Millions of black plastic balls floating on a drinking water tank may seem like an environmental disaster, but for Los Angeles they have become an innovative solution to several major challenges.
The unusual sight has attracted global attention, leaving many wondering why a city would intentionally cover its water supply with 96 million plastic balls. Far from being a publicity stunt, the floating “shadow balls” were introduced by the Los Angeles Department of Water and Power (LADWP) to conserve water during California’s severe drought, protect drinking water quality and meet stringent environmental regulations.
The project has become one of the world’s most innovative and cost-effective examples of modern water management.
What black balls fell into drinking water in Los Angeles?
The black balls are hollow plastic balls known as shadow balls, each about 10 cm (4 in) in diameter. Made from high-density polyethylene (HDPE), a durable plastic commonly used in water pipes and milk bottles, each ball contains a small amount of water to keep it stable and prevent it from being blown away by strong winds.
The Los Angeles Department of Water and Power (LADWP) released all 96 million shadow balls into the Los Angeles Reservoir in August 2015, completing what was then the largest shadow ball project in the world.
The carbon black coating protects the plastic from UV rays, giving the balls a life expectancy of about 10 years.Los Angeles Reservoir covers about 175 acres (71 ha). Engineers have calculated that about 96 million shadow balls would be needed to cover most of the water’s surface.
Instead of forming a solid layer, the balls float freely and automatically adjust as water levels rise and fall. Together they form a protective blanket that blocks sunlight while allowing the tank to continue operating normally.
Reducing evaporation during drought in California
One of the primary goals of the project was water conservation. Southern California often experiences long periods of drought, and large open reservoirs lose significant amounts of water through evaporation under intense sunlight.
By shading the surface, the balls reduce the amount of solar energy reaching the water, which helps reduce evaporation. LADWP estimated that the project could save nearly 300 million gallons (about 1.1 billion liters) of water each year, enough to supply thousands of households annually.
Preventing the formation of bromate
While saving water is important, protecting drinking water quality was an even greater priority. The tank contains natural bromide, which can react during the water treatment process.
After disinfecting water with ozone, exposure to sunlight can convert bromide into bromate, a chemical classified by the International Agency for Research on Cancer (IARC) as a probable human carcinogen.
Since California has strict limits on bromate levels in drinking water, blocking sunlight greatly reduces the likelihood of this reaction occurring.
Keep algae, birds and debris under control
Shadow balls provided many additional benefits. By limiting sunlight, it limited algae growth, which helped maintain water quality and reduced treatment requirements.
They also prevented birds from landing in the tank, reducing the risk of contamination from droppings. Additionally, the floating layer reduced the amount of dust, leaves and other wind-blown debris entering the water, making tank maintenance easier.
Why don’t we just build a roof?
Building a permanent roof or floating cover over a tank of this size would have been technically challenging and very expensive. According to LADWP, the shadow ball project cost about $34.5 million, far less than the estimated cost of constructing a large engineered shade.
The balls also require less maintenance and can be deployed relatively quickly, making them a practical solution that meets regulatory requirements.
How were the shadow balls installed?
Installing the balls was surprisingly easy. Thousands of boxes containing plastic balls were delivered to the tank by truck. Workers released them into the water through large chutes, and the balls spread naturally across the surface. Because they float independently, they constantly rearrange themselves as wind and water movement changes, ensuring consistent coverage without the need to be fixed in place.
Does plastic pollute drinking water?
No, Shade Balls are specifically manufactured for use in potable water tanks and are made from materials approved for long-term use with potable water. The carbon black coating also helped protect the plastic from decomposing in the sun. Throughout their use, water quality was continuously monitored to ensure that the balls did not negatively impact the city’s drinking water supply.
What happened to the shadow balls?
Shadow balls were not intended to be a permanent feature.
As improvements were made to the Los Angeles Reservoir and surrounding water infrastructure, including upgrades to the treatment and storage systems, many of the original balls were removed. Those that have reached the end of their service life have been recycled where possible, while the modern design of the tank has reduced the need for a floating cap.
A simple idea that caught the world’s attention
The image of millions of black balls floating above the tank initially baffled people around the world, but the project has proven to be an innovative response to the multiple challenges facing Los Angeles. By reducing evaporation, reducing bromate formation, inhibiting algae growth, and protecting water quality, shade balls demonstrated how a relatively simple engineering solution can deliver significant environmental and public health benefits.
The initiative remains one of the most distinguished examples of creative urban water management, and continues to be studied as an example of practical and cost-effective infrastructure design.