The True Cost of Anaerobic Digestion – Are You Paying More Than You Should?

The True Cost of Anaerobic Digestion – Are You Paying More Than You Should? Graphic

General Assumptions/Case Studies

The traditional cleaning cost for a 1.2-million-gallon, 90-foot diameter by 25-foot tall anaerobic digester for gas production:

  • 20% grit buildup or 5 feet of buildup in a 25-foot tank = 14-17% gas loss (five-year average on crop residuals and non-sand bedded dairy manure).
  • Grit build up averages are: year one = 5% (1.25 feet), year two = 12% (3 feet), year three = 20% (5 feet), year four = 25% (6.25 feet), year five = 30% (7.5 feet).

  • It could be up to twice this on sand bedded dairy.

Credits are worth approximately $60 a MMBTU at current market.

General Assumptions:

  • 1 to 1.2-million-gallon digester is making 125 MMBTU a day 
  • 125x60= $7,500 a day in revenue 
  • 14% percent loss of gas per day due to lack of retention time = $1,050.00 per day 

Annual revenue potential or model = $2,737,500.00 

  • Annual loss revenue to grit buildup = $383,250.00 
  • Five-year loss revenue to grit buildup = $1,916,250.00 

Traditional cleaning every five years with approximately seven feet of grit 

  • 22 days to drain = $141,900 loss gas sales 
  • 10 days heavy equipment cleaning with removing the top= $220,000
    • Top removal and replacement= $200,000 
  • 22 days to fill and get to gas production = $141,900
  • Average cost for one cleaning per five years = $703,800.00
  • Average loss of gas due to grit per five years= $1,916,250.00 

Over five years, the total cost for just cleaning-related items is $2,620,050.00 per digester running crop residuals and non-sand bedded dairy ($524,010.00 annually). This does not include the equipment cost/replacement for wear and tear due to the added stress from over-sediment buildup. 


The value on a 1.2-million-gallon, 90-foot diameter by 25-foot tall anaerobic digester running crop residuals and non-sand bedded dairy manure, with a goal of making 125 MMBTU a day and selling for $60.00 a MMBTU, over a 5-year period is:

Annualized Traditional Cost = $524,010.00

Includes gas potential loss, one traditional cleaning

Annualized Bristola Cost= $142,125.00 

Includes minimal gas potential loss, five cleanings and no new materials 

Annual SAVINGS OF $381,885.00 per digester, per year. 

The Bristola System

Bristola’s revolutionary system allows for the cleaning of anaerobic digesters online, while making gas. With minimal capital expenditure, effective mixing, and reduced need for pre-separation, the tank maintains its optimal capacity. This setup enables clients to plan for cleaning expenses in advance and facilitates emergency cleaning in case of mixer or separation failures.

Other Solutions 

While there are other methods for routine anaerobic digester maintenance, they do have limitations. These alternatives to cleaning are both less effective and less efficient. 

More Mixing

  1. Reduced Methane Production: Over-mixing can disrupt the formation of anaerobic microbial consortia and hinder the process of methane production. This can lead to a decrease in biogas yield and overall energy generation efficiency.
  2. Viscosity Changes: Excessive mixing can result in a breakdown of particulate matter and cell structures, leading to increased suspended solids and changes in the viscosity of the digester slurry. This can impede the settling of solids and cause operational challenges.
  3. pH Instability: Over-mixing can impact the pH balance within the digester, potentially leading to fluctuations that disrupt the activity of acidogenic and methanogenic bacteria. This can result in a less stable microbial community and compromised digestion.
  4. Digester Upset: Intense mixing can cause digester upset by disturbing the settling of biomass and inhibiting the formation of stratified layers within the digester. This can lead to the washout of biomass and a decrease in overall digester performance.
  5. Energy Consumption: Over-mixing requires significant energy input, leading to higher operational costs and increased energy consumption. This can reduce the overall economic viability of the anaerobic digestion process.

To mitigate the effects of over-mixing, it's important to carefully control mixing intensity and frequency, optimize the design of the mixing system, and monitor the digester performance closely. Balancing the need for effective mixing with the preservation of biological activity is crucial for the successful operation of anaerobic digesters.


Excessive pre-separation before anaerobic digestion can have several harmful effects on the overall digestion process and system performance. Here are some of the key harmful effects associated with too much pre-separation.

  1. Loss of Organic Matter: Overzealous pre-separation can result in the removal of organic matter that would otherwise contribute to biogas production. This can lead to a decrease in the overall methane yield and energy recovery from the anaerobic digestion process.
  2. Reduced Digestion Efficiency: Excessive pre-separation may lead to the removal of easily digestible organic fractions, leaving behind a substrate with reduced accessibility to microbial degradation. This can result in lower digestion efficiency and biogas production rates.
  3. Increased Solid Waste Volume: Too much pre-separation can lead to the generation of larger volumes of solid waste, including fibers and other organic materials that were separated out. This can create challenges for waste management and disposal, adding complexity to the overall process.
  4. Imbalance in C/N Ratio: Pre-separation can result in the removal of specific organic components, potentially leading to an imbalance in the carbon-to-nitrogen (C/N) ratio of the feedstock. This imbalance can affect microbial activity and biogas quality, leading to process instability.
  5. Elevated Processing Costs: Excessive pre-separation increases the energy and operational costs associated with the additional separation equipment and processes. This can impact the economic viability of the anaerobic digestion operation.
  6. Loss of Synergistic Effects: Pre-separation may disrupt the natural synergistic effects of different organic constituents present in the raw feedstock. This can compromise the microbial community's resilience and adaptability, leading to decreased process stability.
  7. Increased Sensitivity to Inhibitors: The removal of certain inhibitory substances through pre-separation can make the remaining substrate more susceptible to the negative effects of inhibitors, potentially impacting microbial activity and biogas production.

To mitigate the harmful effects of excessive pre-separation, it's important to carefully optimize the pre-treatment process, consider the specific characteristics of the feedstock, and balance the need for pre-separation with the preservation of organic matter essential for anaerobic digestion. Implementing a strategic approach to pre-treatment and separation can help maximize biogas production while minimizing adverse impacts on the digestion process. 

A Revolutionary Solution for Cleaning Anaerobic Digesters

When there is a buildup of sludge or other solids at the bottom of an anaerobic digester, it may cause a decrease in biogas production. This can cause them to operate inefficiently and ineffectively. To ensure optimal performance at all times and long-term functionality, anaerobic digesters need to be inspected and cleaned regularly.

Bristola’s revolutionary cleaning technology sends a remote-controlled submersible robot through a patented entrance system directly into your anaerobic digester for cleaning and maintenance. No other technology or cleaning method can protect employees, while also eliminating production downtime. This ultimately saves you time and money.

Contact Bristola today to learn more about cleaning your anaerobic digester.