With a complex community of inter-related microbes, anaerobic require balanced populations of fermentative bacteria and methanogenic archaea. Most textbooks breakdown the microbial communities into three groups:

• ​Hydrolyzing & Acidogenic bacteria are bacteria with anoxic and fermentative biological pathways. These organisms degrade complex organics into forms used by other anaerobic organisms. These microbes utilize any remaining alternative electron acceptors and eventually use organic compounds as the terminal electron acceptor.  This stage can also product nuisance H2S gas
• Acetogenic bacteria are anaerobic cultures that continue the fermentation process and produce the acetate/acetic acids that are the preferred feedstock for developing methanogenic archaeal populations. In both the Hydrolyzing and Acetogenic processes, H2 and CO2 gases are produced.
• Methanogenic archaea - note this is a separate kingdom from bacteria with unique enzyme cofactor micronutrients - are obligate anaerobic cultures responsible for producing valuable methane gas (CH4). Two varieties of methanogens exist based on substrates utilized:
  Acetic Acid Cleavage - CH3COOH --> CH4 + CO2
  Carbon Dioxide Reduction - CO2 + 4H2 --> CH4 + 2H2O
  
  

Traditional Monitoring

• Methane and CO2 percentages in produced gas
• pH, Temperature, ORP
• Alkalinity
• Organic acids
• H2S
• Trace micronutrients used by methanogens that may not be present in the influent
  
  

Adding Environmental Genomic testing to monitoring
Aster Bio uses Microbial Community Analysis (MCA) as a part of our Environmental Genomics testing suite.  The MCA test is a full microbial census for both bacteria and archaeal populations using high throughput sequencing technology. The resulting census gives relative % of the genera present. For monitoring, this shows drifts and changes in populations in response any operational change. MCA also proves useful in discovering problem sources in digesters not achieving desired methane conversion. While not a daily test, routine monitoring with MCA supplements traditional monitoring and an help improve anaerobic digester performance while reducing operating costs.

I had a recent question on why a system was with a history of stable operations was seeing an increase in non-volatile solids which we view as a decrease in the "bug" portion of MLSS.  Effluent quality remains excellent, but the local engineers wanted to think about what was driving this change. To understand MLSS to MLVSS ratio changes, it helps to think about what is happening in the system.

MLSS is total dried solids and includes “everything” after removing the water. This includes living microorganisms, dead microorganisms, extracellular materials including biopolymers, adsorbed organics, organic particulates, inorganic particulates.

MLVSS is the portion of MLSS that is removed during a high heat (550 Deg C) muffle furnace step. The residual contains non-volatile solids - Often benchmarked at 75% of solids being volatile in domestic WW.
How much of the volatile portion is living microbes? Usually less than 15% - longer sludge ages with lower F/M drop the % downward even further.

If you see an increase in the non-volatile fraction – this can mean

• More influent non-volatiles!
• Insufficient wasting leading to buildup of non-biodegradable or inorganic solids. As sludge becomes “old”, you enter endogenous respiration where active living biomass as a % falls
• Influent biodegradable organics drops so a lower microbial population is supported
  ​

What does this MLSS/MLVSS ratio change mean for operations:

• Don’t use the ratio in a vacuum – use settling tests, turbidity, microscopic exam, respiration rates and other available tests to determine operations.
• Monitor & measure
• Look upstream to see if anything has changed