The following is from Closing the loop in wastewater management and sanitation (source)
Resource value of human excreta
Human excreta are comprised of two basic components, urine and faeces. When urine and faeces are kept apart, they have different properties, are produced in different quantities, and require different care in processing. Published figures indicate that more than 1 kg of urine is produced daily, while less than 150 g of faeces, including moisture, is produced daily.11 These figures, of course, vary by type of diet, location, age, activity and health status.
Urine contains nearly 80% of the total nitrogen found in excreta (Table 1). Urine also contains two-thirds of the excreted phosphorous and potassium. The majority of the carbon excreted, up to 70%, is found in faeces. The quantities shown above may suggest that excreta contain few nutrients. Each person urinates annually about 4 kg of nitrogen, 0.4 kg of phosphorous, and nearly 1 kg of potassium; total excretion is 4.5 kg of nitrogen, more than a half kg of phosphorous, and 1.2 kg of potassium. In an urban setting of 10 million people, this equates to 45 million kg of nitrogen, nearly 6 million kg of phosphorous, and more than 12 million kg of potassium. It also represents 10 million litres of nutrient rich and mostly sterile water that is excreted. The water that is not flushed by 10 million people equates to 0.15 km3 of water saved by using ecological sanitation, fresh water that could be used for other purposes, such as food production, without risk of infection. Other elements, such as calcium and magnesium, are excreted in nearly equal amounts in urine and faeces. There are many other nutrients found in human excreta, but they are not shown above. Although using only urine is valuable, both urine and faeces should be recovered and recycled to avoid long term depletion of soils.
|Elements (g/ppd)||Urine||Faeces||Urine + faeces|
Globally, 2 billion hectares have been degraded since World War II, 23% of globally used land.13 If only agriculture land is considered, 38% is degraded. Most of the degradation had occurred in Asia, Africa and South and Central America. The two main causes of degradation are loss of topsoil from water erosion and fertility decline. In Africa alone, 8 million tons of nutrients are lost every year, representing US$ 1.5 billion per year.14 Annual depletion of NPK (N+P2O5+K2O) per hectare from African soils varies from less than 30 kg/hectare to more than 60 kg/hectare. The excreta from 10 people during the course of a year could return more than 60 kg/hectare to soil, restoring fertility. The effects of soil degradation and loss of fertility on food consumption, agriculture income and national wealth are significant.
Failure to restore soil fertility over the last several decades has been speculated as a cause of reduced nutrient content of North American and British foods. A recent sampling of foods showed 20-40% less calcium, iron, and Vitamins A and C than was the case several decades ago.15 Exactly why this is occurring is not known, but modern agricultural methods do return all nutrients it takes from the land. Conventional agricultural practices consider soil a way station for nutrient uptake by plants, not a viable living organism where plants grow and thrive. Recycling a whole range of nutrients, as well as organic carbon, to the land is needed for a healthy, balanced soil.