The Eight Growth Pressures of Wastewater Treatment #3: Toxicity and Inhibition

Growth Pressure #3 Toxicity and InhibitionThe third growth pressure in this series is toxicity with its weaker counterpart, inhibition. For bacteria to thrive and achieve their purpose effectively in a wastewater environment, the treatment system must be free of toxic compounds. The presence of toxic compounds can lead to a variety of symptoms in a biological treatment plant including deflocculation of activated sludge, reduced biochemical oxygen demand (BOD) removal, decreased oxygen uptake rate (sometimes accompanied by increased dissolved oxygen levels), loss of higher biological life forms, elevated total suspended solids in secondary clarifiers, and more.

Confirming whether wastewater contains toxic or inhibitory compounds to a specific biomass is a simple process using treatability techniques such as respirometry. Treatability testing may also be able to identify the source(s) of toxicity and inhibition. This depends on whether streams are combined entering the treatment plant or what makes up the source of the stream. Figure 1 shows how inhibition and toxicity impact cumulative oxygen uptake (BOD conversion) in a typical respirometer study. In the graph, toxicity refers to the bacteria’s inability to degrade substrate. The green line showing toxicity has a very small slope indicating negligible oxygen uptake which represents negligible biodegradation. When EBS discusses toxicity, we refer to the actual poisoning and eventual death of the bacteria. Inhibition refers to a lag at the beginning, where the bacteria are not consuming oxygen due to inhibition from a particular compound. During respirometry, the bacteria may acclimate after some time and eventually reach an oxygen uptake rate near the control.

Since bacterial populations are extremely adaptable, acclimation is an important concept related to toxicity and inhibition. If the other growth pressures create an acceptable environment, a bacterial population may acclimate to toxic compounds over time. However, bacteria will never acclimate to some compounds such as non-oxidizing biocides (chlorine). It is important to know whether acclimation is possible and there are several methods available to evaluate this probability.

Some bacterial species are more susceptible to toxic compounds than others. For example, nitrifying bacteria are much more sensitive to toxicity than carbonaceous bacteria. In the pulp and paper industry, compounds such as turpentine, soaps, and sulfur-containing compounds are known to adversely impact BOD conversion, although the mechanisms which cause toxicity will vary. For instance, turpentine appears to exhibit classic toxicity (poisoning) where soap toxicity appears to interfere with oxygen transfer as the bacteria becoming coated with the soap/surfactant compounds.

Finally, it is important to understand the difference between wastewater toxicity that impacts bacteria in the treatment system and effluent toxicity, which is related to aquatic species in the final receiving stream. A good example of this difference is ammonia. Ammonia is a key wastewater nutrient and carbonaceous bacteria can function with ammonia levels well above 100 mg/L. However, unionized ammonia can result in aquatic toxicity to aquatic species at levels below 30 mg/L depending on pH, temperature, and the affected species.

Here at EBS, we have several methods available for identifying toxicity and inhibition, determining if acclimation is possible, and selecting appropriate treatments for dealing with toxicity in wastewater. If you suspect toxicity or inhibition is adversely impacting your treatment performance, or that a new chemical in your process might be negatively impacting your biomass, contact your local EBS representative or email us at to discuss how to manage toxicity or utilize treatability testing to evaluate your wastewater.

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