Single Pit System
This system is based on the use of a single pit to collect and store the excreta. The system can be used with or without Flushwater depending on the User Interface.
The inputs to the system can include Urine, Faeces, Anal cleansing water, Flushwater and Dry cleansing materials. The use of Flushwater and/or Anal Cleansing Water will depend on water availability and local habit.
There are two different User Interfaces for this system, which include a Dry Toilet or a Pour Flush Toilet. The User Interface is directly connected to a Collection and Storage/Treatment Technology: a Single Pit or a Single Ventilated Improved Pit (VIP).
When the pit is full there are several options. If there is space, the pit can be filled with soil and planted with a tree, as in the Fill and Cover, and a new pit built. This is generally only possible when the superstructure is mobile. Alternatively, the Faecal Sludge that is generated from the Collection and Storage/Treatment Technology has to be removed and transported for further treatment. The Conveyance Technologies that can be used include Human-Powered Emptying and Transport for solid sludge or Motorized Emptying and Transport for liquid sludge.
When the Faecal Sludge is thinner, it must be emptied with a vacuum truck. As the Faecal Sludge is highly pathogenic prior to treatment, human contact and agricultural applications should be avoided. When it is not feasible to empty the full pit, (Semi-) Centralized Treatment can be omitted and the pit can be filled and covered with a suitable material for decommissioning (Fill and Cover). The decommissioned pit can be planted with a fruit or flowering tree since it will thrive in the nutrient rich environment.
Faecal Sludge that is removed can be transported to a dedicated Faecal Sludge treatment facility (Sedimentation - Thickening Ponds, Unplanted Drying Beds, Planted Drying Beds, Co-composting, Anaerobic Biogas Reactor).
In the event that the treatment facility is not easily accessible, the Faecal Sludge can be discharged to either a Sewer Discharge Station or a Transfer Station. From the Sewer Discharge Station, the Faecal Sludge is transported by the sewer and is co-treated with the Blackwater flowing in the sewer network (Technologies: Anaerobic Baffled Reactor, Anaerobic Filter, Waste Stabilization Pond, Aerated Pond, Free-Water Surface Constructed Wetland, Vertical Flow Constructed Wetland, Trickling Filter, Upflow Anaerobic Sludge Blanket Reactor, Activated Sludge).
The Faecal Sludge from the Sewer Discharge station is released either directly into the sewer or at timed intervals. If sludge is introduced directly into a sewer, there must be enough water to adequately dilute and transport the sludge to the treatment facility. From the Transfer Station the Faecal Sludge must be transported to a dedicated Faecal Sludge treatment facility (Sedimentation - Thickening Ponds, Unplanted Drying Beds, Planted Drying Beds, Co-composting, Anaerobic Biogas Reactor)) by a motorized vehicle.
All (Semi-) Centralized Treatment Technologies produce both Effluent and Faecal Sludge, which require further treatment prior to Use and/or Disposal. Technologies for the Use and/or Disposal of the treated Effluent include Irrigation, Aquaculture Ponds, Macrophyte Pond or Discharge to a water body or Recharge to groundwater.
Considerations
This system is best suited to rural and peri-urban areas where there is appropriate soil for digging and absorbing the Effluent from the pit. This system should be chosen only where there is either space to continuously dig new pits or when there is an appropriate manner of emptying and disposing of the Faecal Sludge. In dense urban settlements, there may not be sufficient transportation or access to empty or move to another pit. This system is also best suited to areas that are not prone to heavy rains or flooding, which may cause the pits to overflow. Some Greywater in the pit may help degradation, but excessive additions of Greywater may shorten the life of the pit.
Although different types of pits are common in most parts of the world, a well designed pit-based system with appropriate transport, treatment and use or disposal, is still very rare.
This system is one of the least expensive to construct (capital cost) however the maintenance costs may be considerable, depending on the depth of the pit and how often it must be emptied. If the ground is appropriate, i.e. good absorptive capacity, the pit may be dug very deep (e.g. >5m) and can be used for several years (up to 30 years) without emptying. All types of solid cleansing materials can be discarded into the pit, although they may shorten the life of the pit and make pit emptying more difficult. Whenever possible, solid cleansing materials should be disposed of separately.
References and external links
- Elizabeth Tilley et.al (2008). Compendium of Sanitation Systems and Technologies (low res version). Department of Water and Sanitation in Development Countries (Sandec) at the Swiss Federal Institute of Aquatic Science and Technology (Eawag). (Provides a full overview of sanitation systems.)
