There is only 12 inches of level diff. we have from septic tank to bath floor and drain water level and distence is 53' long. Could I slope 1 1/2" in 53'? (for 6" dia PVC pipe)

I am not sure I fully understand your question. If you have 12" of difference between septic tank and your bathroom than you can run up to:

1. 120' of 4" pipe
2. 60" of 3" pipe

Therefore, you should have no slope issue at all with 6" pipe ....

I would not go to 1/16" per foot slope because it is very hard to maintain over the developed length of pipe. You have no control over slope once you start filling in trenches as well as ground settling. I would lay 6" PVC at 1/8" per foot. Don't forget to include clean out within 24" of the foundation.

Hi Homeweaver...

6" pipe requires 1/8" pitch per foot of pipe... ;) So, you would need at least 6.5 inches pitch to meet code requirements and to be assurred that waste will drain properly!

Let us know if you want to discuss more...

MARK