Hey Mr. Green,

I have five cats. I put Feline Pine sawdust nuggets in their pan, then scoop out the turds and flush them down the toilet into my septic tank. I empty all the urine-soaked sawdust along a fence. Am I doing the environment any harm?

—Sally in South Miami, Florida 

I'm sorry to say that you are doing several kinds of harm. First, it's not a good idea to flush anything from the litter box—even the sweetest, cleanest non-clumping pine—into a private septic tank because it could overburden the septic system. Poop and litter should not be flushed into municipal sewer systems either, because many of them cannot eliminate Toxoplasma, a rather nasty organism sometimes present in cat waste; its release into the ocean has been known to kill sea otters. People with weak immune systems should avoid cleaning cat litter, as should pregnant women. (The bug can cause serious eye or brain damage to a fetus.) 

The most common cat litter contains sodium bentonite, the mining of which tears up land surface (though restoration of mined land is now required). And the disposed litter adds an estimated 4 million tons of material to our landscape and landfills each year. Consider a more organic, biodegradable litter made from corn, corncobs, citrus scraps, grains, wheat, paper, or wood shavings. Whatever litter you use, to avoid spreading Toxoplasma and other dangerous organisms, never dump it outside. Place it in a plastic bag, tie it shut, and put it in your regular garbage. A biodegradable bag will give it a better chance to break down in the dump. 

Your cat should never be allowed outdoors without scrupulous supervision. As I've noted before, they kill billions of birds every year.