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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Resources

Energy Awareness Month – 10 Energy Saving Tips

You have the power to conserve energy.

This October, fulfill the promise you made to yourself earlier in the year to get serious about saving energy and reducing your carbon footprint.

October is an ideal time to address your energy use for a number of reasons. First, you still have plenty of time to put energy saving ideas into action before cold winter weather arrives in earnest and the holiday season diverts your attention. Second, if you enjoy challenging yourself during national awareness days or months, you are in luck because October is Energy Awareness Month (it should be Energy Action Month). Third, reducing your energy use can also save you money.

I realize that switching to LED light bulbs and putting on a sweater instead of cranking up the heat will not stop Americans from burning fossil fuels. However, if millions of Americans take these and other seemingly small actions, it all adds up and can make a significant impact.

For instance, if each American household tackled their energy vampires for Halloween we could save 100 billion kilowatt hours of electricity and use it to provide the annual power needs of 35 million Americans.1

I believe that taking action, even a tiny action, acts as a strong antidote for inertia. The first action may be difficult but each subsequent action is easier because you gain momentum.

Are you ready to take action to save energy and reduce your carbon footprint? If you are, below are ten tips of varying degrees of difficulty and expense to help you get your creative juices flowing. Most of the tips include links to other posts where you can get more information and find useful resources.

Light with LEDs

If you have not made the switch to LED light bulbs yet, now is the time.

Residential LEDs use 75% less energy and last 25 times longer than incandescent bulbs and they do not contain hazardous mercury as compact fluorescents do.2

The cost of LEDs has dropped dramatically over the past several years and now you can purchase an LED light bulb for around $2.00 maybe less (depending on wattage and type).

If you put LED bulbs in your indoor and outdoor light fixtures this month, you may not need to change a light bulb for a couple of decades and you will immediately reduce your energy use. You can even decorate your Christmas tree with a few strings of colored LEDs.

Snug House

Snug House - Scarf Wrapped Around Miniature HouseKeeping cold air outside and warm air inside during winter months and vice versa during the summer is a good idea, right. What you may not realize is how even small air leaks can wreak havoc with your heating and cooling bills. For example, a 1/8” gap under your front door lets in as much air as if the door had a 2 ¼” hole.3

Fortunately, you can shore up your home’s air defenses with a caulking gun, door sweeps, and weather stripping. You may be able to reduce some air leaks with things you have on hand like rolling up a bath towel to minimize door drafts. I folded up a piece of cardboard and stuck in a crack where the weather stripping on the fixed side of our double front door did not quite reach the threshold.

Read more in Seal Air Leaks to Reduce Home Energy Use and Cost.

Take Advantage of Your Thermostat

A thermostat is a useful device for moderating your home heating and cooling system. Turning back your thermostat 7°-10°F for 8 hours a day could save you 10% on your heating and cooling bills.If you frequently forget to adjust your thermostat when you leave for work, try hanging your keys on a hook next to it.

Learn more about thermostats, recommended temperatures, and thermostat options by reading Use Your Thermostat to Save Energy and Money.

Staying Warm Indoors

On average, home space heating consumes a whopping 42% of the energy Americans use in our homes.5 Hot air rises and cold air sinks so during the winter we are living in the coolest layer of our homes.

You probably take care to dress appropriately for the weather when you go outdoors in the winter so why not carry that theme indoors. Instead of ratcheting up your heater consider wearing clothing made of warmer materials or trying one or more of the tips in 7 Ways to Stay Warm Indoors in the Winter and Be Green.

 Shower Power

Low-flow showerheads are water and energy saving devices. Using less water also means using less energy to heat water. A standard showerhead sprays out at least 2.5 gallons of water per minute even when you are lathering up your body or washing your hair.

Low Flow Handheld ShowerheadWhen you switch to a low-flow showerhead that puts out 1.6 gallons of water per minute you can easily cut your water use by 25% and reduce the energy needed to heat your shower water. If you buy a model with a “trickle” button or a shut-off valve you can reduce your water and energy use even more by restricting the water flow while you are soaping up.

Even non-handy people like me can easily install a low-flow showerhead for under $50.00.

Use Your Dishwasher

Washing dishes by hand is not a water or energy saving activity. A kitchen faucet pumps out 2.5 gallons of water per minute so you may be using more water than you think filling up the sink or a dish tub and then rinsing dishes.

Cramming a bunch of dishes in a dishwasher willy-nilly may result in some items not getting clean so do pay attention to where the spray jets are and learn to load your dishwasher efficiently.

Green and Lazy Laundry

Doing the laundry is a habit that you learn and then repeat thousands of times over your lifetime so you may find energy and water savings hiding in your laundry room.

I did not think much about my own laundry habits until my kids went away to college but if you have children at home you do not have to wait that long. If you are interested in evaluating your laundry habits, you may find the posts Laundry – Laziness is Good and Greening Your Laundry Habits useful.

Extra Credit: Using the sun to dry your clothes on a clothesline is a significant energy saving action, but I admit that I do not do it, at least not yet.

 Tackle Your Energy Vampires

Energy Vampire - Cell Phone ChargerAn energy vampire is a piece of equipment that sucks power even when it is not in use; this is called standby power. For instance, a cell phone charger left in a wall socket or a television both draw power just standing by waiting for you to use them.

Our Halloween activity for 2013 was tackling our energy vampires. It was fun, easy, and inexpensive. A few weeks after we completed our energy vampire project I learned the hard way that cable boxes must be on standby power to receive system updates. Our cable television service was abruptly discontinued without notice because our cable box had been going offline each evening. Now I leave it on.

Energy and Water Efficient Appliances

I am not advocating buying new appliances unless you need to replace a worn out or un-repairable appliance or piece of equipment. However, if you are in the market for a new refrigerator, air conditioner, or television, consider adding energy and water efficiency to your list of must-have features.

Look for the ENERGY STAR and WaterSense labels to identify and compare appliances and equipment. I wrote about my search for a high-efficiency replacement dishwasher in Dishwashers – Top 3 Eco-Friendly Features.

Go Solar

There is no better time than right now to go solar. Solar panel prices are low, tax incentives are available, and the summer rush for solar installers is over. You can increase the value of your home with solar panels while reducing or eliminating your electric bills. If you do not want to buy a rooftop solar system, then consider leasing.

Purchasing solar panels for your home is a sound financial investment and even more importantly, it pushes the ball forward in creating a clean renewable energy future for all our children.

You can learn more about home solar panels and our real life rooftop solar experience by reading Go Solar with Home Rooftop Photovoltaics – We Did, Rooftop Solar Costs Less than You Think, and You Can Increase Your Home’s Value with Owned Solar Panels.

My energy saving action for Energy Awareness Month is washing our laundry with cold water. I know, I know, why did I not make this change years ago? My only defense is that old habits stick with you. The good news is that you and I can change our habits today or any day and make a positive impact.

I hope one or more of the above ideas has struck your interest and helped inspire you to take action to reduce your energy use and carbon footprint. Please share what you are doing to reduce energy use with other readers.

Featured Image at Top: Coal-fired power plant looming over a residential neighborhood in West Virginia – Photo Credit Wigwam Jones

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References

  1. Energy Vampires and Phantom Loads – Standby Power, Green Groundswell
  2. LED Lighting – Energy.gov
  3. Energy Advice for Owners of Historic and Older Homes – U.S. Environmental Protection Agency
  4. Thermostats –U.S. Department of Energy
  5. Use of Energy in the United States Explained: Energy Use in Homes – U.S. Energy Information Administration