Azura’s Dave Ellis was invited to join Anna Simet on the Biomass Magazine Podcast to talk about municipal codigestion! Municipal sewage sludge codigestion blends organic waste into existing anaerobic digesters to produce biogas and it is a great option to help reduce the carbon intensity of our fuel system. Watch out the podcast to hear how existing facilities can produce more gas! Scroll down to read the full transcript.
Anna Simet: Hello, and welcome to the Biomass Magazine podcast. I am your host, Anna Simet. I hope you’ve all been well. On today’s episode, I have with me Dave Ellis of Azura Associates, and we are going to get a little into codigestion. Dave, hello, thanks for coming on to the podcast.
Dave Ellis: Hello. Good morning, Anna. Thanks for having me.
Anna Simet: We’re so glad to have you. So, Dave, to start, why don’t you tell us a little bit about Azura Associates and what it is that you do there?
Dave Ellis: Sure thing. Thank you. So, I began working with anaerobic digestion more than 30 years ago, part of my chemical engineering work with a major international consulting firm. And since launching Azura, just over 10 years ago, I focused on industrial waste, high strength waste, which is largely treated by anaerobic digestion. And so building on the years of experience that we have with anaerobic digestion systems, seeing the need for low carbon fuels, that is our main focus at Azura Associates. We are all process engineers, chemical engineers, focusing on that process is, is why we exist. We do that for our shared world to make low carbon projects come to life.
Anna Simet: So Dave, we’re talking about codigestion today. I’ve been hearing that word a lot as of late. So, why adopt codigestion? And you know, what exactly does codigestion mean?
Dave Ellis: That’s a great question. So, the answer first is that digesting waste produces the only carbon negative fuel in the world. I’ll say that again. Anaerobic digestion produces the only carbon negative fuel in the world. Like, that’s the whole answer. It’s the only one. So anaerobic codigestion means blending different waste materials together and putting them into an anaerobic digester tank to produce biogas.
That waste material, it can be from a local sewage treatment plant, it can be organic waste from manure, it can be food processing materials from your local apple pie factory for instance, it can be source separated organic (SSO), banana peels from your kitchen. All of those materials count and blending those material is what anaerobic co digestion is. The biogas that is produced is then purified to produce a biomethane, or it can also be called renewable natural gas or RNG for short. So RNG is clean and sustainable. It’s the energy source that’s the same as what’s used in conventional natural gas, but with a lower carbon footprint.
And. It’s renewable because food waste will exist as long as people keep making and eating food. Anaerobic codigestion and why we should be looking at it more is it’s the easiest opportunity. It’s right in front of us to help us quickly move to reduce the carbon intensity of our fuel system. AD has been in use globally for more than 100 years, and certainly in North America, the sewage treatment community has been using anaerobic digestion for more than 50 years.
The technical issues around running most digester systems have long been solved. So in terms of looking at say for example, a case of a municipal sewage treatment plant. Now, considering why a municipal sewage treatment plant would consider digesting or code digesting sewage solids with food waste there are three main reasons. First of all, revenue generation and the business case; with AD they can generate revenue from the feedstock, from a tip fee, from the biogas itself, and from the digestate. The tip fee for taking and processing the waste, the same as if it was going to a landfill but going to the sewage treatment plant or the publicly owned treatment works.
The biogas produced has value, and that can generate heat and electricity, or be refined into renewable natural gas. They can use that biogas to power the treatment plant itself, or even refine that into renewable natural gas that they can then sell, and they can either sell that at the same price as geologic natural gas, or they can sell the green attributes, understanding the carbon intensity of that material, it’s usually carbon negative in nature, and that has a value also.
And then the digestate, which is the liquid that remains after it’s been through the digester, that can have value too because it has nutrients like nitrogen and phosphorus that can be used as a fertilizer. So, there’s the revenue piece, increased gas production is another great reason. The sewage treatment plant will typically be producing biogas today because they already have an anaerobic digester. Adding in food waste to help them produce more biogas and to get more revenue from an existing asset. The food waste is also rich in different nutrients and high in fuel value. So all of these things help the digesters perform better and produce more gas from the same asset.
And then waste management is a critical piece. So food waste is a significant issue in many cities. By Codigesting food waste with sewage solids or manure, municipalities can divert food waste from landfills, reduce landfill usage, and even extend the life of landfills and the associated greenhouse gas (GHG) emissions.
So, for example, some places in North America are projecting their landfills will be at capacity in less than 10 years. In these places, it can take more than 10 years to get permits and approval to build a new landfill. So these folks are in a jam. In the U. S. alone. 66 million tons of post-consumer food is wasted and 60% of that goes to landfill. So it’s estimated that in Canada, you know, there’s on the order of 155 petajoules per year of RNG potential.
And that’s just in Canada. So in the United States, of course, it’s much, much greater than that. And so the feasibility of Canada’s current natural gas situation being displaced by RNG, it’s not going to be 100% for sure, but it can be a significant percentage, you know, into the single digits. But absolutely, as long as we’re eating food and making food waste, we still need to manage that waste in an economically and environmentally sound approach.
So all of those reasons, waste management, gas production using the existing assets, and the overall business case make good sense for looking at codigestion.
Anna Simet: Thanks for that very thorough answer, Dave. I think my next question is going to be two parts. What are the top two common issues that come from codigestion and then how can you prevent those issues?
Dave Ellis: Excellent point. So, when I said that technical issues with AD have been solved decades ago. That does not mean this is a plug and play technology yet. Codigestion systems still need careful attention to feedstock assessment, engineering design, and operation. So, in the three decades that I’ve been working with AD, I would say consistently the two most common issues I’ve seen are related to foam and grit.
So foam can form in waste receiving areas or in the digester itself. Now because there are more than a dozen different types of foam, I’m talking about it from a chemical perspective, each with a different cause, but ultimately those are all to be avoided. So, looking at foam, the easiest and best way to prevent foam formation is by, I’ll say, engineering a good fit between the feedstock material and the design of the overall system.
So some food wastes are acidic and some food wastes are basic or alkaline. So, Anna do you remember when you were a kid and what happened when you made that vinegar and baking soda volcano, and it foamed all over the place? So that’s where if you add some bacon fat to that and now you’ve got this greasy foamy mess in your feed tank at a digester site; if you’re mixing those vinegar type things and those baking soda type things in your feed tank, and that’s if you’re lucky, and if you haven’t considered these at the outset, and maybe you’ve got some bacon fat in there as well, now you’ve got a greasy, foamy mess that may have foamed over your feedstock tank or come out your overflows, and now you’ve got a greasy, messy spill on the ground.
So, keeping the feedstocks separate is definitely one key approach and being able to meter them into the digester carefully, then there’s no issue. Similarly for grit, almost all food waste and other organic waste have some type of hard solids in them that does not biodegrade particularly well in digesters, and they can settle and build up in the digesters tanks themselves. So these can be everything from tiny bits of eggshells from kitchen waste, or apple seeds from your local apple pie factory, sending their waste to a digester. In either case, just like foam, planning and prevention are always the lowest cost ways to manage these. In my experience, active grit management is essential, and it can be as simply a matter of planning at the outset to take the digester offline for 4 to 6 weeks every 5 to 10 years for a manual clean out exercise. But of course that downtime and that loss of revenue, that needs to be penciled out in the business case up front. Alternatively, including an active grit removal process to clean the feedstock at the front end, that’s another approach. And a third approach is with a focus on robust mixing to help make sure that that material does not settle out or doesn’t build up overly in the digester itself. So those, those approaches all come down to active planning and consideration during the early project development stages.
Anna Simet: Okay. So what should operators consider before adopting co digestion? Excellent. So there is on the commercial side, ultimately, these systems need to be sustainable, and sustainable from an economic viewpoint as well. The food waste industry in North America is still young enough, we are still seeing major food waste digester projects not come off to a great success and some have actually failed and shut down. And within the last two years, we’ve seen this happen. And so, making sure it’s economically sustainable is critical. Looking at the biochemical opportunities in the feedstock. What’s the what’s the fuel value of the fees that are available in my local area? Do we have to truck them far? Are they close by? Understanding the different fuels – that’s really critical as well. And then operationally, what is the different feedstock mean to us? Is there a risk that a feedstock may potentially compromise the biology in my digester at some point?
The general mindset to look at when considering adopting codigestion is more one of farming as opposed to building a machine. And that’s where the farmers, you know, are often at a bit of an advantage to folks perhaps coming out of some other technical area because they tend to intuitively have a good sense of this long view of nutrition and balancing from the different feed blends and how it’s going to affect the biology, or the thing that they’re trying to grow inside their digester themselves. If you focus on growing good biomass, you can’t help but get great biogas out of the system and excellent RNG.
Anna Simet: So Dave, let’s take this a step further. Can you walk through an example of how you would evaluate if you should accept a new feedstock for codigestion?
Dave Ellis: So yes, we get asked this question quite a bit. So the first thing we do is we begin with a quick qualitative screen. Is the material from a source that we would expect to be biodegradable? Did it start as a food or an agricultural material? And because we’ve worked with, the food industry and food waste for a long time. We have a pretty good sense on most waste streams, what they are, what goes on inside the factory, where these wastes get made and some of the risks.
Once it passes that first qualitative screen, the big number to look at is the fuel value. How much fuel is in this material? How much gas can we get? And does it pass the first business case hurdle? And this could be based on reference data, this could be a five-minute exercise looking at reference data all the way out to spending four to eight weeks validating that with laboratory testing.
Then we look at is it degradable? There’s a lot of organic things are not actually biodegradable. So take wood, for instance, wood chips, sawdust, wood waste, it’s natural, it biodegrades in the forest. That’s great. But wood does not actually biodegrade in anaerobic digester. So not everything that we think might or intuitively be degradable is actually degradable. So that’s the next question we ask ourselves.
And then we look at it. Is it nutritious? Is its composition anything that can help the overall balance of the digester biology? Is there any contamination or tramp material? So that can be things from, if it’s a food factory, it can be rubber gloves, it can be hair nets and things that are in the food factory as part of the normal operation that occasionally end up on the floor and might end up in a waste stream coming to a digester facility. Or as we’re talking about grit, for instance, that’s where we can see things from sand and gravel or we can see metals, metal nuts, bolts and washers, again from factory maintenance operations that end up on the floor and then being swept up into part of the waste stream and coming to a digester facility. How can we handle that material?
And then the third of contamination is chemical contamination. So to be aware of what’s going on in a food factory, food safety trumps pretty much everything. And so if there is any concern of food safety that somebody might get sick from eating something, just as we might use bleach at home, there are other chemicals that are used in food factories to make sure that their tools and knives and baking dishes are all clean and safe, but everything they use to kill the bacteria in the food factory has the potential to kill the bacteria in our digester too.So. That’s where being aware of what goes on inside a food factory is really important to understand the risk for contamination.
Anna Simet: Thanks, Dave. All that makes sense. So we’re nearing the end of our podcast. Before we wrap things up, is there anything else that you want to add that we haven’t touched on yet?
Dave Ellis: So I think just to highlight that the key here as we wrap up is that there is no one solution for climate change. Diverting waste is important because we have waste management issues and anaerobic digestion is the lowest hanging fruit, it really is the only potentially carbon negative fuel that we have. So thank you very much for the opportunity.
Anna Simet: Thank you so much for coming on to the podcast Dave and telling us about Azira and co digestion.
Dave Ellis: Yeah. Thank you, Anna. Thanks for having us.
Anna Simet: Absolutely. To our listeners. Thanks so much for tuning in. I hope you enjoyed this episode until next time.