[Joerg]

So, I presume that this question is then targeting the organic fraction of MSW, so organics extracted from a mixed MSW stream.
Besides Plug-flow AD high-solids batch AD is another process type that has been employed on such feedstocks.

Low-solids AD is also a viable process but it requires rigorous pre-treatment of the OFMSW to move it from a physically contaminated and inhomogeneous feedstock into a homogeneous and ‘clean’ bio-slurry ready for digestion.

A caution: do not only think about the generated biogas for beneficial use but also the end-use of the digester effluent. For example, it is prohibited in Europe to land-apply organics derived from a mixed MSW stream.

[Joerg]

I echo Peter’s perspectives.

Bioaugmentation is still an emerging field and has been tried and implemented on a few full-scale AD facilities. What is more common -and often a must- is the addition of micro- and macro nutrients especially where mono-feedstocks such a corn silage is fed to the digesters.

Some facilities dose very specific micronutrients such a cobalt to compensate for nutrient deficiencies in the processed feedstocks.

[Joerg]

From my experience, good and effective odor management (facility design as well as operational SOPs) is often not paid adequate attention to, especially when feedstocks, that are more odorous in nature, are considered for processing. The industry has seen several AD facilities being shut down due to inadequate odor management implementation, which caused huge financial losses to the owners. Once the relationship with the community and its members aka “sensitive receptors” is strained it is difficult to restore and re-build trust.

The same holds true for composting facilities, (also where composting is performed on the digestate on site. It takes experience and solid understanding of facility design and operation to assure successful operation of the facility to meet its intended design life.

My advice: do not take this topic lightly and get experienced professionals on board to navigate through this and implement a robust plan, often requiring a multi-layered approach.

[Joerg]

What qualifies under semi-dry digestion?

Best to differentiate by the level of total solids rather than calling it ‘wet’, ‘semi-dry’ or ‘dry’.

So low-solids AD would be up to around 15-20% TS of the infeed; high-solids above 20% TS. For high-solids AD systems there are various process designs. For example, corn silage digesters can still operate at the higher-end of low-solids or lower end of high-solids TS.
Plug-flow type AD system typically requires TS >30% of the infeed to be able to form the ‘plug’. High-solids batch AD processes operate >35-40% TS of the infeed.

So, the feedstock characteristics play an important part in deciding which AD process type is most suitable to select. For example, food scraps co-collected with yard waste would both be suitable for high-solids plug-low and batch-type processes.

[Joerg]

That is a complex question and difficult to address.

Different places in the world lead to different answers. For example, thermal (in most cases incineration) treatment of MSW is commonly applied in many countries in the EU and also North America. Is it the best/preferred approach?

In the US the most common approach is landfilling -with or w/o prior recovery of recyclables- and possible extraction of the generated LFG for beneficial use. Is that the best/most desired approach?

Other places apply a mechanical biological treatment (MBT) process to recover recyclables and organics for AD/composting -biogas generation and stabilization- prior to landfilling provided that the to be landfilled material has a very low biological residual potential = is close to being considered inert/does not show a notable potential for landfill gas -and toxic leachate formation. Is that the best approach?

(Europe prohibits land application of organics extracted from a mixed MSW stream per EU directive. In the US, there is no such law = land application of organics extracted from an MSW is allowed in principle, as long as it meets certain requirements, e.g., compost standards.)

Others want to chime in here?

[Joerg]

Mechanical pre-treatment would be a feasible option.

May European AD facilities that process more fibrous material have been employing impact crushers sch as offered by MeWa/Andritz.

[Joerg]

Hi Natalia, from my experience it is less a question of which AD process type is more or less popular but rather what are the characteristics of the feedstock(s) that then govern the selection of the process type.

So if you start with a feedstock that has a high TS, let’s say 30+ percent, then high-solids AD processes such as plug-flow or batch would be preferrable.

We did a study for a client back in 2016 that included a survey of all municipal organics AD installations world-wide. So for source separated organics and organic fraction of MSW. We found that the low-solids AD installations were about equal in number to high-solids AD installations. Back then High-solids batch represented about 20 percent of the all high-solids AD installations.

For ‘semi-dry’ AD -I would say between 15 and 25% TS- there are some providers that have specialized on this range, most notably for corn silage applications or an other AG-feestocks. For these applications slow-moving mixer designs are typically used for a CSTR design (e.g., offered by Agraferm, ArgiComp, among others).