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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