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NCRWA.COM|
Winter 2015
feature
iofilm is the basis of biofouling. The term “biofilm” usually generates
a vision in most people’s minds of a wafer thin shinny substance that
lightly coats whatever it is in contact with. Well, for a very brief moment
in its existence, it may have that appearance. In reality, biofilm beyond
its planktonic and initial attachment phases more resembles the growth
of sea coral as opposed to a thin coating.
There are several “pieces to the puzzle” of the impacts to cost created
by uncontrolled biofouling. This article attempts to put at least some of
those pieces together in a useable fashion.
Biofilm bacteria are actually colonies of various bacteria that all have
the characteristic of producing a “slime coat” around the bacteria. This
polysaccharide material is extremely sticky with excellent adhesive
abilities. The size of this slime coat in relationship to the size of the
bacteria is quite large. If a person standing in the middle of an NFL
football stadium were to represent the single bacteria, then the outside
edges of the stadium would approximate the size of the slime coating.
The polysaccharide slime coat serves several functions. All of these
functions combined help the biofilm colony thrive. Some simple
descriptions are listed below.
One function of the slime coat is to trap nutrients found in the water so
they can be brought to the cell for food. Nutrients can be organic and/or
inorganic. As an example, dissolved iron in the water is used by some
of the biofilm colony as food. The slime coat also traps other bacteria,
protozoa, viruses and oocysts. After the biofilm has metabolized its
food, the waste is passed through the slime coat and “stuck” to the
surface material the colony is growing on. In many cases the waste is
composed primarily of iron. But the waste is derived from whatever the
biofilm cells are feeding on. Most commonly, this deposited waste is
observed by utility staff as tubercles and/or corrosion.
This filtering characteristic of the slime coat results in the concentration
of material trapped there. Eventually, this concentrated material can be
ejected from the biofilm colony to become free floating in the water
flow. It then either re-attaches itself further downstream or is delivered
to the consumer. The slime coat protects the bacteria cell from harm by
disinfectants or antibiotics. The slime coat reacts with the disinfectant
reducing its effectiveness. More disinfectant is needed to sustain
desired free disinfectant residuals. This reaction with the slime coat
also increases the level of disinfectant byproducts. The precursor for
the formation of disinfectant byproducts is Naturally Occurring Matter
(NOM). So, the larger the concentration of biofilm colonies afflicting
the infrastructure, the greater the production of disinfectant byproducts;
AND more disinfectant is required to maintain desired disinfectant
concentrations. More money is spent on disinfectant, and even more
money is spent attempting to lower the disinfectant byproduct levels.
The slime coat is also used for quorum sensing. This is where the bacteria
have the ability to recognize other biofilm bacteria and join with them.
Think of a biofilm colony like a city. Its citizens work collectively to
make life better and safer. With quorum sensing the biofilm colonies
work together to ingest more food and protect the biofilm cells. This is
how the biofilm colonies grow and expand.
This cut away of a pipe gives an idea of what
biofouling can do to the interior of a pipe.
The cross section of the pipe is significantly
reduced. The friction loss through this pipe is
severe. So not only is there additional electrical
cost to overcome the reduced inside diameter
of the pipe, the cost is even higher still due
to the friction loss created by the Microbial
Induced Corrosion. In other words, the biofilm has matured to such a
degree it is severely affecting the pipes effectiveness.
It is easy to see the nature of the biofilm colony growth creates pockets
that can protect the biofilm cells from the effects of whatever disinfectant
is used. What might a pipe with no disinfection or interrupted disinfection
look like? Also, how effective can flushing be? Flushing can only remove
loose, unattached material IF sufficient velocities are achieved. Now
imagine the pipe pictured here as the casing of a well. Since a well has
perforations (or screen sections) the biofilm growth occurring on the
inside of the well casing is also happening on the outside of the well casing.
There is typically no treatment being done to the water at this stage. As the
pathway in the screens becomes smaller and smaller, less and less water
passes through the screens. What about the pore space in a gravel pack? Or,
what happens to the water bearing strata outside the well screens?
Reduced flow through the screens causes one or two things to happen.
1. If there are no other sections of screens in the well case, then the draw down
level will continue to increase until it reaches the point where the pump can
pump more water than the restricted well case screens can deliver. The pump
sucks air.
2. If there is more than one section of screens in the well case, as one section
becomes increasingly restricted, more water is pulled from the other sections of
screen which are less impacted. This can change the raw water characteristics
of the well. This shift from pulling water from one layer to another layer can
result in a change in raw water quality. An example of this is where one layer
has an arsenic concentration below the MCL, and another layer has an arsenic
concentration above the MCL. Initially, the water blended from the two layers
resulted in the overall raw water concentration below the MCL. If the flow from
the layer with the lower concentration is reduced, and the difference is made
up from the layer with the higher arsenic concentration, then the blended
arsenic concentration will increase.
By Bruce Baker, Consultant, BRB Consulting Service
IMPACTS OF
BIOFOULING
ON DRINKING WATER INFRASTRUCTURE