16 June 2014

Do the math: Kitchen sink garbage disposal vs. composting service

I am feeling better about the oncoming home canning season now that I've had the kitchen sink disposal unit replaced. It gave up the ghost a couple of months ago, and I hadn't been looking forward to having to deal with the volume of scraps that a batch of home-canned food can produce.

There are mixed opinions on whether a sink disposal is a good idea, whether economically or environmentally. When the city of Philadelphia partnered with a manufacturer in 2012 to install some at no cost to test how effectively their use diverted solid waste from landfills, an environmentalist blog posted an article, with inflammatory and offensive language, calling the effort "greenwashing." One concern is a claim that a household could use 700 extra gallons of water every year to run the disposal. Another is that the solids don't just disappear, but are trapped at the entrance to the wastewater treatment plant and landfilled anyway. So Philadelphia residents should pay a local composting service to come around weekly to gather their kitchen scraps.

I'll counter all three comments. First, that 700 gallons figure seems really high to me, as well as not actually sourced. It doesn't replicate my experience or practice. Unless I have something truly horrifying going down my sink (say, after a fridge clean-out), I generally run my sink disposal only when I'm finishing up hand-washing dishes. The eggshells from breakfast, vegetable trimmings from lunch, and breadcrumbs from dinner all sit in the drain until I flush them through the disposal with a sinkful of dirty dishwater. So I'm not usually running any extra water down the sink disposal over and above what ended up in the sink from washing up after dinner. But even if I am doing a separate run of the garbage disposal, such as during a canning project or a dish that generates a lot of waste, I'll try to run the disposal minimally, clearing out just enough that the sink remains useable, and leaving the last for flushing with dishwater. With this kind of practice, I don't see how I could possibly be using 700 extra gallons of water every year to run my disposal. The article there links to an eHow item about the environmental impact of disposers, but the eHow figure is unsourced to any actual citation. In our home, I would have to use almost 2 gallons of water daily to match that wild-sounding claim. My kitchen faucet is a pretty standard piece of hardware with a flow of about 2 gallons per minute. So for me to use 700 extra gallons of water per year, I would have to run my disposal for nearly one minute per day, every day, with the faucet going. I make my disposer work like that on an apple butter canning day, maybe, but certainly not every day.

Some more realistic math, then. Most days I use my disposal at the end of the washing up, so in those cases I'm using zero extra gallons of water. I would guess that I run the disposal three more times per week, for about 20 seconds each time. That gives us 52 weeks X 20 seconds X 2 gallons per 60 seconds, or just under 35 gallons. That's less than a tenth of that clearly spurious 700 gallons figure.

The second comment goes to the problem of what to do with the solid waste that even a disposal unit leaves in your household's wastewater. I do know that down the road apiece, the Delaware Solid Waste Authority collects and burns for electricity the methane off-gassing from the Cherry Island landfill. Since I don't think that Philadelphia's solid waste goes to Cherry Island, the Green Philly Blog author may have a fair point. In fact, however, Philadelphia does recycle biosolids from city wastewater and uses biogas to generate electricity for one of its plants. Perhaps these measures weren't in place when the blog author was writing.

As for the last comment, that people should hire a composting service, I find that residential compost service costs $15.00/month. But running my disposal is far, far cheaper: Philadelphia water is about $0.04 per cubic foot; a cubic foot is about 7.5 gallons; so water costs about $0.04 per 7.5 gallons, or about a half-cent per gallon. If I use the disposal three times per week, for about 20 seconds each time, then (as we saw above) I'm using just under 35 gallons per year. That costs me just $0.17 per year -- I can find that kind of change on the sidewalk on the way to the supermarket. At this point, brighter minds than mine can look at my disposal's specifications and the electric company's current rates to add in my electricity costs. But I think I can safely say it doesn't add up to $15.00/month.

I'm a day late and a dollar short, so to speak, in addressing that particular author's views against sink disposals. The issue didn't really come up for me, I guess, until I had to decide whether to replace the one that broke and couldn't be repaired. In the end, the blog article hasn't convinced me to move to composting -- and it's not just because of the cost of a service, but also because I can't do any gardening here at the homestead, for architectural reasons. I'm super happy to have a disposal again, and I'm happy to see that I can pay for its use with my "see a penny, pick it up, all the day you'll have good luck" jar.

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