Understanding Wastewater Toxicity and Inhibition

  • MDG
  • October 21, 2015

Toxicity vs Inhibition

It is important to realize that in wastewater processes, there are many genres of bacteria that are responsible for wastewater treatment. The different genres respond differently to system stresses depending on their sensitivity of the organisms and their enzyme systems.  For example, nitrobacter are extremely sensitive to changes in temperature, pH, and low concentrations of substances such as quaternary ammonium, peracedic acid, sulfide, surfactants, certain organic acids, and others. It is possible, that something may be toxic or inhibitory to bacteria such as nitrifying organisms, but do not have a notable impact of BOD removing bacteria.  Toxicity and inhibition is a function of the concentration of the toxin. For this reason, toxicity is generally more common in smaller plants with lower volumes of flow and less available dilution. Inhibition occurs due to stresses and slows down or stops an organism’s ability to function. Toxicity results in death of the organism. Bacteria are very resilient. Some species have the ability to go “dormant” during stressful periods and then carry on their life processes when the stress is eliminated from the system. Our goal in wastewater treatment is to select bacteria that will effectively treat the water and also settle well in the clarifier. There are many obstacles that we must face in order to successfully reach our goal.

Items of Concern in Wastewater Treatment Processes

  • Quatenary Ammonium Compounds
  • Peracedic Acid
  • Chlorine Compounds
  • Hydrogen Sulfide
  • Surfactants
  • Certain Organic Acids
  • Acids and alkaline materials that can bring the pH out of the 6.5-8.5 range
  • Phenols and hydrocarbons
  • Heavy Metals
  • Septage and Leachate
  • Deflocculation and development of pin flocs
  • Initial flagellate “bloom”
  • Disappearance of ciliates (free swimming and stalked ciliates)
  • Decreased oxygen uptake rates
  • Development of “dead bug” foam
  • Decrease in BOD removal and increase in TSS in the effluent
  • Because the actual viable biomass doesn’t reflect the F/M ratio on paper, it is often common to see filamentous bulking from filaments such as Spaerotilus natans upon recovery

Symptoms of Toxicity

Oxygen Uptake Rate (OUR) and toxicity measurement

The oxygen uptake of a sludge that is fed increasing amounts of BOD should naturally increase. If there is inhibition or toxicity present, the OUR curve will show decreasing amounts of oxygen consumed with increasing amounts of the stressful material. In scientific terms, if the endogenous OUR (not substrate fed) is greater than the fed OUR, this indicates death of the microbes. The OUR test can be done in a BOD bottle with a dissolved oxygen probe. For oxygen uptake rates to be valid, baseline measurements for normal conditions should be known ahead of time.  Other methods for determining toxicity include technology such as ATP measurement, Microtox and Deltox, and oxygen uptake rate testing. Having a baseline measurement for normal operations is recommended for these to be useful.  If the wastewater plant has variable frequency drives to control aeration and the incoming BOD is known, the percentage of output to the blowers may decrease if inhibition or toxicity is occurring. (Dead bugs don’t take up oxygen).

Toxicity Control

  • Locate the source of toxicity and eliminate it
  • Promote recovery of the system by maintaining proper growth conditions for bacteria
    • Proper pH, temperature, alkalinity, dissolved oxygen, nutrients
    • Because the viable biomass may be lower than it is on paper, it may be desired to
      • Consider bioaugmentation to accelerate recovery
      • Increase RAS rates to have less sludge in the clarifier and more available for treatment