Various kinds of flies can breed in the chamber of a composting toilet, and come out through the seat into the room. You may get fruit flies if vegetable wastes are put into the toilet, and you may also get fungus gnats – which are like midges but do not bite. it is unlikely that any of these flies pose a health hazard, and if they stay in the composting chamber they are a useful part of the decomposing process. Out in the room however they are irritating and cosmetically unpleasant.

With a well-functioning vent there are no smells in the room to attract the attention of flies, especially if the toilet lid fits well. But it is likely that colonies of flies will establish themselves from time to time, so it is well to be prepared.

Most flies move naturally towards a light source in a dark space. This means that if the lid does not fit well (to close out light), any flies will be in the pedestal area waiting for their chance to come out. For this reason it is not a good idea to have a layout in the room such that direct sunlight comes through the windows and falls onto the seat at any time of day. However, the attraction of flies to a light source can be used to draw them away from the pedestal.

The simplest method is to use the vent, especially in passively vented systems. Ideally, the vent should run vertically upwards from the chamber so that light at the top can be clearly seen at the bottom. Place a metal gauze over the top of the vent to prevent other insects coming in at night, attracted by the smell. Flies then fly up the vent pipe and are trapped at the top; generally a few spiders will be on hand to take advantage of this supply of food, which is a useful arrangement (although you’ll need to clean cobwebs away perhaps once a year to keep it fairly clear).

If the vent cannot be used, an alternative is a simple fly trap made out of a jam jar – particularly suitable on wooden pedestals. For this, drill a 2-3mm diameter hole in the middle of the jar lid, and screw the lid down over a hole in the top of the chamber. The jar is screwed upside down into the lid, with a cone of clear plastic in the lid to help funnel the flies up. Attracted by the light, flies will gather in the jar – which can be emptied as required.

If all else fails and flies are a persistent problem, then whilst an insecticidal strip hung inside the chamber might not sound very ecological, it will control flying insects without affecting the other decomposers in the compost mass.

This advice is taken from the CAT book ‘Lifting the Lid’, by Louise Halestrap & Peter Harper. The book was published some time ago and is now out of print, but the advice is still sound.

If there’s no mains electricity onsite, it may be possible to supply lighting using solar and/or wind power. Using low voltage (12 or 24 volt DC) will reduce equipment costs when  off-grid. To minimise power demand, use a motion detector to only turn the lights on when a person enters.

A suitably sized photovoltaic panel on the roof of the building can be coupled with a battery to supply sufficient energy for lighting during the summer months, but would be inadequate in winter. Year-round power can be supplied by combining a photovoltaic panel on the south facing roof of the building with a wind turbine mounted nearby on a mast.

There is considerable scope for creativity in the design of a standalone toilet cubicle, for example on an allotment. Timber frame structures are popular and attractive. For a robust and vandal-proof loo you might consider a metal clad unit.

An Aquatron separates solids and liquids using the sticking properties of liquid. In the hourglass-shaped device liquids stick to the sides while solids fall down the centre into a composting chamber.

It must be less than 25 metres (ideally less than 15 metres) from the toilet, in order that solids and liquids have not become too mixed. You should try to avoid bends in the pipe, and the waste should enter the aquatron at an angle of roughly five degrees.

A composting chamber may need to be about one-third to one half of a cubic metre per person. That volume is equal to a box about 70 to 80 cm on each side. The larger the chamber, the more forgiving the system. You may need to have provision for drainage of the compost, as more liquid is likely to get through than with a urine seperating toilet. So you’d need to include this in the design and allow for pipes, etc.

Ventilation will reduce problems with bad odours and flies and help prevent saturation of compost mass. A simple unpowered (passive) vent is a straight pipe of at least 110mm diameter, reaching well above the roof line and with a rotating cowl. If you paint the top of the pipe black this assists with thermal draw. In toilets at CAT we have used a small 10 watt fan in the composting toilet.

Nearly all pathogens are broken down by composting if it’s left for at least a year (ideally two) to decompose. The quantity produced is small so as an extra precaution it can just be used where it doesn’t come into direct contact with the edible part of crops.

For more on the Aquatron, see the websites of either the UK distributor or the Swedish manufacturer.

Urine can be used as a fertiliser (on non-food plants), when diluted about 1:10 in water. It is an excellent plant food containing just the right balance of plant nutrients including nitrogen, phosphorus, and potassium. However, don’t store it for too long or it will start to smell. In some cases, diluting with waste greywater may be possible.

Both flushing and composting toilets can have urine diversion. It’s common for composting toilets as diverting urine at source improves the composting process. There are waterless urinal designs that may be suitable for some locations. A more basic alternative is just to collect urine in a container with a funnel.

Urine collected without other contamination should ideally be used within a day or so, before ammonia levels build up and it starts to smell. If you are collecting urine from seated use there is a real risk of contamination from faecal pathogens. Urine collected in this way should be stored for six months, during which time the ammonia level will build up and sterilise the liquid.

Whilst urine is almost always sterile when it comes out, you should avoid using it on any crop where the bit that you eat may come in contact with the urine. So it’s fine round your fruit bushes, shrubs and flowers, but not for salad crops.

At CAT we found good results from applying diluted urine as fertiliser in winter and early spring. At these times of year plants often don’t get enough nitrogen from the cold soil.

At other times, adding urine to compost may be a good option. This is particularly appropriate for a high-carbon compost heap, such as leaves & woody waste. The nitrogen input is then very helpful. Another simple idea is a ‘straw bale urinal’. This is a bale of straw, ends uppermost, for men to urinate on – or women too, with a suitable design. The liquid soaks in and composts the middle of the bale in a few months.