Treatment Plant Operator’s Bug of the Month is an ongoing series that spotlights the organisms present in wastewater microbiology. Each month a new organism is featured, giving readers a profile of the species and how it functions in a wastewater treatment setting. 

In this peek under the microscope, take an up-close look at Microthrix.

Microthrix is a filamentous morphotype that is commonly responsible for bulking and foaming in activated sludge processes. The morphotype “Microthrix” represents the physical characteristics observed in the Ca. Microthrix genus. The Ca. Microthrix genus is classified within the Actinobacteria phylum and includes four recognized species. These species include Microthrix parvicella, Microthrix calida, and two currently unnamed species. Note that with traditional microscopy applications the morphotype Microthrix is scientifically a more accurate description, as multiple species exist within the genus. 

From current research Microthrix parvicella and one of the unnamed species appear to the most common Microthrix filaments observed in full scale operations. For identification of Microthrix under the microscope, gram positive staining reactions are needed or this filament can be confused with filamentous morphotype 0581.

WWTP conditions Microthrix favors

Microthrix grows on long chain fatty acids (such as oleic acid) and favors conditions in which fermentation reactions (septicity) convert  fats, oils and grease into unsaturated or more readily accessible forms. Other conditions that favor the growth of Microthrix include higher sludge retention times, low temperatures and plants with alternating anaerobic/aerobic zones for biological nutrient removal processes. 

While Microthrix has an affinity for phosphorous and stores more phosphorous as a percentage than most traditional floc-forming bacteria, it is not technically classified as a polyphosphate accumulating organism (PAO) because there is no intermittent release of orthophosphate in anaerobic zones followed by luxury uptake to replenish polyphosphate in the aeration basin observed with this filament type.

Control strategies

Common control strategies for Microthrix include operating at lower sludge retention times; selective chlorine application (when appropriate); elimination/reduction of foam trapping; enforcement of grease ordinances to reduce oil and grease entering municipal wastewater plants; primary treatment to remove fats, oils and grease prior to secondary/biological treatment; and reducing known areas of septicity (if feasible). Also, some literature and case studies suggest that poly-aluminum may help discourage Microthrix in certain instances. 

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About the author: Ryan Hennessy is the microbiology and operations specialist at Midwest Contract Operations Inc. He was trained and mentored by Dr. Michael Richard for over 10 years in wastewater microbiology, and serves as a microbiology services consultant. Hennessy is a licensed wastewater treatment and municipal waterworks operator in the state of Wisconsin and fills in as needed for operations at several facilities. He can be reached at rhennessy@mco-us.com.