Part 3 of 8: Toxicity and Inhibition
The third growth pressure in this series that we will discuss is toxicity and its weaker counterpart, inhibition. In order for bacteria to thrive in a wastewater environment, the wastewater stream must be free of toxic or inhibitory compounds. Determining whether or not an influent is toxic or inhibitive to a specific biomass is quite simple using respirometry or other similar techniques. However, isolating and identifying the specific chemical or chemicals responsible for the adverse effects can be very difficult or impossible. The figure below shows how inhibition and toxicity impact cumulative oxygen uptake (BOD conversion) in a typical respirometer study.
One aspect of toxicity and inhibition that must be considered is the concept of acclimation. Bacterial populations are extremely adaptable. As long as the other growth pressures create an acceptable environment, over time a bacterial population will often acclimate to the inhibitive compounds. Some compounds, such as non-oxidizing biocides (chlorine) are examples of compounds to which bacteria will not acclimate.
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 have been shown to adversely impact Biochemical Oxygen Demand (BOD) conversion, although the mechanism of the toxicity vary. For example, turpentine appears to exhibit classic toxicity (poisoning) where soap toxicity appears to be more related to interference with oxygen transfer due to the bacteria becoming coated with the soap/surfactant compounds.
Lastly, one must understand the difference between wastewater toxicity related to the 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 level below 30 mg/l, depending on pH, temperature, and target aquatic species.
If you suspect toxicity or inhibition is adversely impacting your treatment performance or you will be using a new chemical in your process that may be toxic to your biomass, contact Mike Foster at Environmental Business Specialists to discuss using respirometry to evaluate your wastewater.