| NEWEA LAB PRACTICES COMMITTEE
By Tim Loftus, a member of the NEWEA Lab Practices
Committee on Toxicity
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 you wastewater treatment plant. A toxic waste in a treatment plant will
not always kill of 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.
One of the major problems we have at a WTP with toxic wastes is that we often find out
about it too late. The waste is already in the plant and causing the problems. But when we
can identify times that toxicity shows up, we are at the first step in solving the
problem. From here we can formulate a plan to narrow the search for the source of
toxicity. It’s not perfect, but oftentimes it’s all we have to go on.
Through the microscope:
Microscopic observations of the mixed liquor show 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 BODs 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, 2ml of sample into a 300ml bottle with seed results in a BOD5 of
450mg/l, 3ml – 375mg/l, and 4ml – 250mg/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.
Through Respirometry: Many
operators of 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 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 (see2710 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 (lab@narrabay.com)
or Tim Loftus at (508) 949-3865. You can also visit our website at newea.org. Once on the
website, press the Lab Practice button. |