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SCIENTIFIC SUPPORT FOR THE DESIGN AND OPERATION OF VENTILATED IMPROVED PIT LATRINES (VIPS) KM FOXON Pollution Research Group School of Chemical Engineering University of KwaZulu-Natal Durban 4041 ISBN Report No 1630/1/07 EXECUTIVE SUMMARY 1 Introduction Ventilated improved pit latrines (VIP) are an accepted basic sanitation delivery option and are the regulated minimum acceptable level of sanitation in South Africa. For a pit latrine to qualify as a VIP, it must comply with certain requirements; it must (i) provide hygienic separation of human waste from contact with people, (ii) have a vent pipe fitted with a fly-screen to minimise odour and flies; (iii) be built on a secure slab that will resist collapse of the superstructure; and (iv) provide privacy and dignity for the user. Fly-screen Superstructure Vent pipe Door Pedestal with lid Slab Collar Pit Figure 1: Basic structure of a VIP The basic processes occurring in a VIP are filling (with faecal, water and other material), water transfer into and out of the pit, biological transformation and pathogen deactivation. In general, the rate of filling of the pits as a function of local conditions is not well known; neither is there a good understanding (at least at a policy- making level) of what the condition of the material in the pits will be when the pits become full. In principle the rate of degradation or leaching of the material in a pit should be similar to the rate of filling thus providing a long service life for the pit. In practice, it is often observed that pits may fill rapidly, particularly if a significant portion of the material added to the pit is non-degradable. Management of full pits pose a number of complex challenges to policy-makers, local authorities and householders since households or communities with full pits are no better off than those with no sanitation at all. One of the solutions proposed for management/disposal of pit latrine sludges are a range of commercial products hereafter referred to as pit latrine additives. These may be chemical, microbiological or enzymatic in nature and are marketed on their ability to reduce (or even reverse) the accumulation rate in pit latrines, and reduce potential fly or smell problems. Independent scientific investigations into their efficacy are limited and i ambiguous, although there is anecdotal evidence that suggests that they have the ability to significantly reduce the mass or volume of pit contents, fly and odour problems. Equally, a number of informal studies have suggested that there is no significant benefit to the use of these additives over the addition of water or some essentially inert additive (in effect, a placebo). In many instances, municipalities and other service providers are hesitant to sanction the use of pit latrine additives as they have no scientific basis for choosing one product over another, and are concerned that trials with these products may lead to an expectation among communities that the products will be used forthwith. This in turn could lead to a situation where political pressure results in the application of expensive products that may or may not have any significant benefit. It is generally felt that a scientific explanation of the mechanism of pit latrine additives, and proof of their efficacy would provide the authorities with the ability to rationally assess the cost-effectiveness of implementing programmes for treating pits with commercial pit latrine additive products. This project proposed to undertake field and laboratory investigations of VIPs and their contents in and around the eThekwini Municipal area in order to understand the conditions found in the pits and to propose design and operating practice that will extend the life of pits. The study proposed to concentrate on three main areas of interest: • Water balance in pits under different hydro-geological conditions (water draining into the pit, water draining out of the pit, no water transfer in or out of the pit) • Stabilisation rates in pits (organic, pathogens) • Efficacy and action of additives on pit processes 2 Literature Review A comprehensive review of literature relating to the design and operation of pit latrines, processes in pit latrines and pit latrine additives was undertaken. There is not much formal scientific literature in terms of journal publications or conference proceedings that provide much insight into the processes in pit latrines. However there is a wide range of literature relating to design, operation and maintenance issues that shed some light on the nature of pit latrine contents and the processes that may occur there. It was concluded that the dominant mechanism of stabilisation in the unaerated bulk of the pit latrine contents would be anaerobic digestion, while some aerobic degradation would occur at the top layers. In most pits which are not sealed, movement of moisture in and out of the pits depends on the type of soil/rock in which the pit is located, the presence and height of groundwater and the amount of moisture added to the pit. Soluble and to a lesser extent colloidal material move in and out of the pit with the moisture flow. The movement of soluble organic material out of the pit with moisture flow may be a significant mechanism of removal of organic content from the pit. The literature review also highlighted the fact that there is usually a lot of non-degradable material in pits, and that the amount and type of material in the pit depended on user habits and was linked to the solid refuse removal services available to the pit users. 3 Processes in pit latrines A number of studies were undertaken within this project to characterise the material found in a pit latrine and to infer what processes have occurred in the pit latrine from the time when material is added via the pedestal to the time when it is sampled from some depth within the pit. Chapter 3 presented the findings of studies into processes occurring in pit latrines. These include: • A study characterising fresh faecal material ii • A number of studies in which pit latrine contents from a number of pits were characterised • Two studies from which the differences in pit latrine contents at different points within the same pit were investigated • Two studies in which the effect of additional moisture content and alkalinity (in the form of added sodium bicarbonate) were investigated. 3.1 Variability of pit latrine contents It was observed that the nature of pit latrine contents varied widely within a pit latrine and between pit latrines. It was found that pit latrine contents could look very different and have very different chemical and physical characteristics when comparing pits from different communities and even within communities. Many of the variations noted were due to differences in user practices, such as the type of anal cleansing material used, and the practice of using the pit as a solid waste disposal site in some communities. Other variations may have been due to the geographical location of the pit latrine such as moisture content due to presence of groundwater. Differences were observed between samples taken from the top of the pit, and samples buried within the pit. Differences in nature of pit contents affect the type and extent of biological processes that may occur. Equally, the processes occurring affect the nature of the pit contents, particularly of the pit contents located well below the surface of the pit latrine. These observations are important because they emphasise the fact that management of pit latrine sludge is not a one-dimensional problem, but may require different approaches that are dependent on the nature of the pit contents. 3.2 A general theory describing processes in a pit latrine A general theory was presented to describe the fate of organic material that enters a pit latrine. On the basis of measurements of characteristics of pit latrine contents, and observations from the many samples handled during the project it was hypothesised that (i) all readily biodegradable originating from faeces is aerobically degraded by naturally occurring micro-organisms within a very short time of arriving on the surface of the pit; (ii) a significant portion of the remaining biodegradable material is aerobically degraded before being covered over by new pit contents; (iii) the remaining biodegradable material, including organic residual from dead cells from micro-organisms and from the original faeces are slowly converted to soluble products, methane gas and carbon dioxide in the buried layers of the pit contents (the fraction of the original organic material that is converted by this path is not large); and finally (iv) the material that remains at the bottom of the pit latrine or after a long residence time in the pit is largely non-degradable. These stages are presented graphically in Figure 2. The following additional conclusions were drawn from studies on processes in pit latrines: • It was found that the rate of anaerobic digestion of pit latrine contents taken from the surface of the pit could be accelerated by the addition of moisture. Specifically, the rate of gas production increased by a rate of between 0.006 and 0.02 mℓ gas/g total solids/day per 1% increase in moisture content above the baseline moisture level of 76 %. • The effect of increasing the alkalinity so as to increase pH buffering capacity on anaerobic digestion of faeces and samples taken from the top layer of a pit latrine was tested by addition of different amounts of sodium bicarbonate. It was found that none of the treatments with sodium bicarbonate resulted in statistically significant increases in the rate of gas production under anaerobic conditions.

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