The Dalek Loo.
Doctor Who's revenge.

To be blunt - this page is about shit. If you have a problem with this it may be time to leave.
One of the many environmental problems faced by modern humans is the disposal of human excrement. One solution, particularly for sparsely populated areas - is the composting toilet. The basic principle of composting is very simple. Collect the stuff in some sort of container, remove the excess liquid and let it be decomposed by bacteria, fungus and other little critters that have been doing this job for millions (maybe even billions) of years. For infrequent use I made a toilet out of a compost bin.

This is it - the Dalek loo. So far it's working very well but it's only been in use for 3 months (since easter 1998). The loo was made by fibre-glassing a compost bin with an old toilet seat at the user interface end and fly proof vent holes fore and aft. A fan may be fitted in future but doesn't seem to be needed at the moment. This loo is a "batch" type - when it's full some years from now it will most likely be left in place and new toilet built to replace it. Later when the composting is complete the toilet can either be emptied or lifted off the pile and moved somewhere else. The vent holes are standard plastic plumbing fitting and can be sealed off with screw on caps.

In order for a composting loo to work properly it is essential that it remains aerobic - this kills harmfull pathigens, reduces offensive odors and is less attractive to flies. To remain aerobic the system needs oxygen throughout the pile, this means ventilation and removal of excess water but some dampness must remain for the composting to take place.

Four methods are commonly used (in combination) to achieve this.

  • Air is sucked from the chamber by a fan.
  • Excess water is drained.
  • Heat is applied.
  • Urine is seperated before entering the system.

A fan is a fairly standard feature in most composters and is worth some thought. I would expect that a factor-4 improvement would be possible in many existing designs. While the power needed to run a fan may not be an issue in a grid connected house - it sure is for a remote power system (eg. solar). For example our co-op is installing a Rota-loo, this unit comes with a 7.5 watt electric fan. The previous owner was grid connected and the fan was left to run continously. The loo was in continous use by a family and the loo was undrained.

Now we're going to use solar power.

  • The average power we'll get during even on a sunny day will be less than the rated peak power of the solar panel.
  • The sun shines less than 1/2 the time during a 24 hour day in winter.
  • There will be rainy/cloudy periods sometimes lasting weeks.
  • Some energy is lost when stored in batteries.
  • Panels will get dirty.
  • The panels will not be tracking the sun.

My educated guess would be our 7.5 watt fan would require a panel of around 60 watts and 14 days worth of energy storage. Say .624 amp@12 volt by 24 hours a day by 14 days - around 200 amp hours @ 12 volts. The panel would cost around $500 oz, batteries around $700 (plus replacement costs) and maybe a regulator $??
This is serious money just to dry out shit!

How do we improve this?

Well lots of ways....

  • We can reduce or eliminate the need for a fan by not allowing urine into the chamber in the first place, in some settings this could mean not peeing in there at all, otherwise it means fitting the loo with some sort of urine seperator at the user end and some sort of drain for the urine.
  • Have good drainage so liquid can't "pool" in the bottom.
  • Reduce the restrictions that hamper air flow.
  • Aid air flow using convection eg use a black chimney.
  • Buy an efficient fan, not all fans are created equal. It *may* be possible to find a fan that's twice as good at little or no extra cost - this would half the above costs.
  • Fit the correct sized fan, the 7.5 watt fan above may well be overkill.
  • Add manual or electronic control to turn the fan off when
    • The chamber humidity is low.
    • The outside humidity is high.
    • There's no sun.
    • It's cold.
    • The loo hasn't been used for X days.
  • Have a chamber with enough mass/volume to can absorb enough liquid to prevent saturation during *bad* times.
  • Add a wind powered extractor (eg. a whirly-bird).
  • Preheat the intake air with a passive solar collector. For indoor toilets - having the loo on the side of the house facing the equator (North in Oz) makes this easier. But don't over do it - too much heat or too little retained water will stop the composting action.

The fan also has another function - it stops odors from escaping when the lid is raised (and creates a refreshing breeze). It may well be worth fitting a variable speed fan which only runs at full speed when the lid is raised. It's hard to know the appropriate level of control. My guess is one voltage sensor on the battery and one humidy sensor in the chamber plus a $10 micro-controller would be good value for money - if you're handy with micros which fortunately I am. This may be another project for my project page..

As for electric heaters, that's an offensive idea to the energy conscious person. If the loo won't work without an electric heater then it's the wrong answer to the problem. Heaters aren't just to dry things out - the composting action is slowed or stopped under cold conditions. In our climate this shouldn't be a problem. Even if the composting stops in winter - so what - it'll catch up again in spring.

Hits since 26/june/98 =

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