Activists worry that the fracking process itself could contaminate groundwater. But evidence so far indicates that mismanagement of this wastewater is a greater threat.

Start-ups, venture capitalists and large companies, including Veolia and Siemens, see riches in water cleanup and are developing and testing various technologies. They are also working in other areas besides shale gas, including Canada’s oil sands and the use of water to pressure oil out of wells.

One of these companies is Ecosphere Technologies of Stuart, Florida, which uses ozone as a disinfectant to clean water in a process called advanced oxidation. The treatment, which does not use chemicals, can both eliminate the chemicals typically used for bacteria control and scale inhibition during fracking and recycle 100 percent of the water, according to Charles Vinick, the company’s chief executive.

Ecosphere says it has cleaned more than two billion gallons of water and eliminated the need for more than 1.7 million gallons of chemicals at approximately 600 oil and natural gas wells in U.S. shale fields since 2008.

It is now adapting its techniques to oil and natural gas liquids production, which are more profitable than shale gas at present. These operations also produce dirty water that needs to be cleaned and recycled, Mr. Vinick said.

Some companies have created “turnkey solutions” that allow gas companies to clean water on site and to validate the results by testing. The validation step makes it particularly attractive, Ms. Shenkar said.

One of the biggest players in this market is WaterTectonics of Everett, Washington. The company has a global licensing agreement with Halliburton, one of the largest oil services companies, for the frack water treatment market and other applications.