Libris Britannia 4/science library(b).zip/science library(b)/SCIENTIF/1461.ZIP/TEXT.ARC/MCRT.TXT

Text File | 1988-12-22 | 16KB | 237 lines

                              MCRT



        Mcrt has been one of the hardest parameters to come by in waste
treatment.  The reason is because there are so many variable parameters
involved in its calculation...parameters that themselves are dependent on the
quality of data and sampling techniques involved.  Experience shows however
that with some statistical tricks a useable somewhat accurate tool can be
obtained in this number...a number that is perhaps the best tool an operator
would have to control his process.

        Even among engineers, there is some variance as to what is the
best method of calculation is...and there are several.  One method uses
kinetic mass balance equations.  Another method uses the sludge unit as
obtained with the use of a centrifuge.  Still another less used method uses
what is called "Gould age"...which is a relationship between aeration
solids and influent lbs of applied BOD.  Even with the most orthodox
method, the standard EPA perscribed method of Mean Cell Residence Time (a
more correct terminology as to the biology that is taking place and the
reasons for calculating a sludge age) there is a controversy as to how
important the inclusion of Clarifier solids in the calculation are...and
thus we see some equations that include this parameter while in others we
don't.

        Experience relates that the difference in perspective between an
operator and an engineer would show that "not" including clarifier solids
would be wholly inadequate on an operational level while from a design
standpoint perhaps not always necessary.  The question of course is what
percentage of total plant solids could an operator expect at any given time to
be in his clarifier.  Design parameters often equate average static
conditions...ie a set average blanket of one foot for example...while to the
operator it is the real world conditions that he must consider...perhaps a
fluctuating blanket that may be one foot as an average but variant from
nothing to overflowing the clarifier weirs on any given day.  In some
processes, experience has seen that a clarifier can carry over 50% of a
plant's process activated sludge...thusly it is important that the operator
make some effort in coming up with a good calculation of this necessary
parameter and include it in his solids inventory. Experimentation and sample
study including depth profiles of a typical blanket can give some reasonably
accurate ratios of expectant solids content that is some number between the
MLss and the RASss (in most cases).

        The other critical aspect of a good MCRT calculation is keeping track
of one's wasting.  For some reason, this particular number seems to be one
of the most neglected parameters that operators deal with.  Waste sludge is
perhaps some of the thickest sludge an operator has to deal with...and
thusly good flow measurement is hard to come by (unless one is lucky enough
to have the luxury of some expensive instrumentation that is "kept up"!).
By all means, an operator should take extra effort in keeping track of this
very important calculation.  A lot of operators become disgruntled when the
numbers they obtain do not mirror what is happening in the plant...and there
is only one reason for this...and that is bad data.  Granted, wastewater
treatment is such a wide range of activity and the quantities an operator
deals with hard to reduce down to the measurements of a few milliliters or
less...but approaching a statistical profile of one's process "will work" if
only some time and effort is put into it...and an operator will find the
rewards more than worthwhile when all the guesswork is eliminated.  Waste
data should be one of the highest priorities in a process.  One's MCRT is
wholly dependent on this one measurement.

        One problem that occurs in calculating a good MCRT is that operators
try to see it as a pure number...either correct or incorrect.  We all know
that on days that we do not waste one's MCRT would be infinity...or on days
of very low wasting, we'd see our MCRT jump a dozen days or more...somthing
that we know cannot be true as to the real residence time of our biology.
To a design engineer, the idea of a MCRT is relavent to why a plant
works...that is why certain effects take place in the process...and thusly
he designs his plant around certain numbers (be it mcrt or f/m or other
numbers)to obtain required reactions and treatment. To the operator however,
such numbers are "quages"...tools by which he manipulates his
process...hopefully toward what the engineer intended.  Experience has shown
that some operators will actually throw out any MCRT that does not appear
correct or ideal.  The important thing about MCRT to remember that it will
probably take working with...that is, all the parameters that go into its
calculation will have to be "worked with" as to sampling techniques, quantity
of data, relationship ratio constants, and what "tricks" are needed to get
one's MCRT mirroring "real" occurances in his process.  For instance, moving
agerages are highly recommended...but the actual period by which the moving
average is obtained will be up to study.  To plot daily MCRTs on a graph would
probably show a zig-zag of such extremes that it would be unintellegible and
unuseable for the operator.  To plot a three day running average will take out
most of the extreme fluctuations...but it can still be quite erradict.  The
longer the period for the running average, the more the actual daily data will
be lost however...and if the period is too long the objective of mirroring
real events could be lost.  For one thing, the longer the period the longer
the delay in the curve itself in showing real trends. A trend that has occured
in a shift change could have happened two days ago but is only now showing up
on one's graph of MCRT (or any parameter for that matter) if the period is too
long.

       EPA methods quote seven day averages as being a good period...and in
some cases, especially larger plants where change is buffered in the sheer
quantity of treatment, this would be true.  It would be recommended however,
that the period selected should be the smallest possible so that response to
plant trends is quicker and lost information is minimal.  For most small to
small-medium sized plants in the 1MGD to 5MGD flow ranges, a three to five
day running average might do very nicely.

       The next trick involved in a good MCRT calculation is what to do when
one's wasting program is quite variant...especially when there are a lot of
days when there is no wasting accomplished.  Zero waste days alter even
running averages quite significantly and one might see a rise and fall in MCRT
that is is not "real".  Running averages cannot simply skip days.  One method
that will buffer such a rise and fall due to zero waste days is to look back
on historical records and find the highest MCRT that one's plant has run at
(barring zero waste days).  Unless one's plant is undergoing substantial and
unusual change, it would be fairly safe to say that it is operating at least
at or below the highest MCRT that was ever recorded.  MCRT seen as a pure
number is only a relationship between certain other numbers that go into its
calculation.  Zero waste means a zero as a diviend (or a very negligible
number due to effluent solids loss).  In reality, that is...what is really
happening in the plant, we know that the MCRT is not dropping and rising in
extreme discreet quantities...but is in fact slowly changing one way or
another over time.  Logically, one's MCRT is not going to rise above the
highest recorded MCRT overnight simply for lack of one or two day's wasting
(this qualified with the assumption that one's plant is operating in some
normal midrange).  To buffer these unreal MCRTs due to waste variations, it
would be within the logic of the meaning of MCRT (if not within the realms of
the dictates of pure numbers) to set a lower limit of one's "recorded" wasting
gallons (and subsequent pounds) to that required to hold  that maximum MCRT
the plant operates at.  What this means is calculating what it would take to
waste to that highest MCRT and then using that number in one's running
averages on days there is no wasting.  This will buffer one's MCRT curve on a
graph that the rise and fall remain hopefully mirroring what is actually
taking place in the plant.

       This is important.  The object of MCRT on an operational level is a
guage with which to operate the plant.  With a little effort, this can be
accomplished at even the most unsophisticated plants where measurements are
little more than good guesses if only the attitude is taken that the object
is to find what will "mirror" what is "really" happening in one's plant as a
matter of trends.  By "mirroring" what is meant here is a relative rise and
fall...that is, if the mean cell residence time is increasing so also will
the curve of the mcrt show a relative sense of that increase.  It would be
ideal to have the number itself accurate...that is, a rise of 1 day from say
4 to 5.  But what is much more important is the relative change in the
numbers...that say even if your numbers were reading 10 presently, an
increase of one day of mcrt will show a jump of one relative unit.  Every
process will have to be guaged whatever the case...no matter what the
numbers are...for instance, your plant may work best at the number 8...no
matter what the actual mean cell residence time is in reality.  We guage a
control by the numbers we have to work with...no matter their accuracy.
What is important however is their consistency.  Quite literally in fact,
the actual residence time of each microorganism in the process is probably
unobtainable due to the large degree of variance and quantity.  Still, the
relationship of the numbers is logically correct and the thought valid.
What the operator must concentrate upon however is not some elogant answer
provided by the numbers themselves and the equations that are given out of
textbooks...but to be able to understand the principles by which those
equations are based upon...and then to be able to "use" the numbers to guage
"his plant's" relationships. To take the discussion one step further, all
units of measurement have to be relative...that is, are standardized and
thereby relate to some unit of space or time or relationship between space
and time(and force).  A foot may have been an entirely different measurement
of distance(space)if the king's foot had been larger for instance...and an
entire system of English units of measurement may have been wholly different
from what we know it today.  Literally speaking, MCRT is derived from mass-
time relationships...ie lbs in inventory to lbs leaving the inventory that
relates to a time unit.  Often however in the unelogant variance of
wastewater and the lack of time and facility to obtain perfect numbers to
attach to those relationships, the operator may be left as the king
was...having to assign relavent numbers as they are obtained at any given
plant to that plant's unique conditions.  A number after all is only a
number.  Their usefulness to us is to "quantify" space and time (and force)
so that we can intellegently use those demensions.  It is an ideal to think
that there are universal numbers...and indeed we attempt to standardize
things for all of us so that we can communicate.  Thusly, a foot is a
foot...a unit of measurment that we all know and can relate to.  That was
the king's guage that the rest of the English world accepted as a standard
to go by.  The actual distance that a foot represents did not have to be
what we know it as today.  It simply became accepted as a standard.  It is
important to remain within standards whenever there are a great many people
who must communicate with one another.  Thusly on the reportable parameters
of our plants that must correlate say fecal coliform between a plant in
Okeechobee,Florida to fecal coliform of a plant in Oshkosh, Michigan, it is
very  important that set standards are adhered to.  MCRT on the other hand
is the useable control parameter that only has one purpose at any given
plant for the operator...and that is to control that plant to the most
optimum treatment possible.  This is mentioned here not to suggest that a
formal attack on MCRT and the attempt to correlate MCRT to universal
perceptions is not important...but as the case can be, especially at small
plants where good data or quantity of data can be sporadic, it remains very
important to try to correlate that MCRT at the very least to that particular
plant's operation.

       The problem of MCRT as mentioned before is that there are so many
variables involved.  Simply speaking, MCRT is a relationship between total
solid weight in one's process to that of total solids weight leaving the
process on a daily basis.  Of course, the variables are found in the
calculations of those solids weights and involve no less than seven changing
parameters...ie MLss, RASss, WASss, sludge blanket, Gallons of waste,
volatile sus. solids, and the ratio between RASss and MLss.  Reducing such a
wide array of variables down to one number that depicts a time unit is ideal
but not easy to do.  What would be a more common sense objective is to
attempt to find numbers that mirror the relationships jest...that is a
number that will react in comparison to the increase or decrease of these
variables enmasse.  It may in the final outcome have little to do with the
actual time any microorganism is actually spending in the process (five or
six or eight days or whatever)...but will show accurately the "change" in
time that an organism is spending in the process.  This correlated to actual
observed conditions of a plant will be workable and useable tool with which
to control that plant.  It is important however to "standardize" all
measurements and calculations "inhouse".  Your plant may work best say at
the calculated number 9 day mcrt.  What is important is that the same
conditions will arise at that calculated number 9 (as to oxidation level).

    Another thing to keep in mind is that the time unit, that is the
final unit of calculated days is another standard that the operator might
need to change for checking his numbers.  If say, your plant batch wastes
only once or twice a week, it might be an easier parameter to work with
to calculate a MCRT on a weekly basis...that is, to find what percentage
of a week your microoganisms are in the process.  In this way, your waste
would be considered as a weekly sum total and you'd circumvent the
"infinity" MCRT days on which you accomplish no wasting.  That percentage
could be reconverted to days...ie so much percent of a week(or multiples
thereof) would equate to so many days.  It's a bit unorthodox, but
somthing to keep in mind if your wasting is grossly eradict.  Whatever,
some formal attempt should be made to obtain that MCRT no matter what.

  Over time as the numbers are worked with and adjusted, it is highly
likely that a very true depiction of actual mean cell residence time
will be obtained.  The important thing initially is to start...then
to keep the numbers one obtains and try to guage it...and then as one
goes to adjust the sampling and calculations until that "mirror" effect
is in place in the trends.



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