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- 1 A note on terms
- 2 Introduction
- 3 Inception & Past Uses
- 4 Scientific & Engineering Theory
- 5 Design
- 6 Construction
- 7 Operation & Maintenance
- 8 Impacts
- 9 Dissemination
- 10 Re-Design
- 11 References
A note on terms
Strictly speaking, composting latrines refer to the chamber below a toilet system, so one may have different kinds of toilet capture which lead to the composting latrine below. In the general way, however, composting toilets and composting latrines are often used interchangeably and it is very often that a composting latrine is attached to a UDDT. Many use the terms composting toilet and composting latrine to refer to an S-VIP or D-VIP toilet system.
The world over, access to improved sanitation continues to elude development agencies and developing countries (UN, 2010). In 2008, the United Nations Children’s Fund – UNICEF – along with World Health Organization – WHO - reported that 2.5 billion persons, over one-third of the world’s population, lack access to a sanitary excreta disposal facility (UNICEF & WHO, 2008). The direct disease burden from unsanitary excretion practices, water, and hygiene has been shown to be 5.3% of all deaths and 6.8% of all disability-adjusted life year (DALY) (Pruss, et al., 2002). In the last eight decades, a number of sanitary solutions have emerged that aim at reducing bacterial and viral pathogenic infection to humans. One such engineered system is the composting toilet, a relatively scientifically simple system that has continued to gain use in developing countries (Hurtado, 2005; Nimpuno, 1978). The composting latrine is either a stand-alone closet or a toilet system, depending on the engineering and manufacturing. Typically, humans bowel movements are collected in a sealed pit that rests directly below the toilet structure. The organic rich material begins, almost immediately, to be either destroyed by microorganisms, mainly bacteria, in either aerobic or anaerobic environments. The composting toilet and latrine not only removes pathogenic microorganisms biologically from human excrement, but the decomposition of the organic matter is such that the byproduct is a nutrient-rich humus that can be used as a soil additive in efforts to provide sufficient foodstuffs for the growing of agricultural crops. Given the relative ease of constructability, use, and maintenance, as well as the demonstrated health and agricultural benefits, the use of composting latrines has been advocated for use in developing communities, both rural and urban, for over a half century. While there are both structural and social constraints to use, research has shown that the rate of use of composting latrines has increased in the past, and has the opportunity to gain more widespread use in decades to come (Moe & Rheingans, 2006; Hurtado, 2005; UN, 2010).
Inception & Past Uses
Asia has been composting human and animal excrement for centuries (Jenkins, 2005; Moe, 2006). Formal documentation of modern composting became in the later years of the 1930s in Norway and Sweden. The single and double vaulted systems were originally developed in 1938 by Rikard Lindstrom of Norway and dubbed a Multrum, which now has a number of modern variations including the Clivus, Multrumman, Toa-Throne, all manufactured in Sweden, and the Toga Hyttetoalett from Norway (Nimpuno, 1977). The Multrum is either a single vault or double vault composting latrine, above which sits a toilet and enclosure from which the user excretes. In the 1950s the Democratic Republic of Vietnam initiated a five-year plan addressing rural hygiene, and in the process developed a double vaulted anaerobically-digested composting pit latrine that used water-tight tanks to collect human excrement (Rybczynski, et al., 1982; Jenkins, 2005). The double vaulted arrangement was introducted to Central American and Africa in the 1960s and 1970s.
Composting latrines are now readily available on commercial and residential markets, each with individual features and functions. Modern updates to the single and double vault latrines maintain the same underlying scientific and engineering theories developed in the 1930s and 1940s. However, as technology in material sciences and manufacturing practices have improved in the last 70 years, latrines have become more durable, consistent, and lightweight. Various stand-alone toilet compartments and toilet systems are manufactured and sold in markets all over the world. Companies such as Global Inventive Industries (GII) manufacture ECOJOHN-type product line of incinerating toilets, toilet waste combustion systems, composting toilets, and mobile restroom offices (ECOJOHN, undated). The waterless composting toilet billed as part of their BASIC Series, has a solid waste collection box inside of the actual toilet that separates the urine from feces, running the former to a soak pit outside the compartment space. Underneath the waste box is a thermostatically-controlled heating plate that dries out the waste and facilitates expedited kill-off of bacteria and viruses.
Scientific & Engineering Theory
The composting latrine converts human excrement into a soil amendment that can be used to improve the nutrient content of soil to which it is applied. Given sufficient time for the organic matter to be oxidized and for the pathogenic bacteria and viruses to die, the composted organic matter, rich in carbon, will be used to provide nutrient enrichment for agricultural crops.
The decomposition of the excrement is largely a biological process that occurs in both aerobic, meaning oxygen is required to support biodegradation, and anaerobic, or biodegradation occurs absent of oxygen, environments. These conjoint processes occur within all composting latrines, and while both have unique biological features, they produce a decomposed byproduct that is rich in nutrients and largely free of pathogenic viruses and organisms. The decomposition of excrement occurs in three stages, namely stabilization, maturation, and curing. Stabilizing is a high-temperate aerobic decomposition process where much of the easily degradable matter is decomposed by mesophilic and thermophilic bacterica, who in turn release a large amount of heat and cause a rapid rise in temperature of up to 52 Celsius within the latrine. Some have strictly defined composting as the occurrence of this process (Dahi, 1997). Maturation, or mouldering, is a process that occurs over a period of months, is the deliberate decomposition of more complex biodegradable matter. The final stage, curing, can be the longest process, by which a number of viral pathogens die from the immature compost (Jenkins, 2005). Aerobic digestion is a process by which bacterial microorganisms consumer organic matter and oxygen, while producing carbon dioxide, water, ammonia, new bacterial cells other byproducts and heat; the process is commonly referred to as oxidation (Britton, 1999). Ammonia will, in turn, be oxidized through nitrification to nitrate. Specifically, COHNS+O_2→〖CO〗_2+H_2 O+〖NH〗_3+C_5 H_7 O_2 N+energy 〖NH〗_3+2O_2→〖HNO〗_3+H_2 O This process can take place over a number of weeks, or a number of months, depending on the amount of excrement to be decomposed, the nature of the pit construction, moisture content of the composting material, and the amount of pathogenic viruses present in the excrement. Urine must be separated from feces for two reasons. First, urine would slow the decomposition of the feces by allowing it to remain saturated, inhibiting air circulation and thus the transfer of oxygen to the compost pile. The stabilization and mouldering of the feces would, therefore, be inhibited (Mihelcic et al., 2009; Hurtado, 2005). Secondly, urine’s high nitrogen content (carbon:nitrogen ratio of 0.8:1) would lower the carbon to nitrogen ratio of the composting pile, which under ideal conditions is 30:1 (Britton, 1999).
The destruction of pathogens is a critical biological process if the compost is to present limited human and environmental hazard. Organic matter is rich in pathogenic organisms, namely bacterium coliforms and viruses, both of which are of particular interest, given there potential hazard to human health. Bacterium coliforms, which themselves are named for the entheropathogenic bacterium Escherichia coli, an easily distinguishable indicator bacterium found in human reservoirs and frequently used to identify the presence of overall pathogenic bacteria, viruses and protozoa (Mihelcic, 1999; Crittenden, 2005; Metcalf & Eddy, 2003). Anthropological symptoms of distress from bacteria and protozoa include diarrhea, malaise, dysentery, ulceration of the small intestine, and death (Metcalf & Eddy, 2003). Given these negative affects on human health, the presence of both total coliform bacteria and fecal coliform bacteria has been widely accepted to be representative of the presence of fecal contamination in water, and are viewed as a threat to human health (Crittenden, 2005; Pickford, 1995). Pathogens will be removed and die from the compost in a number of complex and interrelated biological processes, including natural die-off, given enough time. Also, the pH of the solution is critical, with a value of 9 being found to lead to a significant reduction of viruses and bacterial pathogens (Stenstrom, 2002). Coliform bacteria and ascaris (roundworms) have been found to survive 6-7 weeks from the start of composting, while helminth ova are killed more slowly (Pacey, 2005). TABLE 1: 6 MONTH REDUCTION EFFICIENCY IN DRY LATRINES
|Parameter||Reduction efficiency (log10)||Data source|
|Bacteria (coliforms)||> 6 log||Chinese|
|Bacterial (fecal enterococci)||4-6 log||Mexico / extrapolations|
|Bacteriophages (index virus)||5 – 6 log||Chinese|
|Ascaris ova (index parasite)||100%||Vietnamese|
Source: Stenstrom, 2002 The removal of pathogens can be expedited by increased temperature, which has been demonstrated to be an exogenous byproduct of aerobic decomposition. While there may be social and cultural impacts to the perception of heat release of the composting pile, the science has held true empirically for over thirty years (Cairncross & Feacham, 1999; Metcalf & Eddy, 2005). Holding all other variables constant, Figure 1 demonstrates that the by exposing the excrement to temperatures above 40 degrees Celsius for extended periods of time will result in the compost present minimal hazard to human health and can be safely handled.
The technology involved with the systems is relatively straightforward. The excrement will, as always, be subject to the force of gravity, and will, upon emanating from the source, fall with force that is equal to the product of the excrement’s mass and Newton’s gravitational constant. One primary engineering design decision revolves around the assumed filling rate of the latrine. It is imperative that the engineer completed a comprehensive assessment of the likely daily users of the constructed system. Is the latrine to be used by a primary household of five people? Or rather, will the immediate family be the primary users? Questions then arise as to the social understanding of an immediate family. Similarly, if the latrine is to be communal, how does the engineer count the users? Essentially, it is imperative that the engineer communicates with the prospective owner(s) as to how often the system will be utilized. As the time required for decomposition of feces is reliant upon a number of physical factors - temperature, oxygen, pH, moisture content – the decomposition rate is, understandably, variable and inherently involves a certain level of uncertainty. Studies have shown that dry sanitation systems are filling at rates faster than expected (Bhagwan, et al., 2008). Studies completed in South Africa found that sludge accumulation rates of pit latrines varied widely from 10 liters per user per year to 100 liters per user per year, and a median rate of 25 to 30 (Still, 2002). Other research has suggested that the average human being daily excreta is 1.18 liters per day (Jenkins, 2005). This variation in estimates behooves the engineer to provide a factor of safety to the sizing of the composting pit or pits. Similarly, the engineer must be have an understanding of how often the pit is drained, as well as the degree to which the pit will be used for disposal of organic household waste. As discussed in the immediate preceding section, the latrine may require the addition of organics, such as rice husks or sawdust, in order to facilitate aerobic digestion of the excrement. Beyond this, the engineering theory of groundwater and surface water, or more specifically the hydrogeological conditions of a particular site, need to be analyzed and assessed in order to ensure that latrines do not allow for the intrusion of water into the composting pit nor the leaching of pathogenic matter out of the pit. Standard design practice suggests a minimum difference in mean elevation of no less than 0.6 meter. Additionally, efforts should be made to ensure that surface water is unable to enter either the pit or the toileted compartment. Most often, the toileted compartment is elevated above the ground, as will be demonstrated in subsequent sections.
These biological considerations have lead to a number of engineering considerations when organizing the key features of the design of the composting latrines. The need to separate urine from feces offers functional opportunities for the engineer. Similarly, a key feature of the composting pit is that it is of sufficient design so as to allow enough time for the maturation of the excrement to compost and allow for pathogens to die. Functionally, this is accomplished a few different ways.
Urine separation is completed in two manners, either by urine diversion or through the installation of a false floor in the composting pit (Mihelcic, et al, 2009; Jenkins, 2005). In the case of a urine diversion system, two practical designs can be employed. First, and perhaps most intuitively simple, is that during micturition, the user utilizes a separate toilet, whereby the urine is piped to a separate soak pit or collected in a container, as was the case with early Vietnamese double vault arrangements. Alternately, the urine diversion structure can be incorporated in a single toilet, placed in such a way – towards the front of the bow - that the user clearly understands its intended use. A second common approach to separating of the urine requires nothing of the user, but rather uses the law of gravity to filter the urine from the feces by installing a false floor above the bottom of the composting pit. Such a design will require that the floor of the composting pit be pitched so as to allow the urine to drain to a piping system, where it is typically transferred to a soak pit that rests below the bottom of the vault. Here, the relative depths of the soak pit and the pit are important, as the system should not be allowed to have urine flow from the pit into the composting latrine.
Batch Systems & Vaulting
The Vietnamese double vault setup was such that one vault was utilized until full, at which time the excrement would be left to decompose while the other vault was utilized. Urine was diverted from the vaults and used for fertilizer, and kitchen ashes were used to aid in the decomposition process. In subsequent decades, the design was utilized extensively in Mexico and Central America, with much success. Similar systems became prevalent in India around the same time, while two bamboo-framed aerobic digesting composting systems became widespread throughout China in the 1970s (Jenkins, 2005).
Continuous systems are characterized by a single-vault that accepts both urine and feces within a sloping chamber that contains integrated air ducts or channels in order to facilitate aerobic digestion of the matter (Jenkins, 2005; Nimpuno, 1977). These systems are often dubbed Multrum-type. In addition to human excrement, organic household waste is typically added to the compost pile, all of which undergoes oxidation over a period of 3 to 5 years. Others have pointed to other operational obstacles, namely the large sub-floor tank required and the need to control temperature in the tank (Nimpuno, 1977; Pickford, 1995). Bearing in mind the shortcomings often found amongst single-vault systems, 300 prefabricated single-vault, solar composting systems were tested in the state of Chihuahua, Mexico. The latrine consisted of a forming pile, collected below the seat, which was pulled down by the user to the composting pile. The pile, stated in the lower compartment, rests below a solar collection that is positioned over the compartment to absorb heat. The units, named Sistema Integral de Reciclamiento de Desechos Organicos (SIRDOs), were studied by Redlinger, et al. (2001) as to the suitability in meeting EPA class A and B standards in regards to fecal coliform counts and were found to very effective.
As demonstrated in “History” there are a number of current single and multi-household composting latrine designs in production and use. However, most of these systems are unavailable in developing countries, be it for economic – no market, or financial reasons. Therefore, this section will focus on the two most prevalent types of composting latrine systems found in developing countries: the Vietnamese double-vault and the Multrum single-vault (Hurtado, 2005; Mara, 1984; Nimpuno, 1977). Lastly, the particulars of the solar composting latrine will be demonstrated, as it is the author’s opinion that such a system could be more efficient and opportunistic for users.
All of the subsequent systems share many similar features. Composting latrines are typically constructed above-ground, although the pit can be partially underground. In all instances, the floor of the pit must be at least 0.75 meters above the water table (Jenkins, 2005; Del Porto & Steinfeld, 2000). The structural foundation flooring, and walls of the pit system are typically constructed of reinforced concrete, ideally sealed watertight so as to prevent the intrusion of either groundwater into the pit, or excrement and urine into the groundwater. The structure can be built with form-in-place concrete or typical concrete block, either 4 or 10 inch. These blocks must be set with and filled with mortar, so as to increase the horizontal and vertical structural integrity when stresses are placed upon the structure (Ching, & Adams, 2000). An elevated false-floor is often installed atop the floor of the latrine pit. The false floor allows for air to circulate beneath the excrement as it decomposes. Care must be taken when removing compost to ensure that the material below the false-floor is removed; else the system will become dysfunctional. In the event that the pit does accept urine, the false floor will allow for the liquid to settle out of the excrement and be removed by drain pipe that goes to a soak pit. The floor of the lavatory space is typically a poured concrete slab, while the walls of the structure can be constructed of either concrete block or wooden framing. Depending on local availability of materials, the wall covering can be concrete block (parged or unparged), concrete stucco, wood, or heavy-duty plastic or metal sheeting. An access door of some manner must be installed so as to facilitate removal of decomposed excreta upon completion of curing of the compost. The placement of the access door within the solar toilet arrangement is more specific. A pitched or sloped roof should be constructed, typically framed with wood, and sheathed in either plastic or metal panels, thatching, or wood with asphalt shingles (Ching & Adams, 2000). The compost latrine seat should be such that the user can be supported safely, and can be constructed of concrete, porcelain, or wood. Various different dimensions are used for the seat structure, depending on the local materials and building skills available. Regardless of the arrangement, a urine separation device should be installed in the toilet seat, so as to not intrude with fecal elimination. There are largely two endpoints for the diverted urine. Either the exudation is removed via piping to a soak pit aside the latrine, or it is collected directly. The urine is then diluted and often used as crop fertilizer.
Vietnamese Double Vault
The Vietnamese double vault allows for one excrement pile to be decomposing while a separate pit space is being used. The setup requires a partition wall, or at the very least, two seats or access holes. The original design utilized open pit style elimination, absent of a seat. The pit structure itself must be partitioned, so to prevent contamination from one decomposing pile to the other. The double vault will, therefore, be more expensive than a typical single-vault arrangement. However, the benefits gained include the possible doubling of production of organic compost, compared to a single vault, and the continuous use of the latrine itself.
Multrum Single Vault
The Multrum single vault has a sloped elevated false-floor that facilitates the movement of the compost down to holding area as it decomposes. The systems are designed to accept both urine and feces, or preclude the diversion of urine from the excrement. When combined, the decomposition has shown to take much longer than the Vietnamese double vault (Jenkins, 2005; Del Porto and Steinfeld, 2000). Therefore, the systems are typically well ventilated, and either have to be more highly elevated aboveground or allow for partial belowground construction.
The solar toilet operates in a manner similar to either the Multrum single or double vault, in that the excrement will slide down a sloped pit from an initial locale to a secondary elevation. At such a spot, a translucent panel will allow for the penetration of sunlight into the pit, facilitating the drying of the excrement and expediting the pathogenic die-off process. The sun-lit hatch can also be used for entry into the composting area in order to facilitate the removal of the compost. Again, an air vent is typically used to remove excess heat and nitrogen gas buildup that occurs during the oxidation process.
Operation & Maintenance
There are three particularly challenging operational processes within the compost pit latrine, all mentioned elsewhere, but summarized again here. The induction of enough oxygen, maintenance of high enough pH levels so as to reduce the number of pathogenic organisms, and the desiccation of the compost are the three primary challenges faced by users of the systems (Del Porto & Steinfeld, 2000; Jenkins, 1999; Hurtado, 2005). Others have shown that the introduction of desiccation-inducing, organic, carbon-rich materials, such as wood ash, rice husks and sawdust, have increased the porosity of compost piles, thereby inducing oxidation and also the removal of fecal and total coliform bacteria, ascia, helminths, and viral pathogens (Hurtado, 2005).
Key Schedule & Events
While mentioned in various places throughout this report, it is worthwhile to demonstrate, in an iterative process, the key schedule of events within the use of the composting latrine. Again, most of the critical points involve the biological activity or engineering design undertaken within the compost pit. The table below summarizes the processes and outcomes of the four stages of composting. Upon completion of composting, the material is of sufficient quality to be used as fertilizing material for agricultural crops.
TABLE 2: KEY EVENTS IN DECOMPOSITION OF HUMAN EXCRETA IN COMPOSTING
|Stage||Biological Processes||Typical Timeframe||Outcomes|
|Mesophilic||Mesophilic bacteria (incl. E coli) proliferate||2 day||Heat energy released|
|Thermophilic||Thermophilic bacteria proliferate||< 2 -7 day||Heat energy released|
|Cooling||Macroorganisms and fungi decompose microorganisms||1-2 months||Resistant organic materials are digested|
|Curing||Pathogen oxidation and die-off||1-2 months||Organic humus with C/N ratio between 20/1 and 35/1|
Compiled by author from Jenkins, 2005; Hurtado, 2003; Bitton, 1999; Crittenden, 2005
Supposing that the decomposition of the excreta into compost was done in sufficient manner, the compost will then be removed from the pit either manually or mechanically, collected, and used as a nutrient supplement to be mixed with topsoil upon which agricultural crops are grown.
In 2010, the United Nations issued a progress report for its Millennium Development Goals, detailing the achievements made towards eight primary goals aimed at reducing poverty, increasing access to education, improving maternal health, and developing a global partnership for development, amongst others. While there is much hope contained within the pages, and recent reports validate the findings (Economist, 2012), the UN stipulates that, with over half of the population of developing regions without sanitation, the 2015 target appears to be out of reach. The data goes on to demonstrate that between 1990 and 2008, improved sanitation coverage for the whole of the developing regions increased by only 5% in urban areas (UN, 2010). The evidence seems to suggest that there are very real constraints that are limiting individual access to improved sanitation. In 2007, Jenkins and Scott published results from a comprehensive survey that studied sanitation change adoption in Ghana. The researchers studied individual attitudinal and structural determinants for households’ decision (and non-decision) to install home toilets. The decision-making model, which drew from cognitive psychology and consumer purchase decision behavior, mapped the surveyed population into three useful categories for understanding behavior change: preference, intention, and choice. There results of 538 rural and peri-urban households from three different socio-ecological zones found that 50% of respondents used public toilets, 14% practiced open defecation, and 25.6% had adopted household or communal toilets (11% - individual household, 14.6% - communal) (Jenkins & Scott, 2007). Furthermore, amongst the non-adoptive households, 61.7% had never considered installing a household toilet, while only 5.8% expressed a high likelihood of building a toilet in the next 12 months. In assessing relevant constraints of the non-adoptive respondents a number of factors were blocking the decision to adopt. Those findings are summarized in Table 3.
TABLE 3: HOUSEHOLDS WITHOUT HOME SANITATION BY ADOPTION STAGE IN GHANA
|Decision Stage||Factors blocking decision to adopt improved sanitation|
|Preference||Lack of awareness of benefits from household toilet|
|Weak or few motivations|
|Satisfaction with existing defecation preference|
|Intention||Lack of preference|
|Lack of priority or competing priorities|
|Permanent constraints related to individual situation, including: limited space, tenancy issues, credit and savings difficulties|
|'Choice||Lake of preference and intention|
|Satisfaction with existing place|
|Temporary constraints related to opportunities: high cost, no one to build, water/soil conditions, and technical complexity|
Source: Jenkins & Scott, 2007
A relevant social factor unearthed by Jenkins and Scott was that while nearly 42% of respondents cited health as a key reason to build a household toilet, only one third of respondents believed that germs were the root cause of ill health, rather believing that the heat, smell, feces, or dirt lead to illnesses, commonly referred to as the miasma theory, was the most prevalent operating principle (ibid, p. 2435). Moreover, tenancy and space issues have been found to be major barriers that seem void of social soltuions (Moe & Rheingans, 2006; Jenkins and Scotts, 2007). The constraints discovered by Jackson and Scott became the framework for a sustainability paradigm posited by Montgomery, et al. (2010), who generally found that the practicality of sustainability rested on three major demand-responsive components: effective community demand, local financing and cost recovery, and dynamic operation and maintenance (ibid, p. 1018). While identifying many of the same obstacles facing the non-adopters of Jackson and Scott, the solutions offered are broader in scope than the concept of social marketing. While the promotion of “person-to-person behavior change messaging” can be thought of as a local form of social marketing, other solutions include access to funding and micro-credit, education, appropriate technology, and formalizing operational procedures (ibid., p. 1019). Clearly, the solution to increased usage of improved sanitation, namely pit latrines, facing a number of macro and micro-socioeconomic hurdles. There are difficult questions to be asked about how to implement solutions when issues of tenancy and ownership, opportunity costs and cost recovery, and misconstrued benefits abound. Therefore, the impact of improved composting latrines has not been as forceful as development agencies would have hoped. Serious consideration to policy options and continued focus on education may help to progress the goals set forth in the Millennium Development Goals (Jenkins, 2005).
The use of composting latrines has occurred the world over, with varying degrees of success. Much has been published in the last thirty years that documents the use of human excreta as a means of agricultural input throughout Southeast Asia. Other have reported that surveyed data suggests that over 90% of farmers in Central Vietnam used human excreta as a fertilizer; furthermore, 84% of survey respondents reported that they 80% composted their excrement in a latrine (Jensen, et al., 2008). Practices have also been observed outside of the farming population, especially in Viet Nam. (Van de Walle, 1996) have continued to see composting latrines a permanent solution to their sanitation and agroproduction needs. In 1996, 8.3% of the rural population, and 6.9% of the urban population of Viet Nam were reported to be using a double vault design. The introduction of the composting toilet in Mexico has maintained sustainability marks, and was still in use in 2005 (Hurtado, 2005). The success seen in Southeast Asia has not been universal, as has been demonstrated by cases of failed diffusion in the Philippines, Argentina and Tanzania (Pickford, 1995). Further evidence of the limited success in the diffusion of composting latrines as a permanent sanitary solution is found in analyzing the implementation of a recent community-based sanitation project in Ethiopia. From the year 2000 to 2006, nearly 22, 400 latrines were installed throughout the Northern highlands of Ethiopia, while only 4-5% of those systems were composting latrines (Loughlin, et al., 2006).
One of the more challenging aspects of the composting latrine is the need to aerate the compost, again in order to allow for the process of nitrification to occur. Some common techniques utilized to induce aeration, mentioned above, is the introduction of organic farm wastes, such as corn husks, or sawdust. A popular method utilized in commercial composting markets in the United States is the use of mechanical augers and manual mixers, or tumblers, to both aerate the compost and provide for a more homogeneous material. However, such practices have not gained widespread attention in developing countries, either for social reasons, or the design of the pit system does not allow such action to be undertaken (Nimpuno, 1977; Mara, 1984). However, the use of sun drying has somewhat alleviated the issues that could potentially arise from the lack of aeration. However, the solar toilets almost always require more space than the conventional single-vault composting latrine. This may pose a problem, especially in congested peri-urban and urban areas where available space is at a premium. With a lack of space as the principle constraint impeding the construction of a solar composting latrine, the author offers an alternative design to encourage the mixing of the compost. A manual paddle mixer with a vertical rod that protrudes through the floor of seating compartment is attached with an offset handle on the end. Upon termination, the user rotates the handle four or five times in order to mix the excrement that rests below. This way, the composting excrement is regularly mixed, allowing for moisture to be released, and oxygen be added to the composting pile. Possible shortcomings of this design may be the release of additional foul smells and odors, which could in turn exacerbate the problem of the miasma theory and lead to the non-use of the mixer. There may also be social issues revolving the handling of the instrument in such a setting, especially in instances of a communal setup. Lastly, the process relies upon the user to implement, thus should not be substituted for standard best management practices of single-vault and double-vault composting latrines.
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