Zoogloea bacteria are a morphotype/phenotype of bacteria that are commonly viewed within flocs in wastewater treatment processes that are large producers of exceocellular polysaccharide (slime). 

Zoogloea bacteria types are traditionally associated with low-molecular-weight organic acids and certain alcohols favoring high food-to-microorganism (F/M) ratio at the beginning of the aeration basin.

Zoogloea bacteria types may appear as fingered zoogloea or globular zoogloea with no currently recognized cause for variations of these morpholgoical traits.  Fingered and globular zoogloea types are found across municipal plants and various types of industrial wastewater processes. 

At low to moderate abundance, zoogloea bacteria types are believed to be beneficial for floc formation due to their polysaccharide (acts like “glue”), while at higher abundance, zoogloea bacteria can create viscous flocs that cause poor settling and contribute to foaming.

The MIDAS field guide recognizes the Zoogloea genus as containing six individual species, including Zoogloea ramigera, Zoogloea oleivorans, Zoogloea caeni, Zoogloea oryzae and two currently unnamed species. Using phase contrast microscopy, zoogloea bacteria cannot be readily differentiated from thauera bacteria types, which are closely related to zoogloea and suspected to occur at similar growth conditions. 

Both the genus Zoogloea and Thauera are classified within the Rhodocyclaceae family within the Proteobacteria phylum. The MIDAS field guide references six known species within the Thauera genus, including Thauera aminoaromatica, Thauera terpenica, and four currently unnamed species.

Thauera are versatile in their substrate uptake, utilizing aromatic compounds, monoterpenes, amino acids and organic substrates such as sugars, acetate, lactate, pyruvate and ethanol, and can use nitrate, nitrite or oxygen as electron acceptor. Thauera can denitrify when using nitrate as electron acceptor.

Zoogloea appear to be less versatile in use of available substrate behaving as chemoorganotrophs (while thauera can also use stored inorganic compounds). Zoogloea are known to use oxygen or nitrate as electron accceptors (when using nitrate they are denitrifiers). While in situ, zoogloea bacteria utilize organic acids, and at lab scale, they have also utilized proteins, amino acids and sugars. Both zoogloea and thauera produce PHA (fat storage), which allow them to compete well in conditions of high F/M ratio (in addition to immediate oxidation treatment, they may also store food for later on).

Process control

From a process control standpoint, reduction of zoogloea bacteria types is often successful upon increasing the MLSS concentration or reducing the concentrations of readily available substrate/organic acids. 

Chlorination is not effective in controlling zoogloea bacteria types and can worsen conditions (killing viable bacteria increases F/M ratio, promoting further zoogloea type growth). 

In conditions of polysaccharide slime settling issues (viscous weak floc structure), increasing the waste activated sludge rate often also worsens the problem. Sludges with high zoogloea bacteria types are often an excellent example of why lowering the MLSS or wasting more does not always equate with improved settling or floc characteristics, and why microscopic evaluation of the root cause of a problem is recommended before any operational changes are made.

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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.