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Short Summary
Main recommendation: Cover open reservoirs with black shade balls to block sunlight, which prevents the formation of Bromate, a carcinogen, when chlorine and Bromide react in bright light. The balls also reduce evaporation by roughly 80 to 90 percent and suppress algae, and they were originally developed as bird balls to keep waterfowl away from ponds.
Key Points
- The problem began with Bromide, a naturally occurring, harmless substance found in salt water intrusions into sources like the California Aqueduct. When water is disinfected with Ozone, Bromide becomes Bromate, a carcinogen.
- Regulators' limit for Bromate is 10 micrograms per liter. The filtration plant measured levels within that limit, but a beverage company customer found high levels after the water passed through the open reservoir.
- Tests showed Bromide plus chlorine in bright sunlight produced even more Bromate than Ozone did, so removing sunlight was the only practical fix.
- Shade balls were chosen over floating tarps, PVC trampolines, and pipe debris fields, which were too costly, too slow, or would have created a warm, plastic-covered Petri dish for algae.
- The balls are a repurposed product called bird balls, originally used at mine-tailing ponds and airports to keep waterfowl off the water. Once deployed at LA Reservoir, the birds disappeared.
Details Worth Noting
- Concrete instructions: Shade balls in this project were a high-density polyethylene, food-grade plastic (the same material as a gallon milk jug), filled partly with water so they resist high winds and stay in the reservoir. The black color comes from carbon black, which lets the plastic last at least 10 years in sunlight. Other shades tested (three blues) were too unstable to last more than a year. The balls have no harm to drinking water, and the project's test used three inflatable kiddie pools, one in sunlight, one under a tarp, and one under shade balls, with the shade balls eliminating the Bromate problem immediately.
- Evaporation reduction is partly explained by the air gap in each ball acting as an insulator, similar to double-paned windows, and the water under the balls was measured to be cooler than under direct sun.
- Cost is about 33 to 34 cents per ball, roughly three for a dollar. The water official estimated that savings in chlorine, chemicals, and evaporation would cover at least half the cost over the balls' life, and the balls have modest salvage value as recyclable material.
- Hexagonal shapes were later proposed by other companies, but they were seen as problematic because they could stack or perch on the reservoir bank as water levels rise and fall; the round shape spreads to cover the water.
- Driving a boat through the balls is very hard, described as like driving through peanut butter, and the balls drift back into the wake behind the boat.
Bottom Line
Shade balls solved a specific chemical problem at LA Reservoir by blocking the sunlight that turns Bromide and chlorine into harmful Bromate, while also cutting evaporation and algae as side benefits.
