Septic alarm going off even when no one is using water

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Apr 3, 2019
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Bellingham, WA
We have a septic system that was installed in 2011. The main floor of the house and the second floor are gravity fed to the tanks and the field, and there has never been a problem with them. The basement, which is furnished and houses 7 people, has a 20-gallon holding tank with a Liberty 382 effluent pump that pumps the sewage up to grade. The head is five feet. Once it reaches grade, the sewage is gravity fed to the septic tank. There is no check valve, at least, we don't see any check valve. About a year ago we added a washing machine and a macerating toilet and a shower to the system. The septic guys said the system is easily large enough to accommodate the increase in load. About three months ago, the septic alarm for the holding tank in the basement started going off, usually in the middle of the night, when no one is using the system, but sometimes when the system is being used (people showering, clothes being washed). When we go into the basement to check the situation out, we silence the alarm, and it sounds like the pump is humming but not pumping. Then we unplug the pump, and replug. There is a wooshing sound, the sewage is pumped out, and the alarm turns off.

So we first assumed the pump was at the end of its life. We replaced it with an identical pump. The same thing kept happening.

We brought a septic guy out. He said the floats weren't set right - one float was hanging on the wall. He adjusted the floats, charged $250, and left.

Didn't help, alarm kept going off about once every 24 hours. We pulled the pump up, decided maybe it was faulty, and replaced it with an identical Liberty. That didn't fix it. So we've been through three pumps - can't be the pump.

We opened the holding tank and completely cleaned it out with a shop vac, thinking there must be something floating around in the tank that is periodically clogging up the pump intake. We found nothing.

We opened the septic tanks outside, and looked in. They look full but they look below the outflow line, as best as we can tell. We're having them pumped in a couple days, but in talking to the septic pumping guy on the phone, he doesn't think that's going to be the problem.

We're pretty sure it's not the floats, and we're pretty sure there isn't anything blocking the pump intake. We're pretty sure it's not the pump itself. We don't know why the pump is humming. I guess the next step is to snake the head and the outflow line from the pump to the sewer tank outside?

It does seem to possibly be related to increase water use, since we added the toilet, washing machine, and shower. Not exactly sure if we are remembering when it started, though. It seems that everything was fine for a good three months with the additions and then suddenly it went crazy.

Edit: Just wanted to add that this last week, the tenants occupying the suite in which the macerating toilet, shower, and washing machine were on vacation. So those items were not used for a week, but the alarm still kept going off.
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