Table 3.

Summary of studies assessing sludge accumulation rates, with different variables

Source	Country	Variable of interest	Study/experimental design	Remarks
DA Still and K Foxon [61]	South Africa	Number of users	Field monitoring and measurements	A decrease in per capita filling rate with an increase in number of users.
		Rubbish content	Sorting and analysis of pit content	Throwing rubbish in a pit almost doubled its filling rate
BF Bakare [54]	South Africa	Number of users	Analysis of amalgamated data documented by DA Still and K Foxon [61]	No correlation (Pearson correlation coefficient of 0.203) between sludge accumulation rate and number of users.
			Field monitoring and measurements	Sludge accumulation rates decreased with increasing numbers of users.
		Degradation	Laboratory experiments on pit latrine samples	50–70 % volume reduction in matter added to the VIP
		Addition of moisture	laboratory batch experiments on pit latrine samples	No evidence that an increase in moisture content of samples from VIP latrines reduced the sludge accumulation rate.
LC Todman, et al. [64]	Tanzania	Seasonal variation	Field monitoring and measurements	During wet periods, large temporary increases in the level (1 m magnitude) of pit content was observed
		Pit latrine Modelling	Modelling pit latrine filling based on model developed by C Brouckaert, et al. [63]	Water inflows and accumulation have an important effect on the filling rate
J Norris [59]	South Africa	Seasonal variation	Field monitoring and measurements	No effect of season variations on the sludge build up
EG Wagner and JN Lanoix [18]	Various	Degradation		A possible volume reduction of up to about 80 % after well-established degradation in wet pits
CA Buckley, et al. [62]	South Africa	Addition of moisture	Laboratory experiments on pit latrine samples	a significant increase on gas production rate was noted
		Increasing Alkalinity	Laboratory experiments on pit latrine samples	No statistically significant increases in the rate of gas production from the samples under anaerobic conditions.
		additives	Laboratory experiments on pit latrine samples	Inconclusive results
C Brouckaert, et al. [63]	South Africa	Pit latrine Modelling	Developing and testing a simple mass balance model	Adding non-degradable material to the pit significantly influenced its filling
K Foxon, et al. [71]	South Africa	additives	Laboratory experiments on pit latrine samples	No statistically significant effect on rate of mass loss
L Taljaard, et al. [69]	South Africa	Bio additives	Laboratory studies on pit latrine samples	Use of biological product is feasible
M Jere, et al. [70]	Zimbabwe	Spore forming bacteria	Pit latrine studies	Efficient in reducing pit content
FF Kassam [72]		Earthworm (Tiger worms)	Laboratory experiment setup	Reduction in human excreta
I Banks [74]	South Africa	Black soldier fly larvae	Laboratory studies on pit latrine samples	Potential in reduction of pit latrine content