Should I install a garbage disposal with a septic system?

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Q: Some garbage disposals are advertised as suitable for homes with septic systems. Is it wise to install one?

A: You can if you want, but plan on pumping — or at least inspecting — your septic tank more often unless you’re using it sparingly.

To understand why, it helps to understand the basics of how homes not connected to a centralized wastewater treatment system clean wastewater from toilets, washing machines, showers, bathtubs, and sinks. This water goes into a septic tank, which typically holds 1,000 gallons or more. Solids gradually settle there and form a layer of sludge at the bottom, fat floats to the top and forms a layer of foam. In between, thanks to the work of bacteria and other organisms, is relatively clear water that is piped to a drainage field, sand heap, or other system that gradually releases the water in a way that doesn’t pollute the environment.

But if the layer of sludge or scum gets too thick, the water going into the drain system isn’t clean enough and can clog it, leading to expensive repairs or even replacement of the entire system. For septic tanks to function efficiently, homeowners are recommended to pump their septic tanks every one to five years, depending on the amount and type of waste entering the sewer.

Of course, cramming food waste into a garbage disposal adds enormously to the volume, and grinding up orange peel, chicken bones and all sorts of other food waste poses a different challenge for the bacteria than when the incoming solids are mostly human waste. People with septic tanks have therefore been recommended for a long time to refrain from garbage collection or at least to have their septic tank pumped out much more often.

But InSinkErator, which dominates the food waste disposal industry in the United States with 70 percent of sales, says all of its models work well in homes with septic systems. (Whirlpool announced Aug. 8 that it would buy the company for $3 billion.)

InSinkErator touts its Evolution Septic Assist Quiet Series model ($299 at Home Depot) as being particularly friendly to septic systems, as it “automatically injects more than 300 million enzyme-producing microorganisms” with each use to help break down food debris, before they arrive the septic tank. (The dose is 1/3 teaspoon, resulting in a cost of about $1.50 per week if the disposal is used three times a day, since a replacement pint of Bio-Charge enzyme product is $19.98 at Home Depot.) But the company offers a rather lame nod to the need for this additive, describing it as a way “to do what’s best for your system,” but notes that “any garbage disposal can help households with septic tanks use leftovers responsibly.”

The Environmental Protection Agency, while not addressing brand-specific products, says flatly that additives, including those with bacteria and enzymes, “are not necessary for a septic system to function properly when treating domestic wastewater.” A website dedicated to common questions about sewage treatment also clearly talks about using a garbage disposal if a home has a sewage treatment plant. “If you have to use a garbage disposal unit, your tank will have to be pumped more frequently.”

To support the claim that septic tanks connected to a garbage disposal don’t need to be pumped more frequently, InSinkErator’s website cites a 2019 study based on a simulation using 20-liter test tanks (about 5.2 gallons each) compared 110 days or just over 3½ months. One tank contained only human waste; the other also received enough food waste to increase total suspended solids by about 30 percent. The food waste was found to be more biodegradable, “suggesting that the addition of FW has an impact on pumping frequency [food waste] will be insignificant.” But how valid is this conclusion for real septic tanks, which typically hold at least 1,000 gallons and operate for years?

When a woman answering a customer service number for InSinkErator was asked if the company could cite research on the effects of using disposal equipment in homes with septic systems, she referred to a 1998 posting on the company’s website that said: a company engineer summarizes and interprets research at the University of Wisconsin.

The study compared five ways of handling food waste, from hauling it away in garbage trucks to disposing of it down drains equipped with disposal facilities. Channeling food waste through disposal into a central sewage system costs local governments less than having garbage trucks haul the leftovers to a composting facility. However, the most cost-effective scenario for the municipality was to dispose of the waste in a house with a sewage treatment plant – in which case the homeowner would bear the costs.

To include disposals in homes with septic systems as an option, the research team enlarged the field for septic tanks and drains by 25 percent to accommodate the additional food waste — an indication of the expected impact of disposals on septic system performance and the additional costs for homeowners . However, the InSinkErator engineer noted that “with a disposer, an appropriate bottom type, and a ‘standard’ sized system, systems have operated trouble free in cold climates for well over a decade.” And he said that the use of bioadditives in a septic system would reduce the required size and therefore the cost of the septic system, as would using an InSinkErator disposal with the Bio-Charge additive.

So what’s the bottom line? Garbage disposers shred food waste enough that it shouldn’t clog the pipe to the septic tank. Non-food waste, like hair and candy wrappers, are more likely to do this. But there’s no guarantee that the mix of food scraps pouring down your drain will match those used in the simulation, which determined that no further pumping would likely be required.

Because repairing or replacing a septic system can get very expensive, simply use it to remove the leftovers after you’ve exhausted most food waste when you want to install a garbage disposal. Or have your system pumped more frequently, or at least have it checked more frequently until you can estimate how often to pump. The test: Is the bottom of the foam layer within 15cm of the bottom of the outlet, or is the top of the sludge layer within 30cm of the outlet?

You definitely don’t want to put off pumping longer than you should. According to homeadvisor.com, an inspection costs about $300 and pumping averages $417. However, installing a new system could cost anywhere from $10,000 to $25,000 or even more — and that’s before you factor in the cost and hassle of replacing the landscaping that will likely ruin the installation.

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