Grinder Pump Before the Septic Tank?
Grinder Pump Before the Septic Tank?
(OP)
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.
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.





RE: Grinder Pump Before the Septic Tank?
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.
RE: Grinder Pump Before the Septic Tank?
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.
RE: Grinder Pump Before the Septic Tank?
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.
RE: Grinder Pump Before the Septic Tank?
I agree with your colleague.
RE: Grinder Pump Before the Septic Tank?
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
RE: Grinder Pump Before the Septic Tank?
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."
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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.
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RE: Grinder Pump Before the Septic Tank?
-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?
RE: Grinder Pump Before the Septic Tank?
RE: Grinder Pump Before the Septic Tank?
RE: Grinder Pump Before the Septic Tank?
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.
RE: Grinder Pump Before the Septic 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.
RE: Grinder Pump Before the Septic Tank?
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
RE: Grinder Pump Before the Septic Tank?
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.