The Montgomery County planning board, dismayed at the state water department's decision to grant a Silver Springphoto processing laboratory permission to dump its waste water into Paint Branch, has requested a reconsideration of that decision.

According to William Chicca of the state agency, state water resources director Herbert M. Sachs may either reconsider the decision or appoint a hearing examiner to adjudicate the appeal of the case.

The dispute over the photo lab discharge has aroused the interest of environmentalists and trout fisherman who say that Paint Branch is one of the few streams in the metropolitan area pure enough for trout to breed.

The water department's permit allows Colofax Laboratories in the Montgomery Industrial Park to dump the 6,000 gallons of water, including water it uses to cleanse developed prints into a streambed that flows directly into Paint Branch near Old Columbia Pike.

Included in this discharge would be tiny amounts of such chemicals as silver and cyanide - chemicals which, trout fisherman fear, could harm or destroy the trout or their eggs.

However, Chicca explained this week, the water resources administration made a technical evaluation of the content of the proposed discharge and set limits on the amounts of the various chemical components that could be dumped into the stream.

A memo from the Maryland National Capital Park and Planning Commission's parks department, written earlier this year, said that the discharge "would appear to lower the quality of this stream, which was rated 'excellent' water quality by the Montgomery County department of environmental protection."

The discharge permit for the Colorfax Laboratories took effect April 1.

The parks department has expressed concern that since Paint Branch flows through Paint Branch Stream Valley Park, the department has an obligation to protect citizens from any potentially hazardous substances in the stream.

In an interview shortly before the discharge permit was granted last month, the environmental consultant for the owner of Montgomery Industrial Park said that, before it was discharged, the 6,000 gallons of water would be treated to remove as many organic and chemical substances as possible.

In addition to the park and planning commission, officials of Montgomery County government had also expressed opposition to the discharge permit before it was actually granted last month.

One environmental protection official for the county said Tuesday that the county isjoining the planning board's fight against the discharge, and that a county attorney has already written a letter of protest to the state.