Understanding Anaerobic Digestion Inhibitors: A Guide to Biogas Stability

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Silent Saboteurs: How Hidden Toxins Can Derail Biogas Production

Introduction

In the world of renewable energy, anaerobic digesters are the unsung heroes, quietly turning organic waste into valuable biogas. However, this delicate microbial ecosystem is vulnerable to silent saboteurs: inhibitory substances that can halt the entire process.

What happens when seemingly harmless materials introduce toxins into the reactor? This excerpt from Dr. Abdul Haq’s research explores the critical issue of toxicity in anaerobic digestion. It breaks down the different types of inhibitors—from ammonia and fatty acids to antibiotics and heavy metals—and explains the complex ways they can antagonize, synergize, or completely shut down the microbes responsible for energy production.

Excerpt

“There are a number of substances which inhibit the anaerobic microbial communities inside the biogas reactor. Inhibitory substances might enter the reactor due to poorly managed or contaminated materials or by using a substances that do not cause any inhibition in initial phases.

All of the toxic substances behave in a certain unique way and the response of process to these compounds depends on certain parameters such as concentration of the toxic compound, retention time, temperature, pH and types of microorganisms.

There are different levels of toxicity which are influenced by different factors as mentioned earlier. They might be antagonistic, synergistic, complexed or adaptable inhibitions (Chen et al., 2008, Chynoweth, 1987), as described below:

1. Antagonism: When the level of inhibition of combined toxic compounds is lower than their individual level.
2. Synergism: Synergism can be defined as the increase in the inhibition level of toxic compounds than their individual level.
3. Complex formation: when the inhibitory compounds combined together with similar or dissimilar compounds and become invisible to microbial communities during anaerobic digestion
4. Adaptation or acclimatization: Sometimes, the microbial communities start to adapt the toxicity of the compounds and use them as substrate or protect themselves by acquiring certain changes in their composition. This phenomena is called acclimatization.

Some inhibitions are reversible and some are non-reversible. In non-reversible inhibitions, the microorganisms are unable to recover from the inhibition even when the inhibitory compounds are removed.

In such conditions, these microorganisms must be replaced by the new actively growing consortia or the process must be re-started. On the other hand, in case of acclimatization, the microorganisms gradually adopt to the toxic compounds and go into lag phase in which they restrict their growth and started learning how to cope with these inhibitory compounds.

In order to avoid the breakdown of complete process, it is mandatory to reduce the load and extend the retention time. Otherwise the microbial communities will be washed off soon. It is also important to reduce the concentration of toxic compounds or to refresh the inoculum from some other efficient anaerobic digester to increase the biogas yield. There are a number of inhibitory substances that have been reported to inhibit the anaerobic digestion process.

1. Inhibitory Substances: Ammonia (NH₃)

Ammonia (NH₃) is released when protein rich substances are biodegraded during anaerobic digestion and usually inhibit methanogenic bacteria. Ammonia and ammonium ions are mostly present in equilibrium during anaerobic digestion. The inhibitory form of nitrogen containing compound is ammonia and increase in its production depends upon various parameters such as pH and temperature.

Ammonia is inhibitory at concentration above 30 mg/L but some studies have reported a stable anaerobic digestion process even at ammonia concentration more than 100 mg/L (Fricke et al., 2007). Normally, the concentration of ammonia is monitored by measuring ammonium nitrogen (NH₄⁺-N) which is the composite form of ammonia and ammonium.

The ammonium nitrogen is considered inhibitory at concentration ranging from 1.5 to 14 g/L (Fricke et al., 2007, Chen et al., 2008). Ammonium nitrogen at the level above 3 g/L have been reported with microbial inhibition in various studies (Schnurer and Jarvis, 2010).

2. Long Chain Fatty Acids (LCFAs)

Fats are energy rich compounds and could be a source of higher energy and methane production but an increase in fats level beyond a certain level could be problematic for microbial communities inside the reactor.

Fats are mainly composed of fatty acids and glycerol. These fatty acids may be saturated, mono-unsaturated or poly-unsaturated. Saturated fatty acids are present in meat and dietary feeds, while poly-unsaturated are present in fish and corn oil and mono-unsaturated fatty acids are present in vegetable oils or nuts.

Triglycerides are the most common type of fats that are hydrolyzed into long chain fatty acids during anaerobic digestion. There are different types of long chain fatty acids but all of them are composed of more than 18 carbon atoms (Schnurer and Jarvis, 2010).

These long chain fatty acids including stearic acid, meristic acid, linoleic acid, linolenic acid, palmitic acids, etc. are released during the lipid biodegradation in anaerobic digestion process and have been reported to possess inhibitory effects on both bacterial and methanogenic communities.

The fatty acids surrounds the microbial cell membranes and stop the transport of materials through their cell membranes both inward and outward. Thermophilic microbial communities have been reported with higher sensitivity to LCFAs compared to mesophilic communities. Long chain fatty acids are able to have an acute or chronic inhibition of microbial cells.

Even sometimes, they kill the microbial communities during anaerobic digestion process (Chen et al., 2008). The toxicity of long chain fatty acids, in most of the cases, is recoverable but the recovery time might be longer (Chen et al., 2008).

3. Antibiotics

Antibiotics are the substances produced by microbial strains that inhibit the other microbial communities. It has been reported that methanogens are more sensitive to antibiotics compared to the other bacterial communities but this concept is mostly related to biogas production i.e. mostly in the presence of antibiotics like chloramphenicol and chlortetracycline the methane production has been reduced 50% at 20 and 40 mg/L.

Thiamphenicol has been reported to reduce 60% methane production at 80 mg/L concentration (Schnurer and Jarvis, 2010). Sometimes the animals also get antibiotics in their feed such as monensin and rumensin that have strong inhibitory effects on methanogenic communities (Zitomer et al., 2007). However, methanogenic communities have also been reported for biodegradation of various antibiotics during anaerobic digestion process (Gartiser et al., 2007).

4. Heavy Metals

Metals are non-biodegradable and can be accumulated in reactor resulting in microbial inhibition. Heavy metals, for example cadmium, uranium and mercury can be toxic while cobalt, nickel, molybdenum and zinc can be non-toxic.

The heavy metals may inhibit the microbial cells enzymes activity. Low concentration of certain heavy metals are required for enzymes activity. To date, there is no specific limit of metals toxicity but the inhibitory concentration of heavy metals mostly lies in 100 mg/L (Chen et al., 2008). Some metals are non-toxic and may appear in the reactor with several hundred grams per litres without causing toxicity.”

Source Citation

• Researcher’s full name: Abdul Haq
• Title: Biotechnological Applications of Jatropha curcas Seeds for Bioenergy Carriers and Bioactive Compounds
• Guide(s): Dr. Malik Badshah
• University: Quaid-i-Azam University, Islamabad
• Completed Date: 2020
• Excerpt Page Numbers: 58, 59, 60, 61