The science of pink public urinals

Oh, sure you’ve seen them — those metal cylindrical enclosures painted bright pink along major and minor roads in Metro Manila. These are men’s public urinals, where men can go do their thing, and not “discharge their liquid waste” (hereafter shortened to “pee”) on hidden corners and walls. The pink cylinders are of interest because of recent discussions of creating some for women. But the idea of these public urinals is also interesting to an environmental engineer and scientist, for it exposes: (1) the lack of understanding of wastewater treatment and the fate of wastewater contaminants, and (2) the lack of systems thinking.

What happens to the urine? These urinals do not incorporate any treatment at all. The urine hits the metal wall, then flows to the ground. About 80 percent of the nitrogen (about 9 g/l as N) and 50 percent of the phosphorus (about 2 g/l as P) in human wastewater comes from urine. These are the two major nutrients that lead to eutrophication of water bodies. These are the two major nutrients that wastewater treatment plants try to REMOVE from wastewater before discharging to rivers and lakes. With these urinals, we now have hundreds of point sources of nitrogen and phosphorus pollution!

If the urinal is built on top of soil, here’s the scenario. The soil has some capacity to absorb and adsorb organics, and also contains nitrogen-transforming bacteria, that hopefully will then convert the ammonia (almost all of the nitrogen is in the ammonia form) to nitrites and nitrates. This ammonia oxidation requires oxygen. The nitrate is then hopefully converted by denitrifying bacteria to nitrogen gas. But this is unlikely, as these point sources overload the underlying soil with ammonia. It is unlikely that there is adequate oxygen to oxidize the highly concentrated ammonia in urine. Bacteria are great, but this is not a designed system. So what you end up with is a stinking mess. 

But the greater irony is that majority of the urinals are built not on soil but on impermeable surfaces (e.g., concrete) where the urine becomes part of the stormwater collection system, or just dries in the baking sun. All that ammonia adds up to stinking urinals. From an environmental standpoint, it is a lot better for men to pee behind trees and bushes. One, it’s on soil. Two, if there are more trees to choose from, you have lower chances of ammonia overloading. And three, it’s better for the trees, as nitrogen and phosphorus in urine act as fertilizer!

What we need is systems thinking. The system is not just the pink urinal, it is the entire watershed. The urine is essentially entering the watershed virtually untreated. The “maintenance” that is done is simply spraywashing the urinals with water, which dilutes the pollution, but increases the amount of wastewater. Most of the water that goes to Metro Manila’s stormwater drains ends up in creeks and rivers untreated (only about 15-19 percent of Metro Manila’s wastewater is collected and treated in centralized wastewater treatment plants).

Separating urine from human feces is a valid wastewater management approach. Since most of the N and P are in human urine and not in feces, then you immediately eliminate a major source of these contaminants. You also get the benefits of less wastewater production. But you have to either treat (in a designed system, and not just let it flow to the ground), or collect and use. This is the idea behind toilets that separate urine from feces (e.g., “NoMix toilets”), one of the technologies in decentralized, sustainable wastewater management.

Urine is virtually sterile (because of the high pH), and has been shown in a number of peer-reviewed publications to be a good fertilizer. Urine can be collected and used to grow plants. We need to worry about pharmaceuticals (most of the drugs, antibiotics, etc. that we humans take end up in urine), but there are ways to reduce the levels through treatment (photolysis, oxidation, constructed wetlands, among others). 

We need to apply some environmental science and engineering here. There are better, more environmentally acceptable alternatives to pink urinals. We need to study the cost-effectiveness and sustainability of collecting, treating, and reusing urine.

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Francis L. de los Reyes III is an associate professor of Environmental Engineering at North Carolina State University. He conducts research and teaches classes in environmental biotechnology, biological waste treatment, and molecular microbial ecology. He is on the editorial board of Water Research, was a 2008 Balik-Scientist of the DOST, and is a 2009 TED Fellow. He is a member of the Philippine-American Academy of Science and Engineering. E-mail him at [email protected].