Carbon “PACs” a Punch Against Taste and Odor Issues

Fall is here. In the Bay Area it brings cooler weather, lovely fall foliage, and blue-green algae blooms. Luckily, the SFPUC’s operators have a powerful new tool at the Sunol Valley Water Treatment Plant.

Blue-green algae are microscopic phytoplankton that are present in natural water bodies. The SFPUC’s filter plants remove the algae during treatment. However, some types of blue-green algae leave behind dissolved compounds, called metabolites. The two most common metabolites that can affect the quality of water are geosmin and 2-methylisoborneal (MIB).  These dissolved compounds are what sometimes present differences in taste and odor in drinking water. Because humans are incredibly sensitive to these compounds, the SFPUC can detect them in miniscule amounts.

A photo of the SFPUC’s PAC building.

Typically, algae blooms occur in fall or spring. When regular sampling of the SFPUC’s reservoirs predicts a bloom, water system operators can switch to other water sources, draw water from different levels in the reservoir below the blooms, or blend the water with other sources to avoid any taste and odor issues. Despite these operational tools, sometimes the SFPUC needs more power to combat these compounds.

The most efficient, long-term solution to address these compounds is a treatment process called ozonation. The Harry Tracy Water Treatment Plant on the Peninsula already uses this process with great success. It will take years to design and construct such a system in the East Bay plant. Because algae blooms are becoming increasingly more common in recent years, the SFPUC needed an interim solution. That is where Powder Activated Carbon (PAC) comes in.

A photo of the SFPUC’s PAC building.
An inside look at the SFPUC’s PAC building.

Construction on the $9 million project to add PAC at the Sunol Valley Water Treatment Plant started in April 2019 and finished in December 2019.  Here is how it works.

PAC is essentially a black powder. When SFPUC Water Quality Staff and Treatment Plant Operators are aware of a geosmin or MIB issues, they inject the black powder into a tank to create a slurry of black water, which they then add to the untreated water entering the plant. Operators also introduce chemicals called coagulants into the water to encourage suspended particles to glom onto each other and then fall out of the water. This is where PAC locks onto geosmin and MIB and pulls it down with the rest of the floc. Operators then filter the water, treat it with chemicals for disinfection, and then send it on to customers. PAC can remove up to 75% of the dissolved compounds out of the water.

One can rest a little easier knowing that the Powder Activated Carbon system is ready to be deployed to protect water quality during this fall bloom season.

The SFPUC’s Water Quality team’s Daniel Kim (left) and Water Supply and Treatment’s Tony Scott (right) work together to ensure high water quality at the Sunol Valley Water Treatment Plant.