Modelling the filling rate of pit latrines

Km Foxon,Cj Brouckaert,K Wood

doi:10.4314/wsa.v39i4.15

Abstract

Excreta (faeces and urine) that are deposited into a pit latrine are subject to biodegradation, which substantially reduces the volume that remains. On the other hand, other matter that is not biodegradable usually finds itsway into pit latrines. The net filling rate is thus dependent on both the rate of addition of material and its composition. A simple material balance model is presented which represents the faecal sludge as a mixture of biodegradable organic material, un-biodegradable organic material and inorganic material. Measurements made on 2 pits in eThekwini, South Africa, were used to determine parameters for the model. Model predictions were then compared with data from 15 other pits in the same area and filling rate data from previous South African studies, which exhibit a 20th to 80th percentile range of 200 to 453 ℓ∙pit−1∙yr−1. These comparisons indicated that the pits studied exhibited relatively low filling rates resulting from orderly disposal practices. The average composition of the pit (COD, biodegradable material and inorganic fraction) changes with age, which will impact on any subsequent sludge treatment process. Pit filling rates are greatly affected by the disposal of solid waste in addition to the faecal material. For the pits studied, the model predicts that the filling time could have been extended from 15 years to over 25 years if all solid waste had been excluded from the pit.Keywords: Pit latrine, filling rate, biodegradation, solid waste disposal

Highlights

EThekwini Water and Sanitation are responsible for the provision of sanitation services in Durban
After the formation of the Municipality in 1999, about 60 000 ventilated improved pit (VIP) latrines were inherited from the incorporated local entities
It seems unlikely that the design of a pit latrine based sanitation service would be driven primarily by the factors described by the model, but rather by considerations of logistics, human resources, cost and the subsequent treatment process

Summary

Introduction

EThekwini Water and Sanitation are responsible for the provision of sanitation services in Durban. After the formation of the Municipality in 1999, about 60 000 ventilated improved pit (VIP) latrines were inherited from the incorporated local entities. After sewerage reticulation had been extended to a number of residential areas and a more formal survey undertaken, it was found that there were 45 000 VIP latrines that had reached or were reaching the end of their service life, in that they were completely full. By June 2011, all 45 000 VIP pits had been emptied and were once again fully serviceable. It is proposed to empty all the VIPs on a 5-year cycle. The municipality proposed that a 5-year cycle should be used for emptying since this was possible from an organisational point of view, and most pits are expected to require more than 5 years to fill. The cost of pit emptying is more closely aligned to the number of pits emptied than to the volume of pits emptied; from an economic point of view, a better understanding of pit filling rates would assist in more cost-effective design of the pit emptying programme

Methods

Findings

Discussion

Conclusion