Urine: Closing the NPK Loop

The Stockholm Environment Institute conducted experiments and collected data that shows the usefulness of a resource every one of us has access to — urine. When utilized as a fertilizer, urine can provide an alternative to chemical fertilizers. The impacts ripple far beyond the nutrient value of the urine; in developing regions, diverting a urine waste stream to fertilizer has a significant economic value. These benefits can easily be recognized at the individual level, and scale all the way up to industrial operations.

Nutrient value

95% of the 0.8-1.5L of urine each person produces per day is water, but the last 5% is comprised of both the macro-nutrients all gardeners are familiar with — Nitrogen (N), Phosphorous (P), and Potassium (K) — as well as some trace micro-nutrients. While the actual content will vary slightly depending on your diet, your urine is generally a well balanced nitrogen rich fertilizer straight out of your body. The average person produces enough urine per year to cover 300-400 m2 of land to a level of 50-100kg/ha of nitrogen.

Some of the yearly values of the nutrients are:

  • 3.5kg of nitrogen
  • 0.5kg phosphorus
  • 1.0kg potassium
  • 0.5kg sulfur
  • 40g magnesium
  • 100g calcium

A family of four can produce the equivalent of a 50kg bag of NPK fertilizer from urine alone every year. This urine has a 10:1:4 ratio of nutrients. This shows a higher nitrogen content than many mineral fertilizers and can be expressed in the lower P/N and K/N ratios of urine. Another positive effect of using urine is that the phosphorous is in a plant usable form, requiring no additional processing before it can be absorbed. The concentration of nutrients in the water can be diluted or concentrated with either drinking more fluid or sweating. This will not have any effect of the total nutrient content and can be diluted just prior to application with water. There is no good reason to concentrate urine destined for use as fertilizer.

How to use

Once collected, urine can either be stored or applied immediately. The application can be done in a variety of spreading methods. The most basic application would be to use a watering can. You can choose to dilute the urine anywhere from 1:1 to 1:15 with water, the most common being ~1:5, or apply it straight. Take care to apply the urine directly to the soil, avoiding foliar feeding the leaves and fruits. To aid in the absorption of the urine it is suggested to dig a small amount of soil up around the leaf perimeter and pour the urine there. Afterward push dirt back over to cover the urine.


Application of diluted urine in early stages of cultivation
Photo: Linus Dagerskog

It is best to apply the urine directly to the soil. This acts as another safety barrier as well as prevents the plants from being burned. Urine applied by foliar feeding, direct application to the plant, will often burn your plants. It will not kill them or do any unrepairable damage, but it is still advised to apply the urine directly to the soil. Some plants are sensitive to the high nutrient content in urine and may burn even if you apply directly to the soil. In the case of tomato plants, which are sensitive to urine, it is best to apply the urine by digging a small trough around the drip line of the plant. The drip line is generally as far as the roots reach; applying the urine here will allow the plant to take what it needs from the peripheral area without burning. As always, if the urine seems too strong for your application, try diluting it just before applying. Dilute as close to application as possible; do not dilute prior to storage.

Other ways to spread the urine are with an irrigation system or animal slurry distribution systems. If irrigation is used, it is advised to flush the lines with water after the urine is applied. This prevents any deposits from building up and clogging the irrigation line. No matter what method of application is chosen, urine should be applied just before it rains or at the end of the day if possible. This is more important in arid areas where evaporation could lead to salt buildup in the soil, but will be beneficial in all regions. If rain is frequent, applications can be increased.

If urine is applied to the soil with little or no storage time, it will break down better and faster than if stored in tanks. This is the preferred use, but only for family farms. The soil, along with its diverse organisms, allows for faster aerobic breakdown of the pollutants picked up in the system. The UV light from the sun also helps hasten the breakdown process — up to 50,000 times more degradation! If an entire community is involved in the urine reclamation, it becomes safer to store the urine.

Handling

For the single family using urine as fertilizer, the handling is very simple. A urine diverting toilet is a great way to make collection painless, but for the developing world some jerry cans are all that is needed. Urine diverting toilets built in Niger have shown to pay back the construction cost when valuing the urine collected as fertilizer, in just under 2 years. The single family does not need to worry about cross contamination as much as a community and therefore is not required to store the urine for as long to make it “safe”. As previously mentioned, urine can be applied directly, without storage — just fine at the family level. These rules are also true for urinals, which have very low cross contamination rates with faeces. It only becomes necessary to store it longer when dealing with large numbers of urine sources. To be extra safe, families can store their urine for 1-4 weeks.

For large community collection several other measures are required. When first implementing any system like this, it is best to contact and involve the farmers that will be using the urine. By involving them you can meet their needs as well as potentially allow them to get involved in the collection process, creating a small income stream for the farmer.

After collecting urine from multiple sources, storage to kill pathogens is required. Large meter cube storage containers are often utilized for their low cost and high availability. If these are not available, jerry cans can be used through the entire process. 20L jerry cans are extremely common. Each 20L container can cover between 4 and 20 square meters of land. These containers are often two colors to indicate their storage life. A yellow container denotes the urine is still in storage, while a green container denotes liquid fertilizer, or birg-koom. Storage time for urine collected from many sources is suggested between 1 and 6 months. The colder the temperature the longer it will take for the pathogens to die down. Ambient temperatures in warmer climates should look toward 1-2 months storage and as the climate gets colder, increase storage time. Make sure not to dilute the urine prior to storage, most commonly with gray water from the sink, or it will take longer for the pathogens to die. Stronger urine is a much harsher environment. Additives and even just a handful of compost are being studied to aid in the breakdown of pathogens as well as increase the nutrient content of the urine. These studies look promising, but have not been published as of yet.


Yellow container for fresh urine,
green container for stored urine to be sold to farmers.
Photo: Linus Dagerskog

One thing to watch out for with the long term storage of urine is the crystallization of phosphorous. The crystals form on the walls of the containers, but can be broken up easily by stirring the urine occasionally. These same crystals can also end up clogging pipes and irrigation lines if not taken care of. For pipes and lines, simply run fresh water through after each urine application.


Storage of urine in one cubic metre tank
Photo: Anna Richert

Safety

While urine is not very dangerous, some precautions should be take to ensure safety. The World Health Organization (WHO) has developed a barrier system to prevent the spread of any disease. The barrier system is quite basic and requires no special skills or tools.


The multi-barrier approach

The first barrier in the WHO process is source separation. This happens at the household level where urine and faeces are separated by a urine diverting toilet. After that, the urine is collected and stored. Everyone involved in handling the urine should wear rubber shoes and gloves. The third barrier is the application technique: applying the urine directly into the soil, preventing it from getting on the above ground parts of the plant. Fourth is selective application. For example, fast growing greens like lettuce are not the best targets for urine fertilizer because they grow and are consumed so fast.


Risk levels in relation to crop and handling strategy

Next is the withholding period barrier. No crops should have urine applied at least a month before harvest. This applies to all kinds of fertilizers, not urine specifically, but it is none-the-less another barrier. The fifth barrier is the gloves and boots, which should be used at all times during urine processing and application. Lastly, hands should be washed frequently when handling urine and all crops should also be washed prior to consumption.

Another safety concern is how urine is stored. Use only sealed containers, otherwise you risk mosquitoes breeding. While urine is fairly sterile when it is fresh, there are some organisms that are alive in it. These will break down when applied to the soil usually, but the storage process is what makes sure these organisms are all dead. One such organism that can live in urine is Salmonella typhi/paratyphi, although it is short lived — requiring only 1 week of storage to reduce their numbers one thousand times.

Cross contamination from faeces is probably the largest concern. While this is not the end of the world, the likelihood of harmful organisms surviving past the application point is higher. Diarrhea is an exception and should be watched for carefully as it is a source of bad contamination.

Again these safety concerns all but disappear at the family level. Family members are more likely to transmit diseases directly to the other members of the family than through urine. The health risks at the family level are very low. Urine can be applied fresh, in its near sterile form as previously mentioned, to allow for aerobic and UV breakdown.

When these safety concerns are compared to using pesticide and animal antibiotics, you are actually at a far less risk of transmitting diseases via urine than you are using “modern” substances and techniques. Urine fertilizer has shown lower heavy metals than waste or sludge water and even farmyard manure. The chemicals introduced through the consumption of pharmaceuticals are much worse, and not properly treated, even in most first world countries. When working with local urine, and even more-so when dealing with a family, your chemical loop will be closed. Only the chemicals you have taken will be present in your urine; no outside sources are used as input. Menstrual blood in the urine has no negative health impacts and can be used normally.


Components of urine collection system Ouagadougou, Berkina Faso
Photos: CREPA

Economics

In developing nations where resources are scarce, being able to produce your own fertilizer can go a long way to economic sovereignty. In Burkina Faso the value of a 20L container of urine is about 25 US cents. The farmers in Burkina Faso can often only afford to purchase one bag of fertilizer per season at a cost of 6-7USD. By harvesting their urine and using it as a fertilizer the farmer can create the equivalent of two bags worth of fertilizer for little to no economic input. The cost of purchasing the chemical fertilizer is also retained, further reducing economic stress.

The economic value can multiply quickly as you scale up to a village-sized collection system. This has side effects of producing jobs all through the cycle from collection, to tracking storage, to sales, and many others. As valuable as this is for the local village, reducing the need for external chemical fertilizers, it is not without its shortcomings. The collection cost of running a urine storage/distribution facility is currently greater than the industrial value of the urine when compared to other fertilizers.

Yield

The true benefits of the system can be seen in the increased yields. In terms of sustenance farming, there is little else to be said. Larger crops mean more income for the farmer, and more food for the community. While the test crops were not measured with scientific rigor, it is clear that the urine treated crops out produced non treated crops.


Field trials from Niger. Urine fertilised millet to the right.
Photo: Linus Dagerskog


Urine fertilised sorghum to the left
Photo: Linus Dagerskog


Spinach (Swiss Chard) fertilised with urine, left.
Photo: Peter Morgan

Conclusion

Urine is a valuable resource that should be harvested more widely. In the developing world, this built-in fertilizer stream is low hanging fruit on the road out of poverty. With even a minimal investment, a family or sustenance farmer can not only reduce their need for importing synthetic fertilizers, but can also reclaim a waste stream. The beneficial impacts stretch far beyond the urine diverting toilet.

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