Reuse of human excreta can be safe or unsafe. Beneficial uses of the treated excreta may focus on using the plant-available nutrients (mainly nitrogen, phosphorus and potassium) that are contained in the treated excreta. They may also make use of the organic matter and energy contained in the excreta. To a lesser extent, reuse of the excreta's water content might also take place, although this is better known as water reclamation from municipal wastewater. The intended reuse applications for the nutrient content may include: soil conditioner or fertilizer in agriculture or horticultural activities. Other reuse applications, which focus more on the organic matter content of the excreta, include use as a fuel source or as an energy source in the form of biogas.
There is a large and growing number of treatment options to make excreta safe and manageable for the intended reuse option. Options include urine diversion and dehydration of feces (urine-diverting dry toilets), composting (composting toilets or external composting processes), sewage sludge treatment technologies and a range of fecal sludge treatment processes. They all achieve various degrees of pathogen removal and reduction in water content for easier handling. Pathogens of concern are enteric bacteria, virus, protozoa, and helminth eggs in feces. As the helminth eggs are the pathogens that are the most difficult to destroy with treatment processes, they are commonly used as an indicator organism in reuse schemes. Other health risks and environmental pollution aspects that need to be considered include spreading micropollutants, pharmaceutical residues and nitrate in the environment which could cause groundwater pollution and thus potentially affect drinking water quality.
There are several "human excreta derived fertilizers" which vary in their properties and fertilizing characteristics, for example: urine, dried feces, composted feces, fecal sludge, sewage, sewage sludge.
The nutrients and organic matter which are contained in human excreta or in domestic wastewater (sewage) have been used in agriculture in many countries for centuries. However, this practice is often carried out in an unregulated and unsafe manner in developing countries. World Health Organization Guidelines from 2006 have set up a framework describing how this reuse can be done safely by following a "multiple barrier approach". Such barriers might be selecting a suitable crop, farming methods, methods of applying the fertilizer and education of the farmers.
Terminology Human excreta, fecal sludge and wastewater are often referred to as wastes (see also human waste). Within the concept of a circular economy in sanitation, an alternative term that is being used is "resource flows". The final outputs from the sanitation treatment systems can be called "reuse products" or "other outputs". These reuse products are general fertilizers, soil conditioners, biomass, water, or energy.
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. An alternative term is "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.
Technologies and approaches
The resources available in wastewater and human excreta include water, plant nutrients, organic matter and energy content. Sanitation systems that are designed for safe and effective recovery of resources can play an important role in a community's overall resource management.
Recovering the resources embedded in excreta and wastewater (like nutrients, water and energy) contributes to achieving Sustainable Development Goal 6 and other sustainable development goals.
It can be efficient to combine wastewater and human excreta with other organic waste such as manure, and food and crop waste for the purposes of resource recovery.Andersson, K., Rosemarin, A., Lamizana, B., Kvarnström, E., McConville, J., Seidu, R., Dickin, S. and Trimmer, C. (2016). Sanitation, Wastewater Management and Sustainability: from Waste Disposal to Resource Recovery. Nairobi and Stockholm: United Nations Environment Programme and Stockholm Environment Institute.
Treatment options There is a large and growing number of treatment options to make excreta safe and manageable for the intended reuse option. 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. Some treatment options are listed below but there are many more: Urine diversion and dehydration of feces (which is done with urine-diverting dry toilets) Composting (composting toilets or external composting processes) Sewage sludge treatment technologies, which is installed downstream of various wastewater treatment technologies Fecal sludge treatment processes, such as sludge drying beds, constructed wetlands. Anaerobic digestion with biogas production Waste-to-energy process Omni processor
A guide by the Swedish University of Agricultural Sciences provides a list of treatment technologies for sanitation resource recovery: Vermicomposting and vermifiltration, black soldier fly composting, algae cultivation, microbial fuel cell, nitrification and distillation of urine, struvite precipitation, incineration, carbonization, solar drying, membranes, filters, alkaline dehydration of urine, ammonia sanitization/urea treatment, and lime sanitization.McConville, J., Niwagaba, C., Nordin, A., Ahlström, M., Namboozo, V. and Kiffe, M. (2020). Guide to Sanitation Resource-Recovery Products & Technologies: A supplement to the Compendium of Sanitation Systems and Technologies. 1st Edition. Swedish University of Agricultural Sciences (SLU), Department of Energy and Technology, Uppsala, Sweden. Further research involves UV advanced oxidation processes in order to degrade organic pollutants present in the urine before reuse or the dehydration of urine by using acids.
Reuse options The most common 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.
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 (fecal sludge) or mixed with much water (domestic wastewater or sewage).
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 (and other organic waste) 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. Nature-based sewage treatment method using aquatic insect life cycle to produce nutrient rich insect biomass for using as fish/poultry meal, and bio-carbon dioxide, ammonia and methane gases. 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.
Resource recovery from fecal sludge can take many forms, including as a fuel, soil amendment, building material, protein, animal fodder, and water for irrigation.
Reuse products that can be recovered from sanitation systems include: Stored urine, concentrated urine, sanitized blackwater, digestate, nutrient solutions, dry urine, struvite, dried feces, pit humus, dewatered sludge, compost, ash from sludge, biochar, nutrient-enriched filter material, algae, macrophytes, black soldier fly larvae, worms, irrigation water, aquaculture, and biogas.
As fertilizer
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.Kratz, S. (2004) Uran in Düngemitteln (in German). Uran-Umwelt-Unbehagen: Statusseminar am 14. Oktober 2004, Bundesforschungsinstitut für Landwirtschaft (FAL), Institut für Pflanzenernährung und Bodenkunde, Germany. This does not apply to excreta-based fertilizers (unless the human's food was contaminated beyond safe limits to start with), which is an advantage.
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.
Urine
Urine contains large quantities of nitrogen (mostly as urea), as well as reasonable quantities of dissolved potassium. The nutrient concentrations in urine vary with diet.Joensson, H., Richert Stintzing, A., Vinneras, B., Salomon, E. (2004). Guidelines on the Use of Urine and Faeces in Crop Production. Stockholm Environment Institute, Sweden In particular, the nitrogen content in urine is related to quantity of protein in the diet: A high protein diet results in high urea levels in urine. The nitrogen content in urine is proportional to the total food protein in the person's diet, and the phosphorus content is proportional to the sum of total food protein and vegetal food protein. Urine's eight main ionic species (> 0.1 meq L−1) 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. The amount of urine produced by an adult is around 0.8 to 1.5 L per day.
Applying urine as fertilizer has been called "closing the cycle of agricultural nutrient flows" or ecological sanitation or ecosan. Urine fertilizer is usually applied diluted with water because undiluted urine can chemically burn the leaves or roots of some plants, causing plant injury, particularly if the soil moisture content is low. The dilution also helps to reduce odor development following 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 nitrogen fertilizers. Urine may contain pharmaceutical residues (environmental persistent pharmaceutical pollutants). Concentrations of heavy metals such as lead, mercury, and cadmium, commonly found in sewage sludge, are much lower in urine.