Digging a trench for a sewer line

[Winslow]

The system will work assuming the pipe is laid with uniform slope on a bed that will not sag. You have to use 4" as code (UPC) dictates that anything under 4" must be 1/4 inch per foot. The reason behind the required slope is to achieve a 2 ft/ sec velocity which is the optimum velocity for the water scrubbing the walls of the pipe.

 

[Leejosepho]

The system will work assuming the pipe is laid with uniform slope on a bed that will not sag ...

Yes, very critical ... and that is why I had mentioned stretching a string along the top of the pipe to be sure it is "straight" (flat in a single plane, and with no sags or "bellies") after being properly bedded and prior to being covered up.

Does it matter which end I start laying the pipes? In other words is it easier to run the pipes from high to low or low to high?

I would begin at the house and then do whatever it takes to get a proper connection at the tank. If the job was being done with thin-wall pipe that already had a bell on one end, I would begin with a nipple at the house and then the bell end of the first piece of pipe to make joints "smoother" along the way (with flow going into a bell rather than hitting the end of the next piece of pipe). However, you need to be using Sch40 here, and that does not come with a belled end as far as I know.