Septic tanks and cistern tanks: what is the difference?

Are you confused about what is a septic tank and underground cistern? Let’s help clarify which the two are and some of the most frequently asked questions about them.

Cistern tanks These tanks are designed for the underground storage of drinking water. The resins used are approved by the FDA for contact with products intended for human consumption, septic tanks are not. They are also heavier than septic tanks, which means they can be routinely filled and emptied without concern for the integrity of the side walls. Cisterns can also collect and store rainwater.

Underground septic tanks These tanks are designed for human waste, both black and gray water. Normally, a bioadditive is introduced into the tank that consumes the waste and the by-product is inert. French drains (pipes with holes) are connected to the tank and the clean inert fluid is discharged over a large area and absorbed by the ground. The nutrients in the discharge are really beneficial for the plant life in the area. They are available in one and two compartment models. Peabody Engineering recommends that you check with individual state regulations to see which model works for your area. These tanks, which are generally 750 gallons or more, are usually pre-installed.

Main 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 tank or above ground? No. Underground tanks are only designed for direct burial applications. There are other tank options available if you need an above ground tank, including holding tanks designed to slide under mobile office trailers.

How deep can these tanks be buried? Peabody Engineering recommends that you bury them no more than 24 “deep. Common culvert extensions are 15” and 24 “long to allow access to your buried tank below grade without having to dig a hole in the ground. to get to him.

Do I have to fill the tank with water during the burial? With standard service (yellow) septic tanks, the answer is yes. This helps stabilize the tank and prevents it from shifting during filling and equalizes the pressure in and out of the tank so it does not warp during installation. It is recommended, but not required, to fill a Bruiser (a heavier septic tank) or cistern with water while refilling.

What size septic tank or cistern do I need for my home? The proper size of a septic tank or cistern depends on several factors. For septic tanks, the average wastewater flow is the most important consideration. Most states require a minimum septic capacity of 1000 gallons. For the size of the cistern tank, you need to determine your home’s water use. You can find a good website to research which size is best for you at For cistern tank size, consider the home water use calculator at

What septic tanks and cisterns are legal in my state / province? The tanks have been approved coast to coast and even in some international locations.

    Ideal for:

    • Holiday homes in rural locations
    • Camps
    • Resorts
    • Construction sites.

    The cistern tanks are FDA approved for safe drinking water storage and will not impart taste or odor to your water storage. They comply with FDA 21CFR 177.150 (c) 3.1 and 3.2 regulations. Septic tanks are IAPMO and CSA approved. If you need help figuring out which tank is approved for your state, Peabody Engineering has a list of these tank model numbers and we will be happy to assist you with that information.

    We have more information on underground septic tanks and cisterns if you want more help.

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