Do food waste disposers really waste water? Certainly disposers use water, which helps transport ground food through plumbing and sewers. But just how much water is needed? Is it a lot of water?
In short, no. Disposers account for only about 1% of a household’s daily use of water, or the amount of single toilet flush. Given that low amount, disposers are not even referenced in graphical displays of household water use provided by the AWWA.
What Studies Say About Disposers and Water Use
A review of ten studies found that water consumption for food waste disposers is less than five (5) liters per household each day. The five liter per household conclusion is the typical finding of half of the studies. The other half concluded that water use decreased where disposers are used, or no statistically significant difference in water consumption could be associated with their use. Further, one recent study by Aquacraft determined that residential disposers save 13 gallons per household per day water and suggested they should not be considered “water wasting appliances” [1].
How can this be? Well, typical flow rates for residential plumbing in the kitchen is about 1.5 gallons per minute. Consider that most people use their disposer only three to four times a day for 10 to 15 seconds each time and it’s easy to see why water usage is low compared to most plumbing fixtures – toilets, showers, washing machines, etc.
(30 to 60 seconds) x 1.5 gpm = 0.75 to 1.5 gallons per day
Water Use Statistics from Various Studies
1 gal/capita/day [2]
3-6 L/household/day [3]
1.01 L/person/day [4]
3-4.5 L/person/day [5]
4 L/person/day [6]
[1] DeOreo, William B., Peter Mayer, Leslie Martien, Matthew Hayden, Andrew Funk, Michael Kramer-Duffield, and Renee Davis. 2011. “California Single-Family Water Use Efficiency Study.” Aquacraft, Inc. Water Engineering and Management.
[2] New York City Department of Environmental Protection. June 1997. “The Impact of Food Waste Disposers in Combined Sewer Areas of New York City.”
[3] Karlberg, Tina and Erik Norin. VA-FORSK REPORT, 1999.“Food Waste Disposers-Effects on Wastewater Treatment Plants. A Study from the Town of Surahammar.”
[4] Bolzonella, David, Paolo Pavan, Paolo Battisoni, and Franco Cecchi. Department of Science and Technology. University of Verona. 2003. “The Under Sink Garbage Grinder: A Friendly Technology for the Environment.”
[5] CECED – European Committee of Manufacturers of Domestic Appliances. Spring 2003. “Food Waste Disposers – An Integral Part of the EU’s Future Waste Management Strategy.”
[6] Waste Management Research Unit – Griffith University. August 1994. Executive Summary. “Economic and Environmental Impacts of Disposal of Kitchen Organic Wastes Using Traditional Landfill – Food Waste Disposer – Home Composting.”
These studies, and more, are available here and here. Happy Holidays everyone!
My last post discussed the use of disposers to boost biogas at wastewater treatment plants. One of the reactions I heard was, “Not many plants use anaerobic digestion.” So what is the current infrastructure framework in the U.S.?
Anaerobic Digestion Treatment Plants Doing Major Work
According to the 2008 Clean Watersheds Needs Survey conducted by the Environmental Protection Agency (EPA), 15,618 wastewater treatment plants in the U.S. serve 284 million people. A new database of these plants indicates over 1200 utilize anaerobic digestion (AD). While this may seem like a small number, these plants are treating a large portion of the U.S. wastewater flows. In other words, a few plants are doing most of the work, not dissimilar to the Pareto principle in which 20% of the people do 80% of the work.
When you consider that most of these plants provide treatment for large municipalities, it’s easy to understand why a small group of plants use AD – it’s where most of the sewage is produced! AD is the most efficient process to reduce the volume of biosolids produced, which at the largest plants is massive. But smaller plants are able to handle their equally smaller amount of solids even though it’s at the expense of more energy.
Biogas=Energy. Just, Not Everywhere.
But the greater question is why many other large plants don’t use the biogas produced to make energy.
The Water Environment Research Foundation (WERF) answered that question in a report last year entitled, “Barriers to Biogas Use for Renewable Energy.” In a nutshell, the conclusion was that it’s very expensive to add cogeneration (combined heat and power) equipment and the return on investment can take longer than officials are ready to tolerate. Even so, simple return on investment shouldn’t be the only rationale for whether or not to make and use energy from anaerobic digestion. WERF also recently published “Reframing the Economics of Combined Heat and Power Projects” to help officials consider other metrics for the business case of cogeneration.
Tackling the Expense of Co-Gen
One way of improving the economics is by utilizing co-digestion feedstocks to boost biogas production. Co-digestion is the process by which food waste and other substrates are added to sewage sludge and manure to improve the digestion process. Cities like Oakland and Des Moines are already net energy producers from utilizing co-digestion, and many more, like Philadelphia and Madison, WI may soon follow. Sheboygan, WI has nearly reached the self-sufficiency goal set for its plant. In talking to folks like Dale Doerr, Plant Superintendent for the Sheboygan Regional Wastewater Treatment Plant and a forward-thinking wastewater professional, short-term payback shouldn’t be the deciding factor in whether or not to invest in cogeneration. His plant will be there for decades to come, he said, and investing in equipment to utilize renewable energy is just the right thing to do. The “unwritten policy” at Sheboygan WWTP is to consider energy use whenever an equipment or process upgrade is needed. The plant now uses 20% less energy compared to a baseline figure in 2003 and ranges from 70-90% energy self-sufficiency due to the energy produced on-site.
Co-digestion and Disposers
While co-digestion normally refers to direct addition of substrates to digesters, food waste disposers accomplish essentially the same thing ‒ most of the solids they grind, though tiny, are removed during primary clarification and deposited in the digesters. An aid to digester health and co-digestion, wouldn’t you agree?
Cooperative Co-digestion. Brewery waste has been found to aid digesters; it’s an excellent feedstock. This humorous old ad for Oshkosh Beer claimed beer was an excellent aid to human digestion too. Who knew. (From the oshkosh beer blog)
