Lab Test Interferences
I tend to go back and forth in my view of just how different industrial wastewater treatment is from municipal treatment. I used to be fixed in thinking there were many significant, complex differences but now I often find myself thinking the differences are not that great. Just recently though, somewhat contradictory, I realized that succesful lab testing in the industrial evironment can be a bit trickier than lab testing in municipal wastewater treatment plants. Let's begin with the simplest case, considering how chloride can interfere with measuring the chemical oxygen demand (COD) in a wastewater sample.
The organic concentration test I always do in the field is COD, not biochemical oxygen demand (BOD). The COD test takes a little over two hours to complete and is easily performed using a "digital reactor" or heating block and a COD reagent test tube (see example COD test procedure below). In contrast to the COD test, the BOD test is more complicated to prepare and requires a five day incubation period. If you have spent any amount of time looking at my different blog posts you will know my preferred vendor for lab equipment and reagents is Hach and it is these I will be covering in this post.
In the table below I have summarized the five Hach COD test kits (or ranges) I use showing the concentration range and the concentration at which chloride will cause an interference with the COD test. There are a couple of things to note from this table. First, I would never need to use the TNT 824 test which measures COD in the range of 5 to 60 g/L (5,000 to 60,000 mg/L) in a municipal wastewater treatment plant. There is just no wastewater stream in a municipal plant, in my experience, that would have such a high COD concentration. Yet, in many different industrial wastewater plants I often need to start testing in this ultra high range. In actual practice I will run both the TNT 824 and TNT 823 COD test, on an industrial influent stream for example, to find the best or most optimal range.
From the table you can also see that chloride will need to exceed a concentration of 10,000 mg/L before it will interfere with the TNT 824 COD test. Yet again, I can't imagine testing a municipal wastewater sample that would have a chloride concentration in excess of 10,000 mg/L with one exception: A municipal treatment plant located on the coast with a leaky collection system allowing for seawater intrusion. In contrast, any number of industrial plants may discharge high concentrations of chloride to their wastewater system and this concentration needs to be known before you start COD testing. The takeaway from this is that laboratory testing in a municipal wastewater plant is most likely going to be more predictable than it might be in an industrial wastewater plant.
Hach's COD test is very robust, being sensitive only to chloride at relatively high concentrations. I will often use Hach's chloride test strips (see photos below) to do a concentration check at both municipal and industrial plants before I start my COD testing. Hach also has titration kits to measure chloride concentrations up to 10,000 mg/L but I don't typically need that level of accuracy. A simple dilution allows me to measure chloride concentrations greater than 10,000 mg/L using the test strips.
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A Little More on Potential Lab Test Interferences
I wanted to start off with an easy example and the COD test provides that. Other lab tests have much greater sensitivities though. For example, another common and frequent test I run, in both municipal and industrial wastewater plants, is ammonia nitrogen. Hach offers seven ranges of test kits for ammonia nitrogen testing, measuring a low concentration of 0.015 mg/L NH3-N to a high range of 1,800 mg/L NH3-N. as shown in the image below. You'll use the two lowest ranges to measure effluent ammonia but why would you ever need to use the highest range that can measure up to 1,800 mg/L of ammonia nitrogen? This ultra high range becomes very handy when you are working around anaerobic digesters. Centrifuge centrate is the perfect wastewater sample that requires the highest test range.
For the Hach TNT test kits that span the ranges from 0.015 up to 130 mg/L NH3-N, the interferences are identical to what is shown below for the TNT 832 Ammonia Nitrogen Test.
It is the highest range (100 to 1,800 mg/L NH3-N) ammonia nitrogen test that has a different (higher) interference level. The interference levels for the TNT 834 ammonia test are shown below. Something important to note when running this test is highlighted in the procedure, below. This ultra high range test requires just 20 micro liters of sample volume so you'll need to have this volume of pipettor to use this test. I made the mistake of only having a pipettor that could measure down to 0.1 mL when I first purchased this test range.
Key Recommendation Before You Test
If you are going to use Hach's TNT reagents because of their ease-of-use and great reproducibility, I highly recommend you download and review Hach's procedures for each test you are going to run. I always have these PDF files on my computer when I'm in the field but I often will have printed copies with me for the range of tests I'll be running. Each procedure is typically described on just two to four pages and they will save you time and trouble when you understand the specific equipment you will need such as a heating block, spectrophotometer, range of pipettors, and other reagents to confirm you don't have concentration levels that will interfere with the test you are most interested in. An example test procedure for Hach's TNT 831 ammonia nitrogen test, spanning the range of 1 to 12 mg/L NH3-N, is available as a PDF below.