Twin Pits for Pour Flush

Twin-pit for pour-flush toilets are improved pit latrines, which allow on-site treatment and transformation of faecal sludge into a hygienized soil amendment. They have been constructed over the past 30 years mainly in India, Bangladesh and Nepal. They basically consist of two pits (similar in design to soak pits), which are linked, using a Y-junction, to a single pour-flush toilet.

The twin pits for pour flush technology can be designed in various ways; the toilet can be located directly over the pits or at a distance from them. The superstructure can be permanently constructed over both pits or it can move from side to side depending on which one is in use. Often pits are placed symmetrically at the back of the latrine in order to have good access an ensure drainage from the user interface to the pits.

If site conditions do not permit this layout, the pits can be placed on the side or even in front of the toilet pan, but this design requires more water for flushing (ROY et al. 1984). Toilets can also be constructed inside the house, while the pits can be situated outside the house. However, the pits should not be situated in drainage lines, the paths of stormwater drains or in depressions where water is likely to collect in order to prevent water from entering the pit which can cause groundwater pollution or destabilize the constructions.

The pit shape can be circular or rectangular, but circular pits are more stable and cost less (ROY et al. 1984). No matter how the system is designed, only one pit is used at a time. While one pit is filling, the other full pit is resting.

During storage, excreta decompose by a combination of anaerobic digestion and composting processes. The end product is hygienised and can be used as soil amendment (i.e. compost) to improve soil structure and increase water storage capacity and fertility. This transformation process takes approximately two years.

As liquid leaches from the pit and migrates through the unsaturated soil matrix, pathogenic germs are sorbed onto the soil surface. In this way, pathogens can be removed prior to contact with groundwater. The degree of removal varies with soil type, distance travelled, moisture and other environmental factors.

The difference between this technology and other technologies such as composting toilets, the arborloo, the double VIP or Fossa Alterna is that it allows for water (flushing water, anal cleansing water, greywater) and it is not necessary to add soil or organic material to the pits. As this is a water-based (wet) technology , however , the full pits require a longer retention time (two years is recommended) to degrade the material before it can be excavated safely.

The pour-flush squatting pan consists of a steep bottom slope and a water seal trap (see also pour-flush toilet). From there, faeces, urine and flushing water are directed to one of two leach pits. The pits are connected via an inspection chamber containing a Y junction allowing the channeling of the blackwater into either one or the other pit (WHO 1992). The pits should be of an adequate size to accommodate a volume of waste generated over one or two years. This allows the contents of the full pit enough time to transform into a partially sanitized, soil-like material that can be manually excavated. For a family of 5 members, a pit about 1 m deep and 1 m around should be enough for three years (HESPERIAN FOUNDATION 2004). After excavation, the soil-like material can be used as soil amendment in kitchen gardens or field or in agriculture (see also use of compost or reuse of urine and faeces in agriculture). In case of humid sludge, it can be composted or dried before reuse (WHO 1992).

The pits are either constructed with a porous material, in open-joint brick lines (like a honeycomb, to facilitate the liquid to flow out) or with perforated concrete tubes (i.e. pre-fabricated). It is recommended that the twin pits be constructed 1 m apart from each other to minimize cross-contamination between the maturing pit and the one in use. If the spacing between the two pits has to be reduced, an impervious barrier should be provided between them. It is also recommended that the pits be constructed over 1 m from any structural foundation as leachate can negatively impact structural supports. Water within the pit can impact its stability. Therefore, the full depth of the pit walls should be lined to prevent collapse and the top 30 cm should be fully mortared to prevent direct infiltration and to support the superstructure. Moreover, the pits should be covered with a wooden or concrete slab.

There is a risk of groundwater pollution when pits are located in areas with a high or variable water table, and/or fissures or cracks in the bedrock. As soil and groundwater properties are often unknown, it is difficult to estimate the distance necessary between a pit and a water source. In alluvial soils (silt mixed with fine sand) and where the pit bottom is at least 2 m above the maximum ground-water level (ROY et al. 1984; WSP 2008), there is genera

lly no risk. But in water-saturated soils, virus and bacteria can travel hundreds of meters. It is normally recommended to have a minimum horizontal distance of 30 m between them to limit exposing the water source to microbial contamination. The table below gives some further guidelines for minimal distances of the leach pits from drinking water sources.

Minimal distance of leach pits from drinking water sources in different soil conditions. Source: ROY et al. (1984)

To ensure that only one of the two pits is used at any time, the idle pipe of the junction connecting to the out-of-use pit should be closed (e.g. with cement or bricks). Alternatively, the pour flush toilet could also be directly connected to the pit in use by a single straight pipe fixed in place with light mortar and covered with earth. The risk of failure and misuse is minimized by ensuring that the junction and pipes are not easily accessible.

As almost any toilet, twin-pit pour-flush can be easily adapted for urine diversion (see also urine diversion components). The aim of such an adapted can either be the collection of the urine as a fertilizer or the prevention of too much nutrients and liquids flowing through the pit. In Nepal, for instance, over 190 such systems have been installed by ENPHO. The toilets are designed the same way as twin-pits for pour-flush toilets, but with a urine diversion slab or pedestal and a urine collection chamber. The collected urine can be easily used as a nitrogen-rich fertilizer. The faeces are flushed with water and are collected along with water used for cleansing in the twin pits. The urine-diversion twin-pit pour-flush toilets have the advantage (compared to urine diversion dehydration toilets) that cleansing water does not need to be diverted separately, but can be flushed down the pan, together with the faeces. This generally results in high social acceptance.

It is a commonly accepted sanitation option; however, some health concerns exist:

• Leachate can contaminate groundwater;
• Stagnant water in pits may promote insect breeding;
• Pits are susceptible to failure and/or overflowing during floods.

The most important operation and maintenance actions are to control the filling of the pit in use in order to change to the other at the right moment and to correctly seal the Y junctions for the pit not in use. The pits must be regularly emptied (after the recommended two year resting time), and care must be taken to ensure that they do not flood during rainy seasons. Pits should therefore be covered and sealed on one hand to prevent surface rainwater from flowing into the pits and on the other hand to protect people and animals from falling into it.Moreover, any fresh blackwater needs to be prevented from entering into the resting pit during the two year storage period. Therefore, the seal in the Y junction must be controlled occasionally. Emptying is done manually using long handled shovels and proper personal protection.

Before starting the filling phase of a pit, a layer of soil (15 cm) should be filled in the bottom of the pit (ROY et al. 1984).

During toilet use, it is important to flush with 1.5 to 2 L (ROY et al. 1984), so that the excreta are transported to the pits and do not clog in the water seal. If dry material is used for cleansing (e.g. toilet paper), the risk of clogging is high.

Fly and odour control can be improved by the periodic additive of ash into the pit. This will also improve the productivity of the maturated sludge as a fertilizer.

As every toilet, the pour-flush toilet needs to be cleaned regularly. It is important not to use too much water for washing to prevent fast filling up. Even though greywater can be poured into the toilet if there is no other reuse option (e.g. in a greywater tower), it should not be added in large volumes because this would cause overflow of the pit or enhanced leaching. Also solid waste (kitchen waste, sweepings etc.) should not be added, as this would cause clogging of the pour-flush pan.

In any case, long-term support facilities to remind and assist the user in changing and emptying pits will greatly improve operational success (WHO 1992).

The construction of twin pits for pour-flush toilets can be rather expensive depending on the local availability of manpower and construction material (about 1.5 of the costs of normal pit latrines, WSP 2004). However, once the twin pits have been installed, they can virtually be used without end.