S.5 Twin Pit Dry System

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 in filtrate 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.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 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.
A sanitation system in which excreta and wastewater are collected and stored or treated on the plot where they are generated.
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 in filtrates 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.

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.Consists of urine and faeces that are not mixed with any flushwater. Excreta is relatively small in volume, but concentrated in both nutrients and pathogens. Depending on the characteristics of the faeces and the urine content, it can have a soft or runny consistency.Refers to (semi-solid) excrement that is not mixed with urine or water. Depending on diet, each person produces approximately 50–150 L per year of faecal matter of which about 80 % is water and the remaining solid fraction is mostly composed of organic material. Of the total essential plant nutrients excreted by the human body, faeces contain around 39 % of the phosphorus (P), 26 % of the potassium (K) and 12 % of the nitrogen (N). Faeces also contain the vast majority of the pathogens excreted by the body, as well as energy and carbon rich, fibrous material.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.The liquid produced by the body to rid itself of nitrogen in the form of urea and other waste products. In this context, the urine product refers to pure urine that is not mixed with faeces or water. Depending on diet, human urine collected from one person during one year (approx. 300 to 550 L) contains 2 to 4 kg of nitrogen. The urine of healthy individuals is sterile when it leaves the body but is often immediately contaminated by coming into contact with faeces.Describes the type of toilet, pedestal, pan, or urinal that the user comes into contact with; it is the way users access the sanitation system. In many cases, the choice of user interface will depend on the availability of water and user preferences. Additionally, handwashing facilities have been included here with a dedicated technology information sheet as a constant reminder that each sanitation user interface needs to be equipped with handwashing facilities for optimal hygiene outcomes.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 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
User interface used for urination and defecation. The organic molecule (NH2)2CO that is excreted in urine and that contains the nutrient nitrogen. Over time, urea breaks down into carbon dioxide and ammonium, which is readily used by organisms in soil. It can also be used for on-site faecal sludge treatment. See. S.18Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

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.

Describes the type of toilet, pedestal, pan, or urinal that the user comes into contact with; it is the way users access the sanitation system. In many cases, the choice of user interface will depend on the availability of water and user preferences. Additionally, handwashing facilities have been included here with a dedicated technology information sheet as a constant reminder that each sanitation user interface needs to be equipped with handwashing facilities for optimal hygiene outcomes.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 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.

User interface used for urination and defecation. Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

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.

The above ground walls and roof built around a toilet or bathing facility to provide privacy and protection to the user.
User interface used for urination and defecation.

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.

The stable remnant of decomposed organic material. It improves soil structure and
increases water retention, but has no nutritive value.
Use of recycled water or other sanitation products.
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.

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.

Describes the type of toilet, pedestal, pan, or urinal that the user comes into contact with; it is the way users access the sanitation system. In many cases, the choice of user interface will depend on the availability of water and user preferences. Additionally, handwashing facilities have been included here with a dedicated technology information sheet as a constant reminder that each sanitation user interface needs to be equipped with handwashing facilities for optimal hygiene outcomes.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 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.

The above ground walls and roof built around a toilet or bathing facility to provide privacy and protection to the user.
User interface used for urination and defecation. Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

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.

Decomposed organic matter that results from a controlled aerobic degradation process. In this biological process, microorganisms (mainly bacteria and fungi) decompose the biodegradable waste components and produce an earth-like, odourless, brown/black material. Compost has excellent soil-conditioning properties and a variable nutrient content. Because of leaching and volatilisation, some of the nutrients may be lost, but the material remains rich in nutrients and organic matter. Generally, excreta or sludge should be composted long enough (2 to 4 months) under thermophilic conditions (55 to 60 °C) in order to be sanitised sufficiently for safe agricultural use.Consists of urine and faeces that are not mixed with any flushwater. Excreta is relatively small in volume, but concentrated in both nutrients and pathogens. Depending on the characteristics of the faeces and the urine content, it can have a soft or runny consistency.Refers to (semi-solid) excrement that is not mixed with urine or water. Depending on diet, each person produces approximately 50–150 L per year of faecal matter of which about 80 % is water and the remaining solid fraction is mostly composed of organic material. Of the total essential plant nutrients excreted by the human body, faeces contain around 39 % of the phosphorus (P), 26 % of the potassium (K) and 12 % of the nitrogen (N). Faeces also contain the vast majority of the pathogens excreted by the body, as well as energy and carbon rich, fibrous material.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.The liquid produced by the body to rid itself of nitrogen in the form of urea and other waste products. In this context, the urine product refers to pure urine that is not mixed with faeces or water. Depending on diet, human urine collected from one person during one year (approx. 300 to 550 L) contains 2 to 4 kg of nitrogen. The urine of healthy individuals is sterile when it leaves the body but is often immediately contaminated by coming into contact with faeces.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. Simple, single cell organisms that are found everywhere on earth. They are essential for maintaining life and performing essential “services”, such as composting, aerobic degradation
of waste, and digesting food in our intestines. Some types, however, can be pathogenic and cause mild to severe illnesses. Bacteria obtain nutrients from their environment by excreting enzymes that dissolve complex molecules into more simple ones which can then pass through the cell membrane.

The process by which biodegradable components are biologically decomposed by microorganisms (mainly bacteria and fungi) under controlled aerobic conditions.
The utilisation of products derived from a sanitation system.
Water that is located beneath the earth’s surface.
The stable remnant of decomposed organic material. It improves soil structure and
increases water retention, but has no nutritive value.
The liquid fraction that is separated from the solid component by gravity filtration through a media (e.g., liquid that drains from drying
beds).
Any cellular or non-cellular microbiological entity capable of replication or of transferring genetic material (e.g. bacteria, viruses, protozoa, algae or fungi).
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.A diverse group of unicellular eukaryotic organisms, including amoeba, ciliates, and flagellates. Some can be pathogenic and cause mild to severe illnesses.
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
The organic molecule (NH2)2CO that is excreted in urine and that contains the nutrient nitrogen. Over time, urea breaks down into carbon dioxide and ammonium, which is readily used by organisms in soil. It can also be used for on-site faecal sludge treatment. See. S.18An infectious agent consisting of a nucleic acid (DNA or RNA) and a protein coat. Viruses can only replicate in the cells of a living host. Some pathogenic viruses are known to be waterborne (e.g., the rotavirus that can cause diarrheal disease).
Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

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.

Describes the type of toilet, pedestal, pan, or urinal that the user comes into contact with; it is the way users access the sanitation system. In many cases, the choice of user interface will depend on the availability of water and user preferences. Additionally, handwashing facilities have been included here with a dedicated technology information sheet as a constant reminder that each sanitation user interface needs to be equipped with handwashing facilities for optimal hygiene outcomes.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 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.

User interface used for urination and defecation. Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

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.

User interface used for urination and defecation.

Fact Sheet Overview

Input Products

Excreta
Faeces

Output Products

Pit Humus

Emergency Phase

Stabilisation + +
Recovery + +

Challenging Ground Conditions

Application Level / Scale

Household + +
Neighbourhood + +

Water-based and Dry Technologies

Dry

Management Level

Household + +
Shared + +
Public +

Technical Complexity

Space Required

Objectives & Key Features

• Excreta containment
• Sludge volume reduction
• Extended treatment time

Strength & Weakness

  • Easier excavation than single pit systems
  • Reduction in sludge volume and pathogens
  • Can be built with locally available materials
  • Pit humus can be used as fertiliser/soil conditioner
  • Double the space required
  • Possible contamination of groundwater
  • Constant organic material supply needed for FA

Selected References

Construction guidelines for Fossa Alterna

Morgan, P. R. (2007): Toilets That Make Compost. SEI, Stockholm, Sweden

Monvois, J., Ganert, J., Freneux, C., Guillaume, M. (2010): How to Select Appropriate Technical Solutions for Sanitation. Programme Solidarité Eau (pS-Eau), Paris, France

Effect of eco-hummus on plant growth

Morgan, P. (2004): Plant Trials Using Fossa Alterna Humus. EcoSanRes/ SEI, Stockholm, Sweden

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