The method uses biogas, which is produced when waste water sludge is broken down by microbes in a process known as anaerobic digestion.

BBC: United Utilities turns human waste to energy at plant

Fueled partly by billions in government incentives, the industry is creating millions of solar panels each year and, in the process, millions of pounds of polluted sludge and contaminated water.

NPR: Solar Industry Grapples With Hazardous Wastes

The state records show the 17 companies, which had 44 manufacturing facilities in California, produced 46.5 million pounds of sludge and contaminated water from 2007 through the first half of 2011.

NPR: Solar Industry Grapples With Hazardous Wastes

What they didn't do was look in the sludge that accumulates when water is treated.

BBC: Smelly socks raise silver stink

Of course, when you drill down there, water comes up, water, sludge and what they call fittings, which are the little pieces of rock and things that come up.

NPR: Drilling for Gas in Dallas-Fort Worth

Sludge settles to the bottom, leaving clean water on top.

FORBES: Muckraker

This is because the waste consists of barrels of sludge containing compounds such as nitrides, oxides and water that are not, themselves, radioactive.

ECONOMIST: Nuclear waste

Opponents of the Tokushima dam argue that dams of similar design elsewhere have destroyed river habitats, blocking the flow of water and leading to an accumulation of sludge on the river bed.

ECONOMIST: Japan

In many cases, a toxic sludge is created when metals and other toxins are removed from water used in the manufacturing process.

NPR: Solar Industry Grapples With Hazardous Wastes

In areas where the water has been removed by 41 operational pumps, thick sludge remains.

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