Grass To Gas Anaerobic Digestion Q&A

This page lists questions and answers from the C-Change GrassToGas project’s PA Extension newsletter.  To be added to the newsletter mailing list, contact

1. Question: Is switchgrass a good feedstock for biogas digesters?

Answer: Not usually. Anaerobic digestion involves microscopic organisms that eat energy-rich components such as sugars. Lignocellulosic biomass, such as switchgrass, contains sugars, but is structured in a way that specifically prevents easy access to its energy-rich components. Because of the limited accessibility to the energy-rich components in switchgrass, it is not considered an ideal feedstock for biogas digesters.

2. Question: Why did my digester fill up with grey sandy material?

Answer: It’s not unusual for a PA farm digester to fill up slowly with a gritty solid material, necessitating a time consuming cleanout once every 10 years or so. This troublesome material is likely a combination of two things: Solids, and Struvite.
Dense Solids, such as sand or other grit, can be carried into the digester along with the regular feedstock, and owing to its higher density, settle down to the bottom.
Struvite is a mineral that actually is created inside the digester when components in the feedstock react to form Magnesium Ammonium Phosphate. According to Robb Meinen of Penn State Extension, Struvite can collect in the digester, and can also coat the inner surfaces of pipes, pumps, and other equipment. It can be a problem.
Ideally internal mixing in a digestion vessel would suspend solids in a manner that allows them to exit in routine effluent. The traditional way to get rid of unwanted accumulated material in a digester, such as dense solids or struvite, is by agitating the digester slurry and pumping it out. However, since most digesters operate on a continuous fill / continuous drain basis, this approach is difficult to make work. As a result, many farmers have been stuck with pulling the cover off their digester and cleaning it out with digging equipment. If you’ve come up with a better way to control solids buildup in your digester, please let us know so that we can pass on the word!

3. Question: What is buffering capacity and why does it matter?

Answer: Buffering capacity is the ability of a feedstock to handle the addition of acids or bases without its pH changing much. This is important in a digester because changes in pH can cause the digester’s microbes to stop growing well, leading to reduced biogas production and sometimes even worse problems such as “souring” (see the previous edition of the Grass to Gas Update for details). Cow manure tends to have a high buffering capacity, which makes more stable as a biogas feedstock. Food waste usually has a lower buffering capacity, which means that food waste digesters are more prone to having pH problems.

4. Question: Is the Hydrogen Sulfide in my biogas dangerous?

Answer: Yes it certainly can be dangerous if it is not properly contained. Hydrogen Sulfide (H2S) concentrations in dairy farm biogas typically range from about 1000 ppm to 3000 ppm. At 700 ppm, one breath of the gas is enough to make you pass out. At 1000 ppm or above, death is almost guaranteed from just one breath. Even if your digester has a scrubber to remove sulfide from the gas, that usually only reduces the Hydrogen Sulfide concentration – it doesn’t eliminate it completely. Also, keep in mind that adding a new feedstock (such as food waste) can increase or decrease your biogas Hydrogen Sulfide level, depending on the sulfur content of the new feedstock.

So be very careful if you have a leak in your system, and be sure to maintain all pipes and fittings so that they are well sealed. Keep any nearby sheds well ventilated so that there is no chance of unexpected leaks causing a buildup of gas. While trace levels up to ~100 ppm are detectible as a “rotten egg” smell, higher concentrations are actually odorless. Low levels of Hydrogen Sulfide can cause headaches and irritated eyes and throats. So, if you notice any of those symptoms cropping up, check for leaks. Be careful out there – we need more farmers, not fewer.

6. Question: What’s the difference between Biogas and Renewable Natural Gas?

Answer: Lately, a lot of people have been talking about Renewable Natural Gas, or RNG. What is it, and is it any different than Biogas? While there’s no “official” definition, we usually use the term “Biogas” to refer to the gas that comes out of a digester (~60% methane, 37% CO2, 3% water vapor and small amounts of Hydrogen Sulfide and other impurities). Renewable Natural Gas, however, is biogas that has been processed to make it “pipeline grade” for injection into the natural gas pipeline network. Upgrading Biogas to RNG usually involves removing the water vapor, removing Hydrogen Sulfide, and removing Carbon Dioxide. The resulting RNG is almost completely composed of methane.

7. Question: Is biochar good for my digester?

Answer: It seems that every time you turn around you hear something about the benefits of biochar. This is true of biogas production as well. Laboratory studies have noted that adding biochar to a digester can increase biogas production and improve methane content of the biochar. The scientists think that this is due to biochar improving the efficiency of the chemical reactions that occur inside the biogas-producing microbes. Is the magnitude of the improvement worth the cost and hassle of adding biochar to your digester? That’s a question that we’re still trying to figure out, and are actually in the process of doing some full-scale testing with partners to see if we can measure improvements to their digester when biochar is being added.

8. Question: What is Siloxane, and should I worry about it?

Answer: Siloxane is a silica compound (silicon+oxygen+alkane, for you chemistry buffs) that is used in a variety of common items, including beauty products and spray sealers used in construction. If present in biogas, they have a tendency to form oxides that deposit in the engine generator, leading to significant wear and expensive premature breakdown of your genset. Siloxanes can only end up in your biogas if there are siloxanes in your feedstock, and the good news is that manure and crop residues normally don’t have any siloxane. To our knowledge, siloxanes never occur spontaneously in nature – they have to be man-made. However, farmers that accept outside materials such as food waste or other organics for their digester need to be careful that there aren’t any siloxanes accidentally included with that material. Something as innocuous as “a bit of nail polish” can cause significant problems in your digester’s engine generator.

9. Question: What is “digester souring”, and should I worry about it?

Answer: “Souring” is a failure mode of a digester, during which the acid-producing microbes outpace the methane producing microbes. This causes the pH to drop, which damages the methane producing microbes even more, and causes the acid producing microbes to become even more dominant. The final result of this “downward spiral” is a sour, goopy mess in your digester that doesn’t produce any biogas. You definitely don’t want your digester to go sour.

While digesters that use 100% manure don’t often have this problem, quickly digestible materials like food waste can be more problematic in that respect. If you are concerned about your digester souring, it might be worthwhile to invest in a pH meter, take daily readings, and be ready to add a basic chemical such as caustic soda if the pH starts dropping.

10. Question: How can I remove Hydrogen Sulfide from my biogas?
Answer: Removing Hydrogen Sulfide (or H2S) from biogas is a good way to make your biogas less corrosive to your equipment and less hazardous to people. While there are many possible approaches to this, the three most common methods that farms use are:

  1. “Iron Sponge” – Pass the biogas through a container filled with rusty iron filings or similar rusty iron material (something with lots of surface area). The H2S reacts with the rust, forming an iron sulfide deposit that stays behind in the iron sponge.
  2. Air Addition – bleed a small amount of air into the digester headspace, to encourage naturally-occurring microbes to consume the H2S without harming the anaerobic bacteria that are in the liquid slurry of the digester.
  3. Bioreactor – pass the biogas through a large container that is filled with a porous medium on which microbes grow that feed on the H2S, removing it from the biogas.

Additives such as Iron Chloride can also be used, but are not a common approach for PA farm digesters. Biochar addition is another possible method that has been talked about lately, but it is not yet clear how well that works or how much biochar would be needed. Which method is best for your farm? That probably depends on the specifics of your digester and your desired level of H2S.

11. Question: What is an eRIN?
Answer: An eRIN is an “electronic Renewable Identification Number”, which is used to provide incentive payments to people who produce bio-based electricity that is used in electric vehicles. The US Department of Energy has recently proposed the creation of eRINs, which could be a new revenue source for digester owners who produce electricity. It remains to be seen how they will relate to the state-regulated “RECs” or “Renewable Energy Credits”, but it could be that a farmer will be able to receive payments for RECs and eRINs from their digester-based electricity.

12. Question: How does biochar help digesters?
Answer: While its benefits in farm-scale digesters are yet to be fully confirmed, many laboratory studies have confirmed several benefits from adding biochar to a digester. This includes:

  • pH control – biochar increases the pH of the digester fluid and provides a buffering effect.
  • Biogas quality – biochar adsorbs contaminants, and can reduce hydrogen sulfide levels in biogas.
  • Biogas amount – biochar improves the environment for the microbes in the digester, resulting in more biogas production.
  • Digestate quality – biochar is known for its value as a soil amendment, so when you spread your digestate on the fields, you can expect to see those benefits translated over to your crops.

Are those improvements to the digester’s performance worthwhile when used in a farm digester? Maybe, maybe not. We’re partnering with several of you to carry out a demonstration project this year, so we’ll be able to provide some first-hand perspective on it before long.