Alice Waters, a pioneering chef and the matriarch of the sustainable food movement, has become an unlikely target in a battle being waged by food activists in San Francisco over a city program that converts sewage sludge into gardening compost.

Several nonprofit groups want the city to stop distributing its “biosolids compost,” which contains solid waste that is treated and removed during wastewater processing, because they say it can contain potentially harmful substances like heavy metals, pharmaceuticals and flame retardants and should not be used on gardens and agricultural land.

Until recently, the San Francisco Public Utilities Commission, which manages the city’s wastewater treatment facilities, gave the compost to residents for free, describing it as “nutrient-rich organic biosolids.” The commission maintains that the compost is safe, complies with federal standards, and has metals concentrations that are “lower than you would find in a daily multivitamin.”

Nonetheless, the agency has suspended the giveaways pending the results of “comprehensive and comparative lab tests.”

“We have an independent lab running a detailed analysis on not only what is required by the E.P.A., but on other emerging pollutants of concerns,” said Tyrone Jue, a spokesman for the commission. “As we have from day one, we want to continue making all our data available for the public in the interest of full disclosure and for people to understand the safe biosolids compost they have received through our pilot giveaways.”

As part of its ongoing campaign, the Organic Consumers Association, a nonprofit advocacy organization, has pressed Ms. Waters and the Chez Panisse Foundation she founded (named after her restaurant) to condemn the city’s compost program – in the same way she once took a stand against the use of genetically modified organisms in food.

The use of compost made with treated sewage sludge is not permitted in organic farming.