Notes

Biomass to Biogas: Anaerobic Degradation 101

Why Treat Wastewater Anaerobically?
The U.S. alone produces over 70 million tons of organic waste every year, which—if mismanaged—poses immediate threats to both the environment and public health, for example through water contamination. In addition to the short-term risks, through natural decomposition, organic waste produces large amounts of methane, a greenhouse gas that traps 86 times more heat in the atmosphere over 20 years than carbon dioxide. By harnessing organic waste to produce biogas, a renewable source of energy, these (and other) major problems are succinctly solved, and it is being done today through none other than anaerobic wastewater treatment.
application of AGS
In addition to the aforementioned benefits of general pollution control by organic matter degradation and energy generation by valuable gas production, anaerobic systems are attractive for reasons like:
Further conservation of energy, thanks to no aeration requirement
Some toxins are only degraded anaerobically
PCE (dry cleaning solvent Perchloroethene)
Chlorinated PCBs (Polychlorinated Biphenyls, e.g., motor oils)
DDT (pesticide)
Low excess biomass generation
Small volume and footprint
Aerobic loading: 0.5 to 3.2g COD/L-d
Anaerobic loading: 2.0 to 40g COD/L-d
Biomass viability after storage, which is ideal for seasonal processes
Disadvantages of anaerobic systems are comparatively few and include:
Sensitivity of the system and influence of many parameters
Need for close monitoring
Long start up
No ammonia removal
How Do Anaerobic Systems Work?
Anaerobic degradation is a biological process that occurs in the absence of oxygen and is divided into 4 steps. Without all 4 steps, biogas production can be significantly impacted.
1. Hydrolysis
Macromolecules (carbohydrates, proteins, lipids) are solubilized by the action of extracellular enzymes excreted by bacteria. Particular compounds are split into monomers or dimers (sugars, fatty acids, amino acids) to be able to be transported within the cellular membranes. The process can, however, be limitative regarding complex wastes with high solids fraction (e.g., cellulose).
Examples of microorganisms involved in hydrolysis are:
Clostridium for cellulose or starch degradation in strict anaerobic systems
Bacillus for proteins degradation in facultative anaerobic systems
2. Acidogenesis
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Once in the cell, the simple molecules resulting from hydrolysis are used as substrates (food) by microorganisms, which produce volatile fatty acids (VFA) and alcohols. Carbon dioxide and hydrogen gas are also produced during this phase.
Examples of microorganisms active in this process include:
Clostridium in strict anaerobic systems
Acetobacter or Streptococcus in facultative anaerobic systems
It is important to note that, in the case of organic overloads, a high growth rate can be responsible for an accumulation of VFA or hydrogen gas, which can inhibit the remaining 2 steps.
3. Acetogenesis
In acetogenesis, microorganisms catabolize the VFA and alcohols from acidogenesis into acetate.
This process occurs only in strict anaerobic systems and requires:
Syntrophic acetogens that produce hydrogen (e.g., Homoacetogens)
Non-syntrophic acetogens
4. Methanogenesis
In the final stage of anaerobic degradation, bacteria use the acetate created during acetogenesis, hydrogen gas, and carbon dioxide to produce methane, otherwise known as biogas.
Like acetogenesis, methanogenesis only occurs in strict anaerobic environments, and involved microorganisms include:
Acetoclastic methanogens (e.g., Archaea), which can produce methane from many substrates having a methyl group
Hydrogenotrophic methanogens, which can produce methane from hydrogen and carbon dioxide
This stage is considered the most limitative regarding the dissolved compounds, and growth is favored by the presence of acetate. Further, some non-methane-producing bacteria can outcompete the methanogens and inhibit their growth, particularly in instances where wastewater has high sulfur content or high proteins/nitrates content, as seen often in food production and rendering plants.
Where Does Anaerobic Degradation Take Place?
Anaerobic degradation can be achieved in many types of bioreactors, with varying organic loading ranges:
Free culture
CSTR (continuous stirred tank reactor): 2–6 kgCOD/m3.day
Contact reactor: 3–7 kgCOD/m3.day
Biofilm
Fixed bed reactor: 8–20 kgCOD/m3.day
Fluidized bed reactor: 30–50 kgCOD/m3.day
Granules
UASB (upflow anaerobic sludge blanket): 10–15 kgCOD/m3.day
EGSB (expanded granular sludge bed): 10–15 kgCOD/m3.day
What Factors Influence Anaerobic Degradation?
As mentioned above, anaerobic systems are relatively sensitive. Influencing factors include:
Temperature: 37° C is optimum for mesophilic; 60° C is optimum for thermophilic
pH: 6,7 <7,3 is optimum
Alkalinity (to buffer the acidification)
Nutrients: DCO/N/P = 700/5/1 is optimum
Micronutrients (nickel, cobalt, zinc, magnesium, potassium)
Inhibitors:
VFA will lower the pH (depending on the buffering effect of alkalinity)
High ammonia levels increase pH and will inhibit acidogenesis
Hydrogen gas will inhibit acetogenesis
High total suspended solids (TSS) content will slow down the hydrolysis
Grease will also disturb the system
Most importantly, the anaerobic degradation of organic matter relies on a precise synergy between many microorganisms, and the right populations of bacteria are needed to reach a complete and stable conversion to biogas. For daily maintenance as well as upset recovery in anaerobic systems, new EZ Anaerobic and Biogas contains a unique blend of bacteria, enzymes, and nutrients designed to maximize the efficiency of organics breakdown and methane production. In fact, regular usage can allow for higher organic load tolerance and increased methane yields of 20-50%.

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