.
So, the project team checked the plant’s
effluent discharge to the Kansas River, because
it discharges through a 180-cm (72-in.) pipe and
flows down over a rock structure, which provides
turbulence, before discharging to the river. During
low flows, the elevation from the pipe to the river
is 6 m (20 ft). During testing, an expanse of
white foam about 15 m wide x 3 km long (50 ft wide
x 2 mi long) developed as a result of the turbulence
and fall. Currently, the plant’s NPDES
permit prevents effluent from being discharged to
the Kansas River if more than a trace of foam is
visible, so the test had to be stopped until no
foam was present.
.To
resolve the foaming issue, the project team reviewed
both the ferric chloride and polymer doses and the
primary basin effluent characteristics. The team
was concerned that the polymer dose was too high,
the primary basin effluent was septic, or both.
The team also did jar tests on primary basin effluent
to determine the optimum doses of polymer and ferric
chloride.
Jar testing (with shaking) was
conducted at various ferric dosages on storage basin
effluent and on raw waste water. Raw wastewater
was also tested without any chemical because of
concerns about the excess-flow storage basin’s
septicity because it had not been cleaned out in
more than a month. During tests, foaming was seen
with both raw wastewater and primary basin effluent
with ferric chloride. With respect to overall solids
removal performance, raw wastewater had better results
than primary basin effluent.
The project team determined
that the polymer dose was not excessive (in fact,
testing established that the optimum ferric chloride
dosage for good solids removal was about 90 mg/L).
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Instead,
foaming was caused by ferric chloride reacting with
biodegradable surfactants in the influent. Testing
revealed that after wastewater solids had been removed
in the ballasted flocculation basin, surfactants
had nothing to attach to, and once flow discharged
over any weir or rock structure that induced turbulence,
the surfactants began to foam.
In an attempt to quantify the
surfactants in the plant’s influent, samples
were sent out for anionic surfactant tests. Raw
influent, excess-flow storage basin effluent, ballasted
flocculation basin influent (after ferric chloride
addition), and ballasted flocculation basin effluent
were tested.
.Results
indicated that anionic surfactant levels at all
locations were very low, under 3 mg/L (normal levels
for wastewater are 1 to 20 mg/L; see Table 3, above).
Unfortunately, the results did not provide any more
infor- mation, for several reasons. First, the surfactants
my not be anionic. Second, if several surfactants
are present, anionic surf actant tests will be inaccurate
because of interference from other surfactants.
Third, city staff did not know any industry that
could be discharging a high level of surfactants
to the treatment plant, making it impossible to
identify the surfactants’ source.
Hunting for Solutions
To alleviate the ballasted
flocculation foaming problems, two treatment alternatives
were developed — using alter native coagulants
at various doses and adding defoaming agents to
ballasted flocculation basin effluent. Unfortunately,
testing with different coagulants (including ferric
sulfate, aluminum sulfate, and polyaluminum chlorohydrate)
at varied doses resulted in significant foaming.
Testing with defoaming agents
proved more successful. Initial jar tests used a
defoamer (Drewplus A-8274) and common vegetable
oil (soybean). Both the defoamer and the vegetable
oil controlled foam immediately. Sodium hypochlorite
was added to ensure that foaming would not occur
after disinfection. Samples with and without the
defoaming agent were tested for BOD to show the
effect of the agent. Initial tests indicated that
the agent did not affect BOD limits.
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