Mercury in biosolids

This indicator tracks the amount of mercury detected in biosolids, or treated sewage sludge, at 155 biosolids management facilities in Washington.

What is mercury?

Mercury is a heavy, liquid metal in its elemental form. Mercury occurs naturally in certain hard-rock and metallic ores, and can enter the environment from both natural emissions and human activities. It reacts with other substances to form organic and inorganic compounds, as well as amalgams with other metals. All forms of mercury can be toxic to humans and other animals, depending on the route and amount of exposure. Mercury is released into Washington's environment from human activities within the state, such as coal-burning power plants, refineries, incinerators, mining, and releases from other businesses, homes, and schools.

This indicator has approximately a one year time lag due to the time involved in gathering, entering, and analyzing data.

What are biosolids?

Biosolids are a semi-solid byproduct of wastewater treatment at sewage treatment facilities. Typically, biosolids are treated to meet high standards that allow them to be applied to the land for beneficial use.

The average concentration of mercury in biosolids has decreased by 66 percent since 1995, as seen in statewide averages for wastewater treatment facilities being tracked. From 1995 to 2003 the statewide averages for mercury in biosolids fluctuated from a high of 2.83 parts per million (ppm) to a low of 1.92 ppm. After 2003, the averages declined steadily from 2.45 ppm to a low of 0.85 ppm and appear to be leveling off since then.

Mercury content of biosolids produced at treatment facilities, 1995 - 2014. View our data for mercury in biosolids.

Why should we be concerned about how much mercury is in biosolids?

Biosolids are used as an indicator to measure the success of mercury abatement programs. Biosolids are not the original nor the primary source of mercury released in Washington every year, but a collection point. Mercury collects from many sources upstream of the wastewater treatment plant. Dental use represents a large percentage of mercury in use. Reducing this source appears to have had significant effects on the overall amount of mercury in our environment.

The amount of mercury in biosolids represents the amount of mercury entering the sewage system from dental offices and other sources. Biosolids can be incinerated or applied to land as soil amendments. Either of these management methods may allow mercury to enter the environment, so less mercury in biosolids means less mercury entering the environment.

Mercury is a persistent bioaccumulative toxic metal. This means it persists when released in the environment. It builds up in humans and other animals, and exposure to excessive amounts can create neurological problems.

What are the benefits of reducing the amount of mercury in biosolids?

  • Keeps mercury out of the environment.
  • Prevents mercury poisoning in humans and animals.

What are some actions being taken to decrease mercury in Washington?

Concerted mercury-reduction efforts began with dentists in 2003-2004. These efforts include the introduction and use of amalgam separators in dental offices as an alternative to sending mercury-laden material down the drain. It is believed that these efforts have been the main reason for the mercury reduction seen in biosolids in Washington.