Coagulation is unquestionably the most
important step in the drinking water treatment process. Coagulation is one of
those mysteries of water treatment. If you read a chapter in a textbook on
water treatment you'll find that the author's like to talk about all the
things that are not coagulation they like to talk about the particles and the
theory and what might not work and what's always different and at the end
of the chapter they'll talk about Jar testing and they'll say Jar testing is a
trial and error approach where you actually optimize the coagulation
process to an individual treatment plant this is all great and good except when
it doesn't work. So traditional jar testing has some
flaws. The flaws include sedimentation. All water treatment plants don't
produce a large settleable floc that would work with the traditional jar testing method. For example if the water
treatment plant has a very low organic carbon concentration and a very low
turbidity it's very unlikely that you're going to produce a large settleable floc
that's going to work with the jar test coagulation controls every process that
comes after coagulation like flocculation and sedimentation
filtration and disinfection because if particles are not properly coagulated
they won't flocculate and if they don't flocculate they won't settle and if they
aren't properly coagulated they won't filter either. Coagulation is controlled
by a number of important parameters as you can see on this diagram the pH
temperature the alkalinity the hardness the concentration of particles and the
concentration of organics all impact the optimum coagulant dose and coagulation
pH plus they all interact with each other and they vary from one water to
the next and from one day to the next or one season to the next
organics are probably the most important part of this diagram and the
concentration of organics varies the hydrophobicity, the molecular weight
and the structure can all change from one water to the next and even from day
to day or season to season Drinking water treatment is challenging
because the particles are small and negatively charged they're too small to settle on
their own they're too negatively charged to flocculate or stick to any surfaces
including the surfaces inside of the filter coagulation has two different
mechanisms for overcoming this negative surface charge of particles the first
mechanism is charged neutralization charge neutralization will produce
positively charged species to offset the negative charge at the surface of the
particle unfortunately this has to be a one-to-one balance or very close to it
if we produce all positively charged particles because we overfeed coagulant
then charge neutralization stops working if we underfeed coagulant and produce
too few positive charges the particles remain negative and this mechanism
doesn't work either the second mechanism of coagulation is sweep coagulation and
sweep coagulation we don't produce any positively charged species we simply
produce a lot of precipitate and the precipitate is neutrally charged the
neutral precipitate then entirely coats the surface of the particle or at least
coats it to the degree that it can be flocculated settled and filtered. Jar
testing is the go to trial and error method for optimizing coagulation in a
drinking water treatment plant unfortunately jar testing doesn't work at
every water treatment facility some water treatment facilities produce a
charge neutralize flaw that is not large and saleable such that the jar testing
method would indicate that sedimentation doesn't work and that the coagulation
process was ineffective when in actuality the coagulation process worked
and the water would filter and produce low filtered water turbidity and meet
all regulations the second limitation of Jar testing is that the pH changes when
the alum dose changes so increments of alum dose also change the pH
because alum is an acid if you look at five different alum doses you also look
at five different pH's and this becomes a two variable optimization problem
you're trying to optimize the pH at the same time you're trying to optimize the
alum dose unfortunately both are changing at the same time jar testing in
general and coagulation specifically is a very complex process there are a lot
of variables to optimize so instead of starting and talking about coagulation
and jar testing, let's do a basketball analogy so let's talk about
an experiment where you're put on a basketball court and you're asked to
make a basket but to make things harder you're blindfolded and you're spun
around several times before the experiment begins and you don't know
what direction you're facing so you get a basketball you're spun around two
times and you've got six shots and you're trying to make a basket so you
shoot the first one it doesn't go in you shoot the second one it doesn't go in
you don't hear anything you shoot the third one nothing fourth fifth six
nothing so what do you do well it becomes very much a random problem you
don't know where the goal is so you don't know which direction to shoot you
don't know how far away you are so you don't know how hard to shoot and it
becomes nearly impossible to make a basket but how could we make this
process easier how could we make it easier for you to make a basket while
still having two variables to optimize well how about this
how about you're put directly in front of the goal you're perfectly lined
up with the front of the rim you just don't know how far away it is
you're still blindfolded and you're given six shots so the first shot you
shoot it easy you don't hear anything it doesn't hit anything
second shot same result third shot same result fourth shot maybe you think you
hear it hit the net fifth shot maybe it bounces off the backboard now you're
starting to get close to making a basket and only five shots
now if you're given six more shots what are the odds that you'll make a basket
if you shoot between where you hit the net and where you hit the backboard you
finally make a basket so we had two sets of basketball trials in the first set of
basketball trials with two variable optimization we didn't get anywhere
close to making a basket in the second set we made a basket weave we found the
optimum we reached our goal and only two more experiments what's the difference
well in the first experiment you were trying to optimize two variables at the
same time in the second set of experiments you're only trying to
optimize one variable at a time we want to do the same thing in drinking water
treatment but we have to redraw the two axes so we need to redraw the basketball
court as the coagulation diagram by Amirtharajah so the the basketball
court becomes the jar tester and the six shots become six jars we'll start with
an alum only situation where the pH is going to change each time we change the
alum dose because the alum is an acid this is another two variable optimization
problem and it prevents us from shooting straight towards our goal which in this
case is the dotted reagent outline in the center of the sweep zone we will
call the optimum sweep to get directly in front of the basket or directly in
front of the optimum sweep zone in the coagulation diagram we would have to add
base each time we change or increase the alum dose such that the pH would stay at
a constant 7.5 and we would be situated directly in front of our goal the only
problem with base addition is it only allows us to move to the right side of
the coagulation diagram to move to the left side of the diagram we have to add
acid particularly if we want to get to the lower left where we would expect
charge neutralization to occur acid is a requirement enhanced coagulation also
occurs on the left of the diagram with acid but typically at higher dosages
the next generation Jar testing procedure has two primary innovations
the first innovation is that the pH and coagulant dose are controlled
independently through titrations so that it's a one variable optimization instead
of a two variable optimization the second main innovation of the Jar
testing procedure is to use granular media filters instead of sedimentation
because all treatment plants don't produce a large settable floc so if you
have a granular media filter instead of a membrane filter instead of a jar for
sedimentation you can actually use the same treatment process that the plants
using and you can optimize the same treatment process with the same
coagulation process to get results that are there ideally suited to your plant now that we've cover the basics of Jar
testing let's get into how to conduct a Jar test and a list of items you'll need
in order to get started we recommend using JTWizard as
this program will guide you through the entire jar test procedure and keep
records of all of your experiments we will begin by selecting the standard
jar test procedure and entering the experimental conditions for this
experiment we will be using a constant pH of 6.5 and varying the alum
concentration as shown the standard Jar test procedure consists of four
mixing stages a 1-minute rapid mix followed by three states tapered flocculation and
concludes with filtration use the time recommended by the jar test wizard for
the final stage as this will depend upon your jar tester always standardize
your pH electrode prior to conducting any jar tests we recommend you measure and record raw
water turbidity alkalinity and hardness prior to each jar test in order to
track changing water quality between experiments before starting a jar test
experiment we need to perform titrations in order to determine the appropriate
amount of acid or base needed to keep the pH constant as the alum dose varies
we will follow the titration steps outlined in the jar test wizard once the jars are filled with water
Center each jar in its position on the jar tester then lower each paddle and
secure the shaft to prevent slippage and to ensure consistent mixing across the
jars now fill the syringes with the corresponding
volume of coagulant as shown in the jar test wizard next we will add the volume of acid or
base determined in the titration step to its corresponding jar prior to
starting the jar test place the beaker at each jar which will be used to
collect post-coagulation pH samples at the end of rapid mix begin filtration by opening the valve of
the first jar at the completion of the final stage of flocculation note that
mixing continues throughout the duration of the filtration process
each subsequent valve will be opened in 30-second intervals the jar test wizard
has a built-in timer which can be used as a guide for opening valves and collecting
samples collect filter turbidity samples approximately 2 minutes after opening
each valve once you measure and record the filtered
turbidity samples the jar test procedure is complete the jar test wizard
generates a one-page summary report of the experiment as well as stores all of
the experimental data within the jar test wizard file thank you for watching
our instructional video on jar testing for more information and the latest
updates please follow the link in the description