When we moved in a new house (our was manufactured) we were "required" to have the septic tank pumped. Prior to that, the tank had been in for 18 years. The man that came to do the pumping said our tank was in great shape and really did not need pumping, but still had to be done (county required)
The thing is, we NEVER had the washing machine use the tank (have 30 acres - and we are in Fl - porous soil)
He also told me that when the new house came, we should separate the washing machine and tub water and use a gray water tank for that. (we had 6 people in the house)
He explained that it is not just the chemicals that go down the drain, but the amount of water itself. What he told me was that the amount of water that the washer and the tubs/showers put into the system so flush the system that the bacteria never gets a chance to grow properly and break the solids down. He went on to tell me that if a tank is properly used, it will NEVER need pumping, since it will liquefy everything that goes into it - eventually.
I might mention, this man had been pumping septic tanks for more than 50 years. He was well known by all the septic companies in the area as the best and most knowledgeable person in the business. (Yes, he was in his 70's and still pumping tanks)
I followed his advise and we have not had any problem with the tank.
I did have to re-work the drain-filed, as the old pipe was compacted with sand and could no longer pass water. Since putting in the new drainage I have not had any problems. :thumbsup: