Anaerobic digestion is the dominant pathway for pit latrine decomposition and is limited by intrinsic factors.

In vitro methods were used to assess the full potential for decomposition (measured as biogas formation) from pit latrine samples taken from the top layer of 15 Tanzanian latrines. We found considerable variability in the decomposition rate and extent. This was compared with decomposition in the same latrines, measured by comparing top layer composition with fresh stools and deeper (older) layers, to assess whether this potential was realised in situ. Results showed a close match between the extent of organic material breakdown in situ and in vitro, indicating that anaerobic digestion is the dominant pathway in latrines. The average potential decrease in chemical oxygen demand (COD) (determined as methane production in vitro within 60 days) and actual measured decrease in situ are 68.9% ± 11.3 and 69.7% ± 19.4, respectively. However in the in vitro tests, where samples were diluted in water, full decomposition was achieved in 2 months, whereas in situ it can take years; this suggests that water addition may offer a simple route to improving latrine performance. The results also allowed us to estimate, for the first time to our knowledge using experimental data, the contribution that latrines make to greenhouse gas emissions globally. This amounts to ∼2% of annual US emissions

Modelling the fill rate of pit latrines in Ifakara, Tanzania

A model of the rate at which pit latrines fill was developed and compared with actual fill rates measured in latrines in Ifakara, Tanzania. Model parameters were derived primarily from data collected during the field study in Tanzania, with one fitted parameter. Although the model did not accurately simulate the fill rate of individual pits, it provided a good simulation of the average fill rate of the pits and aided the identification of pits that were performing poorly. Laboratory experiments conducted on samples of sludge from the pits were used to identify the average hydrolysis constant for biodegradation of the organic material. The average hydrolysis constant (0.046 day−1) was higher than that identified in another modelling study (0.0015 day−1), conducted in South Africa. The higher hydrolysis constant provided a simulation of the profile of volatile solids with lower root mean square error in 17/24 of the pits, however, for the other pits a lower hydrolysis constant was more appropriate. A sensitivity study of the model indicated that the pit fill rates were particularly sensitive to the parameter that quantified water accumulation in the pit, as such quantifying water flow through the pit is a priority for future research.</jats:p