Spring 2021

A Disinfectant-free Floating Pool in New York City

By Julie Lemay, M.P.H., and Amy Dale, Ph.D.

Gradient is helping a non-profit foundation achieve their vision of building a floating pool in New York City’s East River.

Over 10 years ago, a group of designers conceived of an idea to build a floating pool in the inner harbor of New York City. In 2015, the Friends of + POOL incorporated as a non-profit 501c3 to fund their vision of offering New Yorkers a safe and accessible recreational opportunity on the NYC waterfront. Their vision for the + POOL involves a floating, plus-shaped pool filled with filtered river water. The pool includes one Olympic-sized arm that can be used as a lap pool, along with two smaller arms that will be used as a children’s pool and a lounging pool (Fig. 1).

Without the use of chemical additives, the main challenge for the
+ POOL team has been to ensure that water quality would not be compromised by “in-pool” sources of bacterial contamination.”

A central goal of the design was the creation of a pool that is safe for swimming WITHOUT the use of chemical treatments (e.g., chlorine, bromine) or other additives. Gradient joined + POOL’s team of pro bono experts to help identify specifications for water filtration and pool operation that ensure that water brought into the pool meets or exceeds bacterial water quality standards for swimming, regardless of the river’s changing conditions (e.g., due to combined sewer overflows [CSOs] or other point-source contamination). + POOL’s engineering teams determined that microfiltration (MF) or ultrafiltration (UF) membrane systems would be able to treat incoming river water to New York State drinking water standards.

Without the use of chemical additives, the main challenge for the + POOL team has been to ensure that water quality would not be compromised by “in-pool” sources of bacterial contamination. Studies have found that swimmers shed bacteria from their skin (“skin shedding”) into the water. Potential contributions from the bodies of adult and child swimmers include cryptosporidium, E. coli, giardia, enterococci, staphylococcus aureus, and others. (These illness-causing bacteria are the main reason why pools require swimmers to shower before entering the water!)

Proposed + POOL Rendering

Two Images of + Pool Concept

Click to Enlarge Figure.

To ensure that pool water quality would not be compromised by “in-pool” sources of bacterial contamination, specifically skin shedding, Gradient helped the + POOL team develop a mass balance model for the pool using US EPA’s water-quality modeling software, the Water Quality Analysis Simulation Program (WASP), which is recommended for use in predictive modeling of water quality at bathing beaches. Using enterococci as an indicator organism, the Gradient team modeled typical and conservative scenarios for changes in bacteria concentrations in each arm of the + POOL over one week of operation. The model accounted for solar inactivation of bacteria on sunny vs. cloudy days, the unique geometry of the pool, and parameters describing pool design and operation (flow rates, pool capacity, and operating hours). Because of the average salinity of the river, model results were compared to state-level regulatory thresholds for marine bathing beaches. The water quality modeling process identified the flow rate/turnover time, pool capacity, and hours of operation necessary to ensure that the + POOL met relevant regulatory thresholds for bacterial contamination.

An example of the water quality modeling for one arm of the pool is provided above. The hours that the + POOL is open are shaded in blue. Continuous flushing of the pool overnight allows the bacterial concentrations that arise from skin shedding during the day to decrease nightly. The saw-toothed pattern observed during the hours that the pool is open results from short, periodic pool maintenance closures each day.

Comparison of Enterococci Concentrations in the + POOL to Regulatory Thresholds During 1 Week of Operation

Graph with Saw-Toothed Pattern Measuring Bacterial Concentration in Pool Over a Week Period

Hours that the pool is open are shaded blue. The red lines represent the state regulatory thresholds of <104 CFU/100 mL in single sample (solid line) or a geometric mean <35 CFU/100mL over a 30-day period (dashed line) (NYSDOH, 2018). The black lines represent the enterococci concentrations: solid lines for maximum concentrations, dashed lines for mean concentrations. The higher black line peaks are the “high estimate,” while the lower peaks are the “best estimate” using a variety of parameters (e.g., bather load, solar inactivation, bacteria load per bather). Thus, the dashed black lines should be compared to the dashed red line; while the solid black lines should be compared to the solid red line.

Click to Enlarge Figure.

Operational strategies such as requiring bathers to shower before swimming, continuing to operate the pumps outside of pool hours, and enacting intermittent pool closures (e.g., for lifeguard breaks and routine pool maintenance) are all practices that help ensure that enterococci concentrations in the + POOL meet state-level regulatory requirements even under conservative scenarios, such as assuming the pool is at capacity during all hours of operation and for low levels of solar inactivation of bacteria on cloudy days.

Gradient is thrilled to continue contributing our technical skills to the development of the + POOL, which would allow New Yorkers to swim safely in the East River for the first time since 1938.

Contact Info

The authors can be reached at jlemay@gradientcorp.com and adale@gradientcorp.com.


New York State Dept. of Health (NYSDOH). 2018. “State Sanitary Code; Bathing beaches.” 10 NYCRR 6-6.2. Accessed on June 6, 2018 at https://govt.westlaw.com/nycrr/Browse/Home/NewYork/NewYorkCodesRulesandRegulations?guid=Ided74c30b65511ddb903a4af59fec65a&originationContext=documenttoc&transitionType=Default&contextData=(sc.Default`).