Faecal chamber dimensioning


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  • Elisabeth
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Re: Faecal chamber dimensioning

Dear Marijn,

How many cubicles are you planning on building? I guess 40 persons is not a lot, so one toilet for males and one for females? This leaves 20 persons for toilet, which is almost like one extended family. So you could probably build the normal household type UDDTs.

If you like to do a calculation, you might find the calculation example useful which we put in the GIZ "UDDT technology review" which Christian Rieck has been working on for a little while now, and where we recently uploaded an updated version (still not the final version, we are still fine tuning it):

See here: Section 4.2.2 Calculation example for size of double
vaults, page 13

This is the text you find there (nicely formatted of course):

The daily average amount of defecation ranges from 0.12
up to 0.4 kg/day/person (see Section 7.1). As a common
example a rural African family with 10 persons is chosen
(or two households of five people each sharing the UDDT)
with a mainly vegetarian diet in a tropical climate. Adults
are assumed with a daily faeces amount of 0.4 kg per
person per day. Children and old people are also part of
the family who have a smaller food intake and thus smaller
faeces deposits of assumed 0.15 kg per day. The users
deposit faeces, toilet paper and covering materials in the
vaults. In some cases also sanitary items or even solid
waste, which is however not taken into account in this
Furthermore not all members of the household will always
be defecating at home since they are out for work, at
school, are on holidays or otherwise outside of the
household. This will further reduce the daily amount of
vaults deposits by about 15%.
During filling of the vault it has to be taken into account that
a moisture loss of the faeces (through drying) can be
assumed at a factor of 30%.
The vaults should be slightly oversized with a safety margin
to account for airflow, additional visitors, uneven
distribution in the vault (pile) and some distance to the toilet
slab. The faeces pile up as a heap and create a non-even
distribution in the vault which subsequently reduces the
usable volume of the vault unless the heap is manually
flattened. Accordingly the vault needs to be increased by
an assumed safety margin of 20%. This sums up to about
500 liters of volume per half a year (equals 0.5 m³) for a
UDDT that is used continuously by a maximum of 10
persons. This means a volume of 50 litres per person for
dimensioning purposes.
Calculation example household in rural Africa
Covering materials - assumed daily average 0.05 kg ash
Toilet paper - annual average of 8.9 kg *
7 Adults x 0.4 kg/day = 2.8 kg/day
3 Children x 0.15 kg/day = 0.45 kg/day
= 3.3 kg/day
98 kg/month
for absense - 15 % 83 kg/month
toilet paper + 0.7 kg/month =
material + 15 kg/month = 99 kg/month
x 6 months = 592 kg/half year
moisture loss - 30 % = 414 kg/half year
safety margin + 20 % = 497
vault volume = 500 liter
* according to Jönnson, 2004
As a result a vault size could have inside dimensions of 0.9
m long, 0.7 meter wide and a vault height of 0.8 m which
would create a total volume of 450 litres for one vault. This
corresponds already very well with the required dimensions
for the toilet cubicle above the two vaults. However the
exact dimensions of the vaults must be adjusted/ extended
to the desired dimensions of the toilet cubicle and the setup
of either the user facing the entrance door or the side walls
(see Section 3.4.2).
Calculations for schools and public toilets must be adjusted

Hope this helps and please give us feedback with regards to clarity and usefulness should you end up reading more of this document.

Oh and do think about whether you can eliminate the stairs, rather go for the bench UDDT perhaps? (= improved accessibility for people with disabilities)

Dr. Elisabeth von Muench
Freelance consultant on environmental and climate projects
Located in Ulm, Germany
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  • Marijn Zandee
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Faecal chamber dimensioning

Dear all,

One organisation I work with here in Nepal is going to built a UDDT toilet for their students dormitory (approx 40 users). We are currently trying to work out how to dimension the feces chamber.

The diet people there eat is mainly vegetarian, with legumes (lentils) as the main protein source, so this means a relatively large per person amount of poo. Further they intend to use a mixture of ash and dry soil as addition at approximately 1 cup per use.

What we are looking for is a good estimate of required volume per person, to achieve 6 months storage time in the vault.

Thanks for any answers.

Marijn Zandee
Marijn Zandee

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