Composting toilets are waterless (dry) toilets designed for use in applications without a plumbed sewage system. They operate on two principles:

1. dessication of solid waste in an absorbant media to eliminate odor and mess.
2. separation of liquid and solid waste to avoid messing up the first step

The media would compost over time, but composting toilets are emptied regularly, even if infrequently. This means the media is actually not composting to any significant degree in the toilet. Some have argued a better term for this system is “dessicating toilet” but the name is not in widespread use.

While composting toilets are often vented by a fan, this is to remove moisture from the toilet rather than to control waste odors (which are negligible).

In general an RV-appropriate composting toilet is designed for use by 1-2 people. In this use the solids compartment is used for about 2 person-months. The liquids tank fills in a few person-days.

Normal 1-ply toilet paper is safe for the solids compartment though overuse can reduced capacity and over-dry the compost medium.

Commercial composting toilets are a sit-to-pee design for both sexes. The urine diverson systems are designed to work when sitting and keeping urine out of the solids compost is critical to the operation of the toilet.

Commercial composting toilets generally keep the solids container closed by a door when not in active use. There are two approaches:

1. “straight drop” - the cover is removed before use and deposits are made directly into the solids compartment
2. platform - there is a trapdoor mechanism that opens when the deposit is completed (as when flushing a plumbed toilet). A coffee filter can be placed over the trapdoor as a liner to reduce mess.

Some RVers keep a spray bottle of water handy to rinse down traces of liquids or solids that remain in the bowl.

• coconut coir (shredded and compressed husk) - recommended by Nature's Head.¹⁾
• sphagnum or peat moss - recommended by C-Head²⁾, Airhead³⁾, and Nature's Head.⁴⁾
• sawdust - recommended by C-Head⁵⁾
• ground cob

There are several commercial models suitable for RV use. One can also make a simple DIY model.

Features:

• made in the USA
• stainless fittings
• 5 year warranty
• 2.2gal liquid tank with visible level
• relocatable compost crank
• relocatable vent outlet

Features:

• relatively low cost (about $300 less due to simpler components)
• straight drop system.⁶⁾
• a template for making a cardboard mockup
• uses normal 1gal milk jugs for liquid tank

Note: The C-Head's use of an inexpensive, cut-down 5gal bucket for the solids reduces costs but has some side effects the prospective buyer should consider:

1. solids compartment needs to be emptied about 2x as often as other commercial models (every 2 person-weeks).
2. auger-style agitator does not mix in toilet paper in very well; it tends to float on top.

Features:

• designed to mimic domestic toilets
• 5 year warranty
• can remove liquids tank without opening solids compartment

A simple composting toilet can be made with a seat and a bucket. It is very cheap but has significant shortcomings compared to the commercial models:

• no liquid diversion - one must arrange a separate system for urine; it cannot go into the bucket. Female RVers may want to look into clever and collapsible "go girl" funnels which allow them to pee standing up (and into a bottle).
• no agitation - before inital use a liner is installed and a layer of media is added to the bottom. Thereafter each deposit is covered with fresh media (typically sawdust).
• will probably not qualify as self-contained

The Privy 500 is a urine-diverting seat designed for tiny house installations. It might be adaptable to a bucket system, running the liquid into a nearby milk jug or similar. The shipping box has a cut-out template printed right on it.

There are separators/diverters sold on eBay.