Peter
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The five most common operation issues that we see are:
– Foam in Digesters or Receiving Tanks
– Hydrogen Sulfide in Biogas
– Ammonia in Digesters
– Odor Complaints
– Grit Accumulation in the Digester
Foam is an acute problem experienced by almost every digester that must be addressed by operators to avoid upsets, however it is a problem that can often be resolved quickly.
Hydrogen sulfide in biogas is growing as a concern for many facilities as they convert to RNG and must reach substantially lower targets in their product gas.
Ammonia concentrations will only be a problem for certain digesters receiving high nitrogen feedstocks,
however for those digesters managing ammonia will be an ongoing
challenge.Odor complaints are also an acute problem, but can be avoided with the right technology and diligent operation.
Finally grit is not a problem that causes clear issues for an operator on any given day, however all digesters accumulate grit and must have a strategy to deal with it eventually.
We have an article with five more operational issues here on our website:
https://azuraassociates.com/commonly_overlooked_reasons_for_anaerobic_digester_failures/
azuraassociates.com
Commonly Overlooked Reasons for Anaerobic Digester Failures – AZURA
Commonly Overlooked Reasons for Anaerobic Digester Failures – AZURA
If a project is operational and is having issues with gas quality and or getting stable gas production, and is still producing biogas the biological health may be able to recover with some operational optimization looking at factors like feedstock, nutrition, ammonia, sulfide, heating, and mixing.
If a digester is being fed and has stopped producing gas or has become acidified it may have reached a point where the digestate is too toxic for methanogens to recover. In this case the digester may need to be partially or completely emptied and reseeded.
A digester might also need to be cleaned out if a thick crust has accumulated in the digester, if the digester has filled with grit, or if tank walls or submerged mixers need to be repaired.
The seeding and startup process is a very sensitive time for the digester, it can be accomplished in as few as 8 weeks, but only with a clear plan, access to seed biomass, and close bioprocess monitoring to keep the startup on the right track.
One of the best things you can do to improve biogas production and improve stability it is feed a consistent amount of energy to you the digester every day. When the digester gets a really rich meal with lot of digestible solids one day then a small meal with a lot of water the next it is hard for the digester to grow a healthy population of bugs and impossible to create steady gas. Would recommend tracking how much digestible energy is being fed to the digester every day. If the amount of gas is not equal to the energy being fed it is time to look for possible causes of inhibition.
For more information on what information to collect on digester to keep biogas production high and stable check out our article: Top 10 Things to Make Your Digester Biology Happy.
https://azuraassociates.com/digester-biology-top-10/
azuraassociates.com
Top 10 Things to Make Your Digester Biology Happy – AZURA
Top 10 Things to Make Your Digester Biology Happy – AZURA
There are a few categories of digester additives that are on the market to increase biogas production.
Trace-metal nutrients can increase biogas production, but only when the digester is deficient nutrients because of the feedstock it is receiving.
Enzymes can help to breakdown recalcitrant feedstocks like cellulose and fat, because enzyme addition is expensive, we see very few digester operators that see an economic case to use them in North America.
Microbial cultures are marketed to increase biogas production by breaking down more recalcitrant feedstocks, however we have not seen wide adoption of this or clear demonstration that they can be cost effective for a full scale biogas plant.
Digestate has few advantages as a fertilizer over manure.
First anaerobic digesters kill a lot of the human and animal pathogens that might be in the manure. Next while weed seeds can survive in cattle manure, they are typically killed after digestion. Finally, the nitrogen in digestate has largely been converted to ammonia which is will be available to plants faster than the organic nitrogen in manure or compost.
The pre-treatment method depends on the specific crop being digested. Not all lignocellulose can be grouped together.
Grasses such as Naiper grass and certain cover crops may only need chopping,
Wheat straw is not very digestible but the digestibility can be improved with mechanical treatments,
Wood in any form has too much lignin to digest anaerobically and requires complex thermal and chemical treatments to extract a digestible form of cellulose.
Do you have a specific feedstock in mind?
Comparing landfills and anaerobic digestion of a mechanical separated organic fraction, anaerobic digestion captures much more of the potential methane emissions, conserves limited landfill space, and in some cases can recover soil nutrients.
Landfills only capture methane after cells have been closed. When a new cell is being filled the food waste is degrading (some aerobically, some anaerobically) releasing emissions to the atmosphere. At the same time landfill space is limited and permitting new landfills can take a long time due to the environmental risks. That being said, landfills aren’t going anywhere and will remain an important part of MSW management and the biogas landscape even if 100% of organics could be diverted.
While digestion of MSW is not always the best available option, there are several pretreatment and digestion technologies that have been used effectively. The anaerobic digester technology that will be most effective at treating a mechanically separated organic fraction will depend on the characteristics of the local MSW waste stream. Characteristics will be substantially different if there are organics diversion, paper recycling, or glass recycling programs in place. The design decisions will also be influenced by local prices for disposal of residuals, water, and energy.
If you are interested in MSW digestion and feedstock characterization you can reach me at peter.quosai@azuraassociates.com.
While plant operators are familiar with the dangers of poorly ventilated spaces and hydrogen sulfide accumulation, check out our article to learn three common scenarios where H2S
can pose a threat in well-ventilated spaces and at well-maintained
digester sites. Understanding these risks can improve the safety and
efficiency of RNG plants.https://azuraassociates.com/three-risks-of-h2s-at-rng-plants/
azuraassociates.com
Three Unexpected Risks of Hydrogen Sulfide at RNG Plants – AZURA
Three Unexpected Risks of Hydrogen Sulfide at RNG Plants – AZURA
The technologies used for anaerobic digesters and landfills might be different because it is often beneficial to do some hydrogen sulfide treatment in the digester, while this level of control is not possible in a landfill.
Why is in-situ treatment beneficial?
In anaerobic digesters 6,000ppm H2S likely indicates high dissolved sulfide concentrations. Dissolved sulfides can inhibit anaerobic digestion -reducing biogas yields and throughput. Dissolved sulfides can also bind to trace metal nutrients and cause nutrient deficiencies.
What are in-situ treatment options?
Hydrogen sulphide can be consumed by sulfur oxidizing bacteria in the digester headspace or iron can be added to react with dissolved sulfides and prevent them from becoming gas. Air injection paired with a sulfur net inoculated with sulfur oxidizing bacteria can be one of the cheapest options however air injection can also make it more challenging to hit strict RNG quality targets.
With or without air injection iron can be added as iron chloride solution, iron oxide powder, or iron hydroxide powder to react with sulfide in the digester and quickly reduce sulfide concentrations in the biogas.
Would like to push back on the need for AI tools. We work with several the food waste digesters and manure/food waste co-digestion facilities that receive a complex feedstock. These plants are able to predict daily gas production to within 5% with only basic lab testing and excel based tools used by the operators on site.
AI tools still need to get feedstock characteristics and digester health indicators from somewhere and conventional chemical tests currently produce more reliable results than inline spectroscopy-based tools or industry averages pulled from a database.
Additionally, a facility’s knowledge of the local feedstock landscape, how to handle them, and how to best operate their digester is part of the facility owners IP that can help them hold a competitive advantage. Plant owners might not want their operational lessons and data to be used to train the competition.
Finally, the currently available inline sensing technologies cannot screen for all possible toxins and contaminants in feedstock. Without a perfect sensor, plants still need well trained and knowledgeable operators and regular lab testing to make informed decisions about receiving new feedstocks. Since you need a skilled operator checking the AI’s work – I am not sure how investing in such a tool is expected to justify the considerable cost.
Would be happy to be educated about cases where AI tools have generated additional revenue for digesters that are performing well.
Thanks
I am not personally familiar with what technologies are most effective for rice straw.
Mechanical pretreatments to break up fibres increase the digestibility of rice straw, but there may be more effective treatments available.
If you are interested in investigating further, we would start with chemical analysis of the local feedstock, then review the available options and pilot the most promising. When comparing results from studies it is important that digestion trials or BMP tests are performed in a comparable way so that you are sure improvement is really coming from the pretreatment and not differences in digestion conditions.
If you are interested in investigating options further send me an e-mail at peter.quosai@azuraassociates.com.
Would like to add few more:
– pH,
– gas production rate, and
– hydraulic retention time
With weekly monitoring of
– VFA to Alkalinity Ratio
– Volatile solids reduction
– Ammonia concentration
A great point Ryan!
Thanks Tejas, are there chemical or mechanical pretreatments that are common practice in the digestion of paddy straw to improve digestibility?
Thanks Gurkeerat, there is a lot cover on agri residues. Some quick things to keep in mind are:
-What mechanical treatments might be needed to make residues more digestible?
-Do the residues contain high concentrations of lignin or sulfur?
-How will the addition of dry residuals to the digester affect viscosity and mixing?