The Washington Monument Reflecting Pool and the Control of Algae:
By David L Russell, PE, ASP
Here are some of the details on treatments to remove algae:
First: The challenge is to kill the existing growths. This is most often done by shocking the water with a chlorine residual of 5 mg/l for at least overnight. However that can be problematic if there are high levels of nutrients in the water. The principal nutrients are nitrogen and phosphorous (phosphate), added coloring agents such as dyes, and some bit of carbon from bird droppings, animals, leaves, dust, and even human feces. It’s not drinking water, but more like pond water.
When I was a kid, growing up in the Chicago Area, my dad was in the commercial swimming pool business. In his time, he built some very large pools, containing over a million gallons of water.
Commercial pools are regulated by the Department of Health in each state, and in general, all the water has to be filtered every 6 hours, or 4 times per day. Additionally, the pools are required have to have a minimum chlorine residual which varies between 0.5 milligrams per liter and 3 milligrams per liter depending upon the type of chlorine or disinfectant used. One alternative disinfectant is bromine. Bromine is a liquid and a bit easier to handle than gas chlorine, but it is more expensive and not used as widely.
Everyone pees in a swimming pool. That urine contains amines that react with chlorine to give swimmers “red eye” from swimming in chlorinated water. Bromine does not create red eye as often as chlorine and it is a powerful disinfectant but quite a bit more expensive to use.
For certain types of disinfection compounds, the maximum recommended dose is 3 milligrams per liter, but in general should not exceed 5 mg/l as a maximum. Sunlight and warm temperatures are the enemy of chlorine and bromine. I’ve seen a sunny day take a disinfectant levels in a pool from 3 mg/l to less than 0.1 mg/l in a matter of a few hours.
Chlorine and Bromine are both in the Halogen family of elements. Both can be used to kill algae and help prevent it from growing by interfering with its cellular structure. Both can be used to “shock treat” pools with high concentrations of algae. Generally the dose is given overnight at about 10-20 milligrams per liter of water, and at that level, the water is not considered good for swimming or even wading. Bright sunlight will cause halogen disinfectants to be dissipated.
If the water is to be made safe for public use, Not drinking water quality, but free from harmful bacteria and algae, it needs to have some level of a stable disinfectant.
Chlorine will also kill algae, but it can be tricky to maintain a consistent chlorine residual. Chlorine gas is a potent Toxin, and it is something that if accidentally released can cause a deadly gas cloud—not necessarily something one would want to have around a national monument or tourist attraction.[1]
Several years ago the EPA looked into the issue of the storage of liquefied (gas) chlorine at wastewater treatment plants. The EPA program for evaluation was RMP (Remind Me Please). Large users of chlorine get shipments in ton gas cylinders. In an accidental release, from a bullet or a broken valve, the choking chlorine cloud might reach more than 2 miles. The solution was to modify the program. The program has been replaced with Risk Assessment Programs which can perform the same type of assessments for a variety of chemicals and conditions. Any municipality that currently stores more than 2500 pounds of Chlorine (gas) must prepare and file a risk management plan under Section 112( r) of the Clean Air Act[2]. Many municipalities chose to move to hypochlorite (bleach) liquid disinfection systems are much less hazardous
Now to the Reflecting Pool…
I did some research into the history of the reflecting pool. I remember when I lived in the DC area back in the early 1970s, the reflecting pool was always a dark color with no green slime.
I did a bit of AI research and turned up the following:
- First: Size of the reflecting pool:
- The reflecting pool.. The reflecting pool is approximately 2030 feet long, by 167 feet wide and varies in depth between 1.5 feet and 2.5 feet (at the center) and contains around 6.75 Million Gallons of water. That’s enough water to supply a US city of between 33,750 and 45,000 persons with their daily water.
- Second: Source of the water
- The Reflecting Pool used to be provided with DC’s City Water. In 2010-2012 the source was shifted to the Tidal Basin, with evaporative and leaking system losses provided by the discharge from the World War II fountain (estimated at about 50,000 gallons per week.) The initial problem appears to be the water source itself. The Tidal Basin is fed by the Potomac, and the Potomac water quality is barely suitable for swimming, but swimming isn’t allowed. The Potomac also receives some silt and detritus, bird and animal droppings, periodic doses of untreated sewage, urban waste from precipitation & runoff, and the agricultural runoff from farms in Maryland and Pennsylvania. According to the 2025 Potomac River Report Card, the river water quality was rated at a D+. The entering water is screened, sand filtered, and treated with ozone to kill bacteria.
The fundamental problem is that river water contains dissolved nutrients such as nitrate (NO3-) and phosphate (PO4-3). Because these nutrients are dissolved, and in an oxidized form, they are unaffected by treatment with ozone disinfectant, and the sand filtration system is unable to remove them from the water. The river water, even after filtration does contain other substances which algae and bacteria can use as food sources. As little as 0.02 milligrams per liter of phosphate can materially contribute to algal blooms. Add in some airborne bacteria from dust and dirt, and an occasional deposition from a passing bird, and it is a recipe for algal growth. It is important to note that the disinfection system ozone does not leave a disinfectant residual. Without the residual, the bacteria and algae are free to regrow.
The water in the reflecting pool is filtered, about 1/4th of the water is passed through sand filters and re-disinfected daily. But that’s not enough. Even with a larger ozonation unit, the fundamental problem remains, the nutrients are soluble and not removed. New bacteria and algae come in every day to encourage regrowth.
There is a reported problem with the design of the water distribution and recirculation system. Leaking and cracked pipes. That will be an expensive fix because thousand of feet of pipe will need replacement. From an unconfirmed source, it appears that the reflecting pool foundation may also need replacement because soft soils by the reflecting pool may have compacted and cracked the underground piping. The fix may include driving piles underneath the reflecting pool to stabilize the structure when the piping is replaced.
Short of major reconstruction, the fix to the reflecting pool is a chemical water treatment system. Algae Control is something every homeowner with a backyard pool or hot tub knows… algaecides. Algaecides are chemical compounds that are applied to the water specifically to kill algae and prevent it from growing.
The compounds include:
Chelated Copper Algacide, Sodium Carbonate Peroxyhydrate (used to knock down algae), Quaternary Ammonium Compounds, Benzalkonium Chloride, And several commercial algaecides that contain the same compounds. There also is a system that is solar powered and generates ultrasonic sound waves inaudible to humans, that prevents algae growth.
The cost of the treatment chemicals is between $6,000 and $10,000 plus labor. The ultrasonic system might require $20,000 but no chemicals after that.
The fundamental problem is that the chemicals that prevent algae growth are persistent and toxic to other types of aquatic life. They are also banned by the EPA. The only one on the list that can be used in Sodium Carbonate Peroxyhdrate. It does not leave a residual, and decomposes to oxygen. It’s good for knocking down algae blooms but without a residual, there is nothing the compound can do to keep the algae from regrowing in a shallow pool with little filtration and poor circulation.
The idea of using ultrasonic systems has its limitations as well. In a discussion with the manufacturer of the system, the system works best when there is 4 feet or more of water that blocks out the sunlight the algae needs to grow. The sonic systems attach the algae’s ability to float, causing them to sink to the bottom of deeper basins.
In exploring the possible applications with the manufacturer of the sonic system, we came up with an idea about adding a coloring agent to the water that would block sunlight. That’s feasible.
A second idea might be to chemically pretreat the water from the Tidal Basin that is the source, to remove the nutrients. That might work, but it has high operation and maintenance costs.
A third idea would be to a) fix the leaks, b) apply an algaecide to control algal growths, and c) treat the water before it is discharged. That’s a closed system after a fashion. The solution to treatment would be carbon filtration. That’s relatively lower costs and is it protective of the Potomac River and the downstream oyster beds and fish.
A fourth idea would be to up the recirculation rate for the system, providing disinfection and circulating the water every 6-8 hours through filters. Sunlight will knock down the chlorine, but that’s an acceptable cost. There are different types of chlorine compounds that can be added to maintain disinfection levels and prevent algae growth. A residual of between 3 and 6 ppm (mg/l) would be sufficient. A 5 ppm dose would require about 275 lb of hypochlorite ion per day, as hypochlorite. That would require about several hundred gallons of industrial strength sodium hypochlorite (commercial bleach) or the equivalent of calcium hypochlorite powder applied per day. The recirculation rate increase is an expensive option because it would require large pumps, a large filtration system, and probably significant rebuilding of the recirculation system for the reflecting pool. All of that comes with a high energy cost. It’s by far one of the most expensive options but may be the only one feasible when all others are considered.
Or we can consider the option of adding a sunlight blocking dye and considering the ultrasonic system.
In any case, there are few good alternatives.
[1] Chlorine was used in WWI as an asphyxiant (toxic) gas, during the period of trench warfare. It is heavier than air and would settle into the trenches. Chlorine gas can damage the lung tissue and be fatal.
[2] Wastewater Technology Fact Sheet Chlorine Disinfection: EPA 832-F-99-02, Sept. 1999 Available on https://nepis.epa.gov at no charge
Read my previous article in Chemical Processing: https://www.chemicalprocessing.com/voices/from-the-editor/article/55387188/got-a-fix-for-the-lincoln-memorial-reflecting-pools-algal-bloom