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Garbage Grinders or Garbage Disposes and Septic Tank Maintenance
Effects of Garbage Disposer Units on Septic Systems and on Municipal Sewers
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• Garbage Disposer & Food Waste Grinder effects on septic systems
• Garbage Disposal Unit effects on municipal sewer systems
• Garbage Disposer building codes & installation advice
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Garbage disposal units, also called garbage grinders, food waste disposal units, or garbage disposers add to the solid load in the septic tank. A garbage grinder is a mechanical grinder which receives food scraps, typically down a kitchen sink drain, and grinds them into a water/debris slurry that can pass down the building drain. Both electric and water-powered garbage grinders are available.

While garbage grinders are a convenience for the homeowner, the added waste they place in the septic tank means that the tank may need to be pumped more often. Photo courtesy of InSinkErator. The garbage disposal unit shown is their Evolution Septic Assist(TM) that includes an enzyme injector for use with private septic systems. This document is a chapter of "Septic Tank Maintenance: What Can Be Flushed into a Septic Tank - Septic Tank Maintenance and Cleaning Advice". This document explains how to extend the life of the septic system by being careful about what goes into it. Use of this information at other websites, in books or pamphlets for sale is reserved to the author.

This document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

GARBAGE GRINDERS - Garbage Disposal Unit Effects on Private Septic Systems

Some writers estimate that 40% of homes in the U.S. use a garbage disposal unit - but that statistic is not broken down between homes served by a private septic system and homes connected to a municipal sewer. Is a kitchen disposal unit, that is, a garbage disposer harmful to a private septic system? Let's look at what happens when a garbage disposer is used.

Garbage Grinder or Garbage Disposers impact on septic systems: food scraps ground and sent down a drain from a kitchen sink disposal unit increase the solids load in the septic tank. These solids may also be slow to separate and settle into the sludge layer or combine with the floating scum layer in the septic tank.

What is the effect of garbage disposals on septic systems?

Depending on how much use a garbage disposer unit receives, the added solid load it places into the septic tank can mean that the tank needs to be pumped more frequently. Adding to the actual load of ground food waste sent into the septic tank, we've observed that many homeowners are tempted by the presence of the garbage grinder to pour waste household oils and grease into the system. That's a bad idea.

Many septic codes (such as this New York septic code) and other septic authorities state that if a home has a garbage disposal unit installed it should count as an additional bedroom in estimating the load on a septic tank.

Other septic design guides suggest that when a garbage disposal unit is to be installed in a home with a septic tank and drainfield, the average daily wastewater usage level be increased by 250 gallons. To us this seems questionable: the solid load on the septic tank will be increased more significantly than the liquid wastewater load when a garbage disposer is in use.

What can we do to protect septic systems where a garbage disposal unit is installed ?

More frequent septic tank inspection and pumping

Significant steps you can take to protect the septic system from the additional solid load from ground food waste produced by a garbage disposesr also include:

• Inspect the condition of the septic tank, sludge and scum layer to determine if septic tank pumping is needed right now.
• Pump the septic tank now and check its condition again the next time the septic tank is due to be pumped on-schedule. If the solid load in the tank (sludge and scum layer thicknesses) is higher than anticipated, pump the tank on a more frequent schedule. Since some research of the effect of garbage disposers on municipal sewer systems did not indicate that there was necessarily a harmful effect (discussed below), you may find that simply continuing normal septic tank maintenance on a normal schedule is all that's needed.
• Avoid grinding materials that will not break down readily in the septic tank since such items may add unnecessarily to the sludge load.

Refer to our Septic Tank Pumping Frequency Guide but when reading that table, increase the number of occupants or bedrooms by one if the building has a garbage disposal unit installed.

Our advice about use of septic tank additives can be read at Septic System Additives & Chemicals for Septic Tank and Drainfield "Maintenance," "Un-Clogging," or "Repair" - Septic Tank Treatments.

Use of garbage disposer booster enzymes

Some garbage disposal units intended for use in homes connected to a private septic system include a device that sprays an enzyme treatment whenever the disposer is used (such as the InSinkErator unit shown in the photo at the top of this page - www.insinkerator.com).

The enzymes are contained in a replaceable cartridge. In general we oppose use of any septic system additive, and we are looking for research that supports the use of enzymes with garbage disposers which may amend that view. Enzymes, provided that they do not cause agitation in the septic tank, are among the products less-likely to harm the septic system. We're waiting to review evidence thatseptic tank enzymes are needed at all, and we want to see evidence that these septic tank additives reduce the need for more frequent tank pumping for a septic system system serving a home with a garbage disposer.

GARBAGE GRINDERS - Garbage Disposal Unit Effects on Public Sewer Systems

While garbage grinders can cause an added loading on conventional, residential, on-site septic systems, their use is not necessarily bad in municipal sewer systems according to some researchers, as indicated in the article abstract quoted below. Other studies of the effects of garbage disposers on municipal sewers have been conducted by various groups including the University of Wisconsin, Delft University Netherlands, and the Cooperative Research Centre, in Sydney, Australia, and the New York City Dept. of Environmental Protection

Most cities served by municipal sewers `in the U.S. permit the use of garbage disposals and some cities require their use. Using a kitchen disposer or grinder in cities served by a municipal sewer reduce the volume of organic waste which otherwise has to be hauled to a landfill. Cities in the U.S. and other countries are inconsistent in their view of the use of garbage disposers. New York City permits their use. Austin Texas has a rebate program to convert garbage disposer use in food service businesses to a scrap bin system.

Garbage Disposers in New York City: New York City, which used to prohibit the use of these devices legalized their use in 1997, out of concern for the increasing volume of garbage in the city, and in the face of inconclusive test results about their effect on the sewer system.

"The use of garbage grinders is not a usual practice in Europe, but it is in other countries around the world (e.g., North America, Japan and Australia). Sometimes, garbage grinders are accused of producing problems in sewers and wastewater treatment plants and are prohibited by environmental protection laws. In this study, the different impacts determined by the use of this technology were considered to show the positive impacts of its use.

In particular, it was shown that garbage grinders enable the disposal of household organic wastes with advantages for the wastewater treatment processes because of an increase in the carbon/nutrients ratio in the wastewater. This is particularly important for biological nutrients removal processes. Daily specific contributions for person equivalent (PE) due to organic waste disposal through garbage grinders were found to be equal to 75 gCODPE(-1) d(-1) for carbon (as COD), 2.5 gNPE(-1) d(-1) for nitrogen and 0.25 gPPE(-1) d(-1) for phosphorous, respectively.

Those determined a value of 30 for the COD/N ratio. Moreover, no problems with solids settling in sewers were noted. These results were extensively compared with literature data. The economical balance showed that the use of garbage grinders allowed a global saving of some 17 Euro year(-1) for a three people family. Important benefits are also gained from an environmental point of view (e.g., organic wastes disposal nutrients removal in wastewater treatment and increase in biogas production with energy reclamation)."
-- The under sink garbage grinder: a friendly technology for the environment, Bolzonella D, Pavan P, Battistoni P, Cecchi F., Department of Science and Technology, University of Verona, Strada Le Grazie 15, I-37134 Verona, Italy, Environmental Technology, 2003 Mar;24(3):349-59, abstract and article also available from the U.S. NIH at www.ncbi.nlm.nih.gov

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More Reading about Effects of Garbage Disposal Units on Septic Systems

• Building Code Treats Garbage Grinder as An Added Bedroom in sizing septic tanks and estimating septic tank pumping needs. New York State Septic System Design Regulations 75-A.6 Septic tank design specifications
• Garbage grinder output in septic sludge
• Septic System Maintenance advises to count a garbage grinder as one more bedroom
• Septic System Capacity affected by garbage disposal units


• Additives & Chemicals: Septic System Maintenance - Are they even needed? Are they even legal?
• Biomat Formation in the Septic System Drainfield Absorption System - what leads to drain field clogging and expensive drainfield repairs
• Buyer's Guide Home Buyer's Guide to Septic Systems Inspection & Testing - What to Do, Step by Step.
• Cheating on Water Tests - (Chlorine in Wells and Septics) Testing Water for Real Estate Transactions - Water Test Cheating Warnings for Home Buyers and Home Inspectors
• Clogged Drain Diagnosis - is the problem the septic system or the Building drain system?
• What is a Septic System An Engineer's View & Septic System FAQ's
• Contamination in Buildings how to test buildings for bacterial contamination from Sewage and Septic backup, remediation, & references
• Leach Field Inspection ProcedureSeptic Leach Fields - how to inspect and diagnose septic drainfield failures - what causes septic field failure?
• Maintenance: Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
• Sewage & Septic Backups and what to do about sewage spillage in buildings
• Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
• Tank Pumping Frequency Guide for Septic Tanks: When, How, What to Watch For when pumping or cleaning septic tanks
• Water Usage: Capacity: Determining Needed Septic Capacity - Typical residential water usage for people, appliances, and activities.

Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Technical Reviewers

More expert information on this topic

More Information on Septic System Diagnosis, Maintenance, & Repair

• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems


• Cesspools Home Buyer's or Owner's Advice
• Drywells Home Buyer's or Owner's Advice
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location

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