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 process.

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 affecting diet.

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 time.

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 times.