A dual-chamber toilet has two sealed chambers of about 1 m × 1 m × 1 m that fill and rest in turn, and the waste may be emptied only after resting for a full year, which works only if each chamber takes at least a year to fill. At the volumes worked out here, a 1 m³ chamber used by four people fills in about 7 to 10 months, so build each chamber larger, or build a third chamber so each batch rests for two fill periods.
Part of Composting Toilet Systems
A dual-chamber (batch) composting toilet uses two chambers beside each other: one is filled while the other is sealed and left to compost undisturbed. Waste is handled only after it has rested, which is safer than emptying a single chamber that always holds fresh material.
The Layout
The parent page’s design:
- Two chambers of masonry, concrete block or earthbag, each about 1 m × 1 m × 1 m.
- A seat over a hole in the top of the active chamber; the other chamber’s hole is sealed.
- A vent pipe, 10 cm in diameter, from each chamber, rising above roof height and ending in a fly screen (Ventilation & Odor Control).
- An access door at the bottom rear of each chamber for removing finished compost.
- A concrete or plastered floor.
Build above the highest flood and groundwater level and well away from wells, springs and streams. This design needs no pit, so it suits wet or rocky ground where a pit latrine would reach groundwater.
The Rotation Has A Hidden Condition
The parent page’s cycle: fill A, switch to B, seal A and let it rest for 12 or more months while B fills, then empty A. The rest time of A is therefore exactly the time B takes to fill. The cycle guarantees a year of rest only when B takes 12 months or more to fill.
Fill time, for a 1 m³ chamber stopped at 80% full (the point Composting Toilet gives for sealing a vault), so 800 L usable:
- Cover plus faeces per person per day: about 0.63-1.0 L (0.5-0.75 L of cover from the first-month page, plus about 0.13-0.25 L of faeces, a recalled published range).
- Four people over a year = 4 × 0.63-1.0 × 365 = 920-1,460 L per year.
- Months to fill 800 L = 800 ÷ 1,460 × 12 to 800 ÷ 920 × 12 = 6.6 to 10.4 months.
So each batch rests only 6.6 to 10.4 months, not 12, if urine is diverted. If urine also goes in, the chamber fills in a few months and is wet. The parent page’s “6-12 months of use by a family of 4” is consistent with this arithmetic; its claim that the rotation “ensures material always composts for at least a full year” is not.
Three Ways To Get A Full Year
- Bigger chambers. Size the usable volume to at least one year of use (Chamber Sizing): 4 people need about 0.92-1.46 m³ usable, or 1.15-1.83 m³ gross at 80% fill.
- A third chamber. Rotate A, B, C. While one fills, the other two rest, so each batch rests for two fill periods: 2 × 6.6 to 2 × 10.4 = 13.2 to 20.8 months. This keeps 1 m³ chambers workable.
- Fewer users per toilet. Rest time scales inversely with users.
Do not shorten the rest to fit the building. A chamber emptied at 7 months holds material the parent page’s own rule says is not yet safe to handle.
The Floor Drain Is A Mistake In The Parent Page
The parent page slopes the floor to a small drain “directed to greywater recycling”. Do not connect it. The liquid from a toilet chamber is human waste, and the greywater page’s own rule is that the toilet has no connection to the greywater system (Greywater vs. Blackwater). Design the chamber to need no drain: divert urine, cover with dry carbon, and if liquid still collects, add dry carbon. If a drain is unavoidable, route it to a sealed container and pour the contents back into the active chamber.
Operating Rhythm
- Fill A until about 80% of volume. Add 10-15 cm of dry cover on top.
- Seal A: close the seat hole and the access door; label the date on the door.
- Fill B. Keep A’s vent open.
- When B is full, check A’s rest time. Empty A only if it has rested for the full 12 months, by rake or scoop from outside, without entering.
- Switch the seat back to A. Leave a layer of finished compost on the floor to seed the next batch.
Common Mistakes
- Counting fill time as rest time. Rest time starts when the chamber is sealed.
- Entering the chamber to empty it. See the safety block.
- Piping the floor drain to the greywater bed.
- Unscreened vents and unsealed lids, which let flies in and out.
Safety
A sealed chamber of human waste is a confined space with toxic and flammable gas
Fermenting waste can leave the air short of oxygen and can produce hydrogen sulphide, which deadens the sense of smell at dangerous concentrations, so “I can still smell it” is not a safety check, and methane, which is flammable. Never put your head or body into a chamber. Empty from outside with a rake. Keep flames and sparks away from vents and doors. Do not test the air with a flame. Emptied material is handled only after the full rest time, with gloves, a cloth over the face against dust and hand-washing afterwards.
What’s Next
- Learn next: Chamber Sizing — set the volume from users and rest time
- Combine with: Urine-Diverting Systems — keeps each chamber drier and slower to fill