Urine diverting toilets (UDT) work by separating urine and solids at source. The urine is collected at the front and piped away and the solids fall into a container at the back of toilet. It is important to note that a urine diverting toilet does not break the waste down, it merely collects so it can be removed and allowed to break down in a compost bin outside. Urine is piped away to a soakaway. It does not Patrick Boylan present an environmental threat as neat urine (from a healthy person) is relatively sterile. Composting toilets (CT) do not separate urine from the solids. The Nitrogen (from the urine) is required to fuel the composting process. With a composting toilet bacteria will break down the waste, and in so doing, generate enough heat (up to 70 degrees C) to kill pathogens that are in the faecal matter - thermophilic composting. Check out this post on composting for more. Excess liquid from a composting toilet has soaked through the compost pile and so is a mixture of urine and faecal matter. This is called seep liquid.
Seep liquid can be either treated or collected in a container and, ideally, stored for 6 months prior to adding it to the soil. In terms of male urination using a UDT males must sit, whereas with a composting toilet they can stand as normal. Waterless urinals are an optional extra for both types.
Capacity
The great thing about a UDT is that there is no max capacity of the unit. As long as the owner changes the container for a new one the toilet can go on and on. The urine is neatly piped away to a soakaway. In contrast using a composting toilet there is a max capacity, rated in persons. The composting process occurs in the unit and this takes about 6 weeks. So if the amount of material coming in is greater than the amount the toilet's bacterial population can process there will be a build up in the toilet and it will need some time to catch up. So it is important not to choose a composting toilet that is below the usage intended because with a CT you are depending on a biological process to breakdown the waste for you - see post. But with UDT this is not a concern as all one has to do is replace the container.
Follow up
The great thing about a composting toilet is that when compost is removed from the toilet and is safe to place under the bushes or in a flow garden. The thermophilic composting process has killed off the pathogens and so it is safe.
There is no further follow up care required. With the UDT little or no breakdown has occurred in the unit. So that means that you'll need to empty the UDT more often (every 2-3 weeks) than a CT (every 3-6 months). Also the material that comes out of the UDT needs a further 18 month composting prior to burial. This composting needs to take place is a compost bin that is sealed at the base and does not allow rain water in. So there is more follow up care required with the UDT which is up to the owner to manage properly. Some local authorities see this aspect of UDT as a negative as there is the potential for human error and for faecal matter to come into contact with the water table.