There is a clear link between proper excreta disposal and improved health [
20]. The appropriateness of pit latrines at providing improved sanitation thus lies in its ability to safely dispose human excreta in such a way that there is minimal or no contact with humans. Furthermore, the excreta should not be accessible to insects or animals and the facility should be free from odours [
18,
47]. Research directly linking full pit latrines, their smell and insect nuisances to disease and health is limited. However, it has been reported that full and/or over flowing improved pit latrines do not meet the criteria for hygienic, safe and sustainable sanitation systems [
35]. It is not only difficult to use full or overflowing pit latrines as the waste splashes on to the users but also the excreta poses a health risk since it is in closer contact with humans. Additionally, smell and insects nuisances of pit latrine use are the main cause of disturbance of people who come in contact with them. In the past, smell and insects significantly affected the user satisfaction, although the problem did not impact on pit latrine use [
19,
48]. More recently, bad smell has been frequently mentioned as a reason for dissatisfaction with shared toilets [
40,
49], discouraging their use and subsequent use of polyethylene bags [
37]. Foul smell has also been noted as a barrier for acquiring and using latrines [
50]. Smell and insects have been associated with the hygienic nature of the pit latrine. For example, in a survey by IK Tumwebaze and H-J Mosler [
51], respondents considered clean latrines as those free from smell and insects. The subsequent sections detail pit latrine performance in terms of filling, smell and insect nuisances.
Pit latrine filling
Pit latrine filling is currently a problem associated with their performance. Notably the first faecal sludge management seminar was held in March 2011 in Durban, South Africa and brought to light issues related to pit latrine filling [
52]. One of the concerns of pit latrine filling is that a number of the pit latrines within urban areas of SSA have reached their storage capacity. For example, VIPs built in Zimbabwe from 1980–2000 were reported to be full or nearly full [
53]. A study by BF Bakare [
54] reported that the number of pit latrines built across South Africa’s municipalities were full or over flowing. In Durban, South Africa alone, 35,000 pit latrines were emptied by 2011 [
55]. In a study undertaken in informal settlements of Kampala, Uganda I Günther, et al. [
56] noted that 35 % of the pit latrines had been abandoned because they had filled up while 15 % of the latrines were full and still in use. A study by E Appiah-Effah, et al. [
57] undertaken in the Ashanti region of Ghana reported that 31 % of the latrines were found full and needed immediate de-sludging. M Jenkins, et al. [
35] noted that 40 % of the latrines were full or nearly full in Dar es Salaam, Tanzania.
In the past, a pit latrine once full, was covered and a new one dug nearby. Double alternating pits were also proposed for use in peri-urban areas as they sanitize and reduce the volume of human excreta prior to emptying and disposal [
26]. However, due to the high population density in most urban areas of SSA, digging new replacement pits and the use of alternate pits are not practical. Pit latrines can thus no longer serve as a stand-alone solution to human excreta management. A systems approach to sanitation is currently being adopted for urban settings to ensure their sustainability. In this case, the provision of access to improved sanitation is considered a multi-step process where a pit latrine is part of the chain, to be supported by the collection and transportation as well as treatment for safe end-use or disposal [
58].
Attention is currently being focused on the time it takes for the pit to fill, since it is crucial for the management and sustainability of pit latrines. The actual filling times of pit latrines as noted in literature vary (Table
1). The available information indicates that pit latrines are mainly filling faster than expected. This has been attributed to the rate at which sludge accumulates within the pit. Most of the studies determining sludge accumulation have been based on number of users, filling time and the size of the pit. Proposed design accumulation rates range from 40–90 ℓ/capita/year [
15,
18]. More recent field investigations undertaken in peri-urban South Africa by J Norris [
59] found lower rates and thus proposed 25.5 ℓ/capita/year. In another study by DA Still [
60] in South Africa, sludge accumulation rates were found to range between 10–120.5 ℓ/capita/year. Further studies by DA Still and K Foxon [
61] noted filling rates of 1–264 ℓ/capita/year. Available data indicate variable pit latrine sludge accumulation/filling rates by region, even in a comparatively homogeneous environment (Table
2).
Table 1
Summary of studies on pit latrine filling time
| Various | 15–25 | Design recommendations for household properties |
| East Africa | Over 30 | Reported at a house hold level |
| Zimbabwe | Over 30 | Household latrine |
DA Still and K Foxon [ 107] | South Africa | 20 | Design recommendation |
| South Africa | 5–9 | Empting time for most (85 %) pit latrines. Lower and higher filling rates were also noted |
| Uganda | 5 | Study in low income areas of Kampala, Uganda (Slums) |
RN Kulabako, et al. [ 108] | Uganda | <1 | Low laying areas of peri-urban settlements in Kampala |
K Adubofour, et al. [ 109] | Ghana (slums in Kusami metroplis) | 4.2 | Average filling time |
>10 | High income areas |
0.25 | Low income areas |
E Appiah-Effah, et al. [ 57] | Ghana (Ashanti region) | 6–10 | Low income area in Ashanti region |
Table 2
Design accumulation rates and actual excreta filling rates
Design accumulation rates | | |
EG Wagner and JN Lanoix [ 16] and R Franceys, et al. [ 15] | 40 | Wet pits where degradable anal cleansing material is used |
| 60 | Wet pits where non degradable anal cleansing material is used |
| 60 | Dry pits where degradable anal cleansing material is used |
| 90 | Dry pits where non degradable anal cleansing material is used |
Reported pit latrine filling rates | | |
EG Wagner and JN Lanoix [ 18] and R Franceys, et al. [ 15] | 25 (ablution water used) 35 Wet pit | West Bengal, India |
EG Wagner and JN Lanoix [ 18] | 40 (solid cleansing material) | Philippines |
| 20 | Zimbabwe PR Morgan, et al. [ 94] |
| 42 | USA |
| 47 | Brazil |
| 24.1 (mean) | Soshongove, South Africa |
| 69.4 (mean) | JN Bhagwan, et al. [ 110] Bester’s Camp, South Africa |
| 18.5 (mean) | Mbila, South Africa |
| 27.5 (implied) | Gabarone, Dares salaam |
| 29 (median) | Mbazwana, South Africa |
| 34 (median) | Inadi, South Africa |
DA Still and K Foxon [ 53] | 39 (median) | Limpopo, South Africa |
| 48 (median) | Mafunze, South Africa |
| 21 (median) | Ezimangweni, South Africa |
| 19 (mean) | eThekwine, South Africa |
To explain the variation in sludge accumulation rates, studies have assessed different variables (Table
3) some of which are user related, like number of users, other material put in the pit and design related (type of pit latrine, lined or un lined), geophysical and climatic factors. Studies relating sludge accumulation rates to number of users have reported contrasting results. It is perceived that the filling rate increases with number of users. However, some field studies have reported a decrease in sludge accumulation rates with an increase in number of users [
54,
61]. Additionally, BF Bakare [
54] based on a linear model fit to the amalgamated data documented by DA Still and K Foxon [
61], showed no significant correlation (Pearson correlation coefficient of 0.203) between sludge accumulation rate and number of users. However, it is important to note that this study was based on small increments from 5 to about 15 pit latrine users. The case could be different in urban settings where pit latrine sharing leads to higher number of users.
Table 3
Summary of studies assessing sludge accumulation rates, with different variables
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 |
| 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. |
| 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 |
| 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 |
| 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 |
| South Africa | additives | Laboratory experiments on pit latrine samples | No statistically significant effect on rate of mass loss |
| South Africa | Bio additives | Laboratory studies on pit latrine samples | Use of biological product is feasible |
| Zimbabwe | Spore forming bacteria | Pit latrine studies | Efficient in reducing pit content |
| | Earthworm (Tiger worms) | Laboratory experiment setup | Reduction in human excreta |
| South Africa | Black soldier fly larvae | Laboratory studies on pit latrine samples | Potential in reduction of pit latrine content |
Relating sludge accumulation to matter other than human excreta found that the degree of abuse to which the pit is subject affects the filling rate. Throwing rubbish in a pit almost doubled its filling rate in studies undertaken in South Africa [
61,
62]. A simple mass balance model of pit latrine filling developed and tested by C Brouckaert, et al. [
63] using data from VIPs in South Africa, predicted that adding non-degradable material to the pit significantly influenced its filling. A study by J Norris [
59] noted no effect of seasonal variations on sludge accumulation in pit latrines in South Africa. However, in Tanzania, a large temporary increase in pit content was observed in the wet periods [
64]. The ability of the model developed by C Brouckaert, et al. [
63] to simulate data collected in south-central Tanzania and a sensitivity analysis of its parameters was tested by LC Todman, et al. [
64]. The results indicated that water inflows and accumulation have an important effect on the filling rate. In Kampala (Uganda), a study relating the status of pit latrine structures to their performance noted that signs of rain or storm water entry, flooding and cleaning time were significant predictors of pit latrine filling [
65]. This implied that water input into the pit significantly contributed to an increase in the level of pit content.
The rate of filling has also been attributed to the degradation processes occurring within the pit latrine over time. Matter starts to decompose as soon as it is deposited in the pit. Studies have depicted that the process of decomposition in pit latrines is largely anaerobic although aerobic degradation processes may occur [
18,
62,
66]. During decomposition, the degradable fraction of faecal matter will break down into a more stable non-odorous product. Released gases flow into the atmosphere and mineral compounds are assimilated into the ground respectively. Through this action, the volume of matter added to the pit is substantially reduced [
15,
48]. A possible mass - volume reduction of 50–75 % [
54] or up to 80 % [
18,
67] after well-established degradation has been reported. However, literature indicates that the uncontrolled environment within the pit may not be efficient for decomposition under either process which results in slow/incomplete breakdown of organic matter [
68].
In order to quantify the role of decomposition and stabilization on mass loss within pit latrines, laboratory batch experiments have been undertaken. Addition of moisture to samples of pit content in laboratory experiments had a significant increase on gas production rate [
62]. It was thus concluded that increasing moisture content of VIP contents has the potential to increase the rate of stabilisation of buried organic material in the pit. However, in a study by BF Bakare [
54] no evidence was found to show that an increase in moisture content of samples from VIP latrines reduced the sludge accumulation rate. The study proposed that compaction could play an important role on the rate at which pits fill up. The effect of increasing alkalinity (addition of Sodium bicarbonate), thereby the pH buffering capacity of pit latrine samples was assessed by CA Buckley, et al. [
62]. The increase in the rate of gas production from the samples observed under anaerobic conditions was not statistically significant. It was thus concluded that alkalinity was not a limiting factor in anaerobic digestion of pit latrine contents.
Studies on inoculation with additives, which are reportedly a mixture of various microorganisms, some blended with enzymes said to enhance degradation of pit content have also been undertaken. Relatedly, L Taljaard, et al. [
69] reported a feasibility in the use of biological products for the degradation of organic matter. However, the study was inconclusive and recommended field trials to daily monitor contents of newly dug pits. A biological study into the claimed mode of action of the products, to determine the amount and type of microorganisms and enzymes present was also proposed. Earlier, M Jere, et al. [
70] studied the effects of spore forming non-pathogenic bacteria in reducing sludge volume in pit latrines and concluded that the bio-organic breakdown compound proved to be efficient in reducing the pit contents. However, CA Buckley, et al. [
62] obtained no correlation in decrease of faecal matter between the used additives and the rate of change in pit matter content. The results were considered inconclusive due to the difficulty in obtaining representative measurements of any condition and lack of test control sites. Furthermore, K Foxon, et al. [
71] reported no statistically significant effect on the rate of mass loss from the sludge samples under either aerobic or anaerobic conditions by nine additives. It was concluded that commercial pit latrine additives did not accelerate the rate of decomposition of pit latrine contents. Subsequently, DA Still and K Foxon [
61] concluded that sufficient evidence was lacking to prove that pit latrine additives could cause differences in pit latrine sludge build-up.
Earth worms have also been investigated for their potential to reduce pit latrine contents with successful results [
72]. Currently, they are the basis of the tiger toilet, a worm- based sanitation technology aimed at speeding up the decomposition of human waste [
73]. Black soldier fly larvae (BSFL),
Hermetia illucens has also shown potential in reducing pit latrine sludge. Research by I Banks [
74] found the characteristics of faecal sludge from different pit latrines in South Africa to be within the range for BSFL development. Key factors that affected the faecal mass reduction were moisture and larvae density. However, further research is required on the applicability of these organisms in pit latrines.
Pit latrine odours and insect nuisance
The extent of the smell and insect nuisance found in literature has mainly been listed by intensities based on a pre- determined scale (Table
4). Only two studies listed the odour descriptions associated with particular pit latrine smell intensity (Table
5). Of the listed intensities, the strong, unpleasant, repugnant, foul, malodorous smell and any presence of flies are of importance in pit latrine performance.
Table 4
Pit latrine odour intensity and description
| Ghana and Mozambique (Simple pit latrines and VIPs respectively) | No smell (54 and 40) | None/tens (91 and 90) |
Slight smell (9 and 6) | Hundreds (8 and 3) |
Strong smell (37 and 51) | Thousands (1 and 7) |
J Kwiringira, et al. [ 37] | Kampala’s slums | Strong repugnant smell | |
| Kenyan schools | Strong smell (25.6) | Many flies (10) |
| Kampala’s slums | No smell, (2) | No flies (3) |
Slight smell (35) | Few flies (80) |
Moderate smell (22) | Many flies (17) |
Strong smell (39) | |
Very strong (1) | |
| Kusumi, Ghana | Extremely annoying (69 no) | |
| | Very annoying (55 no) | |
| | Annoying (30 no) | |
| | Some annoyance (18 no) | |
| | Definitely not annoying (1 no) | |
Table 5
Pit latrine odour intensity and description
| Durban | VP dry pit | Weak | Sewage, phenol-like |
| strong | Rotten egg, sewage, rancid |
| VP wet pit | Medium | More of sewage than faecal, rotten egg |
| Strong | Rotten egg, sewage, rancid |
| Nairobi | VP | strong: | cheese, manure, horse, farmyard |
| Strong | cheese, manure, ammonia, urine |
| Kampala | VP 1 | weak | farmyard, ammonia slightly urine, geosmin (earthy, moisture) |
| strong | rancid, rotten onion, phenylacetic acid-like |
| | VP 2 | medium | farmyard, ambrinol (earthy, moisture), rancid |
| strong | rancid, phenolic, rotten vegetable |
CJ-Fo Chappuis, et al. [ 113] | Nairobi | | Weak | barnyard |
| Durban | VIP | Weak | Animal, faecal |
Information on the actual composition of the malodorous gases in pit latrine is limited. Methane, carbon dioxide, nitrogen, ammonia and hydrogen sulphide have for long been noted as the smell causing substances in pit latrines [
18,
75]. However, a study by J Lin, et al. [
76] using gas chromatography - mass spectrometry and olfactive analyses found many more odorants. Of the 198 volatile constituents detected [
77], isobutyric, butyric, isovaleric, 2methyl butyric, valeric, hexanoic and phenylacetic acids were responsible for the rancid, cheesy odour/smell in pit latrines. The manure, farmyard, horse-like characteristics of latrine odour were attributed to the combined effects of phenol, p-cresol, indole, skatole, and some carboxylic acids. Dimethyl sulphide, dimethyl disulphide, dimethyl trisulphide, methyl mercaptan, and hydrogen sulphide were contributed to the sewage, rotten egg, and rotten vegetable odours. The sewage malodourous smell in pit latrines has been attributed to anaerobic degradation while the rancid odour was noted to be representative of latrines dominated with fresh faeces [
76]. Fermenting urine resulting from enzymatic cleavage of urea by ureases has been noted to be representative of the smell found in public pit latrines [
78,
79].
Unlike smell, studies characterising insects in pit latrines have been undertaken. Adult and larvae of
Chrysomya putoria,
Chrysomya marginalis,
Musca spp,
Lucilia cuprina,
Sarcophaga spp have been reported [
80‐
82]. S Irish, et al. [
83] identified members of
Psychodidae,
Culicidae,
Calliphoridae,
Syrphidae,
Stratiomyidae,
Sarcophagidae families from pit latrines in central Tanzania. Some types of mosquitoes especially
Culex quinquefasciatus and species of Anopheles are known to breed in wet pits [
84,
85].
Studies have linked the presence of odours and insects in pit latrines to the type and size of the superstructure, and cleanliness. S Irish, et al. [
83] noted that the superstructure minimises the fly nuisance in pit latrines. Absence of a roof for example significantly associated with presence of flies. In addition more flies have been found in latrines with temporary structures. In Kampala Uganda, latrines that were not regularly cleaned were associated with bad smells [
40] and caused disgust among the users [
39]. Another study noted that pit latrine cleanliness, stance length, superstructure material and single household use were predictors of smell. Fly presence was predicted by the superstructure material and status, plus the terrain where the pit latrines were located [
65]. Entomological studies on pit latrines in Botswana and Tanzania [
80] linked the insect nuisance to the smell. The studies showed that insects in pit latrines were attracted by the odours as many flies and mosquitoes were caught trying to enter the vent pipe which indicated they were drawn to the smell source.
Addressing the odour and insect nuisance of pit latrines has involved simple recommendations like the concrete slab that is easily cleaned and ensuring that the pit remains dark during use, which is achieved partly by the use of hole/ seat covers [
18]. The use of inorganic and organic chemicals as larvicides and disinfectants like sodium fluosilicate, borax, paradichlorobenzene (PDB), orthodichlorobenzene (ODB), aldrin, BHC and DDT has been documented [
17,
18,
86]. Muscabac, a
Bacillus thuringiensis preparation containing exotoxin, was tested and showed reasonably good control of flies in latrines in a tropical environment [
87]. Household surveys have also reported addition of oil, kerosene, ash, soil, and disinfectants to control odour and insects [
88‐
90]. Laboratory and field experiments on the use of expanded and shredded waste polystyrene beads to eliminate mosquitoes in pit latrines have been very successful [
91]. Traps placed over the squatting plate hole have also been developed and experimented with success at controlling insects in pit latrines [
82]. Pyriproxyfen, an insect juvenile hormone, and local soap have been found to reduce flies in pit latrines [
92].
Improvements in the design of the pit latrine have also been done to minimise the smell and fly nuisance. Incorporation of a vertical vent pipe with a fly trap and the natural effect of the sun and wind are the principle mechanisms for the functioning of a VIP latrine. The design makes use of circulation of air from outside the latrine, through the superstructure into the pit, then up and out of the vent pipe thereby exhausting any odours emanating from the faecal material in the pit via the vent pipe [
75,
93]. The superstructure is kept dark to prevent flies from going into the latrine. The top of VIP vent pipe is fitted with a wire mesh fly-screen that prevents any flies inside the pit from escaping via the vent pipe where they die and fall back into the pit.
Experiments on the performance of VIP latrines in Zimbabwe showed that they were effective in smell and fly control compared to identical unvented pit latrines. However, the ventilation system was not as effective at mosquito control [
94]. This was because while both flies and mosquitoes were drawn to odour sources in pit latrines, [
80] the latter have a positive phototropism and fly only towards light [
18]. Contrary to the studies in Zimbabwe, field investigations undertaken by A Cotton and D Saywell [
48] in Ghana and Mozambique that were based on a user’s perceptions recorded a higher degree of odour nuisance with the use of VIPs. In a recent study undertaken on pit latrines in Kampala Uganda, VIPs did not provide superior performance (smell, flies) to the simple pit latrines. Additionally, logistic regression showed that VIPs are not likely to smell less nor have fewer flies than simple pit latrines [
65]. This was attributed to the VIPs not meeting minimum design standards, and overcrowding in the slums that could have impeded ventilation within the VIPs to achieve odourless conditions.
In order to understand the mechanisms inducing ventilation in the VIP design, field studies were undertaken in Botswana and Zimbabwe. PR Morgan, et al. [
94] found that the action of the wind blowing across the top of the vent pipe induced ventilation. The effect of solar heating the vent was only negligible [
75]. Additionally, satisfactory odour control in VIP latrines was achieved with a ventilation rate of 10 m
3/h and 6 superstructure air volume changes / h (ACH). A more recent study by JW Dumpert [
95] on VIP latrines in the upper west region of Ghana found out that mechanisms driving ventilation were air buoyancy forces resulting in a stack effect at times in which ambient temperatures are less than temperatures inside the pit of the latrine; and suction wind passing over the mouth of the vent pipe and when possible wind passing into the superstructure. The study further noted that, majority of the latrines (73 %) achieved ventilation flow rates greater than 10 m
3/h. However, the flow rates were not adequate enough to achieve the 6 ACH as to maintain odourless conditions. The larger volume of the pit latrine superstructures in this study compared to those found in Botswana and Zimbabwe was noted to contribute to the low ACH. Additionally the vent pipe sizes were found to be inadequate, while most structures were constructed with openings and entrances facing away from the wind direction.
Other design improvements to the simple pit latrine that have been noted in literature to improve the odour and smell nuisance include the SanPlat pit latrine which consists of a thin circular dome shaped slab of the pit with no reinforcement and has a removable lid cast in the squat hole to ensure it fits tightly. Contrary to the VIP latrine where air is encouraged to flow through the structure, the SanPlat prevents air in and out flows of the pit. The opening into the pit is always kept tightly closed when not in use. Thus most odours remain within the pit and are assumed to be absorbed by the pit walls [
28]. A pit latrine modification with a specially made bowl incorporated in the ordinary concert slab uses a water seal to control odour and insects. About 1–2 L of water is usually poured by hand into the bowl to flush faecal matter into the pit [
15].