One of those nagging questions facing the modern homeowner is whether or not a kitchen disposal, such as an InSinkEratorTM, is good or bad for the environment. Should we buy a house with one? Should we use it at all? A few weeks ago, I heard a woman on television say that she just couldn’t live without her kitchen disposal. One wonders what the rest of her life is all about.
The environmental impact of kitchen disposals is controversial. When we add ground-up organic materials to water, we increase what is known as the Biological Oxygen Demand or BOD of the wastewaters. Decomposition of those materials consumes the oxygen in water, reducing its favorability for fish and other creatures downstream from the point of wastewater discharge.
I question the wisdom of any process in which we purposely put unnecessary materials into wastewaters, only to remove them a few hours later at a sewage-treatment plant. But, nearly all municipalities now have sewage treatment plants that remove organic particles, so the impact of a few more from the kitchen may be small. And if you are on a septic system, the organic materials from the kitchen will decompose along with your other wastes without entering local waterways. Of course, in drought-prone areas, such as the Southwest, the extra water used to flush organic materials down the drain is simply wasted, adding to regional water shortages.
Part of the answer to the question posed by kitchen disposals depends on what your municipality does with the organic debris removed from wastewaters. If it goes to the local landfill, it is likely to decompose without oxygen, thereby producing methane—a powerful greenhouse gas in Earth’s atmosphere. Landfills also contain yard wastes, waste paper, and other organic materials that produce methane when they decompose.[1] A few landfills even capture this gas to use as a fuel. It is better if organic materials are gathered for biosolid fertilizers, but because they smell bad and have the potential for toxic metal contamination, sewage sludge is often considered undesirable to use on agricultural land. In some areas, sewage sludge is burned.
A better alternative is to save your kitchen scraps and compost them. This will produce a rich mulch for your garden and avoid much of the ancillary contamination that results from your disposal. Of course, if you live in a big-city, there may not be an obvious need for compost, and local health regulations may even prohibit a compost pile or bin. What to do?
Bottom line: organic wastes from the kitchen are best not ground up and washed down the drain. Removing them from the sewage stream costs money and not removing them is hard on the aquatic environment. When organic materials are removed from wastewater, they are usually sent to the local landfill. So, why not just put your kitchen scraps in the garbage and land-fill them directly, avoiding the potential contamination of local waters?
Better yet, composting kitchen waste helps “close the loop” to achieve a sustainable human society where wastes are recycled back into the production of useful goods.
References
Bogner, J. and E. Matthews. 2003. Global methane emissions from landfills: New methodology and annual estimates 1980-1996. Global Biogeochemical Cycles 17:
Gunders, D. 2015. Waste-free Kitchen Handbook: A guide to eating well and saving money by wasting less food. Chronicle Books LLC, San Francisco. (Focusing on water pollution, rather than methane, I come to a different conclusion than Dana Gunder regarding sink disposals).
Schlesinger, W.H. and E.S. Bernhardt. 2013. Biogeochemistry: an analysis of global change. 3rd. ed., Academic Press/Elsevier, San Diego.
1] Landfills and sewage treatment account for about 10% of the anthropogenic emissions of methane to the atmosphere. It is difficult to estimate how much of that is derived from kitchen disposals versus other sources of organic materials, such as waste paper, yard waste and human poop. Americans are said to throw out half of their available food every year, but my suspicion is that the fraction ground up in kitchen disposals is relatively small.