Reuse of excreta
Reuse of excreta refers to the safe, beneficial use of animal or human excreta, i.e. feces and urine. Such beneficial use involves mainly the nutrient, organic matter and energy contained in human excreta, rather than the water content. Reuse of human excreta can involve using it as a 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.
Human 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 human excreta or in domestic wastewater 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 several "excreta-derived fertilizers" which vary in their properties and fertilizing characteristics: urine, dried feces, composted feces, faecal sludge, sewage, sewage sludge, and animal manure. However, reuse of animal and human excreta can cause health risk and environmental problems because of pathogen, pharmaceutical residues and nitrate pollution.
Reuse of human excreta is the final step of the sanitation chain which starts with collection of human excreta and continues with transport and treatment all the way to either disposal or reuse.
Background
systems that are designed for safe and 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 human excreta include water, plant nutrients, organic matter and energy content. Reuse of human excreta focuses on the nutrient and organic matter content of human 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 human excreta" rather than "reuse" as strictly speaking it is the first use of human excreta, not the second time that it is used.
It can be efficient to combine wastewater and human excreta with other organic waste such as manure, food and crop waste for the purposes of resource recovery.
Reuse options
Reuse options depend on the form of the excreta that is being reused: it can be either excreta on its own or mixed with some water or mixed with lots of water.The most common types of excreta reuse include:
- Fertilizer and irrigation water in agriculture, and horticulture: for example using recovered and treated water for irrigation; using composted excreta or appropriately treated biosolids as fertilizer and soil conditioner; using treated source-separated urine as fertilizer.
- Energy: for example digesting feces and other organic waste to produce biogas; or producing combustible fuels.
- Other: other emerging excreta reuse options include producing protein feeds for livestock using black soldier fly larvae, recovering organic matter for use as building materials or in paper production.
Multiple barrier concept for safe use in agriculture
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 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 of 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 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.
Comparison to other fertilizers
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. Therefore, reuse can support increased food production and also provide an alternative to chemical fertilizers, which is often unaffordable to small-holder farmers. However, nutritional value of human excreta largely depends on dietary input.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 etc. Thus, they exhibit a slow-release effect and are usually less rapidly leached compared to mineral fertilizers.
Peak phosphorus
In the case of phosphorus in particular, reuse of excreta is one known method to recover phosphorus to mitigate the looming shortage 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.As fertilizer
Urine
is primarily composed of water and urea. It contains large quantities of nitrogen , 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's eight main ionic species are cations Na, K, NH4, Ca and the anions, Cl, SO4, PO4 and HCO3. 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 odor 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, 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. Urine may contain pharmaceutical residues.
The more general limitations to using urine as fertilizer depend mainly on the potential for buildup of excess nitrogen, 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. Important parameters to consider while fertilizing with urine include salinity tolerance of the plant, soil composition, addition of other fertilizing compounds, and quantity of rainfall or other irrigation.
It was reported in 1995 that urine nitrogen gaseous losses were relatively high and plant uptake lower than with labelled ammonium nitrate. In contrast, phosphorus was utilized at a higher rate than soluble phosphate.
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 by using waterless urinals, urine-diverting dry toilets 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 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. There are increasing calls for urine's use as a fertilizer.
Since about 2011, the Bill and Melinda Gates Foundation is providing research funding sanitation systems that recover the nutrients in urine.
Examples
- In Tororo District in eastern Uganda - a region with severe land degradation problems - smallholder farmers appreciated urine fertilization as a low-cost, low-risk practice. They found that it could contribute to significant yield increases. The importance of social norms and cultural perceptions needs to be recognized but is not absolute barriers to adoption of the practice.
Dried feces
Composted feces
derived from composting toilets has, in principle, 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.
The nutrients in compost from a composting toilet have a higher plant availability than the product from a urine-diverting dry toilet.
Fecal sludge
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 dewatering, thickening, drying, composting, pelletization, and anaerobic digestion.Municipal wastewater
can be reused for irrigation, industrial uses, replenishing natural water courses, water bodies, 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 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.
Sewage sludge
The use of treated sewage sludge as a soil conditioner or fertilizer is possible but is a controversial topic in some countries 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
Sludge treatment liquids 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 manure
Animal dung has been used for centuries as a fertilizer for farming, as it improves the soil structure, 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.Health and environmental aspects of agricultural use
Pathogens
Risk management
Exposure of farm workers to untreated excreta constitutes a significant health risk due to its pathogen content. There can be a large amount of enteric bacteria, virus, protozoa, and helminth eggs in feces. This risk also extends to consumers of crops fertilized with untreated excreta. Therefore, excreta needs 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.Treatment of excreta for pathogen removal
The treatment of excreta and wastewater for pathogen removal can take place:- at the toilet itself ; or
- at a semi-centralized level ; or
- at a fully centralized level at sewage treatment plants and sewage sludge treatment plants.
Indicator organisms
Pharmaceutical residues
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. In fact, the pharmaceutical residues in the excreta are degraded better in terrestrial systems than in aquatic systems.Nitrate pollution
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 in groundwater can cause 'blue baby syndrome'. 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.Other uses
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, fodder, fish food, building materials and biofuels.Solid fuel, heat, electricity
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.
Dried sewage sludge can be burned in sludge incineration plants and generate heat and electricity.
Resource recovery of fecal sludge as a solid fuel has been found to have high market potential in Sub-Saharan Africa.