The controversy is indicative of deepening religious conservatism in the American Orthodox world. William B. Helmreich, a professor of sociology and Judaic studies at the City University of New York Graduate Center, said that "in a society where people feel via the Internet and television their very values are under constant attack, there's a need for people to reassert their level of religiosity, and one way this is done is by discovering new restrictions which give people the opportunity to demonstrate their adherence to their faith."

For generations, the most pious Jews -- even revered rabbis like Joseph B. Soloveitchik and Moses Feinstein -- drank unfiltered New York water with no evident concern. But six months ago, a group of Brooklyn rabbis were examining some lettuce imported from Israel that was supposed to be bug-free, but which appeared to have insects on its leaves. After an investigation, they determined that the "bugs" had arrived after the lettuce was washed in New York City water, and said that in the right light they could see the telltale specks with their own eyes.

At some point, a delegation of rabbis took a field trip to the city's reservoirs and asked officials some detailed questions about the origins of the water and the copepods. (Of the three reservoir systems, only one -- the Croton -- is in the process of introducing filtering, with a plant that will cost an estimated $1 billion but will not be completed before 2010.)

The question lingered unresolved by a major communal authority until the Orthodox Union, which certifies as kosher 275,000 products in 68 countries, weighed in last August after checking some water samples.

"When they saw the first sample they didn't feel it reached the threshold of being visible," said Rabbi Menachem Genack, the rabbinic administrator for the Orthodox Union. "What changed people's minds is when they saw a sample taken from a pond and saw them scooting around. Those are beyond the threshold."

The Orthodox Union recommended that restaurants and caterers under its supervision filter their water before using it in drinking and cooking, a policy that quickly was adopted by many homes as well. The policy considered different practical possibilities. Dishes may be washed by hand in unfiltered water, it said, if the dishes are towel dried or left to drip-dry without puddles of water in them.

But it also said water should not be filtered on the Sabbath because one of the 39 varieties of work forbidden by the sages includes "selection," or sifting of food, like separating wheat from the chaff or raisins from a noodle pudding.