Signs of Toxicity
Toxic wastes are not necessarily just those compounds listed by the Federal and
State government as toxic. They can be a host of other compounds, whether by
themselves or in congregate, that create problems at your wastewater treatment
plant. A toxic waste in a treatment plant will not always kill off the whole
system. It may not even lead to an NPDES permit violation. But it will lead to
reduced treatment efficiency and inefficient plants are not only more difficult
to operate, they are more expensive to run.
Through the Microscope
Microscopic observation of the mixed liquor shows a rapid shift in the types and numbers of microlife that you normally see. While it could be a result of a process change, it may not be. Often the filaments that hold the floc together will be the first to die off from a toxic waste. There will be a lot of dispersed growth due to the deflocculation. This is usually followed by a bloom of flagellates (predatory protozoa reacting to a sudden availability of a food source). You may also notice a sudden die-off of ciliates and rotifers. If the toxicity is severe enough, foam will appear with aeration. Check the foam under the microscope, too. If you see lysed cell contents from dead microlife, you are most likely seeing the results of toxicity.
In the BODs
The BOD5 take-down shows that different dilutions for a sample calculate out to different results. Specifically, you notice that the higher the dilution of the sample, the higher the BOD5 results. (For example, 2 mL of sample into a 300 mL bottle with seed results in a BOD5 of 450 mg/L, 3 mL – 375 mg/L, and 4 mL – 250 mg/L.) This indicates a sample that is toxic. In this case, the sample may not be very toxic, but at higher volumes, it will have a toxic effect on the biomass.
Many operators or lab technicians are using some form of respirometry for a quick check on WTP treatment efficiency. The method can be adapted to test for toxicity of an industrial waste or other wastewater sample. It consists of measuring the oxygen uptake rate (OUR) of the mixed liquor in the endogenous stage (where all the food is gone and the bugs are just waiting for more food- usually at the end of the aeration tanks). Then you add a sample of industrial wastewater (or whatever sample you want to test) and check the OUR while the bugs are eating. The OUR should be higher than the endogenous rate. This would indicate that the sample is easily biodegradable. If the OUR is less, then it may be a sign that the sample is toxic. The oxygen uptake rate can be done using expensive equipment specifically designed for this, or it can be done using a simple, although much less accurate, BOD bottle and probe (see 2710 B in Standard Methods 20th ed.).
While these indicators of toxicity may not be totally definitive, they should be seen as warning signs for potential problems.
If you have any questions, suggestions, or comments, please contact LPC Chair Paul Fitzgibbons at (401) 222-6780 ext 118 (firstname.lastname@example.org) or Tim Loftus at (508) 949-3865. You can also visit our website at newea.org. Once on the website, press the Lab Practices button.