TSS, or total suspended solids, seems like an easy and innocent enough test to perform. For the most part, it is. But are you following this test procedure as it should be followed? Would the way you perform this test hold up in the legal system? You may get “workable” results for plant operations, but what about reporting these results to a regulatory authority? If you do not follow approved methods – exactly – then the results, even if they are accurate, may not be legally acceptable. These are not hypothetical situations. I have been through it, and fortunately, my results were not only accurate, they were performed legally. There was no argument.

Under the Federal NPDES program, you have two approved methods for TSS: EPA method 160.2 and Standard Methods 2540D. It is important that you read the methods for the details of the test. But in general, both methods require that you wash, dry and weigh the glass fiber filters until you achieve a constant weight. A constant weight shows a variation of less than 0.5 mg or <4% of the previous weighing. Only then should you use the filter pads for TSS analysis. Believe it or not, a lot of junk gets washed off the filter pads. If it is not washed, a positive interference often results. The last thing you need when measuring TSS on your final effluent is a few extra milligrams that shouldn’t be there. It can make the difference between meeting your discharge limits or failing them. Wash and dry the pads for all your NPDES reporting and any other situations where the results are legally binding.

Sample size is equally important. Say, for instance, that you are running 50 ml of final effluent through the filter in the TSS test. To report the results in mg/l you must factor the measured TSS mg up by 20 times. In doing so, you are also factoring any error by at least 20 times. This can be significant if you have a low TSS limit in your NPDES permit. In this case, it would be better to use a much larger sample volume.

However, too much sample with high amounts of TSS can be harmful. A water-entrapping crust can form on the filter pad and will give a positive interference. This can happen especially with WAS and RAS samples. While these are not considered “legal” samples, the results do affect plant operation decisions. Standard Methods recommends that the final weight on the filter pad be 10 to 200 mg.

Another important aspect of TSS analysis is the filtration apparatus. EPA method 160.2 requires that all filtration apparatus be fitted with a coarse (40-60um) fritted disk. Standard Methods requires only a filter apparatus with a reservoir to have a fritted disk. The fritted disk is a porous glass disk used to support the filter paper. It helps to provide equal suction under the whole filter pad. Many of the filtration funnels out there (Gooch, Buchner, membrane filter funnels) contain perforated disks or bottoms as filter supports. Realistically, perforated disks typically will perform well, but for many samples, they will not.

Finally, don’t forget to repeat the drying and cooling of the filter and sample until a constant weight is achieved.

These TSS method requirements mentioned are often overlooked in many places. However, it is important to do them. Read the EPA or Standard Methods test procedures over for the details. Performing the analysis correctly may take a little more time, but by doing so, your results will be accurate and they will be legal. And that is why we do lab work. Otherwise, why bother doing it?

This article was written under the auspices of the New England Water Environment Association (a chapter of the Water Environment Federation) Laboratory Practices Committee. Please visit the NEWEA website at www.newea.org for membership information and other opportunities.

This article was written under the auspices of the New England Water Environment Association (a chapter of the Water Environment Federation) Laboratory Practices Committee. Please visit the NEWEA website at www.newea.org for membership information and other opportunities.