Troubleshooting Activated Sludge through Microscopic Observations: Five Case Studies


  • Paul Pitt, David Jenkins, Sarah Galst, Mike Lynch, Bryan Atieh, Katya Bilyk, Alonso Griborio

Microscopic analysis of sludge samples at Plant A showed excessive amounts of Microthrix Parvicella. Recommendations for process changes to combat the filament were provided and included operating at higher DO concentrations, lowering SRT, reducing anoxic volume, and initiating RAS chlorination.

This chart shows the impact of the presence of Microthrix on Sludge Volume Indexes (SVIs) at Plant A in this study and the immediate improvement in settling once mitigation measures were enacted.

This chart hows the impact of the presence of Sphaerotilus natans and Nostocoida Limicola II on effluent quality at Plant B and the immediate improvement once DO concentrations were increased.

Plant operators rely on many tools to observe and evaluate their Wastewater Treatment Plant’s (WWTP’s) process. For example, a laboratory analysis showing increasing effluent ammonia may lead an operator to increase the aerobic volume used in treatment. Or a hand-held Dissolved Oxygen (DO) meter showing low DO concentrations may inform an operator to increase the aeration output to the aeration tanks (ATs). These tools provide information that instruct operators on which operational changes to make, and the magnitude of change needed.

One operational issue that is common to many WWTPs but does not have an easy monitoring ‘tool’ is foam. Foaming conditions often arise with no forewarning and can cause many operational difficulties, including odors, spreading beyond the confines of the infrastructure, causing unsafe working conditions, damage to equipment, and add significant maintenance costs associated with clean up.

Foam is caused by the presence of Nocardioforms (filamentous bacteria) which are hydrophobic in nature and have a large surface area to interact with gas bubbles, promoting free-form filament attachment to gas bubbles in the bulk solution which rise to the surface as foam. Once at the surface, filamentous bacteria are not wasted through the settled Waste Activated Sludge (WAS) and remain in the system indefinitely. Operational changes at a WWTP are needed to create conditions non-conducive to the growth of filamentous bacteria in order to recover from a foaming event.

Five case studies are presented, demonstrating examples where microscopic analysis provided either an identification of a filamentous microorganism or forewarning of the presence of filamentous bacteria.

For more information or a copy of the full paper, please contact the author at

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Horizons Fall 2017 (pdf)

Horizons showcases significant water, wastewater, reuse, and stormwater projects and innovations that help our clients to achieve their goals, and can help you achieve yours. Articles are written by top engineers and process group leaders, demonstrating and explaining the beneficial application of a variety of technologies and tools.

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