance also includes some strong smells of excrement so you have to be able to tolerate that. I have seen some mould growing in the finishing drawer.
Another challenge for people who are disabled is that there is some work - there is the buying of the additives and maintenance. Maintenance involves turning a crank every second day for a minute. Unclogging the mesh screen if that ever clogs up, checking to see if the system has overloaded via the emergency drain and other troubleshooting if anything goes wrong. There is a fair amount of troubleshooting so far for me. The fan will have to be replaced or fixed if and when it stops working. You also empty out the bottom drawer of benign humus every few weeks.
I'm having trouble with the Spacesaver as I don't think it has enough capacity for one person let alone two. The tray is filling up too fast, which can be a major problem if you don't have somewhere safe to dump it and it overflows liquids.
If you are building a new house (that's not on a trailer bed) on a site without sewage or septic already in place, you are probably going to be subjected to building codes that require more conventional systems.
Recommended Composting Toilets
Nature's Head Dry Composting Toilet or the Separett.
What's different about these is they separate the urine out. This is crucial. It is the reason the Sunmar is overflowing and going mouldy in the tray. I would go with one of these for sure when I replace the Sunmar.
Other Types of Composting Toilets
This post deals with indoor self-contained units but other types of composting toilets include: an outdoor bucket system, an outdoor dug out (outhouse), indoor central flush, central dry, and indoor bucket systems.
Being off the septic system means you also need to safely recycle your greywater - which is the water that comes out of the kitchen sink, bathroom sink and shower. These contain more bacteria than you think via raw meat and the bacteria on your body etc.
Eco-Sense has a really good free overview and guide including using a worm bin system to remove food scraps from the water. Here are the instructions for the worm bin filter system. There are a lot of different types of systems and for more detailed instruction this book is super user-friendly and outlines the simplest options depending on the number of people you have on the system.
You need both some kind of filter and water dispersion. We tried just burying the pipe so that the water would simply drain into the ground but the water could not absorb fast enough and it backed up through the house. Gross, yes. We dug a small pit and it still backed up. Right now I have a larger pit but it still has no filter on it, it worked fine for years with little upkeep. (NB it's not technically safe to have the water drain above ground).
One more point that needs to be made about greywater and blackwater systems is that any chemical or medication that goes down your sink or toilet will go directly into your backyard. Keep in mind that urban sewage treatment plants do not treat or remove chemicals including those from hormonal birth control and medication, so those are already going directly into our water systems. Here are my complete guides to non-toxic, biodegradable personal care products and cleaning products.