The Washington Post Co. announced yesterday that it has entered into a limited partnership for the purpose of constructing and operating a newsprint mill in Hanover County, Va., about 25 miles north of Richmond.

The Post will have a 30 percent interest in the $100 million mill, which will also be owned by a subsidiary of Dow Jones & Co., inc. (also hold a 30 percent interest) and The Bato Co., a Greenwich, Conn-based firm. Bato will hold a 40 percent interest and will manage and operate the plant.

Post officials said that they hoped to realize 50,000 tons of newsprint annually by the time the mill is up to full operation, which should occur by 1983.

The mill, which Bato vice president Tom Armstrong said would be "as advanced technologically as any in the country at the time of completion," is expected to generate up to 175,000 tons of newsprint a year.

According to Post Co. President Mark Meagher, The Post and the two newspapers it owns, the Trenton (N.J.) Times and the Everett (Wash.) Herald, use about 175,000 tons of newsprint annually.

The plant, which will be constructed near the town of Doswell on the North Anna River, will use a thermo-mechanical refiner process similar to one used by Bato at its plant at Riviere du Loup, Quebec. Dow Jones also has a 39.9 percent interest in that operation.

The thermo-mechanical refiner process is considered environmentally advanced. "It's the only kind of plant we could have built in the United States," said Armstrong. Because it does not rely on chemical treatment, the impact on the environment in terms of pollution and odors normally associated with older paper mills should be practically unnoticeable, according to Meagher.

But environmrntalists and citizens' groups in Hanover County are worried about the mill, according to Margaret R. Miller, co-chairman of the Citizens for Sensible Growth.

Miller acknowledged that at first, the plant didn't sound like a bad idea because it would help bring the Doswell sewage treatment plant, which was having financial difficulties, up to capacity. "We weren't too unhappy," she said. "We decided not to fight it."

But Miller said she and others became concerned when they learned that the plant would be burning coal instead of oil, and as estimates of plant discharge into the river continued to grow - from an early daily dischage estimate of 200 pounds to 825 pounds. The existing Doswell plant, Miller observed, coild handle only up to an additional 200 lbs. daily over its current load.

"The river is just too small," Miller said. "There's just not enough water to accommodate that kind of discharge."

But sources at Bato say that even at the 825-pound level, the discharge would fall well below current federally mandated limits of 2,000 pounds a day.

Environmrntalists contend that the plants is going to ruin canoeing on the river, which apparently has exceptional canoeing rapids. In addition, many claim that water temperatures will rise in the river because of the plant's discharge, and it is feared that the spawning season of certain fish - notably shad and herring - will be disrupted by the change.

Miller is also troubled by the fact that the plant will burn coal, which she claims can cause possible air pollution problems and would mar the scenic vista offered by Rte. 738, the approach road to the facility.

She said that the State Water Control Board issued permits for the new facility in record time, possibly under prodding that if the permits weren't granted in a timely fashion, Bato would be forced to back out on the project.

According to Ray Jenkins a state pollution control engineer, the decision to issue a "no-discharge" permit was expedited because Bato requested it on the grounds that "we'll lose our financial backers if you don't."

Jenkins said that Bato "indicated they needed the permit in a hurry, so we complied."

Officials at The Post, however, denied that there was any such pressure to rush the permits.

It really dosen't matter, according to Jenkins, who said the new plant would not harm the river or even change the water temperature. "The imlact," he said, "will be almost nothing."