The Fairfax County Jail, which has been under attack for its treatment of inmates, has been priased in an outside study as "generally a well managed and operated facility."

The study, commissioned by the Fairfax Board of Supervisors and done by HMB Associates of Fairfax, said the jail "is responding in a professional fashion to currently recognized correctional standards and guidelines. The (jail) ranks highly in this regard and in comparison with similarly situated jurisdictions throughout the country and the Commonwealth of Virginia."

The supervisors decided on the study after 28-year-old Donald Ferguson of Gum Springs died at Western State Hospital in Staunton, Va., after a four-day stay at the jail. Ferguson was the third black inmate to die after confinement at the jail during 1978.

A citizens panel headed by State Sen. Joseph V. Gartlan (D-Fairfax) concluded earlier this month that Ferguson was subjected to "barbarous treatment" at the jail. The panel said Ferguson did not receive adequate medical treatment there, and blamed the detention center for being partly responsible for his death.

The HMB study did not address the Ferguson case specifically -- that will be done in a state investigation yet to be made -- but said "medical and health care (at the jail) satisfies nearly all applicable correctional standards.

Sheriff James D. Swinson, in a memo yesterday to Acting County Executive J. Hamilton Lambert, called the HMB report "an excellent job" and said, "The time has come to quit knocking our jail and to recognize it for what it really is: a fine institution well managed and well run . . . "

While praising the jail's staff and its administration and operation, the report did make three major criticisms. It said the facility was overcrowded, understaffed and needed computerized record keeping.

Deputy Chief M. Wayne Huggins, the retiring Swinson's protege and Republican candidate to succeed him in the November election, said jail officials have in the past brought all three problems to the attention of the Board of Supervisors, but did not get the money to correct them.

The most serious problem, the report said, is overcrowding, the effects of which are "felt throughout the operations and administration of the jail." The only solution, the report said, is more bed spaces, and it called for 68 more beds at a cost of $1.3 million.

The report also recommended that seven more staff members be hired.

To create more space, Swinson wrote Lambert that the county should go forward on construction of a separate prerelease center for 50 inmates so jail space now occupied by the center would be used to add more inmate beds.

Swinson also said the Northern Virginia jurisdictions should build a "regional farm" where those persons sentenced for minor and nonviolent offenses could be kept.