Now comes yet another apparent threat to the home gardener growing his own vegetables, and to the farmer, and to the smoker and to everyone who eats beef liver. It is cadmium, a heavy metallic chemical element found in many places, but most controversially in the sewage sludge that has gained increasing use as a soil enricher.

There was rejoicing in sewerage circles when sewage treatment technology finally made human waste safe for uses as a soil conditioner, like compost or peat moss. With the discovery that cadmium is dangerous and common in sludge, controversy has arisen over the way hazardous substances are defined and dealth with in modern society.

At stake are current sewage-teatment methods, many industrial processes, the lushness of much American pastureland and crops and home gardens, and the overall future of the sludge generated in profusion by every American community.

On one side there are experts such as Dr. Rufus Chaney, noted plant physiologist and cadmium scholar at the U.S. Department of Agriculture, who says high cadmium sludge products used on farms, pastures and tobacco fields may may be at the root of much kidney disease now thought to be purely personal in origin.

Cadmium is taken up by plants and stored, mainly in leaves, without harming the plant. It collects in human and animal kidneys and livers since the body has no ordinary means of getting rid of it. The more cadmium-high sludge used in America, the more kidney disease, Chaney said.

On the other side are experts like Charles G. Wilson, director of sales, marketing and agronomy at the Milorganite division of the Milwaukee sewage district. That district has been selling heat-dried activated sludge under the Milorganite name for 52 years, and it is considered "high" in cadmium: 70 parts per million. (By contrast, sludge produced by sewage treatment in the Washington area is low in cadmium.)

Whilson said it is "crazy" to focus on cadmium in sludge when the heavy metal is found or used in so many other places: ordinary rock phosphate fertilizer is 260 parts cadmium per million. Most metal plumbing pipe-makers use it in galvanizing and it is rampant in the runoff around factories that plate metals. Tires have it and put it on the streets as they wear down; tobacco takes it up from the soil wherever it grows and cigarettes are full of it.

"Why are they picking on sludge? We're building mountains and mountains of sludge and where are you going to put it if we can't put it on the land, just tell me that," Wilson exclaimed.

Cadmium was thought to be the "next environmental cause" in the early 1970s when its prevalence and threat were first noticed in sewerage circles. Chaney alleged that four to 10 times "normal" cadmium intake, defined as less than 25 parts per million of anything, would give a person a 50 per cent chance of kidney failure by the age of 50.

Massive doses of the heavy metal fed to rats caused kidney failure and death. In Japan, cadmium's interference with calcium absorption caused a painful bone disease among those who already had a low-calcium diet: Itai-Itai, or "ouch-ouch" disease, along with kidney failure.

The problem with these discoveries was and is that the experts clashed over whether experimental high-dose results on rats were valid for humans and on whether getting rid of cadmium would be worth all the economic disruption it would cause. Not all the sources have been traced, little if any work has been done on possible substitutes for it in industry and efforts to rid sewage of it are still in the experimental stages.

Concern over possible cadmium and other heavy metal content in sludge caused Maryland to set up stiff regulations in August, 1976, over the statewide sale of sludge from any of the state's 200 or so sewage-treatment plants. Complex and expensive, the regulations are often overlooked, according to Maryland Environmental Services operations chief Robert Pierce.

"The damn thing's gone too far. We're not even sure it's necessary. These guys (sewage processors) just give up on getting the permits when the stuff is backing up in their tanks. They just call up a farmer and say come and get it," Pierce said.

Chaney said fewer than half of all states have any guidelines on cadmium and no one has kept track of where sludge has been used. "I think we will find other cadmium-polluted areas where we can find individuals who ate large amounts of leafy vegetables and who then had kidney problems," he said. "States will have to deal with this in the future."

"We'll have 360 tons to compost coming out every day, 365 days a year, and if we can't sell it in Maryland it'll put is in deep trouble," said Al Cassell, research chief at the D.C. Environmental Services Department's water resources management division.

Although cadmium is not reduced in current sewage treatment, it is diluted in the composting process. Its uptake by plants and retention in human and animal livers and kidneys is also affected by such things as the acidity of the soil and the amount of zinc also present. Zinc is another natural element that occurs everywhere in varying amounts. Lots of lime keeps the cadmium down, while lots of zinc, in Cassell's words, "will kill the plant before the cadmium gets high enough to kill you."

Cadmium content also varies from plant to plant. It generally settles in the leaves, which means that smokers and cattle and those who eat lettuce and artichokes pick up more than those who consume oranges and tomatoes and corn and other non-leaf vegetables. Even so, the dangerous-dose level is controversial and has not been formally determined.

Washington's Blue Plains sewage-treatment plant, which produces 800 tons of sludge every day, gave it away free until 1970, as did Alexandria until recently. Now it goes to haulers who spread it in layers and cover it with dirt to reclaim gravel pits and enrich highway medians and city gardens, notably last year's Bicentennial Garden. Some 450 tons a day is simply buried in trenches.

Washington sludge happens to be very low in cadmium, ranging from 7 to 15 parts per million, because there is little industry in the region. Chicago sludge, by contrast, can range up to 200 parts per million. Reassured by this, Maryland is apparently on the verge of relaxing its rule against uncontrolled sale to the public of a new product, composted sludge.

A pilot project at Beltsville is mixing 50 to 60 tons of sludge per day with wood chips to produce an odor-free soil conditioner that does what peat moss and bagged garden compost do, only more cheaply. Construction has begun on an Oxon Cove, Md., project to compost nearly all the Blue Plains sludge output until 1985, and negotiations for a composting site are under way in Montgomery County.

"Sludges over 25 parts per million (cadmium) should never be given to anybody," said Chaney. Wilson disagreed, adding that he thought the issue had been blown out of proportion.

There are no studies proving that vegetables grown on cadmium-high soil cause problems for humans, Wilson said. Figures comparing Washington's "low" and Chicago's "high" cadmium sludge can be misleading, he continued: One ton of Milorganite per acre, as recommended, puts down less cadmium than the 25 tons of Washington sludge often used per acre here, he said.

"What Chaney wants is for people to get the cadmium out of their sludge and we are taking steps in that direction," Wilson said. He reviewed the history of battles over disposal of sludge at sea, in rivers, in landfills and by burning, all of which caused uproar over air and water pollution.

"The safest place to put sludge is still on the land," he said.