Dispersed growth may be defined as microbes/material present in solution and outside of biological flocs in wastewater treatment processes. Dispersed growth is generally undesirable, as smaller particles lack density and settle slowly, often resulting in suspended solids carryover from the final clarifier. Not all dispersed growth is the same, and through microbial analysis as well as operational data, identifying what is growing dispersed is the first step in troubleshooting.
Single-cell bacteria
Overgrowth of dispersed single-cell bacteria is a common challenge in municipal and industrial wastewater treatment systems. The most commonly recognized cause for dispersed single-cell bacteria growth is excessively high bacterial growth rates or high oxygen uptake rates.
In general, oxygen uptake rates exceeding 65 milligrams per liter per hour in the aeration basin are correlated with undesirable bacteria growth, such as dispersed single-cell bacteria. These issues are most commonly related to the F/M ratio being too high in the initial contact zone (first 15-30 minutes) of aerated biological treatment.
Common control strategies in these instances are reducing the organic loading rate (if possible), increasing the sludge age MLSS, or spreading out the influent (i.e., step feed, putting additional trains online).
Fines or dead cellular material
Dispersed fines or dead cellular material are also a common operational challenge. In phase contrast microscopy the microbes generally appear opaque when they are dead or nonviable. Dispersed fines or dead cellular material are also often accompanied by poor health of filamentous bacteria (dead/damaged cells, empty sheaths etc.) and sometimes changes in the higher life-form organisms (headless stalked ciliates, absence of protozoa, a shift in what is normally observed etc.).
Diagnosing dead bugs is generally straightforward, however, other context such as operational control data and a full view of wastewater microscopy is often beneficial in interpreting these events. For example, if there are occasional pin floc and very common abundance of filament type 1851 (a high SRT filament) then endogeny (starvation) could be a likely cause. Death through toxicity is a more likely potential diagnosis if it is accompanied by loss or disruption of nitrification, breakup of the floc structure, or sudden change of filamentous bacteria health.
Through fluorescent microscopy analysis of viability, it is also common to see inconsistencies in the estimated bacterial viability depending upon which floc is observed when there have been recent toxic events. If there is high dispersed fines or dead cellular material, review of sludge handling side-streams and potential recycling of solids from these processes also generally warrants investigation.
Filamentous bacteria and others
Most filamentous bacteria types are capable of growing dispersed if their kinetic growth rates are high enough. Filaments such as Actinomycetes-Mycolata (Nocardioforms), Microthrix and Type 1863 are hydrophobic in nature and often cause extra challenges with turbidity as there is an upward force (wanting to form foam) versus the density of the filaments and the time available in the final clarifier to settle. If dispersed growth is filamentous, it is recommended to determine which filament type is predominantly responsible through microscopic evaluation and then to work backwards and weight operational logistics to determine what the next steps may be.
Flexibacter (small rod-shaped microbes) are technically classified as filamentous, however they are short and not recognized to cause bulking, and they’re more commonly correlated with dispersed growth. Spirilla (curved rod-shaped microbes) have similar growth causes (most commonly septicity/volatile organic acids) and often also cause dispersed growth. Spirochaetes (corkscrew-like microbes) are commonly seen dispersed in solution in treatment plants that have enhanced biological nutrient removal, and these microbes tend to settle well and are unlikely to contribute to turbidity or total suspended solids.
Other potential causes of dispersed growth
Other causes of dispersed growth include shock loads (rapid changing of the influent characteristics), rapid temperature changes, cation imbalances and microbial inhibition.
In practice, there is often a combination of types of dispersed growth viewed, and while it is inherently subjective, ranking the abundance of each type of dispersed growth is necessary to identify which is the biggest challenge. Once a proper diagnosis has been reached, review of the system, reaching an understanding of urgency of the problem and practical logistics may be reviewed to determine operational changes.
About the author: Ryan Hennessy is the principal scientist at Ryan Hennessy Wastewater Microbiology. 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 ryan@rhwastewatermicrobiology.com. Hennessy's new book Wastewater Microbiology: Filamentous Bacteria Morphotype Identification Techniques, and Process Control Troubleshooting Strategies is now available on Amazon.
