Cambridge's water drinkers, already concerned about the liquid's current unsightly discoloration, gained another reason to worry this week as it was revealed that the city routinely cycles sludge filtered from the water back into a public reservoir.

A recent report released by the Massachusetts Department of Environmental Protection (DEP) says that for several decades, the Cambridge Water Department has dumped waste products removed from drinking water back into Fresh Pond.

The study, conducted by engineer David Erekson, recommends that the city cease the dumping as soon as possible. More than two tons of sludge, composed mostly of organic silt and alum, a coagulant used to separate waste, are dumped each day.

However, city and state officials say that the dumping poses no problem. According to Nancy E. Barnes, director of the water department, the water continues to comply with federal regulations and with the standards set by DEP.

Barnes said there are no easy alternatives to the dumping. The Cambridge sewer system's local limits dictate that "sludge" cannot be dumped in with the sewage. Negotiations are underway to have that policy changed.

Some other options for the sludge are incineration, pelletization or landfill, all of which would requireconstruction of a lagoon to dry it out.

According to Barnes, water quality in FreshPond has actually increased in recent years. Thealum dumped in the pond may be resposible for thatimprovement, Barnes suggested.

City officials also said they have reduced theamount of certain chemical compounds in Cambridgewater to levels that no longer pose a threat tohuman health. In 1990, the city admitted that thewater supply had been contaminated bytrihalomethanes, potentially cancer causingsubstances created when organic waste mixes withchlorine in treated water.

This fall, falling leaves and soil runoff havestained the water supply of Cambridge brown andyellow. However, neither the waste nor thediscoloration, which occurs every fall, signal anyhealth dangers, said Ed Dowling, a departmentchemist.

"Since [1990] we've replaced our filter mediaand also [instituted] pretreatment. The governmentallows a level of .1 milligrams of trihalomethanesper liter, and we've got .06," said Dowling.

Dowling said the color, caused by dissolvediron and manganese from leaves and soil, isdifficult to filter out.

However, he said, the department will soonbegin adding potassium permanganate, which willcombine with the iron and manganese to form asubstance that can be easily removed.

J. Kevin Reilly, an Environmental ProtectionAgency (EPA) drinking water expert, agreed thatCambridge water is safe to use. But he saw twopotential problems with the city's practice ofdumping sludge back into the water supply.

"The main problem is filling up the reservoir,"he said. "Possibly, depending on what they'retrapping in the sludge, viruses and protozoa couldalso become concentrated [in the redepositedwaste]."

But Reilly said the process breaks no federallaws, since the waste contains no metal orsyntho-organic substances.

City officials said that they are hoping tofind a solution by the beginning of next year andthen to start work on the logistics oftransporting the sludge. So far, the city hasbudgeted $40 million to either replace or upgradethe treatment plant once a decision is reached.

In the meantime, Cambridge residents, includingHarvard students, will have to live with re-dumpedwaste and discolored water