The Use of Effluents from Biolatrines in Tanzania (ADF, 1996, 38 p.) Biolatrine technology (introduction...) I. Bio-digestion and biogas generation II. Biolatrine design

The Use of Effluents from Biolatrines in Tanzania (ADF, 1996, 38 p.)

Biolatrine technology

Almost all kinds of organic wastes can be recycled into valuable products, provided the processes for treatment and disposal or reuse of these wastes are well considered in the initial design of treatment facilities. It is also necessary to consider the potential for pollution and the possible diseases associated with handling and recycling of animal and human wastes.

Human waste, in particular include excreta, waste water, discarded food residue, and other solid household wastes. Excreta refers to the combination of faeces and urine, normally of human origin. When diluted with flushing water or other grey water (such as from washing, bathing, and cleansing activities), it becomes domestic sewage or waste water. Another type of human waste, called solid waste, refers to the solid or semisolid forms of waste discarded as useless or unwanted. This includes food wastes, rubbish, ashes, and other residues. Food wastes, which are mostly organic, are particularly suitable for recycling.

The quantity and composition of human excrete, waste water, and solid wastes vary widely from location to location depending upon, for example, diet, socio-economic factors, weather and water availability. Generalized data may not be readily applicable to a specific case, and design of the biolatrine system should be preceded by field investigation at the intended sites.

I. Bio-digestion and biogas generation

Excreta, being an organic waste, is biodegradable. As it is digested, or biodegraded, by bacteria, part is converted into the end products of methane and carbon dioxide and part is converted into bacterial cells. Under anaerobic conditions, i.e. without air, the portion converted into bacterial cells has been estimated to be from 2 to 5 percent (Roscol F. Ward 1985).

In the bio-digestion process, biogas production results in methane and carbon dioxide. The remaining slurry loses most of the smell of excrete, and becomes more liquid than fresh excrete. These changes are caused by different bacteria working on the original matter input.

Figure 1: Biogas Generation

THE BIO-LATRINE INLET TOILET CHAMBERS UNDER CONSTRUCTION.

THE BIO-LATRINE OUTLET OPENING UNDER CONSTRUCTION.

The most important chemical principle at work is the transformation of longer carbon chains such as cellulose, alcohols and organic acids (which form a good part of the fresh excrete) into short carbon molecules like methane (CH₄) and carbon dioxide (CO₂). See Figure 1.

Cellulose takes longer to break up and often stays for a considerable period. The smell of excreta is mainly caused by organic acids (carbon chains with a certain oxygen content) and aromatics (carbon chains in ring form).

The bio-digestion process works best under a restricted range of conditions. The minimum temperature is about 15°C. The maximum is 45°C and the optimum range is 30-35°C. Even more important than the temperature is temperature stability. Changes of more than 2°C per day are harmful to the process, since the bacteria adapt rapidly to prevailing conditions and must readapt when the temperature changes. Acidity is also a significant constraint, with the optimum ph range 6-8. Methane-producing bacteria are negatively affected by acidity outside these limits. Retention times are another significant variable, which will be discussed below.

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.