Every waterfree urinal makes it a point to let you know that it saves “150,000 liters of water every year.” That’s around a hundred 8-person hot tubs. However, exact statistics like this can be very dubious; and in these cases, it may be helpful to try and figure out how reasonable the underlying assumptions are.
The amazing waterless urinal
Ever used a waterless urinal? If you have, then you probably know that it saves some 150,000 liters of water per year, because it doesn’t hesitate to let you know through a large sign right in front of your face. Take a look at this example that greeted me at an office building:
However, if there’s one thing that thesis-writing has taught me, it’s that point statistics are meaningless unless you give some measure of their accuracy. To provide information on the accuracy of a statistic, you can present the methodology, some measure of sampling variability, or confidence intervals (or a range of probable values), in addition to the value of the point estimate. This is why, for me, a bold categorical statement such as “This urinal saves 150,000L of fresh water a year.” is simply too hard to believe.
Why 150,000 Liters?
Nonetheless, let’s try to analyze the assumptions used by the company, as outlined in their FAQ page:
- Water usage at 1.5 gallons per flush - reasonable according to cursory research, as flushes can range from 0.5 to 3 gal/flush.
- Year-round 365-day usage - not reasonable; urinals at working establishments can only be used during working days, and according to the BIR, that’s at around 247 days per year.
- Daily usage at 75 flushes per day or 28,000 flushes per year - not reasonable; the US Environmental Protection Agency and a study by the US Department of Labor Statistics peg the usage at only around 20 and 18 uses per day, respectively.
- So, is the 150,000L figure valid? It can still be valid, provided that the urinal is in a very high-traffic 24-7 area - an airport or mall, for instance. However, for most office buildings, water savings won’t be that high.
Taming the statistic
As you can see, the savings can vary from 25,000 to 160,000 liters. If we try to compute the actual peso savings from installing a waterless urinal using Maynilad Water data, we can compute it as follows:
Whereas the old assumptions paint substantial annual cost savings, tamed assumptions make them a little less signfiicant. Of course, this doesn’t factor in environmental costs, and even a small environmental benefit can be very compelling.
Data presentation is where otherwise truthful data can be used to tell a very different message, often to the data producer’s benefit. Although their business model is noble, there might be better and less misleading ways for the company to present this data, such as:
- This urinal saves 5.5L of water per use. (normalized figure)
- This urinal saves 25,000 to 160,000L of water per year. (range of data)
This entire post is just nitpicking, but you would be surprised at how data can be abused in more serious situations, which I’ll get into in future posts.
So the next time you see or hear a statistic quoted so exactly, be sure to take it with a grain of salt.
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Some caveats, notes, and disclaimers:
- The information presented in this post is purely for curiosity; it is not meant to influence any important decision. The author cannot be held responsible for any harm caused by acting on this information.
- The actual savings may be less than claimed, but these are significant savings nonetheless, especially in a new building. I love that this technology is becoming mainstream as it can have a large environmental benefit.
- The analysis is limited to commercial establishments as they are the only places where urinals make a lot of sense.