Signs of Toxicity
by Tim Loftus
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.
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.
Of course, we
all can recognize a major upset: drastic changes in process efficiency, high
turbidity in the effluent, and so on. But what about milder forms of toxicity
– where no violations occur? Below are a few indicators of toxicity. While
each one in itself does not absolutely point to toxic wastes, they do suggest
that you look into it further.
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.
Through Respirometry
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.
This article was written under the auspices of the New England Water Environment Association (a chapter of the Water Environment Federation) Laboratory Practices Committee. Please visit the NEWEA website at www.newea.org for membership information and other opportunities.