Biologicals and BMPS: A Modern Digester's Blueprint for Success

Ensuring a digester has the right bacteria to work in conjecture with methanogens can lead to higher-than-anticipated yields and a more profitable operation.
By Will Charlton | February 24, 2017

Its 10th century B.C., and you walk into one of several heated baths in the Assyrian Empires capital after a long, hot day. In 16th century A.D., you do the same with Persian baths.  In Victorian London, you find yourself walking along the streets of a lighted city. What do these events in history have in common?  They all utilize that same source of energy, one of the oldest and most renewable sources of energy: biogas from anaerobic digestion. 

Even without technological advances or engineering marvels, the baths of Persia, Assyria and others, as well as the lighted lamps of Victorian London, operated efficiently and without fail.  Of the more common renewable energy sources, biogas is one of the oldest, and the most sustainable.   

The anaerobic digestion process, unlike most any other energy-producing process, relies upon a workforce we do not see—microbes that consist of bacteria and methanogens working hand-in-hand to achieve the best results they can. In their world, perfection is making the most available food sources accessible for the next in their assembly line. To give equally for what they take, the bacteria work to break down the carbon compounds into lower fatty acid chains, or volatile fatty acids, which methanogens use to convert with hydrogen into methane gas. Along with producing low-chain carbons, many of these bacteria also produce hydrogen as a waste product, or a more complex carbon dioxide gas (CO2). Methanogens use these volatile fatty acids and CO2  as food sources, and the hydrogen as an energy source to produce methane gas.

By ensuring we have the right bacteria to work in conjecture with our friendly neighborhood methanogens, we can achieve higher than anticipated yields. Ever wonder why some digesters thrive at 75 percent methane, and other stumble and struggle at 40 percent? Engineering does play a part in this, but not as significantly as one may think. Biology is equally important. Knowing which biology works best for the feedstocks you have is a vital component to your success. To determine this without impairing or risking the success of your current production, we are called to utilize Biochemical Methane Potential Test Systems (BMP), methods by which we can simulate our exact criteria, mixing timings and speeds, and operating temperatures. Using exact feedstocks makes a difference, as the naturally occurring biology is different in one region then it is in another. We may find strong sulfate-consuming bacteria as a part of a recipe in sulfur-rich water regions, but find more aggressive, carbon-hungry munchers like Clostridium in another region. It is vital to use what exists naturally, and find a compliment to help it work more effectively. To do this, one may find that simple enzymes that stimulate current biology may be enough, other times we one may find the need to introduce additional bacteria to help break down those longer chain bacteria. Whatever the solution, in the era in which we live, there are plenty of options, and plenty of opportunities to find the right solutions. It may take 10 attempts, maybe even 20, but if one is patient and continues to pound at it, the most fitting solution for the digester will be found. 

Technology has advanced over the millennium since man discovered anaerobic digestion, but only recently, with growth of our understanding of the microbial world, have we come to understand the use of outside microbial additives. This understanding is new, and even our understanding of the microbes themselves is still evolving.  It has only been in the last couple decades that we understood that Archaea (the class of organisms that methanogens belong to) is not even a true bacterium. The world went through its technical revolution, and is now entering what one could refer to as the microbial revolution. Where we once thought everything needed to be steamed and made void of biology, we now understand that some of these biological entities are our friends. This friendship extends to our energy production within anaerobic digestion.

Often, it’s mistakenly assumed that this biology will evolve to suit our needs, and forgotten that these organisms are subjective to specific living conditions. Overloading with metals or minerals can become toxic. Digesters can obtain upset stomachs and upset biology, just as we do. Just as occasionally we are required to watch our diet, we need to do the same for our digesters.

In the end, we wish to see methane gas, CH4, in the highest possible form. We know from research and practical application that we can see 70 percent CH4 or, on occasion higher, in a healthy digester system. So how do we get there?  How do we get to the Holy Grail that we all know is possible but struggle so aimlessly to acquire?  The thing to understand is this: CH4 is produced by methanogens, and methanogens prefer fatty acid chains that are 1-6 carbons in length, simple foods. These fatty acids are known as volatile fatty acids. For this to occur, enter our trusty bacteria friends. 

The process of breaking down the fatty acid chains is likely the most important step in the entire process. To break these medium- to long-chain fatty acids, and even very long-chain fatty acids, down into simpler fatty acid chains that the methanogens can consume, is a key to every digester’s efficiency.  Whether we break down these chains using ultrasonic or other mechanical methodologies, spend our resources working to ensure the right biology is present, or even a combination of both, this is the key stage for an anaerobic digester’s productivity. How well these fatty acid chains get broken down into more amiable forms is what allows a digester to be successful.

So how can one determine the right bacterial amendment for a digester?  The simplest means is to perform side-by-side BMP analysis utilizing the standard operational procedure and typical feedstocks, against potential biological additives offered in the industry. In this manner, one can dictate the production capabilities of a digester with and without these biological additives, answering whether or not it is worth the cost, time and the effort. Do these additives result in enough increase to justify those costs?  In my experience, I have yet to encounter a situation in which we did not find a product that was the perfect solution to far exceed current production levels. In some cases, these increases were to the magnitude of 200 percent increases. In others, it was a meager 30 percent increase in production, but for over 20 digester simulations, we have seen increases 100 percent of the time over the standard input.

BMPs and biologicals are a very important tool combination for anaerobic digesters. Many debate the efficiency of BMP work as being typically 25 to 30 percent, but the reality is in the way the BMP work is completed.  If it uses a biological inoculant that the digester is not utilizing, then the overall results can easily be 25 percent, or even 50 percent off.

Why are BMP results so different than actual real life results? If you seed your digester only once, the benefits of the introduced bacteria are lost over time, as these populations either wither away or are diluted to nonexistence. A BMP is a batch reactor, working under the assumption that the digester is fed the same constantly, with the right blends of nutrients and metals. Staff should be well-enough educated to look for red flags, and to know the yellow flags when they first appear. Ensure your digester is always kept well-trained and in top mechanical condition, working to its best ability. Don’t shortchange your success by limiting your digester’s biological capability to fully process carbon into energy. Your biology is one-third your success. As in business, the difference between the red and the black can sometimes be as small as a 10 percent factor. 

Where there is a will, there is a way. Find the will to succeed by applying today’s sciences and technologies to your digester’s successful operations.

Author: Will Charlton
President, Digester Doc


CONTRIBUTION: The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of Biomass Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).