Anne Arundel County has suspended waste-water treatment service to Matlack Inc., the nation's largest bulk hauler, after tests showed the trucking firm was dumping toxic wastes into the sewer system.

Matlack has a plant in Brooklyn Park in the northern end of the county that serves as both a truck terminal and tanker-cleaning operation. Matlack's corporate headquarters are in Lansdowne, Pa.

A copy of a cease-and-desist order was taken to the hauler's terminal on Tuesday, while the original was mailed to company's headquarters.

Samples taken from the firm's treatment plant in late November revealed the presence of a variety of prohibited substances, including toxic wastes, said Thomas H. Neel, county utilities director. Neel said the situation posed no serious threat to the community surrounding the Matlack facility.

The county cited Matlack for continuing to clean its chemical tankers without equipment capable of handling toxic wastes. Neel said Matlack could not comply with the order unless it were to build a new treatment plant, hire a contractor to handle its wastes or scrap its tanker-washing operations.

The county's action was independent of a nine-month state investigation that led to a Dec. 18 court settlement in which Matlack pleaded guilty to five charges of violating Maryland's hazardous-waste laws.

A judge put the company on one year's probation and imposed the maximum fine of $125,000 in connection with violations that occurred between Dec. 1, 1983, and Aug. 5, 1985.