See our dry toilet installation for details.
We calculated that for two people for a full week we would need a 31 L tank. Of course, some days we don't urinate as much as other days, but that tank would allow for that odd week where we would exceed normal amounts. We found a 27 L (6 CDN Gal) polyethylene tank. Polyethylene tanks are less prone to retaining odors and are CSA Certified to NSF/ANSI 61. The tank is smaller than we had planned, but because we overcalculated the first amount we think that this will work out fine. The tank has a fitting overtop so we can close the tank while underway.
To empty the tank we'll connect a foot pump to transfer the contents into a smaller 5 L carry container to dump in a public toilet, or in the sea when far from shore. When underway, we might just get a long hose and pump it directly overboard from the forward hatch, we are not sure yet. We could have kept a thru-hull to empty it out, but preferred to close it.
We bought a Forian porcelain urine diverter by Shit and Blossoms to keep the liquids and solids separate. We chose porcelain because of its durable and hygienic properties. Very few companies offer the option of a porcelain separator, it is easy to clean and will last for a long, long time.
We've been living without a holding tank for some time, instead using a sturdy 5 L jug while in marinas or busy harbors. We kept a funnel to help our aim (or rather, Rekka's aim). We would dump its contents in a public toilet once a day, depending on usage. All urine has an odor, we smell it when emptying the bottle and the big liquids tank will be no different. With the jug, we kept a spray bottle with a 1:1 vinegar mix to rinse the funnel after use. The vinegar/water solution helps to break up accumulations, as well as smells. For odors that persist, adding 1 tablespoon of raw sugar or a bit of vinegar to the tank(or jug) will reduce this odor.
We have a 20 L polyethylene tank with a cover (to close it in rough waters). We don't know how often we'll have to empty it yet. For now, we plan to use wood chips sourced from a landscaping company in Victoria, bbecause we aren't filling a truck-load they sometimes give it to us for free. A general rule is to ensure the ratio of medium to human waste is 1:1. We keep the medium in a bag with a scoop. In the winter, we think we may be able to use ash from the woodstove as a medium (we hope).
When the container gets about 3/4 full, we'll have to empty it. Ideally, we'd like to find someone to team up with on land to make a humanure composting pile.
The solids tank has a small 12V 40x40x10 mm fan that vents outside. Without the fan and vent, liquids would evaporate at a much slower rate and cause a bad odor. Make sure the fan is on a fused circuit(use an inline fuse in a non-protected circuit), use that the fuse that is no less than 2 amps to a maximum of 5 amps.
Future Notes: adding diatomaceous earth to the compost will kill and deter insect infestation. The Diatomaceous Earth can be purchased at swimming pool supply stores, Lowes, Home Depot, etc.