Reuse of human excreta

Reuse of excreta (alternative spelling: re-use) refers to the safe use of human excreta or animal excreta in agriculture, gardening or aquaculture. Some sources call this practice "use of excreta" rather than "reuse" as strictly speaking it is the first use of excreta, not a second use. Reuse of excreta is one example of resource recovery of the resources contained in excreta, mainly the plant-available nutrients nitrogen, phosphorus, potassium as well as micronutrients such as sulphur and organic matter. It is one of the options pursued as part of an ecological sanitation concept.

Human excreta

Research into how to make reuse of urine and faeces safe in agriculture was carried out in Sweden since the 1990s. In 2006 the World Health Organisation (WHO) provided guidelines on safe reuse of wastewater, excreta and greywater. The multiple barrier concept to reuse, which is the key cornerstone of this publication, has led to a clear understanding on how excreta reuse can be done safely.

Urine

Urine contains large quantities of nitrogen (mostly as urea), as well as significant quantities of dissolved phosphates and potassium, the main macronutrients required by plants, with urine having plant macronutrient percentages (i.e. NPK) of approximately 11-1-2 by one study or 15-1-2 by another report, illustrating that exact composition varies with diet.

Undiluted urine can chemically burn the roots of some plants which is way it is usually applied diluted with water, which also reduces odour development during application. When diluted with water (at a 1:5 ratio for container-grown annual crops with fresh growing medium each season, or a 1:8 ratio for more general use), it can be applied directly to soil as a fertilizer. The fertilization effect of urine has been found to be comparable to that of commercial fertilizers with an equivalent NPK rating. Urine contains most (94% according to Wolgast) of the NPK nutrients excreted by the human body. Conversely, concentrations of heavy metals such as lead, mercury, and cadmium, commonly found in solid human waste, are much lower in urine (though not low enough to qualify for use in organic agriculture under current EU rules). The more general limitations to using urine as fertilizer then depend mainly on the potential for buildup of excess nitrogen (due to the high ratio of that macronutrient), and inorganic salts such as sodium chloride, which are also part of the wastes excreted by the renal system. The degree to which these factors impact the effectiveness depends on the term of use, salinity tolerance of the plant, soil composition, addition of other fertilizing compounds, and quantity of rainfall or other irrigation.

Urine can also be used safely as a source of complementary nitrogen in carbon-rich compost.

Urine typically contains 70% of the nitrogen and more than half the phosphorus and potassium found in urban waste water flows, while making up less than 1% of the overall volume. Human urine can be collected with sanitation systems that utilise urinals or urine diversion toilets. Thus far, source separation, or urine diversion systems have been implemented in South Africa, China, Sweden and many other countries. Since about 2011, the Bill and Melinda Gates Foundation is providing research funding sanitation systems that recover the nutrients in urine.

"Urine management" is a relatively new way of closing the cycle of agricultural nutrient flows (also called ecological sanitation or ecosan) and - possibly - reducing sewage treatment costs and ecological consequences such as eutrophication resulting from the influx of nutrient rich effluent into aquatic or marine ecosystems. The risks of using urine as a natural source of agricultural fertilizer are generally regarded as negligible or acceptable. In fact, some research has shown that there are more environmental problems when urine is treated (as part of sewage in wastewater treatment plants) and disposed of compared with when it is used as a resource.

It is unclear whether source separation, urine diversion, and on-site urine treatment can be made cost effective; nor whether required behavioral changes would be regarded as socially acceptable, as the largely successful trials performed in Sweden may not readily generalize to other industrialized societies. In developing countries the use of whole raw sewage (night soil) has been common throughout history, yet the application of pure urine to crops is rare. Increasingly there are calls for urine's use as a fertilizer, such as a Scientific American article "Human urine is an effective fertilizer".

If urine is to be collected for use as a fertiliser in agriculture, then the easiest method of doing so is (in increasing order of costs) by using waterless urinals, urine-diverting dry toilets (UDDTs) or urine diversion flush toilets.

Composted faeces

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Faecal sludge == Faecal sludge (also called septage) is defined as "coming from onsite sanitation technologies, and has not been transported through a sewer." Examples of onsite technologies include pit latrines, unsewered public ablution blocks, septic tanks and dry toilets. Faecal sludge can be treated by dewatering, thickening, drying and composting and used as fertiliser.

Animal manure

Animal dung has been used for centuries as a fertilizer for farming, as it improves the soil structure (aggregation), so that it holds more nutrients and water, and becomes more fertile. Animal manure also encourages soil microbial activity, which promotes the soil's trace mineral supply, improving plant nutrition. It also contains some nitrogen and other nutrients that assist the growth of plants.

Manures with a particularly unpleasant odor (such as human sewage or slurry from intensive pig farming) are usually knifed (injected) directly into the soil to reduce release of the odor. Manure from pigs and cattle is usually spread on fields using a manure spreader. Due to the relatively lower level of proteins in vegetable matter, herbivore manure has a milder smell than the dung of carnivores or omnivores. However, herbivore slurry that has undergone anaerobic fermentation may develop more unpleasant odors, and this can be a problem in some agricultural regions. Poultry droppings are harmful to plants when fresh but, after a period of composting, are valuable fertilizers.

Manure is also commercially composted and bagged and sold retail as a soil amendment. Sometimes even human sewage sludge is used, as is the case for Dillo Dirt, a product that has been sold by the city of Austin, Texas municipal wastewater department since 1989.