Twin Pit Dry Systems use two pits in alternating order. Twin pit systems include double Ventilated Improved Pits (VIP), and the fossa alterna (FA). Pit alternation allows for effluent to infiltrate into the soil and sludge to decompose in the one pit, while the other pit is in use. The alternating system reduces the amount of pit humus that needs to be emptied and makes the end product more hygienic.General term for a liquid that leaves a technology, typically after blackwater or sludge has undergone solids separation or some other type of treatment. Effluent originates at either a collection and storage or a (semi-) centralised treatment technology. Depending on the type of treatment, the effluent may be completely sanitised or may require further treatment before it can be used or disposed of.Term used to describe the nutrient-rich, hygienically improved, humic material that is generated in double pit technologies (S.5, S.6) through dewatering and degradation. The various natural decomposition processes taking place in alternating pits can be both aerobic and anaerobic in nature, depending on the technology and operating conditions. The main difference of pit humus compared to compost is that the degradation processes are passive and are not subjected to a controlled oxygen supply and that the carbon to nitrogen ratio, humidity and temperature may be less favourable. Therefore, the rate of pathogen reduction is generally lower and the quality of the product, including its nutrient and organic matter content, can vary considerably. Pit humus can look very similar to compost and has good soil conditioning properties, although pathogens can still be present.Mixture of solids and liquids, containing mostly excreta and water, in combination with sand, grit, metals, trash and/or various chemical compounds. A distinction can be made between faecal sludge and wastewater sludge. Faecal sludge comes from on-site sanitation technologies, i.e. it has not been transported through a sewer. It can be raw or partially digested, a slurry or semisolid, and results from the collection and storage/treatment of excreta or blackwater, with or without greywater. Wastewater sludge (also referred to as sewage sludge) originates from sewer-based wastewater collection and (semi-)centralised treatment processes. The sludge composition will determine the type of treatment that is required and the end-use possibilities.Describes technologies for on-site collection, storage, and sometimes (pre-) treatment of the products generated at the user interface. The treatment provided by these technologies is often a function of storage and is usually passive (i.e. requires no energy input), except a few emerging technologies where additives are needed. Thus, products that are ‘treated’ by these technologies often require subsequent treatment before use and/or disposal. In the technology overview graphic, this functional group is subdivided into the two subgroups: “Collection/Storage” and “(Pre-)Treatment”. This allows a further classification for each of the listed technologies with regard to their function: collection and storage, (pre-) treatment only or both.Refers to the methods through which products are returned to the environment, either as useful resources or reduced-risk materials. Some products can also be cycled back into a system (e.g. by using treated greywater for flushing).A functional group is a grouping of technologies that have similar functions. The compendium proposes five different functional groups from which technologies can be chosen to build a sanitation system:
User interface (U), Collection and Storage/Treatment (S), Conveyance (C), (Semi-) Centralised Treatment (T), Use and/or Disposal (U).
A sanitation system is a multi-step process in which sanitation products such as human excreta and wastewater are managed from the point of generation to the point of use or ultimate disposal. It is a context-specific series of technologies and services for the management of these sanitation products, i.e. for their collection, containment, transport, treatment, transformation, use or disposal. A sanitation system comprises functional groups of technologies that can be selected according to context. By selecting technologies from each applicable functional group, considering the incoming and outgoing products, and the suitability of the technologies in a particular context, a logical, modular sanitation system can be designed. A sanitation system also includes the management and operation and maintenance (O & M) required to ensure that the system functions safely and sustainably. The process of reducing the water content of a sludge or slurry. Dewatered sludge may still have a significant moisture content, but it typically is dry enough to be conveyed as a solid (e.g., shovelled).
The utilisation of products derived from a sanitation system.
The liquid that has passed through a filter.
The stable remnant of decomposed organic material. It improves soil structure and
increases water retention, but has no nutritive value.
Any substance that is used for growth. Nitrogen (N), phosphorus (P) and potassium (K) are the main nutrients contained in agricultural fertilisers. N and P are also primarily responsible for the eutrophication of water bodies.
A sanitation system in which excreta and wastewater are collected and stored or treated on the plot where they are generated.
An organism or other agent that causes disease.The means of safely collecting and hygienically disposing of excreta and liquid
wastes for the protection of public health and the preservation of the quality of public water bodies and, more generally, of the environment.

Waste matter that is transported through the sewer.
An open channel or closed pipe used to convey sewage. See C.3 and C.4
Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

Twin Pit Dry Systems can be constructed as double pit, double VIP or FA. In a double VIP excreta (or faeces, if a Urine Diverting Dry Toilet U.2 is used as a user interface) are converted into pit humus, while in a FA additional organic materials are added to the pit. After every use of a FA dry organic materials such as ash or leaf litter are added to the pit. The FA is built with a shallow pit, with a depth of around 1.5 m, while the double VIP pits can have a depth of up to 3 m. In both systems, the two pits are used alternately. The effluent infiltrates into the soil. When the first pit has filled up it is sealed and the toilet user interface is switched to the second pit. While the second pit is in use the materials in the first pit can decompose and dry, thus decrease in volume and become more hygienic. Due to the extended resting time, the material within the pit is partially sanitised and humus-like. Usually the alternation cycle is 6–24 months depending on the pit volume and the number of users.

Design Considerations

For each system, only one toilet user interface is needed, which is moved from the first pit to the second pit when the first pit is full. Double VIPs are built like Single VIPs S.4 but with two collection pits. Pits should be built next to each other with enough distance between them to avoid cross contamination.

Materials

The latrine superstructure can be made from local materials, such as bamboo, grass matting, wood, plastic or metal sheeting (though this often heats up the interior). Pit lining materials can include brick, rotresistant timber, bamboo, concrete, stones, or mortar plastered onto the soil. The slab can be fabricated on-site with a mould and cement. In the acute emergency phase, pre-fabricated plastic slabs may be used. Other slab materials such as wood or bamboo are also possible, where no other materials are available. For the FA there is a need for constant supply of organic material, such as ash or dry leaves, to be added after each use.

Applicability

Double pit systems are appropriate where there is enough space and reuse potential for the pit humus that is being generated. Therefore, these systems are most appropriate in rural and peri-urban settings and in communities comfortable with handling and re-using faecal material. As the second pit only comes into operation when the first pit is full, which may take between 6 to 24 months, Twin Pit Dry Systems are recommended as longer-term solutions in prolonged emergency situations.

Operation and Maintenance

 Other than the operation and maintenance (O & M) required for the Single VIP, the main operational task for double VIPs is to seal pits when they are full and empty full pits prior to re-use. The FA must always be furnished with dry organic material to add to the pit after every use. If pits are used simultaneously the system does not function. Where there is only one user interface and, for the VIP, one ventilation pipe they must to be switched to the new pit when the old one is full. In some designs, the entire superstructure can be moved from pit to pit.

Health and Safety

By covering excreta or faeces with soil, ash, and/or leaves, flies and odours are kept to a minimum. Keeping the contents sealed in the pit for the duration of at least one year makes the pit humus safer and easier to handle. However, care should still be given when handling the output product. The same precautions that are taken when handling compost should be taken with the humus derived from double VIPs or the FA. Additional health concerns include that the leachate can potentially contaminate groundwater, that the pits are susceptible to failure and/or overflowing during floods and that the health risks from flies are not completely removed by ventilation.

Costs

Construction costs for Twin Pit Dry Systems are usually around double those of single pit systems, except for the user interface that can be switched. However, costs for O & M decrease as the pits need to be emptied less frequently. As the area of the system is doubled compared to single pit systems, any costs associated with elevated land use have to be considered.

Social Considerations

Users should have an appreciation of the advantages of the Twin Pit Dry System and should be willing to operate and maintain it. If users do not appreciate the benefits, the system could fail. Double pit systems are usually built as toilets serving single households, ensuring a clear attribution of O & M responsibilities. If used as shared or public facilities the responsibilities for O & M must be clearly determined prior to the implementation.