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==level 1== INTRODUCTION 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). ==level 1== 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. ==level2== CURRENT USES 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. ==level 1==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. ==level 2==Scientific Theory 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). Pathogen Destruction 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).