Mr. Green is busy on his world-wide publicity tour for his new book. In the meantime, here's a Mr. Green classic column from April 2007.
Hey Mr. Green,
I hate to belabor the issue, but your discussions of garbage disposals make no mention of their effect on septic systems. This question has become the subject of considerable controversy in the 58-unit townhouse complex where I live. What is your considered opinion on the subject? --Joel in Lincoln, Massachusetts
When it comes to environmental quandaries, no issue can be belabored too much. It's rather odd that I failed to mention septic systems, having grown up on a farm where it was a memorable event when we finally replaced the outdoor privy with a septic tank and indoor plumbing! Maybe I'm suffering from the repressed trauma of several unpleasant occasions when our glorious new septic system got plugged up and I was recruited to help root through the sewage in the tanks and pipes to fix the problem. Or maybe I'm showing a culturally insensitive urban bias, assuming that everybody's hooked up to a municipal sewer line, when in fact almost 25 percent of U.S. households use septic tanks or cesspools.
In any event, the biggest problem with garbage disposals is that heavy use of them can double the amount of solids going into your septic tank. (Also, excess grease and oil, which often go down the drain, impair the tank's operation.) These solids come in two categories: sludge that sinks to the bottom and scum that floats to the top. The water runs out to a drainfield. Although the solids are broken down by bacteria, enough remain in the tank so that it has to be pumped out periodically--typically every three to five years. So if you use a garbage disposal frequently, the tank will have to be emptied more often. In my area, the cost of pumping the solids from a 1,500-gallon tank is $400 or $500. Obviously composting is a cheaper and more productive alternative--and maybe your complex would be better off investing in a composting program rather than spending your money on more frequent pumping.
I've talked to sewer guys who pump out the sludge and scum for a living, and they complain, with some frustration, about the public's appalling ignorance of septic tanks and sewage in general. "Some of them don't even know they have a septic operation," one pumper lamented. "It's out of sight, out of mind."
Although garbage disposals themselves don't rank among our worst environmental problems, a lot of the stuff people send down their drains does. "They put all kinds of oddball stuff in there: motor oil, paint, you name it," my source said. He pointed out that some of the chemicals in these substances can kill the septic tank's bacteria, destroying its ability to safely break down the sludge and scum. Other things people dump are just plain toxic. Among the substances that should never go down a drain--whether it flows to a septic tank or a municipal sewage system--are paints, pesticides, oils, varnishes, gasoline, and paint thinners.
Additionally, the sludge removed from septic tanks is often hauled to a municipal sewage-treatment plant. Obviously, if the septic tank material is full of heavy metals and other toxic chemicals, it's that much harder for the plant to produce safe effluent. There is a lot more that can be said about sewage and septic tanks. For more information, download the EPA's comprehensive bulletin, "A Homeowner's Guide to Septic Systems."
You could use it as the basis of a local education program. Finally, if you know any macho do-it-yourselfer who wants to take on septic tank cleaning and maintenance, tell that person to stay inside and call a professional. Messing around with the tank can be lethal. As one maintenance course put it: "Toxic gases are produced by natural treatment processes in septic tanks and can kill humans in minutes."