QUEST Community Science Blog

Unless our sewage happens to end up in the Bay and in the headlines, most of us probably never give a second thought to where our wastewater is headed each time we run the tap or flush the toilet.

To learn more about the travels of sewage, I took a tour of the Las Gallinas Valley Sanitary District treatment plant led by Plant Manager Matt Pierce. The plant has been in operation for about 50 years and serves over 30,000 residents in north San Rafael.

After leaving sinks and showers throughout the District, wastewater travels through a network of pipes and pump stations. Once the sewage arrives at Las Gallinas, it passes through an inlet screen and a grit chamber, which together remove much of the dense, inorganic material-”like diamond rings,” Matt jokes.

A lot of what happens at the plant is not that different from what happens in your compost pile: “It’s basically bacteria at work,” Matt points out. (The much bigger challenge for sanitation districts these days are all the unnatural things we’re putting down the drain: household chemicals, personal care products, pharmaceuticals.)

From the grit chamber the sewage heads into a series of clarifiers, where gravity causes the organic solids to settle out. The biosolids pass through a thickener and then an anaerobic digester-the most, ahem, aromatic stop on our tour. After further thickening in storage ponds, the sludge is injected into a disposal field.

Meanwhile, the liquid from the clarifiers travels through two biofilters, where rotating arms spray the water over rock beds. The organic matter in the wastewater is a feast for microbial slime living on the rocks. In the nitrification tower, more microorganisms break down the ammonia in the water. In the final stages of treatment, the wastewater is chlorinated to kill any remaining bacteria, then dechlorinated since the chlorine is toxic to many aquatic species. Finally, the treated water is sprayed onto District fields or discharged into Miller Creek where it flows to San Pablo Bay.

The District has done a lot to minimize the environmental impacts of its operations. The plant is powered by a field of solar panels. The methane released in the sludge treatment process is captured and used to generate power and heat the digester. Some of the treated wastewater supports acres of fresh and saltwater wetlands-in fact the District’s land is a favorite local gem for walkers and birders. And in a partnership with the Marin Municipal Water District, more than a million gallons of treated wastewater are recycled daily for landscape irrigation and other projects.

There are plans to make even fuller use of the reclaimed water. The Bay Institute-in partnership with the Sonoma County Water Agency, Las Gallinas, and three other North Bay sanitation agencies-has developed a plan to use recycled water for wetland and creek restoration and for agricultural irrigation. Legislation sponsored by Congressman Mike Thompson to establish the program passed the House late last year; Senator Dianne Feinstein has introduced similar legislation that we are hopeful will pass this year.

With California’s growing demands for water, such creative means to conserve and recycle are critical to helping prevent this precious resource from just going “down the drain.”


Ann Dickinson is Communications Manager for The Bay Institute (www.bay.org), a nonprofit research, education, and advocacy organization dedicated to protecting and restoring San Francisco Bay and its watershed, “from the Sierra to the sea.”

When football fans tune in on Super Bowl Sunday next weekend, they’ll be watching a greener Super Bowl, according to the NFL. Demand for carbon credits is booming, with companies from Dell Computer to Enterprise Rent-a-Car offering their customers offsets with their purchases. But critics are concerned that consumers don’t know what they’re buying - or might not be getting what they’re promised.

The 2006 Word of the Year was “carbon netural” in the The New Oxford English Dictionary. But there’s still a lot of debate about what it means. Many people compare the U.S. carbon offset market to the Wild West. Since there is no regulation, how do you know what you’re buying?

There are several guides to carbon offsets that have been created by non-profit organizations, designed to help the average consumer (see related resources). But part of the problem is that many people are still debating what a carbon offset should be. And that’s a debate that can be found in the blogosphere.

One place you can find it is on the Grist.org blog which has many bloggers writing about green issues. Forestry offset projects, which sell credits based on the fact that trees sequester– or hold carbon dioxide, have come under fire. You can read about a few of the critiques here, here and here.

Another blog, Treehugger.com, has followed the issue as well. They posted this comparison of offset providers to help their readers do their homework and this more in depth guide on the issues buyers should be aware of.

Of course, one of the earliest debates over offsets was whether offsets would act as “indulgences”, distracting consumers from making concrete changes in their lifestyles to reduce their carbon footprint. Terrapass, one offset retailer, has tried to investigate this by surveying their customers. They found that the majority of them had already had green habits. Still, the virtues of offsets are a matter of personal opinion.

You may listen to the “Cashing in on Carbon” radio report online, as well as find additional links and resources.

Lauren Sommer is an Associate Media Producer for QUEST.

latitude: 37.325510, longitude: -120.640341