In many applications the choice is between the two types of composting toilet; a urine separating compost toilet (Separett, Wostman, Biolan Separating Dry) or an internal composting toilet (all Sun Mar, other Biolans). Both types are self vented so odour is not an issue with either. But there a subtle differences between the two. Here, I try to explain the pros and cons of each type so that you can make an informed decision.
Urine diverting toilets work by separating urine and solids at source. All use of this type of toilet must be seated. Urine is collected at the front and piped through the toilet to the outlet. The solids fall into a container at the back of toilet. It is important to note that a urine diverting toilet does not break down the solid waste into compost. It merely collects so it can be removed and composted outside. Urine is piped to a container or a soakaway outside as it does not present an environmental threat in most circumstances. Neat urine, from most people, is relatively sterile. Internal composting toilets do not separate urine from the solids. The moisture and Nitrogen from the urine are required to fuel the composting process in the unit. In an internal composting toilet bacteria break down the waste in the toilet, and in so doing, generate enough heat (up to 70 degrees C) to kill pathogens that are in the faecal matter, this is called thermophilic composting. Although some moisture is required in composting, sometimes there can more urine than is needed. Excess urine soaks through the compost pile resulting in a small amount of compost tea. Compost tea is nutrient rich so it can be collected and added to a compost pile or allowed to soak into the soil if conditions are suitable. The advantage of an internal composting toilet is that you never have to deal with raw waste which over the life time of the toilet is quite a major factor.
The great advantage of a urine diverting toilet is that there is no maximum capacity of the unit. Since you are not expecting the waste to break down in the toilet there is no need to wait to empty the container. As long you change the container for an empty one the toilet can go on and on. In contrast using a composting toilet there is a max capacity. The composting process occurs in the unit and this takes time, about 6 weeks. Each internal composting toilet has been designed to process waste from a certain number of people into compost. This rating, in persons, is given on a constant or seasonal basis. For example the Sun Mar Excel is rated for 3 people on a constant basis and 6- 8 people on a seasonal basis. It is important to choose a composting toilet that has a capacity greater than, or equal to, the intended usage so that you get fully broken down compost. There is no issue with using the composting toilet too little and the units are designed to cope with no use part of the year.
The great advantage of an internal composting toilet is that compost is removed from the toilet so you never have to deal with unbroken down waste. The thermophilic composting process kills off the pathogens and transforms the structure of the waste. It also reduces the volume of the waste. Millions of composting microbes consume the carbon in the waste leaving less solid material to remove. With the urine diverting compost toilet, little or no breakdown occurs in the unit so the waste is not reduced or broken down. This needs to be composted outside. This means that the emptying cycle is more frequent with a urine diverting toilet (every 2-4 weeks) than an internal composting toilet (every 2-6 months). The solid material that comes out of the urine diverting toilet needs a further 18 months to break down (can be faster if composting is managed) in line with best international practice. This composting needs to take place in a compost bin that is sealed at the base (pest proof) and has a lid to avoid flooding. So there is more follow up care required with the urine diverting toilet which is up to the owner to manage properly.
Examples of urine diverting compost toilets are the Separett Villa and the Biolan Separating dry toilet. Generally speaking, since the urine diverting toilet does less for you, they are easier to design and manufacture so they are cheaper than composting toilets (Sun Mar range and the Biolan Eco) which have a more thorough design and break down the waste to compost. In the end it is a personal choice based on your application, usage, budget and personal preferences. In this post I match up toilets with typical applications.
I hope this explains this issue of urine diversion somewhat, but if you are totally confused please contact us for more information.