Hold onto that sewage water a little longer

Sewage treatment plants may hold a low-cost key to keeping household chemicals out of surface water, says a new study in Environmental Science and Technology.

What we’re talking about here is a much more insidious problem than the occasional bleach spill. Pollution by pesticides and “personal care products” is coming to be understood as a pervasive problem in fresh water. (The EPA has an extensive fact sheet.)

It turns out that nearly anything you rub on your skin or swallow – lotion, sunscreen, insect repellent, birth control pills, antibiotics – finds its way back into the water. We imagine lotions and medicines entering our cells and fixing tiny molecular problems and we think that’s the end of it.

But unused chemicals wash off our skin or out of our kidneys, then pass through sewage plants and into streams or coastal waters. There, constant replenishment is making the chemicals a new ingredient in the soup of everyday streamwater.

Unused birth control hormones have gotten perhaps the most press, since they lead to “feminized” or “intersex” fish. But flame retardants, insecticides, and perfumes have their share of unpleasant or unknown side effects on aquatic critters (check “Macho Waters” in Science News; resources from Woods Hole Oceanographic Institution, and a news article from Tonya Clayton, a Santa Cruz classmate.)

Sewage plants have never been expressly designed to clear these chemicals from the water, says the EPA. But that’s where researchers led by Joan Oppenheimer of the consulting firm MWH picked up the trail.

One thing sewage plants are designed to do, they noted, is to remove excess nutrients like nitrates and phosphates from the water. They do this by pouring the evocatively named “effluent” into settling ponds. Microbes degrade much of the nutrient supply using ingenious biochemistry they evolved over millennia.

Studying six sewage plants in California and New Mexico, Oppenheimer and colleagues found that a simple lengthening of the settling-out period (to 5-15 days) gave those microbes the opportunity to digest many of the “personal-care” chemicals as well. Now, if federal and state regulators were to make longer retention periods mandatory, it might buy salamanders, minnows, turtles, etc., time while our treatment plant engineers catch up to the chemistry of microbes.

(photo: Greenpeace)