Scientific and management support for ventilated improved pit latrines (VIP) sludge content.
Providing adequate sanitation to all in the form of VIP latrines as proposed by the South African Government Strategic Framework for Water Services does not end with building toilets. All municipalities need to plan for maintenance during the operation and when these toilets reach their capacity. An understanding of the processes occurring in pit latrines will facilitate better management during their lifespan and identifying suitable options for dealing with the accumulated sludge when they eventually reach their capacity. This research aims at providing scientific support for decision making in management of accumulated sludge in ventilated improved pit latrines during their life span and when they reach their capacity under South African conditions. The approach to this research work was divided into two main thrusts: The first was to provide an understanding of the processes in VIP latrines and mechanism of sludge stabilization in pit latrines. The second approach was to provide management and disposal options for pit latrine sludge before and once it has been exhumed in the context of the eThekwini pit latrine emptying programme. Two options were used as case studies, namely: (i) deep row entrenchment of exhumed pit sludge for agroforestry and, (ii) in situ treatment of pit sludge using additives. Three hypotheses were proposed: that (i) significant biological stabilization occurs in a pit latrine with time, such that the disposal/treatment options depend on the inherent ability of the chosen option to accept the load of solids and organic material in the VIP sludge, the residual biodegradability of the VIP sludge, and the health risks, (ii) VIP latrine sludge can be used in deep row entrenchment for agroforestry since the sludge contains nutrients that are available to plants, and that the sludge is sufficiently stable not to cause a negative environmental impact, and (iii) that In situ treatment of VIP latrine sludge using pit additives had no significant effect on the rate of mass loss or volume loss of pit latrines contents. The methodological approach to this research was aimed at addressing the proposed research hypotheses. Thus to test the first hypothesis, two studies were conducted; the first study investigated sludge accumulation rate in pit latrines and the role of digestion processes on sludge accumulation rate in pit latrines. Direct measurement of sludge accumulation rate from selected pit latrines within a community in eThekwini municipality was performed and a laboratory investigation into the effect of moisture content and aerobic/anaerobic conditions on sludge accumulation rate was conducted. The second study investigated the chemical and biological characteristics of pit sludge at different depths within a pit latrine. Research into deep row entrenchment of VIP latrine sludge for agroforestry was conducted to test the second hypothesis. The effect of deep row entrenchment on sludge characteristics and surrounding groundwater at the site was investigated by monitoring changes in sludge characteristics and groundwater quality at the entrenchment site over time. An investigation into the effect of pit latrine additives on pit sludge was conducted to test the third hypothesis. Two sets of trials were conducted; the first was a laboratory trial conducted to investigate the effect of pit latrines additives on collected sludge samples from pit latrine in laboratory scale test units. The rate of mass loss that could arise from the effect of addition of pit additives to sludge in the test unit was determined. The second was a field trial in which pit additives were added to randomly selected pit latrines within a community in Durban and changes in amount of the sludge in the pit was investigated using a laser tape measure and a stereographic imaging technique. The main findings of this research were: • The sludge volume accumulation rate in pit latrines investigated was between 120 ℓ/year and 550 ℓ/year regardless of the number of pit users. The overall average sludge accumulation rate was 282 ± 46 ℓ/year. This converts to a per capita sludge accumulation rate of 56 ℓ/person∙year for an average of 5 number of pit users obtained in this study. Statistical analysis performed indicated that sludge accumulation rate on a per capita basis does not decrease with an increase in number of pit users. • In the laboratory batch experiments, it was observed that by increasing the moisture content the rate of degradation of sludge samples decreases. Over a period of 230 days, mass loss was inversely proportional to total moisture content, and it was found that the mass of solids have been reduced to somewhere between 17 and 64 % of the original sludge mass. This effect was attributed to the exposure of sludge samples in the test units to oxygen, since sludge samples with higher total moisture content in the test units appeared as increased depth of free liquid between sludge sample and air. The calculated mass loss rates observed is expected to be higher than that which will be observed in a pit because the laboratory test had continuous air exposure but pit contents are usually covered over by new materials added to the pit. • Natural stabilization of sludge within the pit does occur if the pit is managed and maintained properly thus providing a long service life for the pit. It was found that the volume of materials have been reduced to between 50 and 75 % of the volume of material added over the 3 years since the pits investigated were last emptied, based on the observed per capita sludge accumulation rate and an estimate of the material added to the pit per person/year. Thus, by comparing the calculated mass reduction in the batch laboratory experiment with the volume reduction in the field investigation of sludge accumulation rate, it can be infered that sludge densification/compaction could play an important role on the stabilization processes in a pit. • The nature of sludge in pit latrines varied significantly within the pit and between different pits. It was observed that below the surface layer in a pit, additional stabilization of sludge does occur and the degree of stabilization within a pit increases with increasing depth from the surface down to the bottom layer of the pit. Sludge samples from the bottom of the pit were well stabilized. • It was also observed from the investigation into deep row entrenchment of pit sludge for agroforestry that further stabilization of pit sludge does occur and as a result of that, nutrients (nitrogen, phosphorus and potassium) locked up in the buried sludge are released as fertilizers. Trees planted near buried VIP sludge showed better growth rate compared to those buried only on soil without VIP latrine sludge and no profound effect on groundwater was observed for the duration in which monitoring was carried out. Further research is needed to develop models for implementing this method cost effectively across a range of conditions. • Neither laboratory trials nor field trials provided any evidence that the use of pit additives have any beneficial effect on VIP latrine sludge. There were no systematic and statistically significant changes in the rate of mass loss on sludge samples in the laboratory test units as well as changes in sludge content of the pit latrines used for the field trials as a result of pit latrine additives. Although, it was observed that there was significant reduction in sludge height in pit latrines in which only water was added compared to those in which additives were added and those in which nothing was added (control) using the infrared distance measure, this effect can probably not be explained completely to be as a result of increasing biodegradation rate caused by higher moisture content, since this explanation would have been observed in the laboratory trials as well as in measurement taking using the stereographic imaging techniques. Instead, flattening of the surface of sludge content in the pit by the addition of water onto the highest part of the pile may play a part in the apparent reduction of sludge height observed. It is therefore concluded from the investigation conducted in this research, that sludge content in pit latrines has naturally undergone significant degradation and that the options for disposal of pit latrine sludge would be limited by the characteristics of the sludge. Therefore disposal options involving biological treatment such as disposal into wastewater treatment plants and anaerobic digestion are not appropriate because the residual biodegradability of VIP latrine sludge obtained was very low (about 30 %) and as such would only result in accumulation of undigested solid; of the options considered in this research, deep row entrenchment of VIP latrine sludge for agroforestry seems to be an appropriate option for the disposal of VIP latrine sludge. There was no evidence to suggest that pit latrine additives have any effect in reducing sludge content in pit latrines.