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close this bookThe Use of Effluents from Biolatrines in Tanzania (ADF, 1996, 38 p.)
View the document(introduction...)
View the documentForeword
Open this folder and view contentsIntroduction
Open this folder and view contentsThe problem defined
Open this folder and view contentsBiolatrine technology
View the documentPublic health aspects
View the documentFertilizer production and usage
View the documentResearch plan
Open this folder and view contentsResearch findings and interpretation
View the documentConclusions and recommendations
View the documentBibliography
View the documentAppendix

Fertilizer production and usage

The application of human excrete to the land to fertilize crops is an ancient practice, well established in many countries of the Far East, where the fertility of the soil has been maintained by excrete fertilization for over four thousand years. It is the only agricultural use option in areas without sewerage.

In developing countries, the majority of households are without sewerage and are likely to remain so, at least in the foreseeable future. It follows that emphasis should be directed toward the implementation of on-site sanitation systems that readily permit the safe reuse of the digested excrete. The primary example is a biolatrine system with a suitable temperature and sufficient retention time, such as 25°C to 35°C for 150 days.

The most important changes in the fertilizer value of excrete bio-effluents, as compared with fresh excrete, can be considered under three headings:

1. the ratio of carbon to nitrogen (C/N);
2. the content of mineralized nitrogen (Nmin); and
3. the biologically harmful effects of excreta.

Table 4: C/N Ratios for Some Organic Materials

Material

C:N Ratio

Poultry Manure

5:1

Human excrete

10:1

Cattle bio-slurry

20:1

Fresh cow dung

25:1

Garden soil

40:1

Straw

70:1

Sawdust

300:1

A substance can only be considered as an N-fertilizer, if the C/N ratio is significantly smaller than that of the soil, i.e., it contains a higher proportion of nitrogen. The range of C/N ratios for some common organic materials can be seen in Table 4.

The values in the table make it clear that one would not add nitrogen to the soil in the form of straw or sawdust. With C/N ratios even greater than ordinary garden soil, they would not add to the proportion of nitrogen content but rather reduce it.

During the generation of biogas, the total amount of nitrogen in the residue stays more or less the same, while carbon is removed in the form of methane and carbon dioxide. The resultant slurry has a lower C/N ratio, and a higher value as a nitrogen fertilizer.

A plant can take in nitrogen only in mineralized form, either as nitrate (NO3) or as ammonia (NH3). The Nmin content of excreta and its bio-slurry is composed mainly of NH3, whereas the Nmin content of soils and compost is mainly NO3.

In fresh excrete, the Nmin proportion of the total amount of nitrogen is about 30 percent. Thus, about 30 percent of the total nitrogen, in the form of NH3, is directly available to plants. In well-digested excrete, the proportion of mineralized nitrogen is doubled. This means that the short-term fertilizing effect is maximized, concomitantly reducing the long-term fertilizing effect. Under tropical conditions in particular, maximizing the short-term effect is more important.

Although fresh excrete as well as bio-digested slurry has a positive fertilizing effect, organic acids in fresh excrete can have a harmful effect on plants and soil organisms by altering the ph value of the soil. This danger, combined with the presence of pathogenic microorganisms, emphasizes the importance of the bio-digestion process.

The slurry produced by biolatrines has a number of advantages for use as a fertilizer. Direct application of liquid fertilizer, covering the soil, conserves both nutrients and moisture. The organic properties improve soil composition and support root growth. Hazards of erosion, as compared with use of artificial fertilizers, are reduced.

The work load can be much less than for an application of artificial fertilizers, particularly when the land topography allows gravity flow distribution through canals and channels. Even when flat land does not allow gravity flow distribution, the effluent can be stored in drying pits, from which it can be drawn in buckets and carried in buckets, wheelbarrows or ox-driven carts to the fields where it is needed. It can also be combined with compost to enhance that process.

Even if one does not optimize the use of biolatrine effluent as a fertilizer, any use at all of properly treated effluent is a net addition to available fertilizer. And it is relatively easy to reduce the risks simply by waiting the appropriate time period for die-off of pathogens.