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Is A Garbage Disposal Bad For A Septic System?

Many people believe that using a garbage disposal with a septic system overloads the system and causes it to fail. However, this is not true! A garbage disposal can be used safely in conjunction with a septic tank so long as you do two things. First, the septic tank should be properly sized. Second, don’t put anything down your drain other than food waste or grease from cooking- these products are safe for your septic tank, even though they may seem like common culprits for clogs. So, in summary;

Garbage disposals can be used with septic systems so long as you have a properly sized septic tank and don’t put anything down your drain other than food waste or grease from cooking.

What is a septic tank and how does it work?

A septic tank is an underground tank that stores liquid waste such as sewage. The main purpose of a septic system is to prevent human waste from contaminating groundwater supplies and surface waters. It consists of two compartments: the primary, or settling chamber, and the secondary, or aeration compartment.

Image of a septic system

A septic tank receives water and wastes from all parts of the house and stores them until the solids settle to the bottom and are broken down by bacteria. The liquids then leave through a pipe called an effluent outlet that leads to a leech field or seepage bed, which disperses pollutants harmlessly into the ground.

If you’re thinking about installing a garbage disposal, then it is important to make sure that your septic system is properly sized. If there isn’t enough room for all of the water and waste, this could cause backups in your plumbing lines.

What if the septic system is too small?

If you have a septic tank that’s too small, then you can add to it with an extension chamber. This is also known as the “auxiliary” or “side-arm” system and will cost around $4000-$5000 for installation.

Or you can simply choose not to use a garbage disposal and choose to compost your food wastes instead.

Does a garbage disposal overload the septic tank?

A garbage disposal works by grinding up food scraps and other materials with water. The grinded-up particles are then flushed down the drain and into the home’s septic system.

As I mentioned before, the food wastes in the septic tank are broken by bacteria. But when garbage disposals are used, it overloads the septic system with loads of food waste which adds an extra load for the bacteria to break down.

When a garbage disposal is used, it’s not just a matter of more scum on top of liquid but also an increase in solids that settle to the bottom that take longer for decomposition.

The good news is that even though the garbage disposal adds more solid wastes into the septic tank, these food wastes are finely ground, so they will quickly decompose without overloading the septic tank.

But all these depend on the size of your septic tank. If the septic tank is too small, the bacteria may not get enough time to break down the extra load of food wastes, and this will cause the septic tank to overflow. So you must have a properly sized septic system to use a garbage disposal.

What size septic tank to use with garbage disposal

There are no particular size calculations for a septic tank with a garbage disposal. A properly sized septic tank should be able to manage the extra effluents coming from the garbage disposals.

The proper size for a septic tank is calculated based on the number of people in a house. For a house with five bedrooms, a 1500-gallon septic tank is more than enough. To estimate the size of the septic tank, the water discharged into the septic tank per day is calculated. Then based on a detention period of three days and the amount of sludge settled down for three years, the final capacity needed for the tank is calculated. Detention time is the length of time water is retained in the tank to allow for the solids in it to settle down.

A general guideline for the right septic tank size is as follows;

Number of bedroomsSeptic tank capacity
Two bedroom house750 gallons
Three bedroom house1000 gallons
Four bedroom house1250 gallons
Five bedroom house1500 gallons
Recommended septic tank size with a garbage disposal installed

Here is a good guide to calculating the size of your septic tank: http://www.civilology.com/septic-tank-capacity-calculation/

How to prevent garbage disposal from overloading your septic tank

1. Use septic tank additives

If you are still worried that a garbage disposal will overload your septic system, there are some other ways to overcome this. Using septic tank additives is a great way to increase the bacteria that break down food wastes and reduce the septic system load.

However, not all septic additives are safe for septic tanks. The government and water companies provide guidelines for what additives should be used with septic systems. They are usually enzymes containing nitrogen, phosphates, and potassium.

2. what not to put in a garbage disposal with a septic tank

For the safety of your septic tank, the additives added to it shouldn’t contain bleach, lye, and phosphates. These are not safe for septic systems. Lye is a corrosive chemical that can eat through pipes, reducing the lifespan of a septic tank. Bleach is toxic when combined with other chemicals, it kills beneficial bacteria in the septic tank, and creates stronger odors, and impairs the functioning of the septic tank. Phosphate additives can also kill beneficial bacteria in the system and can contribute to increased systematic corrosion of metal parts (water regulators).

BioWonder and Septic Saver are two biological additives that are safe to be used with septic systems.

3. Do not overuse the garbage disposal

Remember that your garbage disposal is not a trash can. It would help if you didn’t put every food waste into it. If you have a house party, I recommend putting all the food waste into the trash can rather than sending them to your septic tank. While garbage disposals are safe to be used with septic tanks, they are not meant to be overused.

If you overuse your garbage disposal, it will overload the septic tank and its pipes. Make your septic tank last longer by limiting the amount of food waste you put into the garbage disposal.

4. Do not use bleach or other chemicals that kill bacteria

Septic tanks can handle small amounts of bleach and other chemicals in the water. But if you flush down a lot, it will kill the bacteria that help to break down solids in your septic tank. Heavy use is damaging for this reason- avoid using too much!

Do you have to pump the septic tank more often if you use a garbage disposal?

Many people say that you will need to pump it more often, but there really is no truth to this.

A recent study conducted in a simulated septic tank observed that even though garbage disposals add more solids to the septic system, the impact on pumping frequency is insignificant since the solids are more biodegradable due to their fine size.

Septic tank being emptied by a man

But like I mentioned before, it is important you have the right size of a septic tank for this to work. Having more solid wastes in water increases its detention time and the amount of sludge accumulated. So if you have a smaller septic tank than required, you may have to pump it more often when using a garbage disposal.

best garbage disposal for septic systems

You can use any garbage disposal as long as your septic system is properly sized. Here are some garbage disposals that are the best;

1. InSinkErator Evolution Septic Assist

Septic Assist is a garbage disposal by InSinkErator specially made to be used with septic tanks. It has a bottle of liquid enzyme additives that boost the performance of aerobic bacteria in septic tanks. When the disposal is used, it injects a small quantity of liquid into the drains, eventually reaching the septic tanks.

Septic Assist is also very quiet and is run by a 3/4 HP motor which is ample to handle food wastes from a large household. For more details, please check the amazon product page.

InSinkErator Septic Assist

2. Waste King L-3200

Waste King L-3200 is a good alternative to the expensive Septic Assist model. It comes with a 3/4 HP rating and can grind all disposal-friendly food wastes as well as chicken bones and fruit pits. It doesn’t come with any of the bells and whistles of the Septic Assist disposal, but if you are looking for a garbage disposal to get the job done, L-3200 is an ideal choice.

You can check for more information on its Amazon page.

Waste King 3200 garbage disposal
Waste King 3200

Further Reading:

My name is Thomas Anderson, author of DisposalQA. I have 15 years experience working as a plumber in CA, and this is where I answer common questions about garbage disposals.