What do faeces consist of

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Archaea are bacteria-like microbes that can inhabit some of the most extreme environments on Earth such as hot springs, deep sea vents or extremely acidic waters. Such methane-producing archaea like Methanobrevibacter are associated with harder stools and constipation as methane can slow down intestinal movement. It is believed there are around million archaea per gram of wet faeces. They occur in estimated concentrations of up to a million microorganisms per gram of wet faeces but comprise only a small proportion 0.

Some of the organic material includes carbohydrates or any other undigested plant matter, protein and undigested fats. Faeces does not contain large quantities of carbohydrates as the majority of what we eat is absorbed.

However, undigested amounts remain as dietary fibre. The amount of fat excreted into our stools is highly dependent on dietary intake. Even with no fat intake, though, we do get some excretion of fat into our faeces. Fat in faeces can come from bacteria in the form of short-chain fatty acids when they ferment foods, in addition to undigested dietary fat.

A recent study has found that microscopic plastic particles can appear in our faeces when we drink from plastic bottles or eat foods that have been wrapped in plastic. This small study of eight participants who were exposed to plastics in their food and drink identified up to nine different types of plastics in their stools. Chronic and acute diarrhea within populations could have a significant impact on treatment technology as feces of those with diarrhea showed increases in water content and a change in physical structure.

Global averages of diarrhea prevalence are significant in developed countries; therefore, this should be accounted for and amplified for technologies aimed at low income regions where both the chronic and acute diarrhea prevalence rates are likely to be significantly greater. In contrast to diarrhea, constipation decreases the water content of feces and is equally prevalent in the developed world.

Scales relating to the physical form of feces also provides a further estimation of the solids composition by providing approximate estimations of the TS content of feces across large sectors of populations.

Research being carried out by Wooley et al. Extremes in solids composition may cancel each other out in an averaging effect; however, thermal systems would have to be capable of dealing with this wide range and potential fluctuations in water content. The calorific value can be used as a metric of potential energy that can be produced during combustion of excreta. Human feces therefore could present an economically viable option for energy creation through combustion.

However, humans will consume a much more varied diet then animals, leading to greater deviation from median values than would be seen in manure feedstock. For example, although there is variation in the calorific value of swine manure from different sites e.

This variation is predominantly caused by the varying presence of unavailable carbohydrates in the diet, the larger the quantity of unavailable carbohydrates the higher the energy value of feces voided.

This has significance, as in lower income countries foodstuffs may often have more unavailable carbohydrates, therefore, feces of subjects in lower income countries may have fecal energy values higher than the values presented in this study suggest. The high TS concentration of feces gives a good case for the source separation of feces and urine as the addition of urine could add the further problem of dewatering and could resultantly increase costs of thermal treatment processes.

Other factors that may be significant for thermal process regard the potential emissions from any thermal treatment process. Levels of sulfur are low in feces but slightly higher levels are observed in the urine fraction, this could be significant as sulfur in oxygen starved conditions is reacted in the form H 2 S Kang et al. This review aimed to characterize feces and urine and determine the extent and causes of variation seen and its subsequent impact on technologies treating feces and urine as a fresh waste stream.

Table 12 provides a summary of the key criteria and values that will assist in not only the operation of existing OSS systems but will help advance research and development into new OSS technologies. The generation rate of feces and urine shows significant variation across a wide range of studies presenting difficulties assigning standard design values for treatment technology processes.

The values presented are based upon a large database of values from studies worldwide. The largest factor leading to variability in fecal mass is the indigestible fiber content of dietary intake; this explains the reason why fecal wet mass values were increased by a factor of 2 in low income countries.

A urine generation rate of 1. Variation in the chemical and physical composition of feces and urine was widespread throughout the study; this means that technology developments must be robust and flexible in order to deal with this uncertainty.

It can be concluded however that the composition of feces and urine is highly dependent on the dietary intake of subjects. The predominant factor leading to variation in key parameters in feces was the dietary intake of non-degradable fiber which was shown to impact production rate, stool frequency, TS, fat, protein, and the energy value of feces.

In the urine fraction, protein intake was one of the key factors leading to variation in urea concentration as well as impacting concentrations of P, K, and Ca in urine. Biological treatment processes are likely to be effective at treating feces as a waste stream and a large proportion of the feces are likely to digest readily. However, high non-degradable fiber content of feces may reduce digestibility and with a combined waste stream of feces and urine the anaerobic digestion process may be limited with potential problems such as ammonia toxicity.

Technologies based on separation will predominantly be impacted by the variation in TS concentration as well as fluctuating levels of protein and fat found within the feces. Chemical processes will be largely influenced by variation in the diet consumed by subjects, leading to fluctuations in nitrogen and phosphorus loads which could be influential on pH levels, precipitation, and nutrient recovery.

Thermal treatment processes will similarly be most influenced by variation in TS as well as the energy content of these solids, once again the intake of fiber proved most influential in predicting these factors. The source separation of feces and urine could prove beneficial for biological treatment such as anaerobic digestion where large urea concentrations in the urine stream could prove problematic and cause ammonia toxicity. Similarly, the separation of the two streams could increase the efficiency of the dewatering process and make thermal processes increasingly attractive.

In addition to this the largest proportion of nutrients e. It is therefore evident that source separation could be beneficial to many treatment technologies. This study has illustrated that there is significant variation in both the production values as well as the physicochemical composition of feces and urine. Therefore, there are limitations in using standard design values in the development of treatment technology. Consequently it is important that treatment technology is robust and flexible enough to deal with the variation exposed.

It is however possible to make more appropriate decisions about values of production and composition through the assessment of a target population's diet. Through this a range of dietary factors can be assessed in order to make more informed decisions about design values that specifically target individual populations.

Additional data, especially information regarding how the structure of feces changes over time, would be of further benefit to technology development but there is nevertheless no shortage of data regarding the production and composition of feces and urine. National Center for Biotechnology Information , U. Critical Reviews in Environmental Science and Technology. Crit Rev Environ Sci Technol. Published online May Rose , a A. Jefferson , a and E. Cartmell a. Author information Copyright and License information Disclaimer.

Address correspondence to A. E-mail: ku. Rose, A. Parker, B. Jefferson, E. The moral rights of the named author s have been asserted. This article has been cited by other articles in PMC. Abstract The safe disposal of human excreta is of paramount importance for the health and welfare of populations living in low income countries as well as the prevention of pollution to the surrounding environment.

Table 1 Measured variables for feces and urine. Open in a separate window. Table 2 The geographical location and human development index ranking of studies used in statistical analysis.

Table 3 Daily wet and dry mass produced by humans from low and high income populations. Table 4 The effect of diet type on fecal characteristics. Table 5 Daily loadings and concentrations of elements in feces wet weight. Figure 1. Outliers represent the upper value of the 75th percentile multiplied by the outlier coefficient 1. Factors Affecting Fecal Mass The major factors leading to variation in fecal generation rate are total food intake, body weight, and diet.

Figure 2. Values from 22 studies where fiber intake was recorded. Three large outliers were recorded, however, no reason could be found to exclude these results from the study. Stool Frequency Defecation frequency provides an indication for design parameters relating to treatability as it provides an indication of how often a facility may be used. Figure 3. Ranges of individuals within these studies varied from 0.

Composition Feces are composed of water, protein, undigested fats, polysaccharides, bacterial biomass, ash, and undigested food residues. Figure 4. Daily per capita weights of organic fractions excreted in feces. Lipids Fats contribute between 2. Carbohydrate and Energy Value The carbohydrate fraction is largely made up of undigested cellulose, vegetable fibers, and pentosan Canfield et al.

Inorganic Composition The remaining solids compose the inorganic fraction which is predominantly made up of calcium phosphate and iron phosphate, intestinal secretions, small amounts of dried constituents of digestive juices such as shredded epithelial cells and mucus Guyton and Hall, ; Iyengar et al.

Chemical Nature Fecal pH is neutral with a median value of pH 6. Figure 5. Physical Form For the development of onsite treatment technologies an understanding of the physical form of feces is important; this characterization can be done through the use of visual scales or prevalence rates of diarrhea and constipation. Visual Scale Within the medical literature a number of linear scales have been used to characterize feces e. Figure 6. Data from two separate studies of healthy subjects Heaton et al.

Stool types 3 and 4 make up the most common stool type in both studies, however all types of stool are recorded in both studies. Diarrhea Diarrhea has an impact on stool production, structure, form, and composition. Table 7 Diarrhea prevalence in a selection of six countries. Constipation Constipation has prevalence rates that can range from 1.

Urine In contrast to feces, the characteristics of urine have been studied extensively Diem and Lentner, ; Kirchmann and Pettersson, ; Karak and Bhattacharyya, Liquid Generation Human urine is a liquid that is secreted by the kidneys, collected within the bladder and excreted through the urethra. Composition Urine composition varies due to differences in physical exercise, environmental conditions, as well as water, salt, and high protein intakes.

Chemical Nature The pH of fresh urine is largely neutral with a median of pH 6. Table 9 Concentration of key components in fresh urine. Table 10 Components and generation rate of human excreta waste streams and possible additional inputs.

Table 11 Classifications of broad treatment pathways in wastewater treatment. Biological Processes The predominant factors likely to impact biological processes to the greatest extent are solids loading, energy content, protein, and fat concentration in the feces and the high urea concentrations in urine. Physical Separators There are numerous different types of separating technologies; however, the majority are likely to be predominantly influenced by variation in the solids content, physical form, as well as levels of protein and fat in feces.

Chemical Processes Chemical treatment processes can be wide ranging and are dependent on the end use and initial purpose of treatment and include processes such as chemical precipitation, disinfection, oxidation, neutralization, and stabilization. Thermal Processes Efficient thermal technologies have been the focus of much development because of their potential for energy saving and cost recovery. Table 12 Summary table of feces and urine characteristics providing on-site sanitation design criteria.

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