Sludge is what's left over when waste water from sewage treatment plants and a wide variety of industrial processes is treated to extract clean water. The water is returned to the environment leaving a noxious semi-solid goo full of pathogenic microorganisms and potentially nasty chemicals.

Waste water treatment does what it says -- it treats water. The more efficient the treatment plant, the cleaner the water being discharged into the environment and consequently the more noxious the cocktail of contaminants left in the resulting sludge.

One of the main reason's that sludge is such a problem is that it's incredibly difficult to predict exactly what's in it. The composition of municipal sewage sludge changes all the time depending on what people dump down their drains, and on what industrial waste happens to make its way into municipal waste-water treatment plants. What you're left with is anybody's guess.

According to Cornell University and the American Society of Civil Engineers municipal sewage sludge can contain any or all of the following:

• Polychlorinated Biphenyls (PCBs)
• Chlorinated pesticides such as DDT, aldrin, endrin, chlordane, and 2,4,-D
• Heavy metals from wood preservatives, pesticides, metal plating, and batteries
• Bacteria, viruses and fungi
• Chlorinated compounds
• Flame retardants (asbestos)
• Petroleum products
• Industrial solvents
• Nitrogen
• Phosphorous
• Potassium
• Dioxin

Exposure to the substances listed above have been shown to cause reproductive problems, disease, even death... but as with other environmentally pervasive toxins and carcinogens there is rarely a definite link to the source of the problem.

Sludge has traditionally been disposed of in a number of ways:

• Disposal at sea -- now prohibited
• Disposal by spreading on farmland -- restricted
• Disposal by dumping in landfill -- restricted
• Disposal by incineration -- restricted

Two alternatives for dealing with sludge are currently being touted:

• Composting -- renders the sludge into relatively inoccuous fertilizer, but has some potentially unpleasant by-products.
• Thermal drying -- reduces the moisture content of sludge down to less than 10%, and reduces volume by over six times to produce fuel and fertilizer pellets. There is almost no data available on by-products associated with this process or on the long-term effects of the drying process.