Reuse of excreta (or re-use or use of excreta) refers to the safe, beneficial use of animal or human excreta, i.e. feces (or faeces in British English) and urine. Such beneficial use involves mainly the nutrient, organic matter and energy contained in excreta, rather than the water content (as is the case for wastewater reuse). Reuse of excreta can involve using it as soil conditioner or fertilizer in agriculture, gardening, aquaculture or horticultural activities. Excreta can also be used as a fuel source or as a building material.
Excreta contains resources that can be recovered: plant-available nutrients nitrogen, phosphorus, potassium as well as micronutrients such as sulphur and organic matter. These resources which are contained in excreta or in domestic wastewater (sewage) have traditionally been used in agriculture in many countries. They are still being used in agriculture to this day, but the practice is often carried out in an unregulated and unsafe manner in developing countries. The WHO Guidelines from 2006 have set up a framework how this reuse can be done safely by following a "multiple barrier approach".
There are a number of "excreta-derived fertilizers" which vary in their properties and fertilizing characteristics: urine, dried feces, composted feces, fecal sludge (septage), sewage, sewage sludge and animal manure.
Reuse of excreta is the final step of the sanitation chain which starts with collection of excreta (by use of toilets) and continues with transport and treatment all the way to either disposal or reuse.
Sanitation systems that are designed for safe, effective recovery of resources can play an important role in a community’s overall resource management. Various technologies and practices, ranging in scale from a single rural household to a city, can be used to capture potentially valuable resources and make them available for safe, productive uses that support human well-being and broader sustainability.
The resources available in wastewater and excreta include water, plant nutrients, organic matter and energy content. Reuse of excreta focuses on the nutrient and organic matter content of excreta unlike reuse of wastewater which focuses on the water content.
The most common type of reuse of excreta is as fertilizer and soil conditioner in agriculture. This is also called a "closing the loop" approach for sanitation with agriculture. It is a central aspect of the ecological sanitation approach. An alternative term is also "use of excreta" rather than "reuse" as strictly speaking it is the first use of excreta, not the second time that it is used.
Research into how to make reuse of urine and feces safe in agriculture was carried out in Sweden since the 1990s. In 2006 the World Health Organization (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. The concept is also used in water supply and food production and is generally understood as a series of treatment steps and other safety precautions to prevent the spread of pathogens.
The degree of treatment required for excreta-based fertilizers before they can safely be used in agriculture depends on a number of factors. It mainly depends on which other barriers will be put in place according to the multiple barrier concept. Such barriers might be selecting a suitable crop, farming methods, methods of applying the fertilizer, education and so forth.
For example, in the case of urine-diverting dry toilets (UDDTs) secondary treatment of dried feces can be performed at community level rather than at household level and can include thermophilic composting where fecal material is composted at over 50 °C, prolonged storage with the duration of 1.5 to two years, chemical treatment with ammonia from urine to inactivate the pathogens, solar sanitation for further drying or heat treatment to eliminate pathogens.
There is an untapped fertilizer resource in human excreta. In Africa, for example, the theoretical quantities of nutrients that can be recovered from human excreta are comparable with all current fertilizer use on the continent.:16 Reuse can therefore support increased food production and also provide an alternative to chemical fertilizers, which is often unaffordable to small-holder farmers.
Mineral fertilizers are made from mining activities and can contain heavy metals. Phosphate ores contain heavy metals such as cadmium and uranium, which can reach the food chain via mineral phosphate fertilizer. This does not apply to excreta-based fertilizers which is an advantage.
In intensive agricultural land use, animal manure is often not used as targeted as mineral fertilizers and thus the nitrogen utilization efficiency is poor. Animal manure can become a problem in terms of excessive use in areas of intensive agriculture with high numbers of livestock and too little available farmland.
Fertilizing elements of organic fertilizers are mostly bound in carbonaceous reduced compounds. If these are already partially oxidized as in the compost, the fertilizing minerals are adsorbed on the degradation products (humic acids) etc. Thus, they exhibit a slow-release effect and are usually less rapidly leached compared to mineral fertilizers.
In the case of phosphorus in particular, reuse of excreta is one known method to recover phosphorus to mitigate the looming shortage (also known as "peak phosphorus") of economical mined phosphorus. Mined phosphorus is a limited resource that is being used up for fertilizer production at an ever-increasing rate, which is threatening worldwide food security. Therefore, phosphorus from excreta-based fertilizers is an interesting alternative to fertilizers containing mined phosphate ore.
Urine is primarily composed of water and urea. It contains large quantities of nitrogen (mostly as urea), as well as reasonable quantities of dissolved potassium. The nutrient concentrations in urine vary with diet. In particular nitrogen content in urine is related to quantity of protein in the diet. A high protein diet results in high urea levels in urine. Urine typically contains 70% of the nitrogen and more than half the potassium found in sewage, while making up less than 1% of the overall volume.
Urine fertilizer is usually applied diluted with water because undiluted urine can chemically burn the leaves or roots of some plants, particularly if the soil moisture content is low. The dilution also helps to reduce odour development following application. Applying urine as fertilizer has been called "closing the cycle of agricultural nutrient flows" or ecological sanitation or ecosan.
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 nitrogen fertilizers. Concentrations of heavy metals such as lead, mercury, and cadmium, commonly found in sewage sludge, which is also used as fertilizer, are much lower in urine. Urine may contain pharmaceutical residues (environmental persistent pharmaceutical pollutants).
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. Over-fertilization with urine or other nitrogen fertilizers can result in too much ammonia for plants to absorb, acidic conditions, or other phytotoxicity. 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.
Human urine can be collected with sanitation systems that utilize urinals or urine diversion toilets. If urine is to be collected for use as a fertilizer 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.
The risks of using urine as a natural source of agricultural fertilizer are generally regarded as negligible or acceptable. There are potentially more environmental problems (such as eutrophication resulting from the influx of nutrient rich effluent into aquatic or marine ecosystems) and a higher energy consumption when urine is treated as part of sewage in wastewater treatment plants compared with when it is used directly as a fertilizer resource.
In developing countries the use of raw sewage or fecal sludge 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.
Reuse of dried feces (feces) from urine-diverting dry toilets (UDDTs) after post-treatment can result in increased crop production through fertilizing effects of nitrogen, phosphorus, potassium and improved soil fertility through organic carbon.
Compost derived from composting toilets (where organic kitchen waste is in some cases also added to the composting toilet), has in principal the same uses as compost derived from other organic waste products, such as sewage sludge or municipal organic waste. One limiting factor may be legal restrictions due to the possibility that pathogens remain in the compost. In any case, the use of compost from composting toilets in one's own garden can be regarded as safe and is the main method of use for compost from composting toilets. Hygienic measures for handling of the compost must be applied by all those people who are exposed to it, e.g. wearing gloves and boots.
Some of the urine will be part of the compost, although some urine will be lost via leachate and evaporation. Urine can contain up to 90 percent of the nitrogen, up to 50 percent of the phosphorus, and up to 70 percent of the potassium present in human excreta.
Fecal 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. Fecal sludge can be treated by a variety of methods to render it suitable for reuse in agriculture. These include (usually carried out in combination): dewatering, thickening, drying (in sludge drying beds), composting, pelletization, anaerobic digestion.
Reclaimed water can be reused for irrigation, industrial uses, replenishing natural water courses, water bodies and aquifers and other potable and non-potable uses. These applications however focus usually on the water aspect, not on the nutrients and organic matter reuse aspect, which is the focus of "reuse of excreta".
When wastewater is reused in agriculture, its nutrient (nitrogen and phosphorus) content may be useful for additional fertilizer application. Work by the International Water Management Institute and others has led to guidelines on how reuse of municipal wastewater in agriculture for irrigation and fertilizer application can be safely implemented in low income countries.
The use of treated sewage sludge (after treatment also called "biosolids") as a soil conditioner or fertilizer is possible but is a controversial topic in some countries (such as USA, some countries in Europe) due to the chemical pollutants it may contain, such as heavy metals and environmental persistent pharmaceutical pollutants.
Northumbrian Water in the United Kingdom uses two biogas plants to produce what the company calls "poo power" - using sewage sludge to produce energy to generate income. Biogas production has reduced its pre 1996 electricity expenditure of 20 million GBP by about 20% . Severn Trent and Wessex Water also have similar projects.
Sludge treatment liquids (after anaerobic digestion) can be used as an input source for a process to recover phosphorus in the form of struvite for use as fertilizer. For example, the Canadian company Ostara Nutrient Recovery Technologies is marketing a process based on controlled chemical precipitation of phosphorus in a fluidized bed reactor that recovers struvite in the form of crystalline pellets from sludge dewatering streams. The resulting crystalline product is sold to the agriculture, turf and ornamental plants sectors as fertilizer under the registered trade name "Crystal Green".
Animal dung (manure) 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 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.
Exposure of farm workers to untreated excreta constitutes a significant health risk due to its pathogen content. This risk also extends to consumers of crops fertilized with untreated excreta. Therefore, excreta need to be appropriately treated before reuse, and health aspects need to be managed for all reuse applications as the excreta can contain pathogens even after treatment.
The treatment of excreta and wastewater for pathogen removal can take place:
As an indicator organism in reuse schemes, helminth eggs are commonly used as these organisms are the most difficult to destroy in most treatment processes. The multiple barrier approach is recommended where e.g. lower levels of treatment may be acceptable when combined with other post-treatment barriers along the sanitation chain.
Excreta from humans and farmed animals contain hormones and pharmaceutical residues which could in theory enter the food chain via fertilized crops but are currently not fully removed by conventional wastewater treatment plants anyway and can enter drinking water sources via household wastewater (sewage). In fact, the pharmaceutical residues in the excreta are degraded better in terrestrial systems (soil) than in aquatic systems.
Only a fraction of the nitrogen-based fertilizers is converted to produce and other plant matter. The remainder accumulates in the soil or lost as run-off. This also applies to excreta-based fertilizer since it also contains nitrogen. Excessive nitrogen which is not taken up by plants is transformed into nitrate which is easily leached. High application rates combined with the high water-solubility of nitrate leads to increased runoff into surface water as well as leaching into groundwater. Nitrate levels above 10 mg/L (10 ppm) in groundwater can cause 'blue baby syndrome' (acquired methemoglobinemia). The nutrients, especially nitrates, in fertilizers can cause problems for natural habitats and for human health if they are washed off soil into watercourses or leached through soil into groundwater.
Apart from use in agriculture, there are other possible uses of excreta. For example, in the case of fecal sludge, it can be treated and then serve as protein (black soldier fly process), fodder, fish food, building materials and biofuels (biogas from anaerobic digestion, incineration or co-combustion of dried sludge, pyrolysis of fecal sludge, biodiesel from fecal sludge).
Dry animal dung is used as a fuel in many countries around the world.
Pilot scale research in Uganda and Senegal has shown that it is viable to use dry feces as for combustion in industry, provided it has been dried to a minimum of 28% dry solids.
Urine has also been investigated as a potential source of hydrogen. Urine has more hydrogen molecules than water and is less tightly bonded to other molecules. When electric current is applied to it, more hydrogen is produced at the cost of less energy.
Small-scale biogas plants are being utilized in many countries, including Ghana, Vietnam and many others. Larger centralized systems are being planned that mix animal and human feces to produce biogas. Biogas is also produced in the large anaerobic digesters of centralized sewage treatment plants that process wastewater and sewage sludge. The biogas produced can be used for heating the digesters and for generating electricity.
Pilot facilities are being developed for feeding Black Soldier Fly larvae with feces. The mature flies would then be a source of protein to be included in the production of feed for chickens in South Africa.
It is known that additions of fecal matter up to 20% by dried weight in clay bricks does not make a significant functional difference to bricks.
A Japanese sewage treatment facility extracts precious metals from sewage sludge. This idea was also tested by the US Geological Survey (USGS) which found that the sewage sludge generated by 1 million people contained 13 million dollars worth of precious metals.
Debate is ongoing about whether reuse of excreta is cost effective. The terms "sanitation economy" and "toilet resources" have been introduced to describe the potential for selling products made from human feces or urine.
The NGO SOIL in Haiti began building urine-diverting dry toilets and composting the waste produced for agricultural use in 2006. SOIL’s two composting waste treatment facilities currently transform over 20,000 gallons (75,708 liters) of human excreta into organic, agricultural-grade compost every month. The compost produced at these facilities is sold to farmers, organizations, businesses, and institutions around the country to help finance SOIL’s waste treatment operations. Crops grown with this soil amendment include spinach, peppers, sorghum, maize, and more. Each batch of compost produced is tested for the indicator organism E. coli to ensure that complete pathogen kill has taken place during the thermophilic composting process.
There is still a lack of examples of implemented policy where the reuse aspect is fully integrated in policy and advocacy. When considering drivers for policy change in this respect, the following lessons learned should be taken into consideration: Revising legislation does not necessarily lead to functioning reuse systems; it is important to describe the “institutional landscape” and involve all actors; parallel processes should be initiated at all levels of government i.e. national, regional and local level); country specific strategies and approaches are needed; and strategies supporting newly developed policies need to be developed.
Regulations relating to excreta reuse may hinder export and import of agricultural products.
The European Union (EU) only allows the use of source separated urine in conventional farming within the EU, but not yet in organic farming. This is a situation that many agricultural experts, especially in Sweden, would like to see changed. This ban may also reduce the options to use urine as a fertilizer in other countries if they wish to export their products to the EU.
In the United States, the EPA regulation governs the management of sewage sludge but has no jurisdiction over the byproducts of a urine-diversion dry toilet (UDDT). Oversight of these materials falls to the states.
The reuse of excreta as a fertilizer for growing crops has been practiced in many countries for a long time.