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As Demoflush washes away, formula could be unclogged

(Jan. 23, 2015) Much like the substances it’s based on, Demoflush, the city’s estimate of population via wastewater flow, is likely on its way out, at least in its current form.

After discussion at last week’s Tourism Commission meeting, city council voted this week to investigate “replacing Demoflush with a more modern formula” to gauge visitorship.

For the better part of the four decades in which its been used, Demoflush has met with considerable skepticism from businesses, who find the number to be somewhat arbitrary compared to the economic metrics that have become readily available in recent years.

“Nobody could really come up with the ‘so what’ of it,” said Council Secretary and Tourism Commission Chair Mary Knight. “If we could replace it with some more valuable information involving more metrics, we could make a ‘so what’ out of it.”

For several years, the city has supplemented Demoflush figures with room tax receipts, hotel occupancy reports, bus ridership, and a number of other figures that have become available since 1971, when Demoflush was the only game in town.

Why, exactly, the city has held on to Demoflush as a viable indicator is unclear. No one remembers the exact rationale for the formula the city currently uses, or how to change it. The promulgation of Demoflush after a big event or summer weekend has become institutionalized to a point that putting a critical eye on it now will be quite a hill to climb.

Fortunately, for those with algebra skills, Ocean City Today has dug up the source material on Demoflush.

The method was originally developed in 1971 under the auspices of an organization called the Ocean City Medical Commission, a group that was formed expressly to promote the feasibility of a high-dollar medical complex on the Isle of Wight. The project never happened, ultimately.

However, the Medical Commission decided that sewer flow was the best way to gauge population, and thus how much capacity the medical complex would need. Financial indicators were unavailable. Wastewater was useful because it captures only indoor, domestic water use, whereas water inflow would capture water pumped for landscaping, construction, and other outdoor uses that ultimately drains into the stormwater system and not the sewer.

The original Demoflush calculations, done by a consultant at the Medical Commission’s request, are lost to the pits of time. But in 1976, a scholarly article was written by Drs. Peter Goldschmidt and Andrew Dahl, analyzing the validity of Demoflush for further public health research.

That article still sits in a filing cabinet in the city’s Public Works Department office.

“From reading the article, it seems pretty clear that the formula was designed to estimate medical care capacity,” said Deputy Director of Public Works Jim Parsons. “The method has obviously been extrapolated in the years since then, probably beyond the authors’ original intent.”

In the article, Goldberg and Dahl explain the thinking behind the original formula. The equation used for Demoflush was based around the idea that total wastewater use in a given day was due to four factors added together.

The first was the amount of sewer “infiltration” per day, meaning the amount of groundwater that flows into gaps in the pipes unintentionally.

The second was wastewater from permanent residents, depending on the number of residents and the number of gallons per day the average resident used.

The third was wastewater from overnight visitors, depending on their number and usage. Visitors naturally generated less wastewater than residents, given that they were not washing their clothes, and their household chores were being done on an industrial scale by hotel or condo maintenance personnel.

The fourth was wastewater from daytrippers. Their consumption was assumed to be very low, given that they were not using showers, and were not eating as many meals in the resort, thus generating less wastewater per-person from restaurants.

The last two components are actually functions of the same number, that being total visitor population. The number of daytrippers can thus be expressed as the total visitorship, multiplied by the ratio of daytrippers. The number of overnight visitors can be expressed by the total visitorship, multiplied by the inverse ratio.

If one writes all this out, and then manipulates the equation to solve for total visitorship, it looks something like the graphic that accompanies this article.

This, in Goldberg and Dahl’s paper, was the original Demoflush formula, which, logically, will always give accurate results – as long as accurate values are plugged into the equation.

The problem here, as Goldberg and Dahl note, is that the Ocean City Medical Commission decided to plug in values that weren’t that accurate, even in 1971. One can only surmise how they’ve changed over the past four decades.

Further, as the formula was passed from person to person, it appears that many of the variables were simply substituted for what were being used as the accepted number values at the time, eliminating any future adjustability.

Currently, the city’s method is to take the total wastewater flow for a given day, and then subtract 570,000 gallons. This would represent the load of infiltration and from long-term residents. That number is then divided by 36.04, the average visitor load in gallons per day.

The original report given to the Medical Commission, Goldberg and Dahl state, found that infiltration accounted for 442,000 gallons per day. Under the city’s current formula, that would leave 128,000 gallons of the 570,000-gallon factor being created by resident load.

But assuming the resident load recommended by the 1971 study, 60 gallons per person per day, this means that the city is only allowing for 2,133 permanent residents.

The 60-gallon rate is also probably on the low side, given that average water consumption nationwide is 88 gallons per person per day, and 103 gallons in Maryland, according to the US Geological Survey.

The number of residents accounted for is also extremely low, given that the town had 7,102 full-time residents in the last census. Further, the town hosts several times that number in seasonal residents, who are consuming just as much water as year-rounders.

Additionally, given that the city has expanded greatly since 1971, there are many more sewers for water to infiltrate, meaning the 442,000 gallon allowance is likely low as well. At peak times, the city pumps nearly three times more wastewater today (nearly 12 million gallons) as it did in 1971 (just over 4 million).

The divisor that has traditionally been used, 36.04 gallons, is the weighted average of overnight visitor consumption and daytripper consumption. This assumes that overnight visitors use 40 gallons per person per day, daytrippers use 7, and that 12 percent of total visitors are daytrippers.

Both gallon values are the same as were recommended in 1971, according to Goldberg and Dahl. The 12 percent ratio of daytrippers also dates to 1971, although Goldberg and Dahl note that this was a low ratio selected by the Medical Commission – accepted estimates were between 15 and 20 percent daytrippers.

For those of you who are still reading, let’s get back to the main thrust.

The critical point here is that Demoflush isn’t inherently inaccurate. The original formula allows for a great deal of adjustment. The specific number values that have become the city’s norm, however, are likely long outdated.

One of the points made by commentators is that the percentage of change in Demoflush from year-to-year should be a reliable indicator, regardless of the final tally. But given changes in infiltration, resident population, personal water usage, and the ratio of daytrippers, comparing over a range of more than a few years is a rather moot point.

Whether Demoflush continues to be in the city’s lexicon or not, we invite readers to keep the formula and explanation provided here, come up with their own values to plug in, and see what they get. You might just be right.

Oh, and we worked it out with the full formula and values we believe reflect more accurately water consumption in the modern era. Our results, which are good as anyone’s at this point, show a peak visitorship of 237,000. That’s about 100,000 less than the number being tossed around for the last several years and would mean an annual total population of 5.7 million or so as opposed to the popular 8 million that’s frequently cited.

Then again, there’s the formula, so go figure.



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