Back to the Future... Is that Liquid Gold in those Sewers?
Why separate urine?
Urine represents less than 1% of the wastewater volume generated in the typical American home, but contains about 50% of the phosphorus and 80% of the nitrogen. Urine is generally pathogen free. Many of the compounds of emerging concern, like pharmaceutical products, end up in the urine. Some reasons for the growing interest in urine diversion include:
- Nutrient pollution of our waterways has been an increasing challenge over the past decade and will continue to require treatment facility upgrades to remove nutrients to very low levels at significant capital and operating expense.
- Phosphorus is a critical ingredient of the fertilizers required to grow crops to feed the world. There are world-wide phosphorus shortages predicted to occur later this century.
- It may be easier to manage the future issue of compounds of emerging concern (CECs) in a concentrated waste stream.
- Recovering the nitrogen and phosphorus in urine would reduce the need for commercial fertilizers, including their associated “indirect costs” (i.e. mining phosphorus, petroleum, long-haul trucking, etc.). The average family of four generates about 550 gallons per year of separated urine -- which contains enough nutrients to fertilize approximately 1/3 acre of lawn or other crop annually.
How do you separate urine?
As everyone knows, the standard toilets sold in the US result in the urine being mixed with all the other sewage. There are also a number of manufacturers which produce urine diverting toilets and waterless urinals. A urine diverting toilet requires two drain pipes, which would be easy enough to accomplish in new construction, but a bit more challenging in retrofitting existing buildings. The separated urine would most commonly be piped to a collection tank where it would be stored until it is pumped out and transported to a regional management/ treatment facility or, alternatively, used directly as a liquid fertilizer. As you can imagine, there are a number of practical challenges associated with urine diversion.
What do you do with it once separated?
Once separated, the collected urine can be managed/stored and applied on-site as a fertilizer or it can be transported to a regional processing facility for treatment and nutrient recovery or directly used as a liquid fertilizer. In fresh urine, most of the nitrogen is in the form of urea which quickly converts to ammonia when stored. Much of this ammonia can be lost to the atmosphere if the storage tank is mixed and vented and most of this lost ammonia will be returned to the land with precipitation. Use of the urine directly as fertilizer would be the most economical alternative; however, this approach would require an appropriate agricultural need in close proximity. This would not be practical everywhere and other management strategies would need to be considered. At a residential scale, you may find a fraction of the population willing to learn about managing the collected urine by applying it as fertilizer to their lawns; however, it is unlikely that the majority of the public will be willing to go down this path. Further, the public health and regulatory hurdles are yet unknown. To be successful at a municipal scale, collection services and centralized processing facilities would most likely be required.
One method of processing at a regional facility would be to add magnesium oxide (MgO) to precipitate struvite (MgNH4PO4*6H2O). Struvite can be used as a slow-release granular fertilizer. Struvite production removes about 90% of the phosphorus but only a small fraction of the nitrogen from the urine. Additional nitrogen removal can be obtained by air stripping the remaining ammonia. This ammonia can be collected in a nitric acid scrubber to form ammonium nitrate, which can also be utilized as a fertilizer. The remaining liquid could be further treated if necessary for compounds of emerging concern. There are a variety of other urine treatment strategies available and the best approach is very project specific. Cost effective management of collected urine will require a significant quantity, and therefore urine diverting toilet facilities must be fairly widely used to support this approach.
Where is it being done?
Urine diversion is being applied at small to mid scale in many locations around the world including Europe (Sweden, Norway), Asia (Nepal, China, and Japan), Africa (Ethiopia) and Latin America (Mexico). Urine diversion is by no means “mainstream”, but is a growing trend in industrialized and emerging nations alike.
Is urine diversion the answer to the wastewater nutrient issues?
There are many issues related to implementation of urine diversion — ranging from cultural (personal habits, taboos), onsite management (odor management, pipe scaling), collected product management (treatment and/or nutrient recovery), and regulatory (public health, local zoning, non-point source pollution) — which will result in slow adoption of this approach. However, studies conducted in Europe have shown that nutrient removal efficiency and costs at centralized treatment facilities improve as increasing percentages of urine is removed from the raw sewage.
If influent nitrogen concentrations were significantly lower, there would be only enough nitrogen to support the growth of the bacteria used in secondary treatment, with none left over to remove in a tertiary system. At approximately 70% urine diversion, “nutrient removal” at the treatment facility occurs primarily by uptake for biomass cell growth.
Long term, urine diversion is expected to become another tool to be considered in solving wastewater nutrient issues. While it will not likely be the solution everywhere, it may well be part of the solution in some situations.