Septic Tanks and Cistern Tanks – What’s The Difference?

Are you confused about what a Below Ground Septic and Cistern Tank is? Let us help clarify what the two are and some of the most frequent questions that are asked about them.

Cistern Tanks These tanks are designed for below ground storage of potable (drinking) water. The resins used are FDA approved for contact with products intended for human consumption, septic tanks are not. They are heavier duty than septic tanks as well, meaning they can be routinely filled and emptied without worrying about sidewall integrity. Cisterns can also catch and store rainwater.

Below Ground Septic Tanks These tanks are designed for human waste, both black water and gray water. Normally, a bio-additive is introduced into the tank that consumes the waste and the by-product is inert. French drains (pipes with holes in them) are attached to the tank and the clean inert fluid is discharged over a large surface area and absorbed by the soil. The nutrients in the discharge are actually beneficial to the plant life in the area. They are available as one and two-compartment models. Peabody Engineering recommends that you check with your individual state regulations to see which model works for your area. These tanks that are generally 750 gallons or larger are usually pre-plumbed.

Major Differences Between Septic And Cistern Septic tanks are designed to be full at all times. Cistern tanks can be used as septic tanks, but septic tanks should NOT be used as cistern tanks.

Can I Use My Septic Or Cistern Above Ground? No. Below-ground tanks are only designed for direct burial applications. There are other tank options available if you require an above ground tank, including holding tanks designed to slide under mobile office trailers.

How Deep Can These Tanks Be Buried? Peabody Engineering recommends you bury them not more than 24″ deep. Common manhole extensions are 15″ and 24″ in lengths to allow for access to your tank buried below grade without having to dig a hole in the ground to get to it.

Do I Have To Fill The Tank With Water During Burial? With standard duty septic tanks (yellow), the answer is yes. This helps stabilize the tank and keeps it from shifting during backfill and equalizes the pressure inside and outside the tank so it does not become deformed during installation. Filling a Bruiser (a heavier weight of a septic tank) or cistern with water while backfilling is recommended but not required.

How Big Of A Septic Or Cistern Tank Do I Need For My House? Proper sizing of a septic or a cistern tank depends upon a number of factors. For septic tanks, average sewage flow is the most important consideration. Most states require a minimum septic capacity of 1000 gallons. For cistern tank sizing, you need to determine your home water use. A good website to research what size is best for you can be found at For cistern tank sizing, consider the home water use calculator at

What Septic And Cistern Tanks Are Legal In My State/Province? The tanks have been approved from coast to coast and even in some international locations.

    Ideal For:

    • Rural Locations Vacation Homes
    • Campgrounds
    • Resorts
    • Construction Sites.

    The cistern tanks are FDA approved for the safe storage of Potable water and will not impart a taste or odor to your water storage. They are compliant with FDA regulation 21CFR 177.150(c) 3.1 and 3.2. The Septic tanks are IAPMO and CSA Approved. If you require assistance in finding out which tank is approved for your state Peabody Engineering has a list of these tank model numbers and we would gladly assist you with that information.

    We have more information on Below Ground Septic and Cistern Tanks if you would like further help.