Garbage Disposal Compatibility on Septic Systems

Posted on

Over the past ten years I’ve attended numerous trade shows and conferences, where I network with various environmental professionals, consultants & engineers, academics, and regulators. In talking about the environmental benefits of food waste disposers with my colleagues, and even friends and family, I sometimes hear, “Well, I’m on a septic system, and sometimes they’ll say, “I can’t have a disposer,” or “Disposers are not allowed on septic systems.” My follow-up question usually includes trying to find out who said they can’t have a disposer, because in many cases, there are no actual regulatory prohibitions.

1) Regulations on Disposers and Septic Systems

Health departments, septic installers, plumbers, and some local governments, will often provide guidance on limiting or discouraging the use of disposers, but in my research, most jurisdictions do not actually prohibit the installation and use of food waste disposers. In the U.S., most state regulations are completely silent about food waste disposers. Twelve states do include some language requiring additional sizing of tanks or drain fields, but they are the exception. In fact, over 6 million U.S. homes on septic systems have a food waste disposer according to the 2013 American Housing Survey.

State Restrictions on Disposers

2) The Main Concerns – Organic Loading and Drainfield Performance

So why are people so concerned about having a disposer on a septic system? Much of this originates from perpetuated myths about disposers – their use of lots of water or the additional loading on the system – that this will somehow require more frequent pumping or maintenance of the system. According to a curriculum developed by septic system professionals in 2004 (see page 21 in study below), water use of disposers is minimal.

Food waste processed by a disposer in a household on a septic system does increase organic load, so just how much is the additional load? Unfortunately, much of the research on the additional loading from disposers was done nearly half a century ago. According to the EPA and research from 1951 to 1998 as referenced in “Fact Sheet 2 – High-Organic-Strength Wastewaters (Including Garbage Grinders)” (see table 1), the loading is considerable, although the document makes no assertion about the specific contribution to solids buildup in the tank and simply concludes it may lead to increased pumping frequency. The main concern listed in the EPA’s fact sheet is that increased loading from disposers may compromise the drainfield.

Table 1 – Septic Tank Loadings from Disposers.
Adapted from “EPA Fact Sheet 2 High Organic Strength Wastewaters Including Garbage Grinders” (2002)

3) Impacts to the Drainfield

Wastewater experts Tchobanoglous and Crites quantified the specific impacts to the drainfield in Small and Decentralized Wastewater Management Systems (McGraw-Hill 1998) – see table 2. Their data shows an increase of less than 8% in organic strength (BOD) of tank effluent, the water sent to the drain field, so is this significant enough to compromise the long-term performance of a drainfield?

Table 2 – Septic Effluent Characterization
Adapted from “Small and Decentralized Wastewater Management Systems” (McGraw-Hill 1998)

4) The Basics – Water Content, Density and Particle Size of Food Waste

It’s very important to also think about some of the basics. Food waste is 70-90% water and is essentially the same density of water – 1 g/ml. More importantly, after being ground up in a disposer, the food waste particles are very fine, especially compared to the sewage solids sent to a septic tank. According to research from the University of Sheffield, most food waste particles are 2 -4 mm. The only real difference between food waste particles and sewage solids, is that they haven’t been eaten and digested first! Sending ground up food waste to a septic system is simply sending more food to bugs in the tank instead of your stomach. Finer particles also degrade faster because there is more surface area available to the microorganisms.

5) Sludge and Scum, and Pump-out Requirements 

The impacts of sending additional food waste sent to septic systems is well studied and understood. Comparing homes with and without disposers, the buildup of sludge in septic tanks is very similar (see page 42). There is additional scum buildup in septic tanks, so some studies assert there is an overall buildup of solids (sludge + scum), leading some jurisdictions to discourage disposer use, or at least suggest minimal use. However, given the frequent pump-out requirements and recommendations in most jurisdictions of every 2-3 years, the contribution of food waste disposers to the buildup of solids is negligible and manageable.

6) The Latest Research

Over the past few years, InSinkErator commissioned two research projects at the University of Minnesota to better understand the actual impacts of food waste disposers on septic systems. In our first project using 1 L reactors, we simulated the impact of normal use of disposers based on established textbook values of the additional hydraulic and organic loading.  We followed up with a second phase of research using 20 L reactors. For reference, Metcalf & Eddy’s Wastewater Engineering states grinders increase the organic loading – the chemical oxygen demand, by 28%. In our second phase we increased the loading by 46% using ground up food waste.

In both phases of the research, over a period of 100 days, we increased the organic strength of the wastewater sent to the simulated septic tanks using ground up food waste, and then looked at the solids build-up in the tank and the impact downstream of the tank. Here are the results.

  1. Loading to the tank increased by 31% for total suspended solids and 46% for Total Chemical Oxygen Demand (tCOD) by adding ground food waste, but the effluent quality increased only slightly for Total Suspended Solids (TSS), tCOD, soluble COD (sCOD), Total Nitrogen and Total Phosphorus.
  2. Food waste results in a negligible increase in the nutrients in the effluent, and more importantly, a negligible increase of solids build-up in the tank.
  3. About 76% of the suspended solids from food waste degraded as compared to only 37% of the sewage solids, meaning that the contribution to the effluent and solids in the tank was less from the food waste than the sewage. This demonstrates that food waste breaks down better than sewage solids, it’s simply more biodegradable than sewage solids. Remember, it just hasn’t been eaten yet.

Our research on the impacts of food waste disposers on septic systems is complete and was shared publicly at the annual conference of the National Onsite Wastewater Recycling Association in 2018, and then published in Waste Management & Research in 2019.

7) Basic Guidance

My recommendation to homeowners on septic systems is simply this – check your local regulations. If there is no actual restriction or prohibition, add a modern food waste disposer to your kitchen sink. Used appropriately, you can grind virtually all your food scraps and keep them out of the garbage. Certainly, using a disposer for plate scrapings will not be an issue for homeowners on septic systems, and will help prevent issues in plumbing by grinding the food scraps into very fine particles. Just remember to turn the water on first, then your disposer, and then add your food scraps gradually. When grinding is complete, turn off the disposer and let the water run for a few extra seconds – while you wash your hands for 20 seconds. Used properly, a disposer will not clog your plumbing, and will immediately and efficiently remove food scraps from your kitchen, providing a safe and hygienic alternative for managing the unavoidable portion of your waste.