Anaerobic digestion, for use of biogas, is the resourceful and sustainable conversion of various organic wastes (biomass) into renewable energy. The goal of anaerobic digestion for biogas is to produce as much biogas as possible with the highest possible biomethane content from biomass. With this goal in mind, is there a certain substrate that produces more biomethane (therefore biogas) then others? Short answer is no, but the long answer is yes. Confusing right? Hopefully we can provide some clarity.
To create biogas, the following requirements are essential:
- High methane content
- Low water vapour
- Low CO2
- Low Sulphur
- Optimized steady fermentation process
Biogas consists of 50-75% methane, 25-45% carbon dioxide, 2-8% water vapour, and low quantity traces of O2, N2, NH3, H2, H2S. Biomethane creation is fickle (which is why it is so confusing). It comes down to a calculated regiment that is highly scientific. Although the average person likely doesn’t think about biogas production this in-depth, a lot of the biogas production ability relies on a TON of variables ranging from the feedstock quality to facility design and maintenance.
The way a plant cleans, processes, and maintains their facility can impact their ability to produce biogas, as well as the overall facility design, temperature at fermentation and holding time. Given these variables from facility to facility, it is challenging to pinpoint the best substrate and how they will perform every time – especially since the substrates themselves have their own set of variables which impact how well they decompose and create biomethane. The suitability of biomass as a substrate (to produce biogas) is highly dependant on its nutritional value. Biomass entering the anaerobic digestion process have their own organic makeup, and set variables such as age of product, condition, circumstance, pre-treatment, and PH level. These compositions in feed can influence biogas yield, methane content, biodegradability, and degradation ability.
So all that to say, because there are too many factors at play, there is not a certain substrate that is better then another. Therefore, the answer is no – there is not a superior single substrate. The quality of feedstock and facility maintenance/design differentiate so much that is it quite difficult to say what substrate or mix of substrates are the best for biogas creation.
However, there are a few traditionally used substrates that have proven to provide increased levels of biogas consistently (but not always!). They are:
- Farm Animal (Cattle, Pig, Chicken, etc.) Dung
Animal dung is one of the most suitable materials for biogas because of its methane producing qualities. The best source is specifically cattle dung, because of the methane producing bacteria already found in their stomachs. Once collected, animal dung must be mixed with equal parts water to be used in the anaerobic digester. This is a clean, methane producing substrate, but is generally not as good as manure.
Animal dung is a renewable, cost-effective and environmentally friendly substrate for anaerobic digestion and biogas production. Animal dung that is left unhandled/treated/picked up is a threat to our environment as it contains high levels of nitrogen and phosphorus, in addition to pathogens, antibiotics and heavy metals which can contaminate air, water and soils.
- Animal Manure
Different then fresh cattle dung, manure is the mixture of dune and urine and therefore requires not extra water to mix. The issue with obtaining manure is that urine is nearly impossible to capture in an average farm set up, even though it is one of the most valuable resources for biogas creation (due to its high nutrient content).
The same things apply to manure as they do to fresh dung, it is better for us and the environment to pick up and use for biogas then to leave them untreated.
- Food waste
Food waste and kitchen waste has the highest methane yield for biogas – likely due to the high lipid content of food waste. Substrates that have a higher lipid content create high methane outputs then carbohydrates or proteins, so ensuring yields from food waste contain at least 30% fat is key. The mix of food waste as a single substrate also adds a diverse nutrient profile during anaerobic digestion that others do not possess.
Like animal dung/manure, food waste is also a renewable, cost effective and environmentally friendly substrate. Food waste is often unavoidable, largely due to peelings/bi-products and of course, natural decomposition. Its readily available nature makes it renewable and cost effective. Additionally, if disposed of in a landfill, food waste freely off-gases, emitting unwanted greenhouse gases (GHG) into the atmosphere (and takes up precious space in our already full landfills).
Concluding Thoughts
Biogas plants are often found on farms, as animal manure is a cost effective, clean substrate for biogas production (and dung/manure is a huge methane/GHG emitter, so it is also a win win). With dung/manure readily available on farms, it only makes sense that biogas facilities are near by, or directly, on farm sites. Farms can also have their share of food waste from crops etc. – so this also creates a good feed for anaerobic digestion.
For us at Davidson Environmental, we send all our food waste to Biogas facilities, which ends up being a mix of just about everything under the sun. We take from all industrial, commercial and institutional business, which provides us with a varied input. This gives us a dense nutrient profile to offer to anaerobic digesters for production of biogas, since we have multiple types of food waste inputs. In addition to our mixed, high nutrient food waste, we also are nearly contaminant free, as we run all of our food waste through the
“BioSqueeze 200” to depackage all of our food waste. The BioSqueeze creates a food waste ‘slurry’ – which is an ideal substrate for biogas creation.
Given this, any biomass would work to make biogas, but there are some substrates that naturally create it better (and faster) then others. This blog post really just skims the biogas substrate discussion and provides basic information on the very scientific information out there. It is quite an interesting topic to dive into and it is truly not black and white – it is full of grey. Hopefully we provided you more answers then questions, but if you do have any questions, please feel free to reach out to us!