Use of Dehydrated Faeces

Dehydration Process

In a dehydration toilet, the excreta inside the processing vault are dried with the help of sun, natural evaporation and ventilation. Dehydration is different from composting because the organic material present is not degraded or transformed (e.g. shells and carcasses of worms and insects will remain in the dried faeces); only the moisture is removed. After dehydration, faeces will reduce in volume by about 75%.

The primary motivation for dehydrating the faeces is that dried faeces are a hostile environment for pathogens and help to reduce the amount of pathogens in the faeces significantly over time. The degree of pathogen inactivation will depend on the temperature, the pH and storage time. The addition of calcium carbonates, contained in lime or wood ashes helps to raise the pH, and thus accelerate the process of pathogen inactivation. However, dry faeces may still contain pathogens. Oocysts for examples are spores that can survive extreme environmental conditions and re-grow under favourable conditions.

It is generally accepted that faeces should be stored between 12 to 24 months, although pathogens may still exist after this time. Faeces that are dried and stored between 2 and 20°C should be stored for between 18 up to 24 months before they can be used at the household or regional level without any health concern. At higher temperatures (i.e. greater than 20°C storage) 12 months is accepted to be sufficient for the inactivation of Ascaris eggs (a type of intestinal parasitic worm). A shorter storage time of six month only, is required if the faeces are stored with a pH above 9. However, a secondary treatment (composting, vermi-composting) of dehydrated faeces should always be considered (if possible) as an additional barrier to inactivate pathogens (and here most importantly Ascaris).

The World Health Organization (WHO) has published guidelines for the safe use of wastewater, excreta and greywater in agriculture (see further readings); these should be consulted before using dried faeces. When the faeces are completely dry, they will emerge as a crumbly, powdery substance. This material is rich in carbon and nutrients and fibrous material. If the drying is fast and a low moisture level is achieved, the losses of both organic matter and Nitrogen are small. Compared to composting, dry storage of the faeces recycles more organic matter and Nitrogen to the soil, but the organic matter is less stable. Dried faecal matter is a complete phosphorus and potassium fertiliser, which also contributes considerable amounts of nitrogen (JOENSSON et al. 2004).

While the total amount of nutrients excreted with faeces is lower than with urine, the concentration of nutrients, and especially P and K, is high in faeces. P is particularly valuable for the plant in its early development and important for good root development. In addition to supplying macro- and micronutrients, faeces contain organic matter, which increases the water-holding and ion-buffering capacity of the soil, serves as food for the microorganisms and is important for improving soil structure.

As the breakdown of organic material in dehydrating conditions is slow, toilet paper or similar objects placed in the chamber will not disintegrate quickly. Toilet paper can therefore either be handled separately, or be composted in a secondary treatment process.

Application

Dehydrated faeces can be mixed into soil, buried or composted at home or in a community based composting site depending on acceptance. Nutrients will be available to plants directly or after further decomposition of the dehydrated material. The addition of wood ashes not only enhances the inactivation process of pathogens due to the rise of pH, but can also increase the potassium (K) content of the final product.

The application rate of faeces can be based on local recommendations for the use of phosphorus-based fertilisers and analysis of the phosphorus content of the faecal product. This gives a rather low application rate, and the improvement of the crop due to the added organic matter is hard to distinguish. As the dry faeces can also considerably improve the soil structure and water-holding capacities, they are often applied at much higher rates. If organic matter (wood chips) or ashes are added to the faeces during collection and processing, these will improve the buffering capacity and the pH of the soil, which is especially important on soils with low pH (ECOSANRES 2008).

The average person produces around 50 L of faeces each year. This amount of faeces will fertilise 1.5 to 3.0 m2 of crop if the application is made according to organic content. If application is instead based on phosphorus content, it will be enough to fertilise 200-300 m2 (ECOSANRES 2008).

Because faeces contain large amounts of P, which is important for the development of young plants and roots, faeces should be applied prior to sowing or planting. Faeces must be applied at a depth where the soil stays moist, because the P only becomes available to the plants when it dissolves into the soil liquid, and the water-holding and buffering capacity of the organic matter are fully utilised only in moist conditions. Thus, applied faeces should be incorporated into the soil and covered. However, faeces should not be applied at depths exceeding the rooting depth.

Handling & Health Risk

The handling and use of dried faeces may not be acceptable to some. However, because the dried faeces should be dry, crumbly, and odour free, the use of dehydrated faeces may be more acceptable than that of manure or sludge.

Dehydrated faeces can still contain pathogens and therefore, a number of barriers to minimise the risk of disease transmission on the way from dehydration to crop production and harvesting should be considered. Such barriers could be a secondary treatment (e.g. composting, vermi-composting).

But also simple other measures such as avoiding the emptying of the vaults on windy days to prevent the powder from blowing and being inhaled or incorporation during the dry season, can considerably decrease the hygienic risk.

Furthermore, it also makes sense to apply faeces on crops that grow above the ground (e.g. fruit trees or bushes). To heighten acceptance, it can also be used for non-food crops that need fertilisations (animal fodder, wood etc.).