Biogas: Not letting waste go to waste, San Jose wastewater facilities turn methane into power

At a sprawling sewage treatment plant in north San Jose, engineers are slowly perfecting the ideal temperature for a smelly mixture of bacteria and human waste as the stew churns inside four massive air-tight cauldrons.

The plant, the largest wastewater treatment facility west of the Mississippi to produce water clean enough to be discharged into a sensitive ecosystem like San Francisco Bay, is experimenting with new heat-loving bacteria that excel at turning poop into compost and energy — harnessing the power of dangerous greenhouse gases.

“It’s like making bread or making yogurt,” said civil engineer Mariana Chavez-Vazquez, who oversaw the renovation of the cauldrons. The bacteria “need to be happy. They need to be a certain temperature. If you put in something that they don’t like, they will go sour.”

In the global effort to reduce methane, a greenhouse gas that’s 72 to 84 times more potent than carbon dioxide, municipalities across California are facing increasing pressure to comply with laws designed to keep the dangerous gas from escaping into the atmosphere.

Many wastewater treatment plants, including the San Jose-Santa Clara Regional Wastewater Facility and the Santa Cruz Waste Water Treatment Facility, are already doing their part by trying to ensure that their waste doesn’t go to waste, and instead turning it into biogas, a blend of gases dominated by methane, that can be used to generate power.

In San Jose, that process is done with finely tuned cauldrons, also called digesters. From the odorous, bubbling mixture sealed within the digesters, the plant already extracts enough fuel to power half of the facility. But that amount will increase once the higher-temperature bacteria, introduced in October, settle into their natural, methane-creating rhythm.

Until the 1970s, facilities burned the combustible biogas into carbon dioxide and nitrous oxide. In fact, wastewater treatment plants were easy to spot because of their blue flame hovering at the top of a tall stack, resembling a lit candlestick.

That changed after the federal Clean Water Act of 1972, when facilities were made to extract some use from methane before it was burned. The result was cogeneration — the creation of electricity from an engine powered by biogas. Hundreds of facilities in California now do it.

As its name implies, wastewater isn’t just water. It’s a mixture of road runoff, raw sewage, toilet paper, wipes deceptively advertised as flushable, and other dirt and plastic flowing through the sewer system. When treated with bacteria, the sewage and other organic solids in wastewater turn into sludge and biogas.

At the San Jose plant, the benefit of finding the ideal temperature for bacteria at around 130 F, Chavez-Vazquez said, is that the new bacteria will eat more organic material from the sludge than their lower-temperature predecessors. This creates a compact soil that requires fewer trucks to haul it away. And the higher-temperature bugs are capable of generating a larger amount of biogas, composed of a higher proportion of methane, which means more electricity for the plant.

The facility is also primed to digest food waste, should Santa Clara County start sending the compostable material to the plant. In 2016, California first began an effort to reduce methane emissions by 40% by targeting organic waste sent to landfills, which accounted for 20% of the state’s methane emissions.

At least half of California’s landfill-bound food waste could be processed at the state’s wastewater treatment plants and serve as an innovative power source, according to a 2020 report by the California Department of Resources Recycling and Recovery. The Inflation Reduction Act, signed by President Biden last summer, is expected to rapidly accelerate these efforts by offering a system of tax incentives.

Oakland’s treatment facility is a prime example of its effectiveness: Since 2008, the facility has turned 20 to 40 tons of restaurant food scraps into electrical power each day.

While co-digestion of food sludge and wastewater sludge is still on the horizon for most plants, the digestion process itself is not new. Facilities have long partnered with microscopic organisms to break down solid waste from wastewater to create gaseous fuel and soil.

Similar to San Jose’s plant, Santa Cruz’s wastewater facility has powered itself from methane produced by bacteria during waste digestion for several decades. The digesters — 30-foot tall circular tanks, stretching roughly 90 feet across — act like a stomach, where the bacteria mixture is sealed from the air. The gas rises to the top of the digester before being bubbled back through the sludge to spur more fuel creation.

What appears as waste to us still contains plenty of nutrients left to satisfy small, hungry bugs, said David Meyers, interim operations manager at the Santa Cruz plant, which uses the lower-temperature bacteria. Meyers said this cuts down on the energy needed to heat the digesters. The plant houses two internal combustion engines that can produce a total of 1.33 megawatts of electricity per day.

At the San Jose facility, biogas is piped from the digesters to four SUV-sized internal combustion engines inside the sprawling complex. Each engine, by turning a generator, can produce 3.5 megawatts of electricity per day. Combined, the four engines create enough electricity to meet the needs of a small town, Chavez-Vazquez said.

In the meantime, the wastewater treatment facilities will continue treating waste 24-7 — a commodity often overlooked in an area known for searching for the next big thing.

“You cannot put a price on sanitation,” Chavez-Vasquez said.