Thursday, November 20, 2014
My Account  | Login
Nashua;33.0;http://forecast.weather.gov/images/wtf/small/nbkn.png;2014-11-20 21:20:19
Monday, February 10, 2014

My septic tank melts snow; where’s that heat coming from? (Maybe not bacteria)

David Brooks

When my kids were in school, they’d badger me every year for ideas of good science projects – and every year, I’d suggest they look at how bacteria heats up our septic tank.

This wonderfully weird phenomenon is really noticeable when there is snow on the ground, as you can see in the accompanying photo of my front yard taken before the recent storm. My septic tank lies under a couple feet of insulating earth, yet it produces enough heat to melt snow on several square feet of ground! ...

Sign up to continue

Print subscriber?    Sign up for Full Access!

Please sign up for as low as 36 cents per day to continue viewing our website.

Digital subscribers receive

  • Unlimited access to all stories from nashuatelegraph.com on your computer, tablet or smart phone.
  • Access nashuatelegraph.com, view our digital edition or use our Full Access apps.
  • Get more information at nashuatelegraph.com/fullaccess
Sign up or Login

When my kids were in school, they’d badger me every year for ideas of good science projects – and every year, I’d suggest they look at how bacteria heats up our septic tank.

This wonderfully weird phenomenon is really noticeable when there is snow on the ground, as you can see in the accompanying photo of my front yard taken before the recent storm. My septic tank lies under a couple feet of insulating earth, yet it produces enough heat to melt snow on several square feet of ground!

Great topic for a science project, I think you’ll agree. But did my ungrateful progeny do anything with my idea? They did not, unless you consider rolling eyes and muttering sarcastically as doing something.

So I’m going to do something with it right here – especially because questions have been raised about my hypothesis that heat from bacteria is melting the snow. Maybe it’s actually the heat from the warm water we send down the drains.

Bacteria do generate heat when they digest organic material, as is most easily seen in compost piles. Those piles can get really hot – 130 degrees Fahrenheit is not unusual.

Septic systems receive the – well, let’s call it the material – that we send down the drains and toilets and holds it in concrete underground tanks that fit several hundred gallons. Water leaches out through pipes into the surrounding soil (hence the term “leach field”), but the solids settle, creating a sludge where heat-generating bacteria thrive. (Hordes of tiny flying insects also live inside the tank, as I learned when I saw one being opened.)

A lot of heat is generated. Check my photo: About 5 square feet of snow has been largely cleared of roughly an inch of snow. With a 10-1 water ratio, that’s 72 cubic inches of water, weighing about 2.5 pounds.

It’s not trivial to melt 2.5 pounds of snow. I’ve tried to melt snow on a woodstove during power outages and gave up because so little was happening.

Melting ice or snow requires not just heating it but also breaking molecular bonds – causing a phase change, as physicists say. Raising the temperature of ice requires one-half a BTU (British Thermal Unit) per pound per degree, but changing 32-degree ice to 32-degree water (creating phase change) requires a whopping 144 BTU per pound, or almost 300 times as much energy.

So we know a ton of heat is coming out of the septic tank. But is it really from bacteria?

A friend of mine, environmental engineer Peter King, thinks not.

He says septic sludge is mostly home to anaerobic bacteria (those that don’t require oxygen), which don’t generate much heat – unlike aerobic (oxygen-breathing) bacteria in compost piles.

A bigger source of septic-tank heat, he suggested, is the water sent down our drains. There’s hot water (at least 110 degrees F) from showers, sinks and maybe the washing machine – there’s no reason to waste money by washing clothes in hot water, but many people do – as well as cold water, which is warmer than the 55-degree temperature underground below the frostline.

Pete, a senior engineer with Geosyntec consultants, did some back-of-envelope calculations – assuming 63 gallons used a day, one-third of which is hot – and estimated that the heat carried into the septic tank each day is equivalent to the energy in eight-tenths of a gallon of heating oil.

Burning that much fuel in a furnace in your septic tank would certainly heat it up.

So what’s melting the snow in my yard – busy little bacteria or my own wastewater?

I asked Mario Leclerc, superintendent of the Nashua Wastewater Treatment Facility, for insight. They have a gigantic “egg,” an anerobic digester that can hold 1.3 million gallons at a time, which generates heat, but they haven’t done the non-trivial number-crunching to measure this heat and pin down the specific source – it takes enough time and money just to cope with it.

Their situation is complicated because Nashua uses methane generated by bacteria to fire a power plant that produces an average 200 kilowatts of electricity, some of which is used to generate heat in the egg to keep the processes going.

Further, it’s not clear how well you can correlate heat data between my little septic tank and the egg, which gets 60,000 gallons of pre-treated water daily, and holds material an average of 20 days before the water heads out to the Merrimack River.

So he couldn’t help me in my bacteria-or-water quandary. I’m going to have to mark this question as “undetermined” for the moment.

Hey, you know what? Answering it more completely would be a really cool science project. Why didn’t I think of that before?

Granite Geek appears Mondays in The Telegraph. David Brooks can be reached at 594-6531 or dbrooks@nashuatelegraph.com. Also, follow Brooks on Twitter (@granitegeek).