Composting Chamber

Composting toilets are toilets systems, which allow to minimise water use and to recycle nutrients contained in excreta and faeces. Compost is a valuable soil amendment which increases soil fertility (see also use of compost). There are various different systems (i.e. pits or vaults; urine diversion or not; low-tech and high-tech; single-vault continuous or multiple vault batch). The functioning of the various different composting toilet systems is basically the same. This technology usually requires four main parts: (1) a reactor (storage chamber); (2) a ventilation unit to provide oxygen and allow gases (CO2, water vapour) to escape; (3) a leachate collection system and (4) an access door to remove the mature product. The leachate has very high concentrations of nutrients, organics but also contain pathogens. They need to be collected, treated and if possible reused (see also leach field, soak pits, evapotranspiration beds, fertigation). Urine diversion usually reduces leachate production (GTZ 2006), see also urine diversion components.

Excreta, food waste and bulking material (such as wood chips, sawdust, ash or paper) are mixed in the chamber. There are four factors that ensure the good functioning of the system: (a) sufficient oxygen, provided by active or passive aeration; (b) proper moisture (ideally 45 to 70% moisture content); (c) internal (heap) temperature of 40 to 50 °C (achieved by proper chamber dimensioning); and (d) a 25:1 C:N ratio (theoretically) which can be adjusted by adding bulking material as a carbon source.

Dry material, which contains carbon (such as sawdust or ash) increase composting properties. It regulates moisture and the carbon to nitrogen ratio (C/N) and enhances the composting process. If ash and lime are used as adding material, this has the additional beneficial effect of raising pH, which leads to improved pathogen die-off (JOENSSEN et al. 2004, see also SCHOENNING & STENSTROEM 2004 for more information on alkaline treatment).

In practice, these optimal conditions are difficult to maintain. As a result, the output product (compost, or also called humanure) is often not sufficiently stabilized and sanitized, and requires further treatment.

A composting chamber can be designed in various configurations and constructed above or below ground, indoors or with a separate superstructure. Examples of chambers that are built as vaults above ground are the clivus multrum, the double vault Vietnamese composting toilet or the skyloo, see picture below). Pit composting toilets have vaults underground. This factsheet focuses on above ground composting toilets (vault composting toilets). You can find information on typical pit composting toilets in the factsheets for the arborloo and the fossa alterna.

In continuous vault composting toilets, such as the clivus multrum, compost can continuously be harvested on one end, while faeces fall into the other. In batch systems, the composting chamber is changed once it is full and the next one is used during which the other one is closed and left aside for maturation. Batch systems, especially in low-tech models, are much safer as they prevent mixing of fresh and matured material (WHO 2006). Typical batch systems are double vaults or movable containers (WHO 2006).

Composting toilets rely on the aerobic degradation of organic matter, resulting in a hygienic product that can be used as soil fertiliser. Composting is possible at high temperatures (thermophilic composting) and at low temperatures (ambient or mesophilic composting). Thermophilic composting is faster and more efficient to inactivate pathogens. The optimal operational conditions for thermophilic composting are (GTZ 2006):

• Good aeration
• A moisture content of 50 to 60 %
• A C/N ration of 30 to 35

The carbon to nitrogen ration (C/N) of excreta (including urine) is about 7 to 8, but for optimal thermophilic composting, it needs to be 20 to 35 (WHO 2006). The addition of paper, wood or bark chips, sawdust, ash or other similar substances will help to increase the C/N ratio. However, if the C/N ration becomes too high (> 30 to 35), then the composting is slowed down, impairing the attainment of required temperatures. Adding bulk material is also important to reduce humidity in the chamber and thus the potential of odour and fly breeding problems (WHO 2006). The addition of organic household waste can also help to raise the C/N ratio (WHO 2006). It can be added to a composting toilet through the toilet itself or through a separate chute.

A design value of 300 L/person/year can be used to calculate the required chamber volume.

Ventilation channels (air ducts) under the heap can be beneficial for aeration. Ventilation maintains low moisture content of the compost and prevents odour. Best ventilation is designed similar as for urine diversion dehydration toilets. More complex designs can include a small ventilation fan, a mechanical mixer (to homogenise the compost) or multiple compartments to allow for increased storage and degradation time. Mechanical ventilation requires a fan or another mechanical device and power/solar energy.

For natural ventilation, a difference of pressure (or temperature) is required inside and outside the vaults. This can be given by wind or a stack effect. The stack effect can be achieved by installing the ventilation pipe outside and expose it to the sun (it may also be painted in black). When the air in the pipe heats up, it rises upwards out of the vent; a downward draught of cooler air of higher density then flows in through the squat plate hole, replacing the vacuum space created after warm air rising (OKETCH 2005). A sloped bottom and a chamber for compost withdrawal facilitate access to the final product. A drainage system is important to ensure the removal of leachate.  

Excessive ammonia from urine inhibits the microbial processes in the chamber. The use of a Urine-Diverting Dry Toilet UDDT or Urinal can, therefore, improve the quality of the compost (see also urine diversion components). With urine diversion, less bulking agent is needed and the C/N ratio is naturally enhanced.

Due to its complexity, thermophilic composting can be difficult to manage and often, the operating range of composting toilets is varying within the thermophilic, mesophilic and ambient composting (WHO 2006). Even though the pathogen content is considerably reduced in composting toilets, complete pathogen destruction can only be achieved if good process conditions can be guaranteed. This can be done by using an advanced toilet design with insulation for maintaining a high temperature within the whole composting chamber (GTZ 2006). If thermophilic composting cannot be guaranteed, longer maturation times or a secondary treatment may be required (see also co-composting at small and large scale or drying and storage of faeces). In practice, thermophilic composting of faeces at the domestic level is questionable, as only slight elevation of the temperature was recorded in some trial (SCHOENNING & STENSTROEM). Therefore low-cost composting toilets at the household level may only be adapted if a secondary treatment is provided to ensure safety of the system. As a secondary treatment on a larger scale, where the process can be insulated and monitored, composting might be more effective (SCHOENNING & STENSTROEM 2004).

If the composting chamber is well designed, the users will not have to handle the material during the first year.

A well-functioning composting chamber should not produce odours. If there is ample bulking material and good ventilation, there should be no problems with flies or other insects. When removing the final product, it is advisable to wear protective clothing to prevent contact with (partially) composted material.

If operation conditions for thermophilic composting are adequate (moisture content 50 to 60 %, carbon to nitrogen ratio 30 to 35 and mixing with bulking material), the temperature will rise to between 50 and 65 °C (WHO 2006). Such temperatures will effectively inactivate pathogens (WHO 2006). Otherwise, secondary treatment will be necessary (see above).

There are many designs and models offered by manufacturers all over the world with a large range of prices (GTZ 2006). Low-tech composting toilets can also be self-constructed with locally available material.

Although simple in theory, composting chambers are not that easy to operate. The moisture must be controlled, the C:N ratio must be well balanced and the volume of the unit must be such that the temperature of the compost pile remains high to achieve pathogen reduction. After each defecation, a small amount of bulking material is added to absorb excess liquid, improve the aeration of the pile and balance the carbon availability. The immediate coverage of the fresh faeces with an additive material also lowers nuisances caused by odour or flies. Turning the material from time to time will boost the oxygen supply. 

A squeeze test can be made to check the moisture level within the chamber. When squeezing a handful of compost, it should not crumble or feel dry, nor should it feel like a wet sponge. Rather, the compost should leave only a few drops of water in one’s hand. If the material in the chamber becomes too compact and humid, additional bulking material should be added. If a UDDT is used, some water should be added to obtain the required humidity.

In multi-vault/container systems, filling is generally stopped when the vault is 2 to 3 quarters full (WSP 2007; WHO 1992). Then content is covered with soil and the vaults or containers are sealed. The maturation periods depends on temperature and the local climate. Under optimum thermophilic conditions, it may only take some weeks until the faeces or excreta are decomposed and hygienised. However, batch systems should be designed such as each filled vaults/containers can maturate for at least during two years (WHO 1992). That means that each vault of a double-vault composting toilet should be large enough to hold at least two years’ accumulation (WHO 1992). 

Emptying composting toilet constitutes a critical handling point. The emptying frequency depends on the size of the chambers, the feeding rate and the composting rate (volume reduction, pathogen removal) (GTZ 2006). Depending on the design, the composting chamber should be emptied every 2 to 10 years (TILLEY et al. 2014) . Only the mature compost should be removed. Proper protection measures, mainly personal protection, should be taken, especially if the material is not fully hygienised (WHO 2006). In addition to protective clothing (e.g. gloves and boots), normal hygiene and washing after the emptying operation are important (WHO 2006). The material may require further treatment to become hygienically safe (e.g., Co Composting large scale, Co-Composting small-scale and drying and storage of faeces ).

With time, salt or other solids may build up in the tank or drainage system. These can be dissolved with hot water and/or scraped out.