Septic system design question

[RobA]

I need to get a septic system approved for our new house and hired a company yesterday to take care of the process. Without getting into expensive solutions are there practical limits as to how far the drain field can be from the septic tank in a basic system?

 

[dooleysm]

Our drainfield starts about 150' from from our tank, and includes 5 lateral lines that run another 100' away from the septic tank.

 

[PILOON]

Distance should be no problem as long as you have gravity on your side.
The field is basically gravity fed from the tank.
That being said, many systems due to terrain elevations need to pump uphill to the drainage field and then require a pumping station as well as a pump.

I have a 16 ft rise to my field .

I know of one recent installation that is using 2 stages of pumps as he is on a waterfront at the base of a mountain and to meet our new environmental standards he needed to pump up the side of the mountain and then they mandated an 'equoflow' system (big fiberglass dome filled with peatmoss) as there is virtually no topsoil for a proper field.

 

[polo1665]

Our leach fields are 700' from the tanks. It is a gradual downhill slope to the leachfields. The leach fields cover an area of 70' x 150'. It is a dual system, allowing me to switch from one leachfield to the other. I make the switch once per year.

 

[Highbeam]

I like a lot of fall between the house and the tank and then the tank and the drainfield. This often requires distance. Distance is no issue so long as you have enough elevation change to provide the required slope in the pipe. Especially after the septic tank, where solids are removed, the liquid is clear and can travel a long ways.

The beauty of a drainfield being much lower than the tank is that if there is ever a restriction in the drainfield the effluent will still leave the septic tank. It may surface somewhere but it won't screw up the tank or backup into the home.

I actually prefer, and so does our state, the pumped systems where the standard septic tank dumps into a pump tank where a pump turns on and off to deliver a couple of hundred gallons at a time through small diamter lines into a fairly traditional drainfield. The pressure systems are superior due the "doseing" of effluent, the even utilization of the drainfield since the gravity systems really only dump into the first pipe perforation, and the diregard for proper pipe grading since you're pupming. They do need power though and another pump and tank.

 

[Egon]

[the gravity systems really only dump into the first pipe perforation,/QUOTE]

On some tanks with a siphon system you will get batch dumps.

 

[schmism]

our septic guy said the tank could be no more than 100 feet from the house.

the drain field has no practical distance limit. The difference is your relying on the 1.6 gallons of water that went down with the solids to carry those solids to the tank. To far away and the likely hood that the water and solids separate goes up, leaving a pile in the middle of the pipe which eventually leads to a blockage.

the drain field on the other hand is leaching only liquid water so as long as its headed down hill, it will drain.

 

[skidoo]

A little sidetracked, but what happens to a system that stops being used, i.e. vacated for a long time?

 

[Highbeam]

Yeah, that's bogus. We run sewer mains that have flatter slopes and generally only get plugged up with grease on rare occasions. I go more than 300 feet between manholes on these mains. If you don't have proper slope on the sewer lines then you can get problems. The important point though which I agree with you on is that it is better to have a long run of septic tank effluent than to have a long run of sewage. Just be sure that the septic tank is able to overflow onto the surface before backing up into your home in the event of a cloggage in or upstream of the tank.

The siphon systems are very cool and rely on a good bit of elevation change. The resulting batch that is dumped to the drainfield has actual pressure head and behaves much like a pumped system. If you can get a siphon system to work then they are superior to traditional gravity and in some ways superior to pumped systems since they need no power and have no moving parts.

 

[Dmace]

Typically the only limiting factors to septic tank placement are elevation change, sill height, wetland buffers and protective well radius. If your land drops too fast then the tank will have to be closer to the house since the pipe to the tank cannot be over 5% slope. Too much of a drop will cause the house waste to "stir" up the effluent that is settling in the tank and the solids will go to the leach bed instead of staying in the tank to break down. At over 2% you need to put a "T" fitting on the line into the tank to slow the speed.

If you are within a wetland buffer, you can usually get away with specifying that the tank be "sealed" so that it cannot leak out into the ground.

For this area (NH), the typical protective well radius is 75ft so the tank and leach bed cannot be within 75ft of the well. This can change depending on septic type, well type and waivers granted.

I have not seen any problems with typical septics staying stagnant for months at a time like seasonals. It may even allow the growth of more good bacteria that is helpful with the break down of the effluent.