Troubleshooting Fixed Film Systems
The goal of this article is:
1. 1. To illustrate
through actual case studies how to use the Four Basic Concepts of Wastewater in
actual practice for fixed-film systems.
2. 2. To illustrate
the importance of using the O&M Manual to gain an accurate understanding of
the design parameters of the system in operation.
Learn Theory! Learn Theory!
Learn Theory! If you know theory you can use it! If you don’t know theory you
can’t use it!
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The Georgia Rural Water Association is
always available to assist any municipality that serve a population of
<10,000 with process control, troubleshooting, training, lift station
maintenance, smoke testing, leak detection, standard operating procedure
development, asset management development, or any other compliance and
operational issues that may crop up. To request their assistance, please
contact them at Contact Us -
Georgia Rural Water Association (grwa.org).
The next few posts will focus on Activated
Sludge Concepts. After that, I will focus on some water related topics of
interest.
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This first troubleshooting
article is going to cover fixed film devices. Fixed film technology was the
original concept behind trickling filters. The concept has evolved to the point
where a thin film of biological “slime’ is grown on a wide variety of ‘media’. The media is either submerged in the
wastewater to be treated or the wastewater is poured, trickled or sprayed over
the media in a constant stream. Over time the biological ‘slime’ thickens to a
point where it will slough off and can be separated from the treated
wastewater.
The media can consist of rocks,
wood slats, flat plastic, corrugated plastic, plastic balls (similar to ping
pong balls), and various configurations of plastic media that can be produced
in sheets (like an air conditioner filter) or small aggregate material placed
into a sump.
Years ago, when I worked for a
chemical company, I was asked to come and evaluate chemicals for a wastewater
system that served a religious camp. The camp was used at various degrees
throughout the year, and the waste stream was, obviously, very erratic. Wastewater
flowed from cottages around the camp into large sumps that captured paper and
other solids before flowing to a large box. The box was equipped with submerged
corrugated plastic media for slime growth. There was a space of around three
feet under the media for the sloughed off material to settle and a sludge pump
for pumping off the settled sludge. It was also equipped with a bower and the
diffuser headers were located just underneath the plastic media. Air was
constantly blowing through the headers to assist in controlling the thickness
of the biological growth on the media.
The concept is simple: during the
warmer months and periods of higher organic and hydraulic loading, the blower speed
would be increased to increase agitation of the media to maintain the biological
growth at a specific thickness on the media. During the colder months and
during those periods when the organic and hydraulic loadings were less, the air
flow would be reduced to reduce the agitation and allow the bacteria more time
to develop the proper thickness.
When I arrived on the scene, the
first thing I did was to inspect the facility to get and understanding of the principal
components. The second thing I did was to review their flow and BOD loading.
The third thing I did was to ask for the M&M Manual and compare the design
specifications to the actual flow and BOD loadings of the plant.
During my walkabout of the plant,
I noticed the SCFM air flow of the air distribution system was at the maximum
20 SCFM. When I looked at the O&M Manual, the recommended SCFM for their hydraulic
and organic loadings was 5 SCFM. I asked the operator if he had ever adjusted
the air flow of the unit. Of course, the answer was no, that the air flow was
where it was at when they took over operation 13 years earlier. I asked if we
could turn the ari flow to 5 SCFM, and the answer was no. I then asked if they
felt comfortable if I adjusted it back to 10 SCFM. They were agreeable to doing
that. Som while there, I adjusted the air flow back to 10 SCFM.
Two weeks later I called to see
how the system was operating and was told this was the first time in 13 years
the plant met permit. What was happening?
The way the system is designed to
operate is this: the amount of turbulence in the treatment chamber is conditioned
upon the relationship of hydraulic water flow and air flow. There must be
enough turbulence to slough off the biological growth and maintain the correct
thickness of the growth, but not too much to keep the sloughing’s from
settling. The problem in this case is the turbulence was too high to allow a
thick enough growth and to permit settling. So, they always had a high TSS
leaving the plant during the winter, and a high BOD and TSS leaving the plant
during the summer. Turning down the air reduced the agitation in the chamber, permitting
a thicker growth, which produced a heavier slough that settled better, and reduced
the turbulence in the tank enough to allow the sloughing’s to settle.
This is similar to what we had to
do when operating a high-rate trickling filter. During the winter months we had
to reduce the internal recycle to allow the biological growth to thicken on the
filter walls. This is equivalent to increasing the sludge age in an activated
sludge system. During the summer months we increased the internal recycle to
speed up the sloughing so we did not get snails, which is an indication of too
high of a ‘sludge age’ on the filter.
CONCLUSIONS
The O&M Manual should provide
an accurate understanding of the design parameters of the system in operation. The
design parameters are provided at 100% of flow. Most systems do not operate
anywhere near full capacity, so the operator must make adjustments to the
system to ensure the system is operating according to the correct design
parameters. In some cases this may be taking process units out of service. In
others, adjusting the air requirements or recycle rates based on organic,
hydraulic or environmental (temperature) conditions.
Feel free to contact me should
you serve a population >10,000 or have any questions or issues that the GRWA
is unable to assist you with. Also, if you would like to contribute an article
feel free to email me at the address below. I am always looking for
contributors that have an interesting perspective, topic or has an interesting
case that they would like to share – especially if the solution is a direct
result of applying the principles from this forum.
Dennis Brown,
Wastewater Specialist and Trainer, Retired
678.750.3996
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