There is no reason not to use a dishwasher with a septic tank, just as there is no reason not to use a washing machine. There are perhaps several precautions however. A dishwasher add two types of loading on a drainfield. It add more water to the field and it add more dissolved solids in the form of surfactants and detergents. It would perhaps be best to run the dishwasher after the household had gone to bed in order to maximize the time the drainfield has to absorb the water. You should also look to treatments with Septic Scrub on an anual basis. This is because you are adding waste to the drainfield which can use up the available oxygen in the ground. Septic Scrub can help keep the drainfield aerobic. You should also perhaps pump your tank a little more often. The additional water can decrease the amount of time the waste has to seperate in the tank. Depending on where you live, another alternative would be to install a dry well for the dishwasher and washing machine. This relieves all stress on the drainfield.