In a gravity system the drainfield gets liquid whenever we use water in the house. Make sure your drainfield isn't overloaded by excess water drained from roof tops, hot tubs or anything else under your control. Keep any irrigation system at least 10 feet from the edge of your septic system. Try to maximize the exposure of your drainfield to sunlight and wind currents to improve its performance. Surrounding the perimeter of your drainfield with tall trees not only blocks winds, but roots can invade the drainfield lines, interfering with uniform delivery of effluent.
Tree roots can plug up or divert sewage from receiving proper treatment.
There are no magic additives to routinely clear tree roots from the lines or your tank.