2015 March Pipeline

Making your plant
your own.
“Experience of
your operators
can be a vital
asset to an
efficient system.
Although
textbooks and
training
manuals can
give an
unexperienced
operator a wide
overview
as to how
Wastewater
Treatment
Plants are
meant to
operate, there
are features in
every plant that
must be
improved on to
suit the
situation.”
Nothing ever is as black and white as it
seems. There is a feeling-out process that
accompanies any new application. If you’ve
spent enough time at one position or in one
place, you’ll agree that at some point, you’ve
thought to yourself, “I know what might work
better”.
The very same has been evident since the
Wastewater Treatment Plant was
constructed and put into service in Swift
Current, Saskatchewan. Designed by Stantec
Ltd, the SCWWTP has been producing a high
quality of effluent since its commissioning in
April of 2006. Utilizing a Modified
Johannesburg process of biological
treatment, the plant is capable of removing
BOD’s, ammonia, and phosphorus by
simultaneous nitrification and denitrification
within the bioreactor cells. Because the City
of Swift Current never had a treatment plant
in the past, there were a lot of assumptions
made during the design stages. The plant
capacity was designed with historical flow
rates in mind, and the strength of the
wastewater was derived directly from
samples from the collection system.
Overall the SCWWTP is a very successful
project, especially in terms of effluent
quality. What cannot always be predicted,
however, is the way that certain systems or
specific stages will react when the science is
applied in real life. No two treatment plants
are exactly the same, nor are two collection
systems, nor are two small systems identical.
For example, depending on the strength of
the raw influent, each plant will have a
specific way of treating it. The amount of
solids that a plant brings in on a daily basis,
as well as the nutrients that accompany it,
will have a direct impact on what happens in
the middle stages of treatment. Sometimes
the intended design does not always work as
efficiently as expected. This is the point
where an operators’ experience and
ingenuity takes over.
Design Issue: In Swift Current, the
bioreactors have a tendency to develop
foam during certain points of the year due to
factors such as mixed liquor consistency and
dissolved oxygen variations. When the
foaming problem occurs, not only does the
foam back up into the cells, but because of
the recycling of the material to the on-site lift
station, foam (and the microorganisms that
help cause the foam) can be recirculated to
the Headworks building and subsequently
back into the system. The solution: Tie into
an existing discharge pipe that takes WAS
(waste activated sludge) directly to a nearby
lagoon. The WAS discharge pipe was meant
to act as a means of sludge disposal if the
centrifuge was unable to operate as
intended. By simply adding two valves, the
plant now has the option of pumping all onsite
sewage from our lift station (including
foam from the bioreactors) into the lagoon
rather than recycling it into the system. An
unexpected result of this amendment was
that the end of the pipe at the lagoon
remained thawed during the winter due to
continuous, rather than intermittent flow.
Design Issue: Another foam-related problem experienced
at the WWTP was getting the foam to end up at the
intended location. At the end of the bioreactor train, in the
mixed liquor trough, there is a foam removal system. It
consists of a trough that is maintained at a level that allows
a small amount of mixed liquor (ML) to flow into the
trough, carrying any foam build-up with it. The problem
was that sometimes the foam wasn’t being carried away
into the trough with the mixed liquor, but instead floated
on top like a large mat while the ML continued to flow into
the trough. The solution: Create a system that would help
push the mat of foam towards the trough. Operators tried
several versions of water jets that would direct the foam to
its destination. The water spray was effective in the
summer, but tended to freeze in the winter. The idea also
called for a lot of potable water to be used, which in the
long run was not cost effective. Operators contemplated
use of treated effluent to reduce the use of potable water,
but a temporary solution could not be developed due to the
distance from the effluent trench to the ML trough. The staff
finally decided to use an air bubbler process that would
utilize air (from the nearby DO piping) to create a wave of
mixed liquor that would force the mat of foam into the
trough and ultimately away for disposal. It was tested for
12 months using temporary parts & piping to ensure that
the new system would work, and then built permanently
once operators were convinced that all of the bugs had
been worked out. This system is currently still in operation
and maintains the foam level at an absolute minimum
without wasting any potable water.
Design Issue: Waste activated sludge (WAS) that was
pumped to a holding tank until such time as it could be
dewatered was causing problems in the centrifuge.
Because of a good-settling sludge, the WAS in the sludge
tank would settle before it could be dewatered. The sludge
would be very thick at the bottom of the tank, virtually clear
in the middle, and a medium thickness at the top (due to
some sludge floating rather than settling). The plant was
built with a recycle line that was supposed to divert some of
the flow back to the sludge tank, thereby mixing it and
maintaining consistency. The result was an uneven
consistency in the flow to the centrifuge causing highs and
lows, and variable polymer dosing rates. The Solution: A
better mixing method. Staff installed a submersible mixer,
sized according to tank dimensions, and voila, the sludge
was maintained at a consistent thickness, and resulted in
more efficient polymer dose rates and a more consistent
sludge cake. At the same time the mixer was added to the
sludge tank, the Operators also changed the point of
polymer injection from upstream of the RAS pump to
downstream. This reduced the chance of the floc being
sheared while it travelled through the RAS pump impeller.
Although these are just three of a dozen changes
implemented to improve the operations at the SCWWTP,
you can see how experience of your operators can be a vital
asset to an efficient system. Operators in Swift Current have
taken their own ideas, as well as ideas from other plants, and
improved on the original design of the plant. No design is
perfect, we all understand that. Some things cannot be
foreseen, and those things need to work properly for a plant
to function. We encourage our staff to keep thinking up
ways to make our jobs more efficient with as little impact on
the successful operation of the plant itself. Some changes
work wonders, some not at all, some changes are on a much
grander scale, some are so simple that we often ask
ourselves, “Why didn’t we think of that before”. In any case,
your plant will only work for you if you suit it to your needs.
Tim Cox
Superintendent Swift Current Wastewater Treatment Plant.
6
March 2015
| the pipeline
the pipeline | March 2015 7