Question:

We are purchasing a house with a new 1,500-gallon septic tank. As part of the home inspection, our inspector gave us the rundown on septic tanks. On the topic of our garbage disposer, he rightly recommends against using it for anything other than very light duty. But he went on to further stress never put onions or cabbage down the disposer, as it will (somehow) harm the ecology of the septic system. My wife and I have been scratching our heads as to how/why this could be. Do you have any insights? Also, any tips on when/how to use a diverter valve installed between the main tank and the leachfield?

Answer:

I have never heard the onion or cabbage story in regard to affecting the bacterial action in a septic tank. I can’t imagine what impact either cabbage or onions might have. Any raw vegetables will not break down completely and so will accumulate more solids in the septic tank. I see no reason why the bacterial action should be adversely affected.

AVOID CABBAGE

Your story is interesting because my wife and I recently had an experience with a plugged garbage disposer. We moved to a new home with a garbage disposer. We were disposing of cabbage leaves when the unit became badly plugged.

The serviceman who cleaned out the disposer and waste piping stressed that cabbage should not be placed in the disposer. He also included onion skins, celery and banana peels as items to keep out of the disposer. So, I do know onions and cabbage can plug a garbage disposer. Perhaps this is the problem your inspector was referring to rather than in the septic tank.

Grinding bones or other materials that will not decompose with bacterial action will also add solids to the septic tank sludge layer. Other items that should not be added to a septic tank, with or without a garbage disposer, are cooking fats and coffee grounds.

SIMPLE ADVICE

Any organic material that has not been through the human digestive system will fail to break down in the septic tank. This is why using a garbage disposer is not recommended with a septic system. We used to present this message at our Onsite Sewage Treatment Workshops in Minnesota: “Do not deposit anything into the septic tank unless you have eaten it first!” We did exempt toilet paper from the list.

The frequency of cleaning the septic tank will be increased with the use of a garbage disposer. The quality of the effluent going out of the tank will not be as good. This is because the organic materials have not been through the human digestive system and do not decompose as much in the septic tank.

I would also recommend a filter be installed in the outlet tee of your septic tank. This is especially important when a garbage disposer is being used. The outlet filter will prevent large solids from leaving the septic tank and getting into the drainfield. Some onsite codes now require septic tanks to have outlet filters.

DIVERTER VALVE

In regard to your question on using the diverter valve, I would assume the valve has been installed to use only half of the drainfield while the other half is not being used. I believe it is sound design to divide the drainfield and use one part while the other part is resting and recovering treatment capacity.

The designer and/or installer of the onsite sewage treatment system should have made recommendations on how to use the diverter valve in your system. Your inspector also may have some suggestions on the operation of the diverter valve.

In general, I would suggest a diverter valve be switched at least on an annual basis. There are some other considerations depending on the size of the drainfield. Also, the drainfield could be a series of drainfield trenches or two seepage beds.

If the drainfield has trenches and if there is an inspection pipe at the end of each drainfield trench line, a pumping professional can quickly determine how much of the drainfield is being used. If the drainfield trenches are on a slope and sequential distribution is used, it will only be necessary to check the effluent level in the last trench.

It would be a good idea to have each of the trenches checked at least once to see if effluent was reaching the end of the trench. If all the trenches on one side of the drainfield become full before the end of a year, then the diverter valve should be switched to allow half of the field to rest.

The portion of the drainfield that no longer receives effluent from the septic tank will dry out and oxygen will enter through the soil pores. The biomat will be oxidized as will some of the fine sewage solids. If the resting time is adequate, the drainfield will likely recover most or all of its initial treatment capacity.

The same procedure should be followed with the other half of the drainfield, which is being used. Once a time period or frequency has been established for switching the diverter valve, frequent use of the inspection pipes should not be necessary.