There has occasionally been talk of placing portable restrooms near the Rt. 153 beach along the Maine shore of Province Lake near Shore Acres Road as a way to reduce phosphorus in the lake. As part of my review of the Province Lake Watershed Management Action Plan, I decided to calculate the technical viability of this approach.
My first question was, when someone pees while swimming, what volume of fluid are they actually contributing to the lake? Fortunately, I was not the first to have this question, so I was able to find a compilation of five different sources HERE. Looking at the table, the volume at micturition (presumably in adults) is 150 to 400 milliliters (mL). Assuming that many of the contributors would be children, I used 200 mL (6.8 fl. oz.) as the volume in my calculation.
The second question was, what is the phosphorus concentration in urine? The best source I found was a 2006 journal article "Phosphorus recovery from human urine" by Gethke, et al.
This article gave a phosphorus concentration of 164 milligrams per liter (mg/L). The phosphorus was in the form of phosphate (PO4). This was an average based on collecting urine in 1000 L (264 gallon) tanks.
The same paper also gave the nitrogen concentration as 190 mg/L, with the nitrogen in the form of ammonia (NH4).
The nutrient contributions for a single bladder voiding would therefore be:
(164 mg P/L) x (0.2 L urine) = 32.8 mg P
(190 mg N/L) x (0.2 L urine) = 38.0 mg N
The steering committee for the Watershed Management Plan set a goal of reducing the amount of phosphorus getting into the lake by 25% over the next 10-15 years. The goal is to prevent 113 kg (250 lbs.) of phosphorus from entering the lake annually.
Since we are therefore concerned with kilogram (kg) quantities of phosphorus, not milligrams, the more relevant question in this analysis is how many bladder voidings directly into the water would be required to contribute 1 kg of phosphorus?
The answer is:
(1 voiding/32.8 mg P) x (1000 mg/g) x (1000 g/kg) = 30,488 voidings per kg of P.
This calculation includes several estimates of values that necessarily cover a range, but it seems safe to assume that a portable restroom at the Maine stretch of beach would need to be used 20,000 to 40,000 times during the beach season to avoid the contribution of 1 kg of phosphorus into Province Lake. If beach season were assumed to be 100 days from Memorial Day to Labor Day, that would be 200 to 400 uses per day, 7 days per week. This number would need not to include those swimmers who would have "held it" until they got back to a toilet during past years, non-swimmers, or those driving by who stop only to use the facility. The probability of there being that much qualified demand and the likelihood of capturing it need to be considered.
Another issue to consider is that if restrooms are available, some people may stay longer at the beach. If they stay longer, their car will be parked longer and their parking spot will not be available for new cars arriving. Those new arrivals will need to park further along the road, causing more of the disturbance connected with parking. Any small decrease in urine phosphorus to the lake might be partially or completely offset by increased erosion phosphorus.
Given the cost, liability, maintenance, and private property issues also involved, the placing of portable restrooms near the Rt. 153 beach along the Maine shore near Shore Acres Road to reduce phosphorus input to the lake is probably not a viable solution to water quality problems.
It is likely that very few people know what is in urine besides warm yellow water. A simple low-cost education campaign could let folks know that urine they may choose to pass while swimming fertilizes their swim area with phosphorus, which encourages cyanobacteria, algae, and weed growth. Nobody wants to encourage that, especially in their own swim area. They could then be encouraged to stop or at least minimize doing it.
In fact, I have taken a break from writing this document to add a bullet on this subject to the "Clean Living" list on my web site.
Education campaigns could be particularly effective at the campgrounds, where there are large concentrations of swimmers who DO have toilets very close by. Since they have existing facilities close by, unlike at the Maine beach discussed above, they only need a little motivation to hop out of the lake for a minute or two.
A campground could do an education campaign where maybe they could have a contest for the best slogan or sign, e.g., when I was a kid, I remember my friend's parents had a sign that said "We don't swim in your toilet. Please don't pee in our pool!" Contests are popular at campgrounds, so much the better if they are slightly naughty. Once the kids become aware of all this, peer pressure might prevail.
Another thing to consider: When we tell people to keep phosphorus out of the lake, many people might think "Where the heck would I get any phosphorus? And why would I put it in the lake?" By letting them know that some of it comes out of them if they pee while they are swimming, then they are empowered to do something positive to help the lake. Even small children can understand that they are doing something to protect the lake they love. Once engaged in protecting the lake in this small but extremely personal way, some folks will undoubtedly get engaged at a higher level, whether at Province Lake or somewhere else that needs protection, either now or in the future.
Norm Dudziak, P.E., I.M.
7 March 2014
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