Very few people have written about digestate monitoring for productive biogas plant operation, but careful sampling and analysis of the digestion process is essential for any commercial anaerobic digestion process. It's essential, to reliably achieve high methane yields and the lowest downtime periods.
By referring to digestate monitoring in this article we also make the assumption that the term is synonymous with “digester substrate monitoring, which in principle it is for the most common digester type which is the CSTR type of anaerobic digestion plant process.
The CSTR (Continuously Stirred Tank Reactor) using a single tank is simple to monitor in this way by analysing the digestate as it leaves the reactor.
For other reactor types such as:
- Plug Flow Biogas processes and
- Dry AD Systems,
there is a similar level of importance but multiple monitoring points may be needed.
The Importance of Digestate Monitoring
Understanding the anaerobic digestion process as an ongoing and ever-changing dynamic process. It is the key to reliable AD plant operation, even for very constant and seasonally non-varying feedstocks.
Operators need to know when important parameters are out of the normal range before a biogas reactor shows a significant slowdown in yield. That way, the operator can act to correct them before the health of the productive methane-producing microorganisms declines.
A purely responsive approach to digestate monitoring may a tempting strategy to keep monitoring costs down but has been shown to incur a net loss when biogas yield slow-downs occur. As the value of the methane produced by the AD industry rises, this fact becomes ever more relevant.
Would You Ask any Doctor for a Reliable Diagnoses of Any Illness Without a Knowledge of What's Wrong?
How can you as the biogas plant operator make it better if you don’t know what the ailment is?
The analogy here is that for diagnosis of any stomach complaint, your Doctor would need to know about your:
- Diet; What and how much are you eating? – which for an AD plant is the feedstock
- General health and any other medications that are taken by the patient – AD plant dosing, enzyme addition etc
- The exercise you are taking – Digester agitation etc
- Full details of the illness.
Knowledge of the above through digestate monitoring and pro-active process management allows successful corrective action before the process output suffers significantly. The worst of all scenarios of a sour digester, and the need to remove part of all of the digester contents, before resuming methane production is avoided.
Let's look at in more detail at all important “digester diet”, firstly from the point of view of feedstock quality and throughput quantity, and secondly by considering what we call “quality pointers”:
1. Digester Diet: Feedstock Quality and Quantity Data
Using our “human” analogy “diet” doubles for the biogas digester's “feedstock quality and quantity”. Feedstock quality is a lot about its consistent organic content and quantity.
Get these two right and the biomass will operate at a consistent temperature and will tune itself to optimally digesting that material. This is the industry's general experience. But it is best to monitor to ensure that all the most important parameters that affect methanogen viability, are present in concentrations that allow healthy digestion.
Good productive “healthy digestion” in the digester needs a consistent feedstock at a controlled viscosity or at least viscosity within known boundaries of biomass toleration.
For this to occur:
- Particle size and maceration (chopper length) need to be controlled and the equipment working as intended.
- Contaminants present in the incoming feed need to be held within limits that are tolerable for the mechanical operation of the process.
- Plastic input needs to be largely removed especially bags/ films that can seriously impede stirring.
- Other unwanted items need to be removed before maceration/ mechanical processing. And, to make it clear what can go wrong. Almost unbelievably, there have been occasions when even large objects such as tyres etc., have been found in feedstocks!
In short, any diagnosis needs a full understanding of the feedstock at the time of the methane yield problem and for some months previously.
2. Digester Diet: Quality Pointers – Testing and Observation
In effect to understand the biomass growing conditions is essential to be a biogas “doctor” and continually monitoring the process should, for most biogas businesses, include:
- Observing what’s coming in
- Silage sampling
- Testing tanker loads
- Retaining samples for comparison
- Changing Dry Matter (DM) in feed
- Changing C: N ratio
- pH and available pH buffering capacity within the digestate
- Avoiding ammonia build-up to cause possible methanogen inhibition.
3. Key Chemical Parameters to Monitor Regularly in Digestate
Monitoring work should be based upon flagging changing composition in the digestate as warning signs of digester methane yield problems developing. We list some of the most important to monitor in the digestate, below:
- Ammonia concentration
- Trace elements
- Sulphur loading
- pH and available alkalinity.
Rapidly rising ammonia concentrations, should they occur, need to be addressed before they worsen.
Trace elements should be reviewed regularly for many types of biogas plant, especially for waste digesters. If trace element deficiencies are found there are products available that can be used as additives to the feedstock.
Sulphur loading should be monitored in plants where a historical problem has occurred.
Conclusion to “Digestate Monitoring for Productive Biogas Plant Operation”
In this article, we have explained the importance of regular digestate quality monitoring of key parameters coupled with regular review of the data obtained, leading to corrective actions taken.
The list of analysis requirements should be closely attuned to your type of feedstock and digester design. The frequency of sampling should be based on your situation.