Montana Earth Science Picture of the Week

Sludge Injector Truck

From Sewage to Fertilizer . . .
This photo, taken about 8 miles east of Helena, shows a special truck that is used to inject sludge into farmland soil. Sludge is a byproduct of sewage treatment. Finished sludge can be dried and disposed of in a landfill, or it may be put to use. In some cases dried sludge is burned as a fuel. Since it is rich in nitrates, sludge from some cities is used as fertilizer. During the summer, sludge from Helena is hauled to this farm east of town where it is transferred into the yellow truck shown above. The truck is equipped with a device that injects the wet sludge into the soil, providing nutrients for crops to grow. During the winter it is added to composting material that the city collects. In other cities such as Missoula and Kalispell, the sludge is sold to companies that make compost. These businesses mix the sludge with wood chips and other "secret" ingredients, compost the product, and then sell it in bulk or as individual sacks that can be purchased in hardware/home improvement stores.

Here's how sludge is formed. . .
When a toilet is flushed in one of Montana's larger cities, the sewage ends up at a wastewater treatment plant (a.k.a. sewage treatment plant). Treatment plants use a variety of processes to break down the sewage. In the first step, called the "headworks", big screens filter out sticks, rags, and other larger objects. Gritty solids like sand fall to the bottom. Material removed at the headworks is taken to a landfill. Next, the sewage sits in a tank called a "clarifier" where the organic waste sinks to the bottom, while oils and grease float to the top. Organic waste is anything that comes from plants or animals (including human waste). The stuff that sinks to the bottom, called sludge is removed and sent to a "digester" where bacteria eat much of the organic waste. Air is mixed in (aeration) to help these bacteria thrive. When the bacteria are finished the sludge looks a lot like mud. From the digester, sludge may go to a lagoon; a large pond that holds sludge while it basks in the sun until much of the water evaporates. Or, it can be sent through a big press that squeezes out the water.

That's not all . . .
Two other important byproducts of sewage treatment are methane gas and a clear liquid called "effluent". Methane is a gas formed when any organic material decays. Landfills, swamps, and sewage treatment facilities all release methane. It is a strong greenhouse gas, so methane is burned it as it comes out of the sewage treatment plant. Ideally it would be burned as fuel but that often is not economically feasible. Below: Methane is being burned (flaring) at Helena's Wastewater Treatment Plant near the Costco store (Scratchgravel Hills in background).

Below: Effluent is treated with UV rays to disinfect it before it empties into Prickly Pear Creek. However, like sludge, the effluent still contains plenty of nutrients, which can be problematic for streams and lakes. The nutrients cause excess algae to grow and bacteria that consume the dead algae use up dissolved oxygen in the water, making it difficult for fish and other organisms to live there. Watch the video near the bottom of this page to learn more. Helena discharges its effluent into this canal, which takes it to Prickly Pear Creek, then into Lake Helena, and onto the Missouri River (closer view).

Terms: compost, eutrophication


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By Rod Benson
Earth Science Teacher at Helena High School

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