Anaerobic Digesters are Good for the Environment

Don’t waste your green waste.

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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All Americans Should Visit a Landfill

There is no “away.”

I double dog dare you to visit your local landfill to see what happens to your garbage. Afterward, ask yourself how you can improve your own waste habits.

Think of touring a landfill as a cross between an interesting field trip and a civic duty. Add to the fun with a side trip to a materials recovery facility also called an MRF (pronounced “murf”).

Each week, you, I, and everyone else toss our solid waste into garbage, recycling, and green waste bins, perhaps without much thought about what goes where. Then, once a week, while we are at work, school, sleeping, running errands, or otherwise going about our day, the contents of these bins are magically whisked away.

U.S. Total Solid Waste Generation 2015 Pie Chart
Percentages of materials that made up U.S. municipal solid waste in 2015 – source U.S. EPA

The problem is that there is no “away” and the environmental impact of landfills is enormous.

Environmental Impact of Landfills

Nowadays, at least in the U.S., landfills operate under more stringent regulations than in the past. Environmental issues vary widely depending on a number of factors including landfill size, location, design, contents, age, and how it is operated and managed.

U.S. Disposition of Solid Waste 2015 Pie Chart
Disposition of U.S. solid waste in 2015 – source U.S. EPA
  • Landfills take up large amounts of land displacing plants, animals, and even people.
  • Slowly decomposing plant, animal and human waste produce methane, which is a greenhouse gas 20 times more powerful than CO2. According to the U.S. EPA, landfills are the third-largest source of human-related methane emissions in the country.
  • Rainfall and moisture can cause toxic chemicals to leach into streams, lakes, and groundwater killing aquatic life and endangering drinking water.
  • Air pollution is created by trucks collecting and transporting garbage and by dust and other particulate matter churned up at the landfill.
  • When reusable and recyclable materials are buried in a landfill, the energy, resources, and people power that went into making them is wasted.

To me, the biggest environmental impact of landfills is that they enable and even encourage consumerism and wasteful practices. Our waste is conveniently tucked away out of our sight so we do not have to think about it.

The Trash Trifecta

I acknowledge that I might be a wee bit more fascinated by garbage than many people. For instance, one of my favorite nonfiction books is Garbology by Edward Humes. I have also written at least 35 posts that have some connection to waste.

So, you can probably imagine my excitement when, recently, I had the opportunity to visit a landfill, MRF, and an anaerobic digester all on the same day. I was in solid waste heaven.

Below, you will get a glimpse into what happens at a landfill and a materials recovery facility. We will take a look at the anaerobic digester in the next post.

I live in a small town on the Central California Coast in San Luis Obispo County, CA where the entire population is about 285,000 people. My household’s garbage goes to Cold Canyon Landfill the largest of the three landfills in the County. Based on the permitted capacities of the three landfills I estimate that Cold Canyon Landfill takes in 60-70% of the County’s solid waste.

Aerial View of Cold Canyon Landfill and Materials Recovery Facility
This is an aerial view of Cold Canyon Landfill in San Luis Obispo County, CA using Google Maps. The red rectangle shows the location of the Materials Recovery Facility (MRF).

Although landfills and MRFs operate in a similar manner across the U.S. there are differences and capabilities may vary considerably. For instance, some MRFs have equipment that enables them to process difficult to handle items like single-use plastic bags, but many like the one where I live, do not.

At the Landfill

At the Cold Canyon Landfill, my spouse and I joined students from the California Naturalist Program at Cuesta College. Our host was Mike di Milo. He is the owner of Science Discovery the company that runs education programs on behalf of the San Luis Obispo County Integrated Waste Management Authority.

After donning electric yellow safety vests, we bundled into several vehicles for a short trip into the landfill and stopped on a section of road that overlooked the day’s activities. Mike had us stand in a group while he shared facts about the landfill and talked about how it works.

Engineering

You may think a landfill is just a pit, but that is not the case, at least at Cold Canyon. The landfill is carefully engineered and monitored.

Cold Canyon Landfill Garbage Dumping Deck
As soon as we stopped on the side of the road, I jumped out and took this photo of the Cold Canyon Landfill garbage dumping deck.

The excavated area currently being used was lined with plastic, clay, and gravel before any garbage was put in it. A drainage system diverts stormwater runoff to a settling pond to keep sediment out of nearby Pismo Creek. The compacted garbage is arranged into layers about 15-20 feet high and then covered with about 2 feet of compacted soil. The top is seeded to grow plants and grasses to reduce erosion.

Operations

Cold Canyon Landfill takes in about 1 million pounds (500 tons) of garbage a day six days a week. Our visit was during the morning before most of the municipal garbage trucks had come by to dump their loads so the pile of garbage was small.

We watched as pickup trucks pulled up alongside big garbage trucks at the edge of the garbage pile and dumped their loads. The bulldozer operator maneuvered nimbly around the trucks staying out of the way but constantly pushing the pile toward the pit. Another heavy equipment operator focused on getting the garbage compacted in a designated area.

On windy days, large litter screens are strategically placed to stop garbage from flying out of the landfill.

At the end of the day, the garbage is covered with tarps until the next day. This practice eliminates the need for a daily layer of dirt allowing the landfill to extend its useful life because there is room for more garbage.

Mike pointed out the above ground pipes running across the top of the covered landfill sections that capture and transport methane gas produced by the landfill. Burning the methane generates power for the landfill and the Arroyo Grande Oil Field in Price Canyon.

We learned that there is a section for collecting separated demolition and construction materials. Lower dumping (tipping) fees encourage contractors to do this, but it is not mandatory. There is also a place on site where residents can drop off their household hazardous waste like used motor oil, paint, and pesticides.

Environmentally Friendly Pest Control

After Mike filled us with information, we had the pleasure of meeting a falconer named Aaron and a magnificent Harris’s hawk.

Falconer Aaron with Harris's Hawk at Cold Canyon Landfill

Aaron cares for and works with this hawk and four other birds of prey. They keep the volume of seagulls flying around the open garbage pile in check. Swarming seagulls can make working conditions unsafe and present a health hazard when they pick up garbage, fly away with it, and leave it somewhere else.

After talking with Aaron and admiring the hawk, we got back in our vehicles to drive over to the MRF.

At the Materials Recovery Facility (MRF)

As we drove up to the MRF, I spotted a truck pausing momentarily at a weigh station. Recycling and garbage tipping fees are based on weight. The truck is weighed again after it is empty.

Visitors are not allowed to go into the actual MRF so we gathered in the education center. This space has a large glass window where you can watch the action, interactive displays, and rows of chairs in front of a large movie screen.

Cold Canyon Landfill Materials Recovery Facility Education Center
Education Center at the Cold Canyon Materials Recovery Facility (MRF). The window on the left looks into the MRF. Photo – Science Discovery.

Mike lured us away from the window to watch live action feeds on the screen from various cameras mounted inside the MRF.

A heavy equipment operator used a big clunky grabber mounted at the end of an articulating arm to pick up materials from a huge pile and move them to the conveyor belt intake area. The operator also delicately selected unrecyclable materials like plastic bags, box strapping, and garden hoses and dropped them over a low wall into another pile. Later these materials would be picked up and transported to the landfill.

The facility was a maze of conveyor belts, machines, and bins. Materials were being separated by machines and people. Workers standing in small areas (kind of like waist high turrets) adjacent to the conveyor belts removed trash and recyclable materials and tossed them in nearby chutes or bins.

At the end of the process, separated materials enter a baler and come out as strapped cubes of materials like cardboard, plastic bottles, and aluminum cans. I did not see what happened to glass or paper.

The amount of garbage mixed in with recyclable materials was shocking. Mike said that as much as 25% or more of the materials that come into the MRF are not recyclable. In this facility, it was easy to see that it is not equipped to handle things like plastic bags or little bits of anything.

U.S. Recycling Percentages 1970-2015 Table
U.S. recycling percentages 1970-2015 – source U.S. EPA

The California Naturalists were on a tight schedule so we thanked Mike, carpooled back to the entrance, and dispersed.

My Takeaways

Seeing what actually happens to the contents of my garbage and recycling bins made a big impact on me. Faced with the reality I cannot hold on to my wishful thinking anymore. Here are some of my thoughts.

  1. The people who work at the landfill and MRF work in dirty, noisy, and smelly conditions and they know what they are doing.
  2. There is no one sorting through the garbage mound looking for and setting aside reusable or recyclable materials. Everything dumped on the landfill deck goes into the pit.
  3. The MRF is not a precision operation. Some recyclable materials end up accidentally going to the landfill. Some recyclable materials are contaminated by substances or materials that do not get separated out making them unusable and unsellable.

I came away committed to continuing to find ways to reduce the amount of solid waste our household produces and to sort it into the right bins.

It occurs to me, that every week, you, I, and everyone else have an opportunity to positively impact the environment when we take out the trash.

Featured Image at Top: A woman tossing trash over her shoulder is being followed by a wave of garbage – photo credit iStock/ pick-uppath.

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