Dehydration and Storage

Direct use of excreta, human faeces and urine results in the beneficial use of plant nutrients to agricultural land (SCHOENNING & STENSTROEM 2004). These products usually do not contain industrial chemical contaminants that may hamper the reuse of municipal wastewater, but should be treated to reduce the levels of human pathogens to a safe level (SCHOENNING & STENSTROEM 2004).

The treatment of faeces from dehydration sanitation systems (such as urine diversion dehydration toilets, or UDDTs) generally requires a primary and a secondary treatment in order to transform them into compost, even if the distinction between these treatments is often diffuse (JOENSSON et al. 2004).

The primary treatment takes place during the collection of the faeces in the dehydration vaults. It aims to reduce the volume and the hygiene risk (i.e. to reduce the number of potential pathogens in the faeces), and to decrease the risk of odours and flies (JOENSSON et al. 2004).

When faeces are not mixed with urine and other liquids, they dry quickly. In the absence of moisture, organisms cannot grow, pathogens are destroyed and smells minimized.

The use of two alternating vaults in allows the faeces to dehydrate in one vault while the other vault fills. When one vault is full, the seat of the Urine-Diverting Dry Toilet (UDDT) is moved to the second vault. While the second vault fills up, the faeces in the first vault dry and decrease in volume. When the second vault is full after approximately 6 month , the first one is emptied and put back into service.

To prevent flies, minimize odours and encourage drying, a small amount of ash, lime, dry soil or sawdust should be used to cover faeces after each use.

The factors influencing the primary treatment are time, temperature, dehydration and pH.

The secondary treatment can take place in the toilet itself (e.g. in a double-vault urine diversion dehydration toilets)) or off-site on drying beds or in air-permeable bags protected from the rain (JOENSSON et al. 2004). The main objective of the secondary treatment is to render the faeces hygienically safe and transform them into humanure, which is odourless and visually non-repulsive (it should no longer be possible to recognise pieces of faeces or toilet paper, JOENSSON et al. 2004).

There are several options for secondary treatment: further storage, high-temperature composting, anaerobic digestion, alkaline treatment (raise of pH by adding alkaline material such as lime, ash or urea) or incineration. The thermophilic treatments (composting, digestion, incineration) for sanitation rely on all material reaching a sufficiently high temperature for a sufficiently long time to ensure pathogen die-off. This time ranges from seconds for incineration to days or even a few weeks for thermophilic composting. To achieve similar sanitation levels with dehydration and storage only, more time is required and the die-off depends not only on temperature but also on a number of other parameters, such as humidity (ventilation), pH, competition with other microorganism for nutrients etc. (WINBLAD 2004; ESF 2009).

A common practice for the treatment of faeces form urine-diversion toilets is to either dehydrate and store the faeces for a very long time, or to first store them before composting them in order to achieve pathogen die-off and prepare them for reuse (see also co-composting at small or large scale, terra preta treatment or terra preta toilets). If urine-diversion were not wanted, the alternative would be to install composting toilets (e.g. composting toilet, arborloo, fossa alterna).

Primary treatment can be achieved on-site. In UDDTs the process takes place in the dehydration vaults beneth the toilet slab and seat (see schematic above.

In dehydration toilets, the faeces are dried with the help of heat, ventilation and the addition of dry and alkaline material. Further dehydration and volume reduction is achieved by diverting urine (and anal cleansing water, if applicable) away from faeces to a separate collection chamber (WINBALD 2004). This also makes it possible to use the urine as a fertiliser (after urine storage and hygienisation or further processing such as struvite precipitation)

Dehydration vaults can be constructed indoors or with a separate superstructure. A vent pipe is required to remove humidity from the vaults and control flies and odours. The chambers should be airtight for proper functioning of the ventilation. They should be made of sealed brickwork or concrete to ensure that surface runoff cannot enter.

The main design parameters influencing the treatment performance are briefly described below.

Temperature

The temperature in the dehydration vault mainly depends on the ambient temperature. At higher temperature, the faeces are dried faster and if very high temperatures can be achieved over a long period, this also kills pathogens (similar to pasteurisation). High temperature in the dehydration vaults can be achieved by exposing the vaults to the sun or by using special design. The Tecpan model of the UDDT, mainly used in Africa, provides such an option. Metal sheets are used as vault chamber doors, which are exposed to the sun in a 45° angle, in order to absorb the heat, transfer it into the vaults and speeding up the drying process. However, the inclination of the doors make them more complicated to construct and rain can enter more easily into the chambers.

Ventilation

If single-vault UDDTs with movable containers are used, the dehydrated faeces can be directly collect in rice bags or other air-permeable low-cost material and then directly exposed to the sun away from the toilet protected from the rain. If correctly piled up (e.g. provide a grid on the floor to allow air circulation) this can also enhance ventilation.

Ventilation is the process by which fresh air is introduced and ventilated air is removed from an occupied space (OKETCH 2005). Ventilation can be natural or mechanical. Mechanical ventilation requires a fan or another mechanical device and power. If correctly designed, dehydration toilets can also be sufficiently aerated naturally. For natural ventilation, a difference of pressure (or temperature) is required inside and outside. This can be given by wind or a stack effect. The stack effect can be described as follows: as warm air is lighter than cool air, it generally rises up being replaced by cooler air. In UDDTs for instance, the solar radiation heats the vent pipe outside the chamber (the pipe can bepainted in black to enhance radiation absorption). When the air in the pipe heats up, it rises upwards out of the vent; a downward draught of cooler air then flows in through the squat plate hole, replacing the vacuum space created after warm air rising (OKETCH 2005). This creates a continuous airflow from inside towards outside of the toilet housing. The rate of ventilation is directly proportional to the size of openings and the height difference between inlet and outlet of the vent pipe. 

Addition of Dry and Alkaline Material

The immediate coverage of the fresh faeces with an additive material can considerable lower nuisances caused by odour or flies. The faster drying means also that the biological degradation is small if sufficient additive is used and thus, the losses of organic matter and N from the faeces to the air are small (JOENSSON et al. 2004). Ash and lime has the additional beneficial effect of raising pH, which leads to improved pathogen die-off (JOENSSON et al. 2004). The addition of ash and lime should therefore be preferred (SCHOENNING & STENSTROEM 2004). The addition of lime, ash or urea can also be used for a secondary alkaline treatment. Dehydrated faeces, which have been treated in such a way, will have an elevated pH (> 8). This may be beneficial for many soils when it is applied as compost, but it may affect crop production in already alkaline soils adversely (WHO 2006).

Another option to enhance pH is to add urea. This kind of alkaline treatment has been considered for large-scale treatment of faeces on municipal level (SCHOENNING & STENSTROEM 2004). Besides enhancing the pH urea also adds fertiliser value and further inactivates pathogens by the combination of high ammonia concentrations and pH (SCHOENNING & STENSTROEM 2004).

Time

The WHO recommends a minimum storage time of 6 months if ash or lime are used as cover material (alkaline treatment), otherwise the storage should be for at least 1 year for warm climates (>20 °C average) and for 1.5 to 2 years for colder climates.

In case of alkaline treatment, each vault is sized to accommodate at least 6 months of faeces accumulation. This results in a 6 month storage and dehydration time in the out-of-service vault. The vault dimensions should account for cover material, airflow, the non-even distribution of faeces, and possibly visitors and dry cleansing materials. It can be assumed that one person will require around 50 L of storage volume every 6 months. A minimum chamber height of 60 to 80 cm is recommended for easy emptying and access to the urine pipes.

Treatment performance

The pathogen reduction increases with increasing ambient temperature and pH. Several studies report the pathogen die-off rate in dehydrating toilet (see table 5.2 in WHO 2006).

In small-scale systems (household level), the faeces can be used after primary treatment if they have been stored for at least 1.5 to 2 years at ambient temperature of 2 to 20 °C or 1 year at temperature of 20 to 35 °C. In the case of an alkaline treatment, pH should be above 9 for at least 6 months (see table below, SCHOENNING & STENSTROEM 2004).

Suggested requirement for the treatment of dehydrated faeces. Source: SCHOENNING & STENSTROEM (2004)

Larger systems (at municipal level with transport infrastructure and centralised treatment) require alkaline treatment, composting or incineration.

Shorter storage times can be applied for all systems in very dry climates, where moisture levels less than 20 % are achieved (SCHOENNING & STENSTROEM 2004).

If the moisture level is kept low (< 20 %, JOENSSON et al. 2004), the degradation is low and so are the losses of N and organics. Furthermore, since the final product is incorporated in moist soils with planted corps, the risk of ammonia or leachate losses is virtually eliminated (JOENSSON et al. 2004).

Dehydration vaults can be a clean, comfortable, and easy-to-use technology. It is crucial, however, that the users are well trained to understand how the technology works and appreciate its benefits.

When the vaults are kept dry, there should not be any problems with flies or odours. After the recommended storage time, the faeces should be very dry and relatively safe to handle, provided that they did not get wet. However, a low health risk remains. Single dehydration vaults or bins do not allow faeces to sufficiently dehydrate. When the full container needs emptying, the faeces on top are still fresh. Hence, the risk associated with the handling of faecal matter is inherently higher in single vaults compared to double vault designs. The use of alternating chambers is, therefore, recommended. However, research and field tests of sealed faeces containers (or cartridges) for safe transportation and easy cleaning, along with the corresponding logistics, are on-going.

There are many different designs of dehydration systems, which can be separated into two categories, single- and double-vault UDDTs. Double-vault models require slightly more space and are slightly more expensive than conventional latrines. However, the removal of dried faeces is not frequent and can be done safely by the user him-self. Single-vault UDDTs can be constructed with little investments needs, but a movable container should be available and space for off-site drying. But for both UDDT options, very little surface area is needed compared to the treatment of sewage or faecal sludge and no energy at all is required.

Operation and maintenance or UDDTs includes regular control of the moisture content, addition of alkaline material (lime, ash etc.) the emptying of the vault approximately every 6 month and the emptying of the urine collection container. Further addition of absorbent material may be needed when diarrhoea is prevalent.

Just like the faeces, which are dried, but not degraded in the vaults, dry cleansing materials will not decompose in the chambers. Whenever the material is intended to be applied onto fields without further treatment, it is recommended to separately collect and dispose of the dry cleansing materials. The materials can be incinerated. Toilet paper can be thrown in the faecal compartment if the material is to be composted or incinerated. 

If anal cleansing is practised, a separate collection basin needs to be installed for anal cleansing. The diverted anal cleansing water can be infiltrated on-site (directly outside or beside the dehydration toilets) in a mulch or soak pit for treatment (or used for direct fertigation of trees or uneatable plants). Vegetable materials used as cleaning material can be put in the faecal compartment.

Occasionally, the faeces that have accumulated beneath the toilet should be pushed to the sides of the chamber.

Care should be taken to ensure that no water or urine gets into the dehydration vault. If this happens, extra ash, lime, soil or sawdust can be added to help absorb the liquid. 

To empty the vaults, a shovel, gloves and possibly a facemask (cloth) should be used to avoid contact with the dried faeces. Faeces should be worked into the soil as soon as possible and not be left on the soil surface (they should be incorporated into the soil). Faeces should not be used for vegetables, fruits or root crops that will be consumed raw, excluding fruit trees. After the last application of dehydrated faeces for fertilisation, a withholding period of one month should additionally be applied before harvest.