Why I Dislike the Diluted Settleability Test 

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Ryan Hennessy

I have been to treatment plants that have had settleability issues and seen tables full of settleability tests with various dilutions of MLSS. I believe the intended objective of the diluted settleability test is to see if less biomass would correlate with improved settling; the general idea that if less biomass settles better, then wasting should theoretically improve settleability.

 There is a tremendous factor that is missed in this test, being that factor such as the MLSS concentration, the SRT (sludge retention time), and the F/M (food to microorganism) ratio (particularly in the first 15-30 minutes of aeration, which I refer to as the “initial contact zone”) are all significant components for what drives the selection of the bacteria that grow within the plant. Contrary to popular belief, changes to biomass may occur very rapidly depending upon environmental conditions present. Therefore, if the MLSS concentration is greatly reduced, it is likely that the microbes that compete with the lower MLSS concentration may be greatly different than what were initially present, often worsening problems rather than improving settling.  

Lower MLSS Does Not Always Correlate with Lower SV30 Value

 The idea that “wasting out the sickness (filaments, zoogloea etc.) appears to be prevalent among many wastewater treatment professionals. Our natural intuition is that if the sludge blankets in the final clarifier (s) are high, wasting aggressively would create more room for sludge to settle. Issues such as zoogloea bulking, bulking from low DO filament types, and bulking from various organic acid filament types all have the potential to worsen with increased wasting rates. While these listed issues are separate, the commonality is that selection of undesirable bacteria is based on the bacteria’s high kinetic growth rates and their ability to store food (“hoard it from floc forming bacteria) when there are higher F/M ratios in the initial contact zone of the aeration basin. For example, “low DO” is generally a function of the oxygen availability in proportion to the oxygen uptake rate of the bacteria. A way I like to think about it is that the bugs always want to achieve their desired MLSS concentration based upon the incoming organic loading rate. Therefore, rapidly increasing the wasting rate has the potential to increase bacterial growth rates, increase corresponding oxygen uptake rates, and further increase competitive advantage for Low DO filament types, basically creating a negative feedback loop resulting in higher aeration costs, additional sludge production, risk of loss of nitrification (if desired), and often further weakening the floc structure. 

Importance of Microscopic Evaluation Diagnosis 

Determining the predominant problem of settling issues through microscopic evaluation can be predicted with an extremely high amount of accuracy when it is done by a trained professional. The first (and often easiest step) to is to identify the root cause of the issue. Major problems may arise when the root cause of the problem is mis-diagnosed and the “wrong path” for troubleshooting is chosen. Due to the complexity and overall subjectiveness (sometimes more “art the science”) with wastewater microscopy, it is often cost effective and practical to send a split sample to an expert prior to choosing a troubleshooting strategy. 

Practical Application 

The truth is that there are some problems in which increasing the wasting rate is warranted, and other problems in which decreasing the wasting rate may be warranted. A quick example of when increasing the wasting rate is warranted would be if bulking is caused by filamentous bacteria types that gain a competitive advantage at higher SRT values/ lower food availability on the back end of the aeration basin. 

There is no “one size fits all” approach and each situation must be considered. For these reasons, I do not offer operational suggestions without a discussion with operational personnel about assessing the level of urgency, the treatment plant goals, and other logistics etc. I would personally be highly skeptical of any person or company who is offering advice and/or a product without obtaining intimate knowledge of the individual circumstances of each situation beforehand. 

Furthermore, it is common that more than one change may need to be made at a time for recovery after a treatment plant upset. For example, decreasing the wasting rate when the treatment plant is organically overloaded sounds good in principle, however if the sludge isn’t settling because of filamentous bulking by organic acid filament types, an intervention such as selective RAS chlorination may be needed in conjunction with increasing the MLSS concentration, trying to decrease the incoming organic acid concentrations, changes such as step-feed, etc. 

Source: https://rhwastewatermicrobiology.com/wastewater-microbiology-book/

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