Grinder Pump Before the Septic Tank?

[EngWade]

All,

I have a colleague who wants to design a septic system discharging from a small commercial building (Peak Daily Flow ~ 1000 GPD) using a grinder pump/septic tank/absorption field system.

The field is located approximately 600 feet away from the grinder pump, and at an elevation about 30 feet above. This is the only location on site suitable for a wasterwater disposal field. My colleague mentioned to the client that we could put the grinder pump immediately outside of the building (to take the building's sewer lateral), as opposed to going from the building into the septic tank, and then into the pump. The "benefit" of this would be to not have the septic tank close to the building, and not having the septic tank vent pipe "ruining the site aesthetics."

So instead, the pump would pump sewage immediately from the building, up to the septic tank located adajcent to the absorption field. The septic tank would then deliver effluent to the absorption field by gravity.

I have indicated that I did not think this was a good idea, for a few reasons, most notably, likely septic conditions within the force main, no dosing volume for the absorption field, the inconvenience of getting septic tank maintenance equipment up the hill to service the septic tank (and preventing them from driving atop the absorption field itself), and that it was my opinion you should have vent pipes for grinder pump stations also, so you should be stuck with a vent pipe near the building anyway. My colleague disagrees. Also, the State Standards do not recommend this practice, and in fact, recommend against it.

Your thoughts are much appreciated. Thanks.

 

[Ron]

Placing a grinder pump after the septic tank is worthless. What is there to grind?

Dosing to the drain field is controlled by the septic tank volume...any influent creates effluent.

Yes, the conditions can and probably will go septic in the wet well, so you'll have to cycle the pumps accordingly (I would use a duplex pump system, without regard to the size).

The wet well should be vented, but could also be vented such that the odors do not permeate the building.

I see nothing wrong with this approach. It has been used many times. Septic tank maintenance should be very infrequent, so I wouldn't see that as a big deal.

 

[bimr]

There does not seem to be a problem with your colleague's comments.

The outside lift station recommended by your colleague is nothing more than a standard residential grinder pump station. E/one makes such a station. I have attached a drawing that shows a manhole type cover, not a vent pipe.

There should be no concern regarding septic conditions in the force main.

The force main should discharge into a septic tank 600 feet away. If you are concerned about someone driving over the septic tank, install pipe bollards around it.

I don't believe Ten State Standards addresses the installation of septic tanks.

 

[EngWade]

Thanks for the comments. While you are all convincing in your statements, let me continue a bit more from my side.

I've always been under the impression that absorption fields should be designed with a minimum dosing volume equal to 75% of the field's volume. I've even been held to this standard on previous jobs, by other regulators. By having the septic tank after the pump station, you essentially lose your ability to dose. My colleague's arguement was "water in = water out." Sure, I'll buy that, but water in at a velocity and (gauge) pressure won't exit at that exact rate when introduced to the pressure drop when it enters the septic tank (atmospheric pressure), and the velocity will be lessened further by the outlet tee.

Also, is it not true that each pump cycle would result in a "surge" load on the septic tank, with the potential to cause turbulent conditions in the septic tank, thereby resulting in possible resuspension of solids and the possibility of depositing these solids in the absorption field - leading to premature field failure? Now, I'm aware there would be an inlet tee and an outlet tee, but with a pump cycle volume of say 30 gallons, I thought it was a valid concern to resuspend/deposit septic solids into the absorption field.

I'm rather aware I'm making more of this than I probably need to be, but going this far helps me gain a much more comprehensive understanding, and so I learn better.

Thanks again for your input.

 

[Ron]

There are numerous things you can do to mitigate your velocity concerns. You could put a stilling basin in prior to the septic tank. It doesn't have to be large, just something to stop the turbulence; however, with a 600 foot run of pipe, there's a good chance the head loss in the pipe will solve those issues.

I agree with your colleague.

 

[beej67]

"Water in = water out" --> continuity equation, a fundamental underpinning of fluid dynamics, and holds true no matter what the pressure drops are in the line. I'm not sure what you're getting at there. If the pressure drop is high in the force main you may need a bigger pump, but for an incompressible fluid with no accumulator tanks or other weirdness in your system the flow rate "in" is guaranteed to be the same as "out." So is the velocity if the pipe size doesn't change.

I'm no expert on turbulent boundary layers within septic structures - seems like some PhD stuff there, given the variable viscosity of sewage, the potential for biofilm accumulation on the boundary surfaces, etc. But you could easily assume a viscosity and calculate a Reynolds number for your tank if you're that concerned, given tank geometry and flow rate. I highly suspect it's laminar. I also highly suspect it doesn't really matter at the far end of the tank.

Short answer - I personally think you're over thinking this.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[bimr]

As you can see from this reference, you can install the next larger size septic tank if you have a concern.

One would think with the small volume of your system, that the pumping should not be an issue.

"If a lift-station pump will be installed in the septic tank to allow pumping to an upslope absorption field, the regulatory agency may require the next size larger tank to offset the displacement volume of the pump."

http://extension.missouri.edu/publications/DisplayPub.aspx?P=EQ401

Here is another reference:

"Figure 7. Septic tank effluent pumping chamber. Working capacity should equal about one-fourth the daily sewage volume. Reserve plus working capacity should equal about one day’s sewage volume. This allows time to correct any pumping problems.

http://www.ag.ndsu.edu/pubs/ageng/structu/ae892-1.htm#sizing

 

[CarlB]

I'm familiar with primarily small gravity systems, and have questions not answers for EngWade.
-Is a vent over the septic tank mandatory? In our locale tanks do not have vents, just sealed risers. Venting occurs through the builing's vents.
-You mention lack of dosing being a problem with the grinder system. Standard gravity fields locally do not use dosing, just gravity flow-through. Is there some demonstrated performance increase with dosing, especially with small fields such as this? The grinder pump setup achieves some dosing action, nearly the same as a pump after/in the tank?

 

[jartgo]

You might also consider locating the septic tank at the building and pumping its effluent to the field (STEP). Otherwise you could put a small solids pump at the building (in lieu of the grinder) and pump everything up to the septic tank. Although what you've described would likely work with much headache.

 

[chicopee]

Is the grinder to act as a macerator?

 

[jgailla]

The scenario jartgo describes is the standard way of doing septic systems in north Florida.
There is typically a high groundwater table and the drainfield is often elevated above natural ground by a couple of feet. Therefore a small pump, usually a grinder, is necessary.
You could look at Florida's code (google FAC 64-E) for more information.
I don't see why there would be a problem pumping into the tank, but your lift station will also need to be vented and could develop an odor over time if the cycling time is not right or there are long periods between usages.

 

[Crystaltanks]

Firstly, here in the UK we do not pump into septic tanks. They are designed for gravity flow only and any pumping should be done after the tank.
Venting is required to prevent the gases causing a problem by escaping through the lids, and these vent pipes must go vertical within 10 yards of the tank, with a minimum height of 4 metres.
All pumping stations must be able to hold 24 hours sewage volume.

 

[bimr]

It does not make any difference to a septic tanks whether the flow is pumped or not.

Pumping stations are required in situations where the effluent cannot flow by gravity and must be lifted to a destination. Many new houses are using lift stations in the basement (with overhead sewers), and outside the house. The wastewater flows by gravity through the septic tank to the drainfield.

http://www.ag.ndsu.edu/pubs/ageng/structu/ae892-1.htm#sizing

 

[Spartan5]

jartgo definitely has the right idea.

Place the septic tank near the building and pump the effluent to the drainfield.
The septic tank should not require its own vent. It is typically ventilated via the buildings plumbing and vent system.

Placing a grinder pump ahead of a septic tank in a 1,000 GPD system is a bad idea unless the septic tanks are oversized to increase the retention time.

Septic tanks function by separating the solids in the waste via settling; and the fats, oils, and greases via flotation. These wastes are then broken down anaerobically within the tank. The relatively low strength effluent passes on to the drainfield.

Grinding the waste stream into a slurry prior to discharging it into a septic tank greatly affects the processes of settling and flotation which results in a higher effluent strength and decreased drainfield life. And you are correct, the pumping of the waste in the tank does create turbulence which can resuspend the small solids that were created during the grinding process. If I were designing a system with a grinder pump, I would oversize the septic tanks by 1-1/2 to 2 times depending on the site conditions at the drainfield.

Using an effluent pump instead of a grinder pump has additional benefit. An effluent pump for this system would be on the order of a 1/2HP, 10 or 20 GPM pump with a 20 year lifespan discharging to the drainfield using an 1-1/2" or 2" forcemain. Maintenance of this system would also be significantly less than that of a grinder pump system (clogs, pump rebuilds, etc.). Dosing can be controlled with a simple timer based control panel. And the same pump would be capable of distributing the wastewater equally around the drainfield using a network of small diameter pipe (1") and 1/8" orifices instead of "pump and dump" into 4" perforated drain pipe. This also improves drainfield life and treatment by using small discrete doses spread out over the course of the day which results in increased aerobic conditions in the field. Some regulators will allow you to reduce the size of the drainfield when using this technology.