Use of Dehydrated Faeces

In a dehydration toilet, the excreta inside the processing vault are dehydrated  with the help of sun, natural evaporation and ventilation. Dehydration is different from composting because the organic material present is not degraded or transformed; only the moisture is removed. Faeces will reduce in volume by about 75% after dehydration. Completely dry faeces are a crumbly, powdery substance. The shells and carcasses of worms and insects in the faeces also dehydrate and becomes part of the dried material.

The primary motivation for dehydrating the faeces is to significantly reduce the amount of pathogens in the faeces over time. The material is rich in carbon and nutrients, but may still contain protozoan cysts or oocysts (spores that can survive extreme environmental conditions and be re-animated under favourable conditions) and other pathogens. The degree of pathogen inactivation will depend on the temperature, the pH (using ash or lime raises the pH) and storage time. The addition of calcium carbonates, contained in lime or wood ashes added as drying material, helps to raise the pH, and thus accelerate the process of pathogen inactivation. It is generally accepted that faeces should be stored between 6 to 24 months, although pathogens may still exist after this time (refer WHO guidelines on excreta use in agriculture (Volume IV) for specific guidance). In some cases (e.g. centralized storage and reuse), a secondary treatment (e.g. small scale composting or centralised composting, or terra preta sanitation) of dehydrated faeces should always be considered (if possible) as an additional barrier to inactivate pathogens (and here most importantly Ascaris).

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).

The material can be mixed into soil for agriculture (depending on acceptance) or safely mixed into soil or buried elsewhere. Extended storage is also an option if there is no immediate use for the material (see surface disposal and storage). 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.

Faeces that are dried and kept at between 2 and 20° C should be stored for 1.5 to 2 years before being used at the household or regional level (see also dehydration and storage of faeces). At higher temperatures (i.e., greater than 20° C), storage over 1 year is recommended to inactivate Ascaris eggs (a type of parasitic worm). A shorter storage time of 6 months is required if the faeces have a pH above 9 (i.e., adding ash or lime increases the pH). WHO guidelines on excreta use in agriculture (Volume IV) concerning the use of excreta in agriculture should be consulted beforehand.

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 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.

The handling and use of dried faeces may not be acceptable to some people. However, because dehydrated faeces should be dry, crumbly, and odour free, using them might be easier to accept than manure or sludge. Dry faeces are a hostile environment for organisms and they do not survive long in it. If water or urine is mixed with the drying faeces, however, odours and organisms may become problematic because bacteria easily survive and multiply in wet faeces. Warm, moist environments are conducive to anaerobic processes, which can generate offensive odours.

Dehydrated faeces should not be applied to crops less than one month before they are harvested. This waiting period is especially important for crops that are consumed raw.

For additional precaution  faeces may also applied to 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.).

When removing dehydrated faeces from dehydration vaults, care must be taken to prevent the powder from blowing and being inhaled. Emptying of vaults on windy days must therefore be avoided. Workers should wear appropriate protective clothing.

Faeces should be kept as dry as possible during storage . If by accident, water or urine enters and mixes with drying faeces, more ash, lime or dry soil should be added to help absorb the moisture. Prevention is the best way to keep faeces dry.