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Generating Biogas: The Best Feedstock Materials

A great variety of organic material can be used in anaerobic digesters as a feedstock for generating biogas. However, there are scientific, engineering and legal limits to what can be added successfully to a digester, according to research from Michigan State University.

16 April

16 April

5 minute read

5 minute read

The feedstock needs to be a liquid mixture with an appropriate moisture content. For example, mesophilic complete mix tank digesters (the type most commonly used today) typically operate best with a mixture of four to eight per cent solids in water. Digesters require various moisture contents, depending on the design and operation of the system.

Feedstocks for Anaerobic Digestion

Most easily biodegradable biomass materials are acceptable as feedstocks for anaerobic digestion. Common feedstocks include livestock manure, food-processing waste, and sewage sludge. The energy production potential of feedstocks varies depending on the type, level of processing/pretreatment and concentration of biodegradable material. Listed below are feedstocks that can be commonly used in anaerobic digesters:

• Livestock manures
• Waste feed
• Food-processing wastes
• Slaughterhouse wastes
• Farm mortality
• Corn silage (energy crop)
• Ethanol stillage
• Glycerine as the product from biodiesel production
• Milkhouse wash water
• Fresh produce wastes
• Industrial wastes
• Food cafeteria wastes
• Sewage sludge

Livestock manures are generally lower-energy feedstocks because they are predigested in the gastrointestinal tracts of the animals. Manure, however, is an easy choice for anaerobic digestion because it generally has a neutral pH and a high buffering capacity (the ability to resist changes in pH); contains a naturally occurring mix of microbes responsible for anaerobic degradation; provides an array of nutrients, micronutrients, and trace metals; is available in large quantities; and can be transferred by pump.

Animal wastes containing bedding such as chicken litter with substantial quantities of wood chips or sawdust can be used successfully in anaerobic digestion. The woody material, which degrades very slowly because of its lignin structure, is essentially passed through without digestion, and retention times are based on digestion of the manure.

Blending of energy-dense feedstocks with livestock manure is a common practice to maximize biogas production by optimizing nutrient levels and providing buffering capacity. The use of manure as a base for anaerobic digestion is important because many of the energy-dense feedstocks, such as food-processing waste and ethanol stillage, are acidic, contain little if any naturally occurring microbes, and oftentimes lack the nutrients (nitrogen, trace elements, vitamins, etc.) necessary for anaerobic digestion. Potentially, farms operating anaerobic digestion systems could take on additional wastes and benefit from increased gas production as well as tipping fees.

Materials to Be Excluded from Anaerobic Digesters

Materials that should be excluded as feedstock from anaerobic digesters include those containing compounds known to be toxic to anaerobic bacteria, poorly degradable material, and biomass containing significant concentrations of inorganic material. Poorly biodegradable materials require higher retention times, meaning they must spend more time in the anaerobic digester to be broken down and converted into biogas.

Inorganic materials, on the other hand, contain no carbon and cannot be converted into biogas. Materials such as sand bedding do not contribute to the biogas potentialo and may cause operational problems such as pipe clogging, premature equipment wear and volume reduction due to sludge accumulation. Also, the feedstock containing too much ammonium or sulfur should be avoided, because ammonium and sulfur inhibit anaerobic organisms.

Evaluating Feedstock Biogas Potential

The biogas potential of different feedstock materials or feedstock combinations is often difficult to predict due to differences in the source, processing, volatile solids concentration, chemical oxygen demand, moisture content, and/or inclusion of toxic compounds. The total biogas potential assay, also known as the biochemical methane potential (BMP) assay, provides an efficient and economic method for estimating biomass conversion and biogas yield of feedstocks or feedstock blends.

BMP assays are a multifaceted approach to evaluating the potential to produce biogas. BMPs are a practical, lab-based approach to identifying and evaluating potential feedstocks for anaerobic digestion. Potential anaerobic digestion feedstocks are commonly evaluated by three criteria.

Feedstock characterization: Both before and after BMP assay, includes pH, chemical oxygen demand (COD), total solids (TS), and volatile solids (VS). Characterization results found prior to the experiment are used to determine the quantity of feedstock needed to maintain the BMP assay for as much as 30 days. Characterization results following the completion of the BMP assay are used to evaluate the anaerobic digestion process in terms of the destruction of the organic material.

Total biogas production: Is measured throughout the BMP either through manual means or continuously by commercial software designed for tracking gas production. Biogas can be scrubbed of the carbon dioxide by running it through a potassium/sodium hydroxide solution to monitor only methane production or can be left unscrubbed to monitor the total biogas production.

Biogas analysis: Biogas composition can be investigated by means of a gas chromatograph during the BMP assay. Though the capital investment is large, gas chromatographs provide accurate measurements of the constituents of the biogas produced during the BMP. Gas chromatographs can be set up to determine the concentrations of methane, carbon dioxide, nitrogen, and hydrogen sulfide gases.

The BMP assay is a combination of a single feedstock or feedstock blend, inoculum, and stock solutions in a batch system. Inoculum is used to seed the feedstock with an active anaerobic culture to initiate activity and reduce any lag time required for establishment of a culture. Stock solutions are added to assure that macronutrients, micronutrients, and vitamin deficiencies do not limit biogas production. BMP evaluations should always be completed in replication and results should be verified at pilot or full-scale, and it is strongly recommended that full-scale designs not be based on BMP results because full-scale digesters are often run at continuous mode while BMP tests are batch mode.

Considerations

The biogas potential of feedstocks is an important factor when considering anaerobic digestion on your farm. But other considerations, such as economics, regulatory issues, feedstock availability on and off the farm, and end use of the biogas, should also be evaluated.

References Chynoweth, D.P., C.E. Turick, J.M. Owens, D.E. Jerger and M.W. Peck. . Biochemical Methane Potential of Biomass and Waste Feedstocks. Biomass & Bioenergy 5:95-111.
Liu, Y., S.I. Miller, and S.A. Safferman. . Screening co-digestion of food waste water with manure for biogas production. Biofuels, Bioproducts, Biorefining 3:11&#;19
Owen, W.F., D.C. Stuckey, J.B. Healy, L.Y. Young, and P.L. McCarty. . Bioassay for monitoring biochemical methane potential and anaerobic toxicity. Water Research 13:485-492.
Steffen, R., Szolar, O., and Braun, R. . Feedstocks for Anaerobic Digestion. Institute for Agrobiotechnology Tulln, University of Agricultural Sciences, Vienna.
Speece, R.E. . Anaerobic Biotechnology for Industrial Wastewater. Archae Press, Nashville.

April

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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:

1. High methane content
2. Low water vapour
3. Low CO2
4. Low Sulphur
5. 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:

1. 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.

2. 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.

3. 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!

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