Septic lift pump

EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
The septic tank is gravity fed from cabin drains.  The field is uphill from both the cabin and tank, roughly 15' higher and ~100' distant from the tank, with a sewage pump in the last tank chamber.  The system has been in mainly summer seasonal use for ~4yrs.

It works, but pretty much since day one the pump seems to run longer that I think it should.  It takes ~800w running, so I've tended to run it as an opportunity load after batteries get into absorb.  Obviously, this can sometimes allow the tank level to get a bit higher than if I let the float switch run it whenever, but letting it do so can end up with it running all night, draining batteries.

I haven't pulled the pump to get specs yet, but I'd guess it's 1/3 or maybe 1/2hp and I'd expect a pump like that to be able to handle 100's of gallons/hr, and empty the tank completely in the hours it 's been running, and get to the float shut-off level in an hour or so.  The float switch on/off distance is ~18", which I's guess is maybe 20% of the tank, say 400 gallons.

I'm wondering if anyone here has experience with this kind of system and can tell me if my expectations are realistic (1-2hrs to dose the field).

One thought I've had is maybe the pump doesn't efficiently pump given the total head.  There's a check valve and a couple of ells to prevent draining back in summer, and a bypass to allow drainback in winter.  I've relaced the check valve, and it makes little difference running via the bypass..  There's also the head / friction losses in the pipe to the field, and the field itself.  Would this be enough for the pump to run for so long?  Maybe a pump with higher total head capacity needed?

Another thought was maybe the field can't take the flow all at once and would be better on an on/off schedule, or maybe dosed from another tank uphill from field to gravity feed more gradually.

Thoughts?
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Comments

  • icarusicarus Solar Expert Posts: 5,241 ✭✭✭✭
    My guess, and it is only a guess is that the field is not taking water as fast as the pump can deliver, so it is constantly fighting a head of water with no where to go.  A couple of things to consider, some easier to implement than others.  How about putting the pump on a temple timer (or do it manually for a while) so that it runs for 10 minutes then stop for a hour to let the effluent drain into the field.  Or you could consider a much smaller pump that runs at way fewer liters per minute, but pumps at a rate that the field can absorb.  

    I might talk to a knowledgeable septic guy.  (I hope that is not an oxymoron?) 

    15’ of head translates to ~7 psi roughly.  Add in the friction losses (how big is the pipe?) and the load is still pretty small.  100’ is not that much pipe given the size of the pump.

    Tony
  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    I think the pipe is 1.25". 

    Will try the 10 minutes/hr dosing.  It does make sense that it may not handle the load all at once, although you'd think it should be designed to  handle whatever the loading is between the high and low float switch levels.
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  • icarusicarus Solar Expert Posts: 5,241 ✭✭✭✭
    It (the entire system) is designed to take X amount of water into the drain field and disburse it a day.  My guess it the pump run cycle (and the level between low and high) was probably not a major design criterion.  Design criterion used to be ~250 gpd/per person.  Assuming you have a 1000 gallon holding tank, and the volume in the last (pump) chamber is ~150 gallons (1/2 of 1/3?) pumping 150 gallons at a time into a field is a lot of water to absorb, compared to that same 150 gallons over say 4-8 hours in a pure gravity system.

    All that said, this is WAG on my part.  Good luck and see how it works.  

    Tony

    PS The other thing you could do, but probably are not inclined to do, would be to add a second tank, higher than the field, pump into that and let it drain by gravity?
  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    Adding a second tank above the field is definitely a possibility if it is a loading issue. 

    OTOH, if it turns out the pump has trouble with the head, it might still be a problem?  I'm not sure how much head the field adds, so I guess it should be only slightly higher than the field.

    Our overall load should be well under design load.  It's a 3 bedroom cabin, so design would be 6 people.  Normal occupancy is only 1 or 2, and pretty water miserly at that. 
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  • icarusicarus Solar Expert Posts: 5,241 ✭✭✭✭
    edited September 10 #6
    Try this for an example...Let the system go “fallow” for a couple days if you can.  Before you do, drop the pump chamber as much as possible.  Then let chamber fill to near 100% after a few days.  Then, assuming dry weather/ground, pump the thing full bore and see how long it takes to pump down to the low float level.  That might serve to confirm that it is an infiltration rate issue.

    Tony

    PS.  As a test, you could check the current draw when the pump first starts pumping (not starting current) and then again 1/2 way into the cycle.  Some increase might be expected simply as the motor and pump heats, but a significant current increase would indicate (to me anyway) that the pump is working harder because of increased head due to infiltration lag?
  • BB.BB. Super Moderators, Administrators Posts: 29,508 admin
    Pump/motor loads do depend on the type of pump.

    The pictures of random sewage pumps seem to be centrifugal pumps... Motor current/load is maximum at maximum GPH flow. As flow rate drops (higher back pressure, or even using a valve to reduce flow volume)--The less current the motor will consume (down to a minimum with zero flow).

    If the pump is a positive displacement type, the lowest power/current is with no back pressure... And as back pressure increases, the current will increase. And you can stall the pump (and cause damage) if you restrict flow to zero (and very high back pressure).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,241 ✭✭✭✭
    ^You know...you hang around this forum, and you learn something new everyday.  (Not that I could tell the difference between the two pump systems!)

    T
  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    I'm pretty sure it's a centrifugal type... it looks pretty much like other septic pumps I've seen.  I've rented sailboats with diaphram pumps for off-shore discharge, which I assume are displacement type.  They'd have very little back pressure, so would be pretty easy on a house bank.

    After initial startup, the pump seems to be relatively constant current draw.  Pretty gloomy here today so I'm being especially miserly with water. If there's sun tomorrow, I'll experiment a bit more.
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  • Marc KurthMarc Kurth Solar Expert Posts: 476 ✭✭✭✭
    My system was designed and and specified to run the ejection lift pump for 5 minutes, every 2.5 hours. A low level float in the tank locks out the pump if the tank level is satisfied. While I really do not like so many start/stop cycles from an electrical/mechanical perspective, the intent is to avoid exceeding the capacity of the field.
    In my case, this approach feeds the field a maximum of 37 gallons per 5 minute run, so it is setup for 360 gallons per 24 hour period,
    My system is comprised of a lower and upper half because my home is built on a slope, with the leaching field and another tank system on the uphill side.

    I always have more questions than answers. That's the nature of life.
  • icarusicarus Solar Expert Posts: 5,241 ✭✭✭✭
    EST...

    yea I know about gloomy...we had nearly 100mm (4”) of rain last night, and it has been low fog and drizzle all day, infact for 10 of the last 15 days!

    I think Marc has it about right.  Try the timing and see how it works.  Tony
  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    Low fog and drizzle all day here too, but not 100mm overnight.  That's a lot of rain for this neck of the woods.  My son's considering Vancouver for more school.  Personally, I don't think I could hack the grey and rain.

    @Marc Kurth ; how much vertical is there between the lower and upper tanks?  I'm also on a hill, with the field higher.
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  • Marc KurthMarc Kurth Solar Expert Posts: 476 ✭✭✭✭
    edited September 11 #13
    @Estragon ; - About 16 feet of vertical, with 120 lineal feet of 2" pipe between the tanks. The field is another 7' to 8' higher than the upper tank.
    BUT, my septic system is an aerobic processing type, with the effluent distribution field being 1750' lineal feet of 1/2" drip line, buried 16" deep. I live on the shore of a drinking water reservoir, so the rules are pretty tight, hence the complexity of the septic field.

    I always have more questions than answers. That's the nature of life.
  • wellbuiltwellbuilt Solar Expert Posts: 391 ✭✭✭
    Hi estragon , 
     in my are most septic systems are rated to Except so many gallons a minute based on size of your leaching field. 
     You system is Probably throttle back to deliver A determined amount of water to the field . 
     Check in your pump tank , there should be a pvc ball valve in there to control the flow of water.
     We pump to leach fields 100s of feet away , we test the flow at the field by filling 50 gallon drums of water for x Amount of time,to get the flow rate . 
     I take the handles off the valve so it can’t be changed . 
      They could restrict flow with a small pipe also . 
     We use mostly 1hp or 11/2 hp pumps . 
     You could get a smaller pump but 1/3 hp is not that big 
     
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  • wellbuiltwellbuilt Solar Expert Posts: 391 ✭✭✭
    Another thought , 
     if you have the septic plan it will have the flow rate on it . 
      Also it should tell you how they want you to restrict the flow, there could be a valve at the field. 
      Some times the flow is controlled by the leaching  manifold  , the plan would read some thing like this , 11/2 sch 40 drilled at 12” OC 5/16 th hole . Yes the hole size is different on every job .
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  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    Hi wellbuilt.

    I don't think there's a ball valve in the tank, just a check valve after the elbow fitting on top of the tank.  There could be a valve or pipe restriction at the field though.  The installer probably did a perc test, but it's pretty much just a thin layer of duff over rock around here, so the fields are almost always made imported sand etc., with known drain properties.

    I'll check back on the paperwork I got after the install to see if there's a plan or something specifying flow restriction.

    Thanks
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  • mike95490mike95490 Solar Expert Posts: 8,373 ✭✭✭✭✭
    Or something may have failed in the system and debris is plugging things up.  All sorts of things get inadvertently flushed into the system
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  • jonrjonr Solar Expert Posts: 1,297 ✭✭✭✭
    edited September 15 #18
    A pressure gauge on the pump would make it clear if there is unexpected back pressure (vs intake restriction).  I agree that the options are an upper tank, a smaller pump or rapid cycling of the current pump.

    I assume that the pump intake is from some middle level in the septic tank?  Or in a small, dedicated chamber to achieve the same thing?

    I am available for custom hardware/firmware development

  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    mike95490 said:
    Or something may have failed in the system and debris is plugging things up.  All sorts of things get inadvertently flushed into the system
    Anything is possible.  Once or twice I've found the tank access open (likely a rather stupid bear), so leaves or whatever might have got in.  We're pretty careful about what gets flushed.  Visitors get a spiel covering what gets flushed, but do they always comply?  Dunno.
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  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    The pump is in a separate chamber (the last  of 3, I think).  There's a cylindrical plastic mesh filter before that chamber that should catch anything larger than ~ 1/8".

    I think my next step will be to pull the pump to check for any obvious intake clogging, and if possible to get specifics on this particular pump.  Looking into replacement pumps, it appears many in the 1/3-ish hp range spec a max head of ~15'.  I'll try to estimate the elevation more accurately.  With elbows and pipe added, I'm thinking maybe that's the problem.
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  • wellbuiltwellbuilt Solar Expert Posts: 391 ✭✭✭
    did You notice the pump running longer ? 
     The pump could be going bad , they really don’t start to pump less water  95% of the time they just die .  (I’ve only replaced a few ) 
     You could feel around on the bottom of the tank to see if there is sludge build up .
     There should be no sludge in the pump tank .
     The pumps we use here could grind up just about any thing that could make it over the dividers in the tank .
     If your field was clogged you would have spluggie bubbling up out of the ground before  you had enough back pressure to slow the pump . 
      The manifold is usually only 12 to 16 inches below the grass .
      If you are pumping more water at one time then you system is designed to handle this could cause more back  pressure . 
       Luck John 
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  • wellbuiltwellbuilt Solar Expert Posts: 391 ✭✭✭
    One other thought , 400 gallons is way to much water for one watering my last pump system had a 1200gallon pump chamber with 4 11/2 hp pumps that discharged to 2 huge leach fields 40x96 feet and 36x50  these field where getting 180 gallons and 220 gallons . 
     This was for a apartment complex . 
     I don’t think your system should dump 400 gallons at a time , it should be more like 100gallons 
     2 hours  is a long time to water a field . 
     Could your check valve be stuck ? 
     A 2000 gallon tank is very big for 3 bed rooms here we would use a 1000gallon tank with a 250 gallon pump tank 
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  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    The tank may not be as big as I guessed.  Code here for a 3br is a flow rate of 1600l/day, and minimum tank size is the greater of 2x flow or 3600l.  3600l is roughly 1000gal.  They may have installed larger than that code minimum, but likely not twice as big.

    Assuming 1000gal tank, the dose size could well be ~100gal.  This makes it even more surprising to me that it can take many (6-8+) hours per cycle.  Looking at a flyer for a  1/2hp pump rated at 3600gal/hr (max head not spec'd), should only run a couple of minutes for 100gal?

    I don't think the check valve is stuck, as opening a bypass (which I installed to allow winter drainback) doesn't seem to make much difference.  Also, I've had to replace the original check valve (freeze damaged), and the problem predates that.
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  • wellbuiltwellbuilt Solar Expert Posts: 391 ✭✭✭
    I guess all you could do is pull the pump and see if it’s working , check to see if the  pump blades are good . 
       If the pump tests good try to to find the end of the line ,  then you could test flow there could be a rock stuck preventing full flow . 
     Guys put the pipes together and never look in side to see if there is a plug up , things get stuck in the elbows and even couplers . 
       
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  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    It's possible.  I'd like to think the guy who installed it wouldn't be so careless, but who knows?
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  • icarusicarus Solar Expert Posts: 5,241 ✭✭✭✭
    Can you break the outfall line (with a repair coupling) and measure the flow rate with no head?  Might give you an idea if the pump is running correctly.  Pump your outlet chamber dry first into the field if you can, then fill the outlet chamber with clean water?

    Tony
  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    Not a bad idea.  Weather is nicer, and the admiral will be gone tomorrow.
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  • Marc KurthMarc Kurth Solar Expert Posts: 476 ✭✭✭✭
    edited September 17 #28
    Estragon said:
    The tank may not be as big as I guessed.  Code here for a 3br is a flow rate of 1600l/day, and minimum tank size is the greater of 2x flow or 3600l.  3600l is roughly 1000gal.  They may have installed larger than that code minimum, but likely not twice as big.

    Assuming 1000gal tank, the dose size could well be ~100gal.  This makes it even more surprising to me that it can take many (6-8+) hours per cycle.  Looking at a flyer for a  1/2hp pump rated at 3600gal/hr (max head not spec'd), should only run a couple of minutes for 100gal?

    I don't think the check valve is stuck, as opening a bypass (which I installed to allow winter drainback) doesn't seem to make much difference.  Also, I've had to replace the original check valve (freeze damaged), and the problem predates that.
    At the risk of being repetitive, I can't help but feel like you are thinking of a dosing rate that is very high. My system isn't your system, but the concepts are similar.
    For example, my system was designed for an average usage of 360 gallons per day, so the system is sized for 500 gallons per day. The tank capacity is 1,000 gallons to buffer occasional high usage periods.
    Currently the effluent pump is set to dose the field slowly with only 37 gallons, every 2.5 hours in order to avoid over saturation.  This equals 360 gallons per day. As you can see, my dosing rate is 7.5 gallons per minute, for only 5 minutes at a time.
    Marc
    I always have more questions than answers. That's the nature of life.
  • jonrjonr Solar Expert Posts: 1,297 ✭✭✭✭
    edited September 18 #29
    > The pump is in a separate chamber

    Make sure that flow into this chamber exceeds the expected pump output rate.

    I am available for custom hardware/firmware development

  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    I'm going to give the short dosing a try over the next day or two, ~5-10mins run every hour or so during daylight hours.  There is no timer designed into the system (just hi/low float switch), but it wouldn't be hard to add one if it solves the problem.
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  • EstragonEstragon Registered Users Posts: 3,962 ✭✭✭✭✭
    jonr said:
    > The pump is in a separate chamber

    Make sure that flow into this chamber exceeds the pump output rate.
    Why?
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