There is no official designation for septic safe laundry detergents. Liquid detergents break down faster and do not have the clay component that can add to the sludge layer. Using the correct amounts as listed on the product for effectiveness is a best practice.

It isn't necessarily doing laundry that causes the issues with the system—it is doing all the laundry in one day and overwhelming the system by not allowing the bacterial breakdown and degradation that happens naturally in the tank, but forcing too much effluent through the tank into the drain field.

The rule of thumb is two loads with a standard washer or three loads with High-Efficiency washers per day to maintain a more regulated system.