cleaning roof

[buyag]

We are power washing our roof and amazed by the amount of mold growth on it. It has been 4 years since last power washed, when it was painted. We are on catchment. We are using only water for power washing (no soap etc) and diverting all the wash water away from catchment. Is there anything that can safely be used to retard mold/mildew growth on roofs that are used for catchment/drinking water? ie. something applied and washed off that would kill the mildew so it does not grow back so quickly, but not be harmful for catchment/drinking.
Mahalo!