Anaerobic Digesters are Good for the Environment

Don’t waste your green waste.

Kompogas Process Ecological Cycle Infographic

An anaerobic digester can magically transform your yard trimmings and food scraps into electricity and other good stuff so please do not send it to a landfill.

A reasonable question is “What the heck is an anaerobic digester?” In short, it is a giant tube that uses an anaerobic (without oxygen) fermentation process to convert the contents of your green waste bin into renewable energy (electricity or vehicle fuel), liquid fertilizer, and compost.

Kompogas Anaerobic Digestion Process Infographic
Kompogas anaerobic digester plant process infographic – source Hitachi Zosen Inova.

You cannot imagine my amazement and delight, when several weeks ago, I spotted a social media post from SLO Natural Foods Co-op offering a tour of the anaerobic digester plant in San Luis Obispo, CA. I had wanted to visit it for months, but I did not expect that my Co-op membership would be my ticket in.

My spouse and I were already scheduled for a long-awaited tour of the Cold Canyon Landfill and Materials Recovery Facility (MRF) the morning of the same day. Fortunately, the anaerobic digester tour was in the afternoon.

In the previous post entitled, All Americans Should Visit a Landfill, I covered our visit to the landfill and MRF. This post will focus on the anaerobic digester.

First, let’s talk about your green waste bin.

Green Waste Bin

The waste industry refers to the stuff you put into your green waste bin as organic waste because it comes from a plant or animal organism and contains carbon compounds. Examples include tree branches, leaves, grass clippings, fruit and vegetable peelings, meat bones, coffee grounds, eggshells, and cooked, processed, and spoiled food.

Depending on where you live, you may or may not even have a green waste bin. If you do, you may or may not be allowed to put all or only some of the items listed above in it. Check with the company that provides waste removal services for your household.

U.S. Solid Waste Generation by Material 2015 Pie Chart

According to the U.S. Environmental Protection Agency, in 2015, organic waste (wood, yard trimmings, and food) accounted for a whopping 34.6% of the total solid municipal waste generated in the United States.

The purpose of a green waste bin is to keep organic waste out of landfills where it emits CO2 in the early stages of decomposition and methane after it is buried and deprived of oxygen. Methane is a greenhouse gas twenty times more potent than CO2 and is a significant cause of global warming.

San Luis Obispo Kompogas Plant

The San Luis Obispo Kompogas Plant uses the Kompogas® patented dry anaerobic digestion technology owned by Hitachi Zosen Inova (HZI).

Aerial View of Kompogas Plant in San Luis Obispo, CA
This is an aerial view of the Kompogas Plant in San Luis Obispo, CA. The rounded rectangular building houses the anaerobic digester –source Hitachi Zosen Inova.

Bringing the anaerobic digester to San Luis Obispo County was a multi-year effort initiated by Bill Worrell, the former general manager of the San Luis Obispo Integrated Waste Management Authority. He first became aware of the Kompogas technology during a trip to Europe in 2010. At the time, the company that owned the patent was not interested in doing business in the United States.

HZI acquired Kompogas in 2014 and they did want to expand into North America. In 2015, HZI and Waste Connections collaborated on a proposal to build an anaerobic digester in San Luis Obispo.

Over the next several years, the project was approved, underwent environmental review, obtained grants and funding, and was constructed. It opened for business on November 15, 2018.

Revenue is generated from several sources.

  • 65% – tipping fees based on the weight of the green waste each truck delivers and dumps
  • 30% – electricity generated by burning the biogas produced in the anaerobic digester (enough to power about 600 homes)
  • 5% – liquid fertilizer and compost (that remain at the end of the process)

Touring the Plant

After my spouse and I finished our landfill and MRF tour, we stopped by SLO Natural Foods Co-op to grab lunch before heading over to the anaerobic digester plant.

Truck Carrying Green Waste on Weight Scale

While we were waiting for our group to assemble in the parking lot, this truck pulled onto the weigh scale. The plant receives about 100 tons of organic waste a day five days a week.

Thomas Gratz U.S. Sales Manager HZI at San Luis Obispo Kompogas Plant

Thomas Gratz, the U.S. sales manager for HZI was our tour guide.

He knows every inch of the plant and did an excellent job explaining its operations in a way non-technical people like me could understand.

Intake Area at San Luis Obispo Kompogas Plant

In the waste receiving building, Thomas talked about how various machines screen out non-organic materials.

As you can see from this pile, most of the green waste currently received at the plant is yard waste (about 90%).

Green Waste Chip Storage and Automated Crane at San Luis Obispo Kompogas Plant

After screening, everything is chopped into 2″ feedstock pieces and stored in concrete bunkers.

The yellow automated crane (it reminded me of the claw in Toy Story) grabs chips and deposits them on a staging platform for a conveyor.

Chip Conveyor Intake Area to Anaerobic Digester at San Luis Obispo Kompogas Plant

This is part of the conveyor that transports the feedstock chips from the intake building on the right to the anaerobic digester building on the left.

Pipe Feeding Chips into Anaerobic Digester at San Luis Obispo Kompogas Plant

The black tube structure delivers the chips from the conveyor into the anaerobic digester.

The digester has a plug-flow design meaning that the chips being fed into the tube push the material down the digester.

Motor that Turns Agitator Blades in Anaerobic Digester at San Luis Obispo Kompogas Plant

This motor, which is lower on the building than the tube above, turns agitator blades that run the length of the anaerobic digester to keep the contents mixed up.

I am sure Thomas told us the dimensions of the anaerobic digester but I did not record them. I estimate it is about 140 feet long with a diameter of 30 feet or so. This construction photo depicts its scale – source Hitachi Zosen Inova.

Inside the anaerobic digester bacteria and heat ferment the feedstock chips turning them into biogas and digestates (more on this later).

Anaeraboic Digester at San Luis Obispo Kompogas Plant
This shot is from the door of the building that houses the anaerobic digester. The digester is the black structure running the length of the building.
Earthquake Footings and Heating Pipes for Anaerobic Digester at San Luis Obispo Kompogas Plant

Thomas explained the seismic features of the digester like the footing show here.

You can also see some of the pipes and tubes that connect the heating system to the bottom of the digester to keep the bacteria happy during fermentation.

After walking up several flights of metal stairs, we reached the top platform from which we could survey the grounds of the plant and the hills surrounding San Luis Obispo.

The plant has several safety measures to ensure that no pressure builds up inside the anaerobic digester.

  • The first line of defense is a domed storage tank that can hold several days of produced biogas if for some reason it cannot be burned in the combined heat and power plant on site.
  • If the tank is full, then the excess biogas would be burned inside a concrete flare tube.
  • As a last resort, a gas overpressure valve would burst to release the methane-containing biogas into the air.

An environmentally friendly feature of the plant is that everything is surrounded by curbs and drains. Stormwater runoff is collected in stormwater ponds. Cleaning and wash down water are contained on site and reused in the anaerobic digester.

Combined Heat and Power Equipment and Pipes at San Luis Obispo Kompogas Plant

The combined heat and power plant burns the biogas (methane) produced by the anaerobic digester.

The heat is used to keep the inside of the digester at the proper temperature. Electricity not used to run the plant is sent to the electric grid.

Various impurities are removed prior to and after burning the biogas. For instance, hydrogen sulfide, a highly corrosive chemical compound is converted into sulfur that can be used to make fertilizer.

Tour Group and Conveyor to Compost Building at San Luis Obispo Kompogas Plant

The pipe on the right returns about 30% of the liquid digestate to the digester. The remaining liquid is stored in a tank for later sale.

The conveyor on the left moves solid digestate to the composting building.

Tanker Truck Pumping Out Liquid Digestate at San Luis Obispo Kompogas Plant

This tanker truck pulled up while we were admiring the back end of the anaerobic digester building.

The liquid digestate being pumped from the storage tank was destined for a local vineyard to be used as fertilizer.

Compost Bunkers at San Luis Obispo Kompogas Plant

The solid digestate is stored in these bunkers while it is aerated to create compost for sale.

Inside the composting building, it was extremely humid and the air felt heavy to breathe. Negative air pressure keeps any odors inside the building.

Tree Root Air Filter for Compost Building at San Luis Obispo Kompogas Plant

Microorganisms growing on this mass of tree roots absorb the volatile organic compounds (smelly stuff) in the exhaust air from the composting building.

Dan Kallal in our group took this photo.

Lastly, Thomas showed us how the plant is monitored 24/7/365 via an online system linked with the home office overseas.

My impression of the Kompogas Plant is that it has been carefully designed and constructed to safely take in green waste and food waste and convert it to biogas, fertilizer, and compost. The process is both straightforward and complex.

I know I barely scratched the surface of the anaerobic digestion process in this post, but hopefully, you got the gist. There is more information in the resources section.

A Few Words about Food Waste

I cringe whenever I read or hear the words “food waste.”

Growing, transporting, processing, distributing, and preparing food requires a tremendous amount of land, resources, water, energy, and people power.

Our first option should always be to eat the food we buy and to make sure everyone else has enough food to eat. Sending food to an anaerobic digester or a composting facility should be the last option.

You can do your part by eating the food you buy and putting your yard trimmings and food scraps in your green waste bin.

Featured Image at Top: this infographic shows the Kompogas process ecological cycle – source Hitachi Zosen Inova.

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Author: Linda Poppenheimer

Linda researches and writes about environmental topics to share information and to spark conversation. Her mission is to live more lightly on Earth and to persuade everyone else to do the same.

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