II. Biolatrine design
Biolatrines are integrated units, consisting of ventilated
improved pit latrines, with septic tanks attached. The septic tanks, which serve
as bio-digesters, differ from normal septic tanks in that processing is carried
out in an anaerobic environment. The treatment of waste is more thorough than in
a normal septic tank, and there is an outlet for the biogas produced in the
a. Size Requirements
Biolatrine units differ in size depending on several factors
including the quantity of feed stock or the number of users; the climate, since
temperature differences may require variation in retention time of the feed
stock; the nature of the user community, and social and economic conditions
The nature of the community may require different-sized units.
Schools and army camps, for example, normally have break periods during the day
when many people visit the toilets at the same time. Such peak moments make
necessary installation of more seats, although the total volume over a day's
period may not be correspondingly increased.
THE COMPLETED BIO-LATRINE UNITS.
More generally, the quantity and the composition of excrete is
directly related to the social and economic conditions and living habits of the
community, via the effects on diet and health. Literature surveys by Feacham et
al. (1983) found the quantity of faeces production in Africa and other
developing countries to be between 130 and 520 grams (wet weight) per capita
daily, while that in some European countries and North America to be between 100
and 200 grams (wet weight) per capita daily. Most adults produce between 1 and
1.3 kg of urine daily depending on how much they drink and the local climate.
These figures are consistent with those obtained from the biolatrines installed
in Oljoro Military Camp and Biogas Extension Service in Arusha, Tanzania.
The content and nature of pathogenic microorganisms found in
faeces also depends on such background factors. The food consumed, food handling
practices, the quantity of water available and other similar factors produce
feed stock for biolatrines with distinctive characteristics.
b. Retention Time
The retention time is the period for which the digester feedstock
has to stay in the bio-digester before the first overflow (effluent) can be
safely extracted. Calculating this time period appropriately is a key factor in
successful management of biolatrines.
The necessary retention time can be affected by temperature. High
temperatures speed up the microbiological processes and shorten the retention
time. Lower temperatures correspondingly lengthen the recommended retention
In anaerobic processing in developed countries, the digesters are
often artificially heated to allow fast treatment of large volumes of waste.
This is an expensive exercise, and in developing countries bio-digesters
normally depend on natural prevailing conditions to establish the temperature.
In Tanzania, average ambient temperatures range between 10°C (in colder
parts and cold season) and 38°C in coastal areas and hot seasons.
Under these conditions, the recommended retention time for human
excrete is about 100 days. CAMARTEC designs including an additional safety
factor are premised on an average retention time of 150 days. This allows for
unpredicted feed additions. In addition, as Roscol F. Ward (1985) points out, if
improved health is a consideration (i.e. destruction of most pathogenic
micro-organisms), then it is advantageous to err on the side of caution with
higher temperatures and longer retention