The Fairfax County Board of Supervisors voted yesterday to establish a citizens committee on child abuse after receiving a task force report that said the county has improved its record in identifying and treating victims and offenders but needs to do more.

The committee will follow up the work of the Child Abuse Prevention Task Force, which was formed in 1984 to review the county's programs, funding requirements and possible legislative initiatives. The committee also will seek private funding for prevention and treatment programs.

"Fairfax County has a long way to go before we can feel comfortable with our response to the needs of abused children," said Mary E. Collier, who served as chairman of the task force and is School Board chairman. The average number of child abuse cases reported each month to the county's Child Protective Services rose from 465 in 1984 to 653 in 1986, Collier said.

The task force report -- a summary of progress on 29 recommendations made in an initial report in March 1985 -- said that although persons are seen immediately in crisis situations, they sometimes have to wait to receive treatment and "often are 'fitted' into existing treatment programs that may not be tailored to their special needs."

Waiting lists exist at two programs, according to the report, which criticized "the lack of local residential/hospital programs for sex offenders, many of whom cannot be adequately served on an outpatient basis."

The report also said that the county had not proceeded quickly enough to develop a computer system to coordinate information and programs among county agencies and had been slow in implementing a county policy on child abuse.

Nonetheless, Collier praised the county for allocating $800,000 to hire new employees for child abuse programs, hiring a child abuse program coordinator, embarking on a four-year program to increase the number of after-school day care slots at county schools from 3,060 to 4,620 by fall 1990, and commissioning a $110,000 consultant's report to assess child protection services.

Verdia L. Haywood, deputy county executive for human services, said the county promptly and thoroughly investigates reports of child abuse, but acknowledged that "on the treatment side, we are going to have to beef up our capability."

Haywood said the county had set aside $500,000 to implement changes recommended in the consultant's report, and funding increases are under consideration for the next fiscal year for child abuse programs. Haywood said that in addition to improving efficiency through better management and computer systems, eight to 14 positions would be needed to eliminate waiting lists.

County Executive J. Hamilton Lambert said the county wanted to streamline services, possibly by consolidating some programs, and the staff hopes to have formed a countywide child abuse policy by July.

In other action yesterday, the board voted 7 to 1 to approve final financing for a controversial $252 million incinerator near Lorton that would burn about 3,000 tons of trash a day. Supervisor Gerald Hyland (D-Mount Vernon), in whose district the plant will be located, voted against the measure, which was the last hurdle before full construction of the facility can begin.

Lorton residents, concerned about toxic ash, had fought the incinerator in a court case that ultimately went to the U.S. Supereme Court, which refused to block its construction. Hyland said yesterday he voted against the incinerator because the county had relaxed its emission standards, options for future expansion at the site discouraged recycling and any new incinerator should be built farther west in the county, where new growth will generate more trash.