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 a fruit or ornamental tree can be planted, which will thrive in the nutrient rich environment (D.1), 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 (C.2) for solid sludge or Motorized Emptying and Transport (C.3) for liquid sludge. A vacuum truck can only empty liquid faecal Sludge.
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).
Faecal Sludge that is removed can be transported to a dedicated faecal Sludge treatment facility (T.13-T.17)(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 a Transfer Station (C.7).From there, it will be transported to the treatment facility by a motorized vehicle (C.3). A technology selection tree for faecal Sludge treatment plants is provided in Strande et al., 2014 (see Sector Development Tools, p. 9). (Semi-) Centralized Treatment technologies (T.1-T.17) produce both Effluent and Sludge, which may require further treatment prior to Use and/or Disposal. For example, Effluent from a faecal Sludge treatment facility could be co-treated with wastewater in Waste Stabilization Ponds (T.5) or Constructed Wetlands (T.7-T.9) such as a Free-Water Surface Constructed Wetland, a Horizontal Subsurface Flow Constructed Wetland, or a Vertical Flow Constructed Wetland.
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. After adequate treatment, Sludge can either be used in agriculture (D.5) or brought to a Storage/Disposal site (D.12).
Considerations
This system should be chosen only where there is either enough space to continuously dig new pits or when there is an appropriate way to empty, treat and dispose of the faecal Sludge. In dense urban settlements, there may not be sufficient space to access a pit for desludging or to make a new pit. This system is, therefore, best suited to rural and peri-urban areas where the soil is appropriate for digging pits and absorbing the leachate. It is not recommended for areas prone to heavy rains or flooding, which may cause pits to overflow.
Some Greywater in the pit may help degradation, but excessive amounts of Greywater may lead to quick filling of the pit and/or excessive leaching. All types of Dry Cleansing Materials can be discarded into the pit, although they may shorten the pit life and make it more difficult to empty. Whenever possible, Dry Cleansing Materials should be disposed of separately.
This system is one of the least expensive to construct in terms of capital cost. However, the maintenance costs may be considerable, depending on the frequency and method of pit emptying. If the ground is appropriate and has good absorptive capacity, the pit may be dug very deep (> 5m) and can be used for several years without emptying (up to 20 or more years). However, the groundwater level and use should be taken into consideration when digging pits in order to avoid contaminating it. 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 rare.
More on the Functional Groups: U, S, C, T, or D
