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TSS: Do you cook twice?
Posted: Wednesday, April 28, 2010 1:56 PM
Joined: 4/20/2010
Posts: 1

Greetings out there in lab land,

I recently took a new position and am finding that they have been doing things a bit differently than I have done in the past.  The thing that concerns me the most is the practice of cooking the TSS and TVSS samples twice.  I understand the theory behind this; you cook, cool, and weigh twice to make sure you are getting a stable weight reading.  So far, I am seeing no difference in my first and second weights, and I feel like it's just a big waste of time.  I was told that they started doing this because an old auditor told them to do so years ago.  Fast forward to present-day, and I have been brought in to run this lab and do things "my way."  Now I'm starting to wonder if my way is actually the wrong way, and maybe I should have been doing this double cooking/weighing procedure all along.  I'm interested to see how others out there do their solids testing.  I'm seriously thinking about rewriting the SOP to exclude the second cooking/weighing procedure.  Any thoughts? 

Posted: Wednesday, April 28, 2010 4:00 PM
Joined: 12/31/2009
Posts: 40

Standard Methods specifically says to repeat the drying, dessicating, weighing cycle until consecutive weights are within 4% or 0.5 mg (whichever is greater) of the previous weight.  So, yes, you must do it twice.  I understand the frustration of getting the same thing twice and feeling it is a waste of time, but occasionally you will find the odd one that needs extra drying time.  Of course we usually find that in dissolved or total solids, not suspended.

James Royer
Posted: Thursday, April 29, 2010 8:49 AM
Joined: 9/21/2009
Posts: 98

We did a study of drying and cooling times to establish constant times written into our SOP so that we get constant weights. The perodically recheck some by redrying and cooling to verify that the times still work. The auditors have not had a problem with that.

The only problem where redrying was necessary was a sample where considerablly more residue was on the filter and it did not get completely dry. So you should set a maximum amout of residue per filter to meet your SOP so that redrying is not necessary.

This is a wastewater plant so all our samples a reasonably the same.

Posted: Thursday, April 29, 2010 10:13 AM

I run a blank by filtering 300 mls of water and treating it as a sample.  I dry for a minimum of one hour.  Blanks never show a need for additional drying so I use that as justification for only one drying cycle.


Posted: Thursday, April 29, 2010 10:34 AM
Joined: 12/31/2009
Posts: 40


You must have considerably more reasonable auditors than ours, I've tried going that route and was told I had to adhere strictly to the method.  Gregg,  

No offense, but that is no justification whatsoever for not doing replicate drying cycles.  The reason for replicate weights is to show that the residue left on the filter is completely dry.  If you just run a blank all you show is that your filter dries out but it has no bearing on any samples at all.  Also, shouldn't your blank consist of 1000 mL water?  That is the maximum volume required by the method for filtering.  If you only filter 300 mL for your blank you will only be able to report non-detect of less than 6.7 mg/L.

Posted: Friday, April 30, 2010 3:16 PM
Joined: 1/8/2010
Posts: 4

In Wisconsin, we are allowed to only do a reweigh quarterly if we dry overnight or at least 8 hours and document our times.

James Royer
Posted: Monday, May 3, 2010 7:53 AM
Joined: 9/21/2009
Posts: 98

We also run a blank but this is more to correct if we had any drift in the balance or if the crucibles we changing weight thru the TSS run. Most times it is zero or within the reproducability of the weighing.

You do need to be able to document that the residue is dry to a stable value. The actual time depends on the type of drying oven, gravity or forced air. It depends on the residue, organic or inorganic, especially sulfates as they will hold water. The amount of mass that is typically on the filter is crucial.

As for volume of water used in the blank it is important to get the filter wet. There should be no difference if you filter 300 ml or 1000 ml as you DI water should contain no TSS. There should be no reduction in weigh if the filter is proberly rinsed when initially seated. We filter between 25 and 500 ml for real samples so any amount for a blank should give the same zero weight.

Your written SOP should just be followed and QC good and auditors should be satisfied.

Posted: Tuesday, May 4, 2010 7:25 AM

The blank is not an instrument blank, per se but to check if the filter handling and filtration process has any effect on the filter weight.  Like Jim said, if the filter is handled the same in way in the blank as in the sample it shouldn't matter how much water is filtered.  Dsmith, please show the math on your blank calculation.  I can't get the numbers you came up with.  If you use 2.5 mg as lower limit of weight and 300 mls water you get 8.3 as mg/L.

I will be checking drying times to see how long it takes.  The method doesn't have much about QC in it so blanks and standards are something I have come up with on my own with helpful input from others such as on this forum.



Posted: Wednesday, May 5, 2010 8:24 AM

We use USGS I-3765-85 for TSS.  The method states drying overnight and does not require any re-weighing.  Mark

Posted: Wednesday, May 5, 2010 8:12 PM
Joined: 12/31/2009
Posts: 40

We use a lower limit of 2 mg/L, which is what our state bodies require for the reporting limit.  I think there is some math to work that out from the weight difference and 1000 mL volume, but I don't have access to our SOP right now and am way to lazy to rederive it.  I'll see what I can come up with when I get back to the lab on Monday.

Charles Lytle
Posted: Friday, May 7, 2010 7:52 AM
Joined: 10/5/2009
Posts: 49



Here in the cold, miserable, rainy NW, we dry overnight w/ only one weighing.  But we do check periodically.  Over the years, we haven't had a problem.


Chuck Lytle

City of Portland WPCL


PS:  It was actually clear at 5 AM this morning.  So we do get to see the sky on rare occasions.  happy

Posted: Tuesday, May 11, 2010 3:33 PM
Joined: 5/11/2010
Posts: 3

Yep, all depends on your auditors.  In my State, you cannot deviate regardless of how reasonble your case.  Many years ago, we got an ATP from EPA to allow for a single drying based on experimentally determined times and limitations on mass on filter.  Now with the new ATP rules (40CFR Part 136.6) you "should" be able to produce that kind of data to be stored on site only (no ATP) and be in compliance (as far as EPA is concerned).

Posted: Thursday, May 27, 2010 4:10 PM
Joined: 1/21/2010
Posts: 9

Drying for 2 hours and weighing, then repeating that 3 times- is the equivalent of drying overnight and weighing once. Therefore, dry it overnight and you only have to weigh it once. Just make sure your dessicator is working properly when you're cooling the papers. 

Posted: Wednesday, July 21, 2010 10:14 AM
Joined: 7/21/2010
Posts: 4

It sounds like you can do this several ways.  If  you dry overnight, you should have little worries about residual moisture and inconsistent weights.  This is the method we chose for TS and TDS. 


As for TSS, we have process control samples that we run daily in addition to our reportable samples.  Operations wants these numbers sooner than a day later.  In order to meet their needs, we dry and cool our samples twice and report them the same day, as this is what SM requires.  To me, you could do it either way based on the way the data is being used and what your auditors feel is appropriate.

Posted: Wednesday, July 21, 2010 2:05 PM

We only cook and dry once.  We annually verify that we don't need to cook twice.  Our auditors have been happy. 


We are a WWTP that runs the same samples daily.