What to do with all that gas? That was the problem facing the San Mateo Waste Water Treatment Plant. Yes, your waste material that flushes into the sewers has to end up somewhere – and even after processing into dry ‘cakes’ there is plenty of dirty gas to get rid of. Mostly methane, this gas is normally burnt or ‘flared’ into the atmosphere, contributing to greenhouse gas emissions and pollution.
Your Gas is Our Gas!
In 2014, the City of San Mateo decided to set up a pilot program with the help of a grant from the California Energy Commission. The goal was to clean the gas to covert to CNG (Compressed Natural Gas) and use it to power city vehicles. This solution is not only green, but is the most cost effective use of digester gases, with a payback period of just 4 years. It is also far more efficient than producing electricity due to the low processing needs of converting gas to gas.
Converting the Treatment Plant
The treatment plant already had two large egg-shaped anaerobic digesters that use bacteria to digest sewage sludge and produce methane gas. Each digester produces 1.2 million gallons of digester gas each, giving 100 cubic feet per minute flow.
In May 2016, after a lengthy permitting process, the project installed gas cleaning treatment, gas and fuel storage, and a modern waste gas burner. The biogas operations were up and running just 4 months later in August. The gas ‘cleaner’ removes pollutants such as sulfide, moisture, VOC and carbon dioxide to create bio-methane for use as CNG. The CNG gas is stored in a giant rubber bubble until it is needed at the vehicle fueling pumps. Any waste gas is burned in an enclosed chimney, with reduced methane and CO2. At full capacity, the project saves around 1800 tons of CO2 equivalent per year.
Need More Cars!
So, what are the results of this pilot program? Firstly, the project runs successfully and produces biofuels as planned. But, the big problem has been procuring enough vehicles to use the full gas capacity. Originally, the plan was to power the city’s fleet of school buses but this has run into legal and insurance problems.
Currently, the city owns two CNG hybrid trucks and one car, used as a ‘clean energy’ model by the Chief of Police, Susan Manheimer. Most likely, the project could fuel up to 50 vehicles. There are another 17 vehicles on order, but there are no current production models available. Each vehicle has to be converted to add a second CNG fuel tank and switching mechanism.
In the future, it is planned to use the biogas to power the admin building in the new treatment plant next door.
Sustainable San Mateo County enjoyed an exclusive tour of the Biogas facility – with thanks to our two tour guides Gogo Heinrich and Stephen Wu.