Penetrometer tests on 109 pit latrines in Kibera, Nairobi, Kenya

Abstract

Pit latrine contents can have varying consistencies that make the pits challenging to empty. It has been assumed that solids sink to the bottom of the pit with a more liquid layer above this and a scum on the top. This implies that it would be possible to remove the uppermost layers with a pump. However, recent studies have found alternative sludge profiles, including a thick crust on the top. This paper presents penetrometer data from a large number (109) of pit latrines in Kibera, Nairobi, Kenya. The penetrometer consists of a weight, dropped onto an anvil that drives a cone into the pit. The depth of penetration following each impact was measured. This was converted into plots of depth in the pits against penetration. Each pit was analysed using Spearman’s rank correlation coefficient, and categorised into positive or negative correlation, significant or non-significant. Significant, negative correlation was assumed to correspond to pits where the solids had sunk to the bottom. 38% of pits were classified in this way, which suggested that while it is common that the number of impacts required to penetrate the sludge increases with depth in the pit, this is not always the case. Pit emptiers should be prepared to deal with pits with a variety of sludge profiles, including those with a solids layer at the top.