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in this problem we're going to learn how to solve for biochemical oxygen demand or Bo D for short well let's go to the board and see how to work out this Bo D problem well the problem reads a secondary effluent has a total Bo D of 45 milligrams per liter a Bo D test is run on this same secondary effluent at the same time using a nitrification inhibitor using the data for this test as given below calculate the nitrogenous bo D we're supposed to assume that a 300 milliliter Bo D bottle is used to run this test well they told us that our total Bo D was 45 milligrams per liter now there's two types of Bo D that make up the total Bo d we have carbonaceous Bo D and nitrogenous Bo D and when you add this nitrification inhibitor you're determining the carbonaceous Bo D so what we will do is we will calculate the carbonaceous Bo D using the data given and then we'll take the difference between our carbonaceous Bo D and the total Bo D and that'll be the nitrogenous Bo D well some things we need to know before we work this problem are we have to make sure that the test is a valid test and the first thing we look at is our blank when you set this up you'll set up a blank bottle to see if there's any depletion without adding any sample well in our problem here our blank dissolved oxygen at the beginning was 9 milligrams per liter and after the five-day test we still had 8.9 milligrams per liter in the bottle well we need our depletion in the blank to be 0.2 or less 0.2 milligrams per liter or less so in this case going from 9 to 8.9 we see a depletion of 0.1 milligrams per liter so that's acceptable so now we have to look at each individual bottle to see if it meets the other two criteria when we look at the 50 mil and the 30 mm samples we need to see that we have at least 0.2 milligram per liter of dissolved oxygen depletion from beginning to end and we also have to have at least 1 milligram per liter left in the bottle at the end of the test well looking at this data both the 30 mil and the 50 mil have both depleted 0.2 milligrams per liter and there's at least one milligram per liter of dissolved oxygen left in each so we're going to use both of these sample bottles to calculate our carbonaceous Bo D what we'll do is we'll calculate the 30 milliliter sample Bo D then we'll calculate the 50 milliliter sample Bo d add them together and divide by 2 to find the average then we can subtract that from our total and we'll have our answer well the formula we're going to use is our Bo D formula to find Bo D and milligrams per liter we use the formula that says our initial dissolved oxygen - the final dissolved oxygen times 300 milliliters which is the size of the sample bottle divided by the milliliters of sample added to the bottle so in this first case we use the 30 milliliter data so our initial dissolved oxygen was 8.9 milligrams per liter and our final was 6.0 so we subtract the 6 milligrams per liter from the 8.9 we multiply that by the 300 and then divide by the 30 mils and we see that our Bo D in this instance is 29 milligrams per liter well now we need to do the other sample the 50 milliliter sample set up the initial dissolved oxygen was 8.8 milligrams per liter and then the final was 4.2 milligrams per liter so we take the difference of those two multiplied by 300 and then divide by the 50 milliliter sample size and when we do that we see that this Bo D was twenty seven point six milligrams per liter well now that we have both of these samples we're going to average them to get our result so we're going to add the 29 milligrams per litre to the twenty seven point six milligrams per liter and divide by two and when we do that you see that our result is twenty eight point three milligrams per liter now this twenty eight point three milligrams per liter represents our carbonaceous VOD well they told us that the total B OD was forty five milligrams per liter so if we take our carbonaceous VOD of twenty eight point three and subtracted from that forty five we'll have our nitrogenous bo D so the formula looks like this nbod or nitrogenous bo D is equal to our total Bo D minus our carbonaceous bo D and when we do that we see that our nitrogenous bo D is sixteen point seven milligrams per liter and that my friend is how this problem is solved you you