Facts and fables concerning manure digestion

Facts and fables concerning manure digestion

CCS recently gave a presentation for the municipalities that are part of the RES region of West Overijssel. The presentation was about green gas and everything that is involved in its production and sales. An important issue that came up was the criticism that many municipalities receive when they start talking about biogas and green gas production. But, is this criticism justified? We don’t think so. Especially when we talk about mono-manure fermentation. That is why in this article we explain a few frequently heard fables, but also a few interesting facts.

Nuance difference

When giving criticism or expressing fables and facts, it is important to be clear about the difference between mono-manure digesters and co-digesters. Because differences in nuance sometimes create ambiguity. In mono-manure fermentation, only 100% manure is fermented. A co-digester must be fed at least 50% with manure. Crop residues or residual flows from the food industry (co-products) may be added to this.

Myth: manure disappears through a digester and you dissolve the manure surplus

Manure fraud is an existing and proven phenomenon. It’s just unclear how often this happens. The reason that so many farmers and manure traders commit fraud is that the Netherlands has a manure surplus. This means high costs for the redistribution and export of all manure flows. The higher the costs, the more interesting fraud becomes. A digester, regardless of whether it is a mono-manure digester or a co-digester, converts part of the manure into biogas. As such manure “disappears” and this seems an interesting way to solve the manure surplus or possibly to commit fraud. During the fermentation process however, only 3-4% of the ingoing mass is converted: that doesn’t make a dent of course.  Therefore fermentation is unattractive as a solution for your manure surplus.

Fact: fraud with residual flows for co-digesters not yet proven.

Subsequently, there are indications of fraud with residual flows, which only applies to co-fermentation. This is all still under investigation, but it seems that a number of suppliers of residual flows for co-digesters also trade in waste flows that may not be fermented. On paper, these waste streams are “recycled” into residual flows, for example when they are transported from Germany to the Netherlands. For example, waste is dumped in digesters. Here too it is difficult to assess the extent of the fraud, but the Public Prosecution Service is currently investigating the sector as a whole.

Myth: fermentation installations cause odor nuisance

In addition to fraud, there is often criticism of the odor nuisance caused by fermentation installations. Mono-manure fermentation is also an exception to this. Odor is caused by the formation and diffusion of various gases (such as ammonia and sulfur compounds). This is often caused by the manure on a livestock farm, with differences in odor production between different animal species. Manure stores and cellars are the main source of these gases, as the same process takes place here as in a manure digester. The advantage of a digester is that it is sealed gas-tight, so that the gases that arise here are collected and are therefore not spread in the environment. The use of a manure digester therefore decreases the odor production rather than increase it.

Fact: co-fermenters can cause odor nuisance

This is very different for co-digesters that are placed close to the city or villige limits, for example in an industrial area. Here the same smell is produced in an environment that is not used to it. In addition, the installations are often larger in scale, which produces more odor. The storage of (degrading) co-products can also cause odor nuisance.

Fact: subsidy is needed to make biogas production profitable

There is also criticism of the subsidy that almost all fermentation installations need to be able to run profitably. Biogas production would not be profitable without the gigantic amounts of subsidy that go into this every year. This comment is correct, but this applies to almost all renewable energy sources. The EU acknowlodges this, it concerns about 8 billion euros annually, according to Milieudefensie. By way of comparison: the SDE++(Dutch subsidy for stimulation of durable energy) budget has been around 5 billion euros for years. There are also examples of digesters that cannot continue to operate profitably despite the subsidy they receive. Again, this often concerns co-digesters. These depend on the prices of co-products, which have risen sharply since the business cases were drawn up. In addition, most of these installations use their biogas by converting it into electricity and heat using a CHP. Electricity prices have also fallen. As a result, a large number of these installations are now running at a loss or even have shut down.

Myth: digesters have a high chance of exploding

There are even more myths surrounding fermentation. For example, some interested parties point out the danger of explosion in digesters. For an explosion with a methane mixture (such as biogas and natural gas) between 4.4% and 16% methane must be present: the explosion limits. In a digester, however, the methane content is much higher, around 55%. This means that there is too little oxygen present for an explosion, thus biogas is not an explosive mixture. Although an explosive mixture can arise in the event of a leakage around fermenters, this will not lead to an explosion. Explosions are only possible if this mixture is ignited in an environment in which enough pressure can be built up. The last possibility is if, for example, outside air ends up in the digester, causing the methane content to fall below 16%. Most digesters have a flexible roof of a gas-tight tarpaulin. This sail tears relatively quickly, which again rules out an explosion: sufficient pressure cannot be built up. This does not alter the fact that the combustible gas that is produced in the event of a fire still poses a real risk. That is why the fire safety rules for such installations are also very strict.

Myth: fermentation maintains intensive livestock farming

Another myth is that fermentation sustains intensive livestock farming and depends on it for its raw materials. To start with, less than 5% of all manure in the Netherlands is currently fermented. If, as an extreme example, we were to halve the livestock, there would still be more than enough manure available to produce a large amount of biogas. Whether intensive livestock farming should disappear in all its forms and what exactly falls under intensive livestock farming are discussions in themselves. In this example I assume that the so-called “mega stables” is a part of intensive livestock farming that we do not want to maintain. These are locations with more than 250 livestock units, for example 250 dairy cows or 1,250 fattening pigs.

Some solutions require a certain minimum amount of fertilizer to be profitable. Think of green gas production, for which a mega amount of manure is needed. An alternative, however, is a collaboration between smaller farmers: a biogashub (Bio-HUB). Small companies with small digesters jointly supply their energy (heat, electricity, green gas). No dragging around with manure, no large installations, but a healthy revenue model and sustainable energy. An additional argument is that small companies, which we therefore want to maintain, have a relatively much greater interest in a sideline such as energy production. This can form a substantial part of the company’s turnover.

Myth: burning biogas contributes to the greenhouse effect

Finally, the CO₂ myth: when biogas is burned, CO₂ is released, just like when natural gas is burned. The use of biogas therefore still contributes to the greenhouse effect. If we look at the combustion of natural gas, this energy and the carbon it contains has been stored underground for many millennia. If we raise this and burn it, we add CO₂ to the carbon cycle. As a result, the CO₂ content in the atmosphere becomes increasingly higher making it retains more heat. Burning biogas from fermentation also produces CO₂. The difference is that this is carbon that has been taken up from the atmosphere shortly before (in almost all cases less than a year). As a result, the CO₂ content remains the same and the cycle intact.

In the case of mono-manure fermentation, there is even a positive effect on the greenhouse effect. This is because the carbon from grass ends up in manure via the cow. It escapes from this in the form of CH₄ (Methane) and this has a 25 times stronger greenhouse effect than CO₂. By pumping the manure into a digester as quickly as possible and producing as much methane as possible in a short period of time, the methane emissions that would otherwise have occurred are greatly reduced. So win-win: sustainable energy to replace fossil energy and less methane emissions.

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9 June 2021 / Author:

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