Understanding Biochemical Oxygen Demand (BOD)

So this class we're going to continue our discussion on the BOD or the biochemical oxygen demand. And also we may be able to introduce some new contents regarding the water quality in rivers. Right. So so basically. So regarding the reading assignment, it's still going to be 9-2 and 9-3 of them. of your textbook. So if we do a quick recap, so last class we learned about the final contents about the hydrology. So we discussed basically what is the Darcy velocity and also what is the actual or linear velocity. So we need to consider when the water flows by a certain area, the soil that the underground water is going by, it's not an open structure, right? So it's always, there's going to be rocks or sands. So the actual velocity of the water is going to be higher compared to the Darcy's velocity, right? So we need to consider what is the porosity of the soil that's in the, basically for the water to flow by. So regarding that, I have a quick quiz. So maybe we can do a... really quick calculation here. all right i'll give it 20 more seconds Okay, we'll stop here. So the final answer or the actual, the correct answer should be two millimeter per hour. Okay, so we mentioned that, so for the porosity, and when you divide the the Darcy velocity by the porosity to get the actual linear velocity, right? So for example, this is the soil, basically the water is going this way, right? So the soil is composed of all the sands or the rocks here. So the water velocity that's going through the void is going to be higher compared to the average velocity going through this cross section here, right? So what we do is we need to divide the Darcy velocity by the porosity, let's say eta here, all right? So if the porosity is 50 percent, then what that means is that the actual linear velocity is going to be twice of the Darcy's velocity. So this is how we consider this problem. So we also mentioned or discussed what is the major sources and types of the water pollutants. So in terms of the sources, we have the point sources and non-point sources. Point sources are just like the sewage discharge. We directly put it. put them into the river and then for non-point source it can be fertilizers or some chemicals from the farmlands right so they infiltrate into the soil and then further get emitted into the river or into the ocean right so we also discussed what is oxygen demand and dissolved oxygen right so for the for these types of the water pollutants we mentioned that Actually, the most important one is the oxygen demanding materials. So we said when we see some water is heavily polluted and then there are no fish inside, it's not because those trash are toxic. It's because those trash demands oxygen to decompose them into carbon dioxide and water that requires oxygen. And if there are not enough oxygen in the water, then the fish will not be able to survive in the water system. right? So because of that, we need to discuss what is the oxygen demand and what is the dissolved oxygen, right? So the oxygen demand, you can treat it as the required oxygen, basically the demand for oxygen to oxidize those organics, right? So you can treat it as a reactant. So basically, it requires a certain amount of oxygen. But the dissolved oxygen is basically the concentration of oxygen in water, right? So we said that the dissolved oxygen under normal condition, normal temperature, and normal pressure, the dissolved oxygen has to be below 8 or 8.5 milligram per liter, right? This is the maximum amount of oxygen that's available in the water. So because of that, if there are too much of the organic waste in the water, then it's going to consume far more of the oxygen. or is going to have a far larger oxygen demand than the dissolved oxygen, then we may have some trouble. So if we just pick up the discussion from there, so basically if the oxygen demand is larger than the dissolved oxygen, then we know for sure that there are no fish inside. And if the oxygen demand is smaller than dissolved oxygen, then maybe there are fish, but definitely there will be some lives. for example, microorganisms being survived in the water. So in terms of the oxygen demand, there are different types, right? So last class, we also talked that there are theoretical oxygen demand, which is basically, if you know what is the composition of the wastewater, then you can calculate what is the oxygen demand. For example, if there are glucose, right? And then we know that the glucose is going to react with six number of oxygen. So this is the oxygen demanded by this chemical here right to form carbon dioxide, six number carbon dioxide and six number of water right. And then if we know what is the compost what is the concentration of the glucose in the water, then we can calculate what is the corresponding amount of oxygen that's required to oxidize these organics, right? But we also mentioned that to determine this theoretical oxygen demand is almost impossible, mainly because there's no such a wastewater that you know what is the composition of the waste is in there and also you know what is the concentration of the waste. It's going to be very difficult for us to tell what is the composition of wastewater, okay? So it contains all different types of organics. So basically the composition is so complex. that we cannot do this calculation to determine the theoretical oxygen demand. So that's why people come up with other methods, for example the chemical oxygen demand, to kind of find a tracer or a substitute for the oxygen demand. For example, the chemical oxygen demand, what that means is if we collect a wastewater sample, what we do is we introduce some highly oxidizing species. For example, here the chromic acid. So the chromic acid can oxidize anything that's either organic or that's living organisms. So anything that meets with this oxidizing species, they will get oxidized into carbon dioxide and water. So what we can do is we can measure, we can know what is the concentration of the chromic acid in the beginning. and know what is the concentration of the chromic acid at the end after we mix this solution here. Then basically the amount, let's say, the change of the concentration of the chromic acid would correspond to the oxygen demand, right? But there is another problem associated with this, it's because there are many species in the nature that are not biodegradable. for example, the plastic bag, okay, plastics. So if we just throw plastic bottles into the water, although it's still polluting the environment, but these plastic bottles are not going to consume any oxygen, mainly because they're so stable, right? They're not being consumed by any of the microorganisms. So because of that, it's not going to reduce the dissolved oxygen. So it's not affecting the oxygen demand. But what happens is that if you have these plastics in this solution here, and then you add in this. chromic acid, they're still getting oxidized into carbon dioxide and water. So by using this chemical oxygen demand, it is kind of like an overkill or like an overestimation of the oxygen demand because there are many things that theoretically are not reducing the dissolved oxygen in the wastewater. So that's why people came up with this final approach, which is the biochemical oxygen demand. or the BOD. So I think you guys did the BOD lab last week, right? Hopefully you got some some basic ideas about the BOD, but we're going to talk more about how to measure these BOD later this class, okay? So what this means is that we're trying to replicate what's happening in the nature. So after we collect these wastewater bottles, wastewater samples here, So instead of introducing these highly oxidizing species, we're introducing the microorganisms, which are bacteria, right? All different types of bacteria as an assay or as a seed into the water sample. Okay. So theoretically, if there are waste, organic waste inside, right? So these seeds are going to consume oxygen. So similar to what's happening in the... river or the lake, right? And then what we can do is we can just measure the oxygen or the DO, dissolved oxygen, at the beginning when we collect the sample and at the end, let's say after a few days, after five days or seven days, right? So we have the initial DO and the final DO. So the BOD is just going to be equal to the DO initial minus DO final, right? So theoretically, if there is no dilution of this sample here, so this is very simple. You just measure the oxygen concentration before and after to determine what is a BOD. But for a lot of the times, the BOD for the sample is too high. So there's no way we can get this accurately. So that's why the equation will also get quite complex if you consider the dilution and also if you're adding controls. for these experiments. So the advantage for this method is that it reflects biodegradability. So we know that these seed or these microorganisms, they're not so dumb that they're going to decompose the plastics, right? They don't have that capability to do so. So this reflects what's happening in the nature, right? And also the bile acids or the seed we use, also it's quite similar. They're quite similar to the natural water, right? So in this way, the BOD is a more appropriate way to describe what is the level of pollutants, what is the amount of organic pollutants that's in the water, right? So we introduce three types of oxygen demand. We have the theoretical oxygen demand, the chemical oxygen demand, and the... biochemical oxygen demand. So what are their relationships? So first, regarding the theoretical oxygen demand and chemical oxygen demand, what we can know is that under rare circumstances, these two are going to be equal to each other. So for example, just like the case or the example we gave earlier, if we know that the wastewater is composed of glucose, which is very simple sugar, So if it's composed of simple organics, right, then these two are going to be equal to each other, right? So first thing is that these sugars are very easy to decompose and also we know exactly what is the composition of the wastewater. But if the wastewater is getting very complicated, which is the normal case, let's say from a wastewater facility, wastewater treatment facility, then the wastewater composition is so complex that there's no way we can do this theoretical oxygen demand calculation, then these two are definitely not going to be equal to each other, right? So under rare circumstances, these two are going to be equal to each other. And in terms of the BOD, the BOD never equals theoretical oxygen demand or chemical oxygen demand. So typically the BOD is smaller than these two. So the reasons are two ways, okay? So if you consider The theoretical oxygen demand, or the COD, what it assumes is that all the carbon are getting consumed into carbon dioxide. But for the microorganisms, it's not the same thing. So we can think of human, right? So we eat a lot of things every day. We eat a lot of organic carbon, right? For example, apple or cake. So not all of them are converted into carbon dioxide. So some of them are being converted into, let's say, muscle, right? And some of them are converted into, let's say, fat. So they can basically convert it into a part of their body. So in that case, not all of the carbon are getting converted into carbon dioxide. And because of that, the BOD is going to be smaller than COD or theoretical oxygen demand. And at the same time, not all of the carbon are getting converted into part of their body, mainly because these organisms are going to generate... waste, right? These wastes are not pure carbon dioxide. So there can be some other soluble organic matter that are just generated by these microorganisms, right? So in this way, the BOD should be smaller than the theoretical oxygen demand or the COD. But because the BOD represents what's happening in the nature, so the BOD is still the most widely used. parameter to describe how polluted this water is. So now how do we measure the BOD? So a simple way, as I have introduced earlier, is just to measure the dissolved oxygen concentration, the DO, before and after we add in these microorganisms. So basically we measure the dissolved oxygen as a function of time. right? So we will also add in the microorganisms into the body, into the bottle, right? So that they can consume the organic matter inside. So now the thing is, if we just measure these dissolved oxygen as a function of time, what people are really interested in is the total amount of BOD, right? So for example if you can measure what we do in our lab is we either measure five days or or 10 days or seven days or 10 days right but you don't know when does it reach to the final bod right so we name it bod5 bod7 and bod10 but what we are really caring about is the ultimate bod So this ultimate BOD can happen, let's say, after several days, after one month, or after one year. So it's not reasonable to do these measurements like day by day after the entire year, right? So that's why we need to understand what is happening in the kinetics. So whether we can just use the BOD, let's say BOD5, to calculate what is the ultimate BOD. So is there a way to do so, right? So then we need to understand what is a kinetic scene. So as we have introduced earlier in this class, we assume that most of the reactions are first order reaction. So it also applies for these organics. OK, so what happens is, let's say if you if you plot the concentration of the organics in the wastewater, let's say this is L. So. what you know is that at the time equal to zero, when you take out this wastewater sample out, then organic concentration is going to be the highest, right? And then after some time, these microorganisms inside are going to compose these organics, right? Then its concentration is going to decrease, right? And then we assume that it's going to decrease according to the first-order reaction. So what that means is that the dL is going to be negative k multiplied by L. So k here is the rate constant, right, reaction rate constant. So because of that, we can calculate that the concentration of these organics are just going to be L0 multiplied by exponential of minus kT, right, L equal to L0. exponential minus kT. So basically the concentration of these organics are going to decrease exponentially. All right. So what about the BOD, right? So this is the concentration of the organics. What about the BOD? So the BOD is quite different from the concentration of organics, right? So basically the definition of the BOD is the oxygen that's consumed, right? So ultimate BOD is the final amount of oxygen that's being consumed. So that's why the BOD at the ultimate condition that's also going to be the highest, right? So if we just draw the L here again, right, this is the concentration of the organics. Then the BOD is going to be reverse of it. So just think about it. At the beginning, when we introduce the microorganisms, the oxygen that's being consumed is just zero, right? We haven't consumed any oxygen yet. But as time goes on, first the Oxygen are being consumed at a fast rate, and then it's going to get slower and slower, mainly because the organics are being consumed, right? And when all the organics are gone, they reach to the highest BOD value, right? So this is the BOD, right? And this is the organics, okay? So basically, the BOD is going to follow a trend that's reverse of this concentration of the organics, right? So we can write it out as BOD equal to L0 minus L0 exponential of minus kT, right? So you can think about this basically. What this means is that when T is equal to zero, then BOD is equal to zero, right? When T is equal to infinity, then BOD is equal to L0, right? So this is going to basically the BOD and the organics, they're reciprocal to each other, right? So if they add up together, they're going to be equal to the ultimate BOD, right? So because of this, then we know that now we have the kinetics of the BOD. And normally people will also name this as BODT. as a subscript because this is valid for any time, right? So for example, if you know what is a BOD at three days, right? And then you can just plug in this value, L0 minus L0 exponential of minus k multiplied by three, right? So let's say if you know what is this value, let's say this is equal to a, right? And if you further know what is a k here, then you can calculate. what is the L0. So in this way, even if you just did the BOD measurement for five days, seven days, or ten days, you can calculate what is the ultimate BOD. You don't need to wait for an entire year to determine the BOD value of the wastewater. So here I'm just showing the figure on your textbook. So basically it's talking about the same thing. Again, the concentration of the organics, the L here, is going to reduce. It's very rapid at the beginning, mainly because the organics concentration are the highest. And also, you just introduced these microorganisms into the water, right? So it's going to reduce very rapidly and then finally go to zero because there are no more organics in the water anymore. And then in terms of the BOD, it's going to increase. from zero right at the beginning there is no oxygen consumption and then increase very rapidly at the beginning and then at the end the BOD is going to stop since there's no more organics it's going to it cannot consume any more of the oxygen from the water right and then regarding the quantitative behavior of the BOD you can see that can be calculated by L0 minus L, right? It's going to be just L0 minus L0 exponential of minus kT, right? And then the ultimate BOD is going to be equal to this L0 here, right? So this is how we can calculate the ultimate BOD from the measurement at a single day. Right, so here I have a short example. It's also the example from your textbook. Maybe we can use a minute or two to do this calculation here. Okay, you can use this time to read this question here. Okay, so let's try to solve this problem. Okay, so the problem says that if the three-day BOD of a wastewater is 75 mg per liter, and we know what is a BOD decay constant, which is a rate constant, k equal to 0.345, what is the ultimate BOD? It is quite straightforward, right? So we have the BOD3 is equal to L0. minus L0 exponential of minus kT, right? And that's going to be L0 1 minus exponential of what? So k we know is 0.345, right? And then the time, let's see, and the time is 3, right? And this equation turns out to be equal to 75 milligram per liter, right? So you can use your calculator and then find out is that what you can find out is L0 is equal to 75 divided by 0.645, which is equal to 116 milligram per liter, okay? So this is going to be the ultimate BOD. This is the total amount of the oxygen demand that's required by these organic waste. So you can use this simple equation here to calculate or to convert from one another. So the same thing, if you know what is the ultimate BOD, you can convert to see what is the BOD after three days or after five days, seven days, and so on. So here you may notice that in this set of equations here, so we can measure what is a BOD for different time, right? We know what is time. So the ultimate BOD is also a kind of a parameter that describes how polluted the water is. So this factor k here is quite important, right? So let's see, if you have a polluted water, but you have a very large k, a very large BOD decay rate, basically a very large decomposing rate, right? So what that means is that all of these organic waste are going to get consumed very fast and then it's easier for the water to resume back into the the clean state, right? And if this rate here is so slow then what that means is that the organics are going to just stay in that way, the concentration barely moves after a long time, right? So basically this K here is going to determine how quick this water can resume to the to the unpolluted state. Right. So there are several factors that can determine this K values. So basically this K, the rate constant determines how fast the oxygen is consumed. Right. So it depends on the following conditions. The first is the nature of the waste. Right. So if you have simple sugars, for example, the glucose, they're very simple. They can be directly. consumed by the microorganisms, then the K is going to be larger. But if you have very complex organics, right? So for example, the cell walls, what they call the lignins of the bowel mass, right? So it's very difficult for the microorganisms to use them, then the K value will be smaller, right? So also depends on the ability of the microorganisms to use the waste. It depends on the type. and also depends on the amount. So you have a lot of the microorganisms in the water, then they can consume these organics very fast, right? And also it depends, it will affect how fast the oxygen is consumed. So it also depends on temperature. So if you recall, when we discussed the reaction kinetics, right? So the k here, if you remember the equation, k is equal to k2 is equal to k1 multiplied by theta t2 minus t1, right? So what this equation means is that the rate constant is really dependent on temperature, right? So if you know the rate constant at one temperature, you can use this equation to calculate the constant at another temperature. And specifically for this class, the theta here, they have fixed values. So for example, typically the k in this equation here, let's just use this equation, and typically people will determine the rate constant at the temperature of 20 Celsius, okay? But we know that in the nature, we have different seasons, right? For example, a few weeks ago, the temperature in the Midwest is very low, right? So under that situation, the rate constant is going to be definitely different compared to the lab determined rate constant at 20 Celsius. So we can still use this equation, but we need to convert the rate constant to a new temperature. Right. So generally we will use this equation and then plug in the theta values. So, for example, if the theta is between 4 and 20 Celsius. then we can use a theta value of 1.135, okay? And if the temperature is between 20 and 30 Celsius, then we can use the theta value of 1.056, okay? So what that means is, let's say, under the lab condition, under 20 Celsius, we found that this k value here is one per day, okay? And then Let's try to determine what is the rate constant at 30 Celsius. Okay, so what we can do is basically k 30 Celsius is equal to 1 multiplied by theta 30 minus 20. Okay, so the theta here you can find that if it's between 20 and 30 then the theta is 1.056 to the power of 30 minus 20. Okay, so that's basically 1.056 to the power of 10, right? So basically you can use some simple methods. You can either use your calculator or use the Taylor series expansion, right? This term is close to around 10.10. It's around 1.106, okay? So this is how we can determine the rate constant at a different temperature. So here I have another example to let you practice. How does the temperature affect the calculation for the BOD? Let's use another minute to read through the problem and think about how to solve this. Okay, so let's try to look at this problem, right? So it's saying that a waste is being discharged into a river, and we're looking at this problem at a different temperature. It's not 20 Celsius anymore, it's 10 Celsius. So it's asking what fraction of the maximum oxygen consumption has occurred in four days if the BOD rate constant k has certain value, okay? So basically, the fraction is asking for what is the fraction of the maximum BOD has been consumed. It's basically it's asking for what is the BOD for divided by the ultimate BOD, right? The fraction of the maximum oxygen consumption, right? So there are already four days passing by, right? So basically it's BOD4 divide by L0. And we know that for BOD of any time, that can be solved by L0, 1 minus exponential of minus kT, right? Divide by L0. This is basically 1 minus exponential of minus k multiplied by 4, right? We know that the time is equal to 4 days. So now it's just a matter of... calculating what is the k value here. So we cannot directly use the 0.115 per day, mainly because this is determined at 20 Celsius. We're trying to look for 10 Celsius. So now we have to use this equation here. So kT is equal to k20 theta to the power of T minus 20. So basically is going to be equal to k20 theta to the power of 10 minus 20, right? So that turns out to be 0.115 multiplied by theta is 1.135 times minus 20, right? So if you see, if the temperature is between 4 and 20, then we should use 1.135, right? So this is how we can... plug in these values and then calculate and find out that the rate constant is 0.032 per day. Right, so this is the new rate constant at 10 Celsius. So now since we already have k10, we can just plug it in and then we can find out that this fraction here is 0.12. Okay, so basically this tells us how the temperature is going to affect how fast these organics are consumed, right? So let's think of two scenarios. Let's think of the condition under hot summer, right? If you go to a nearby river that has a lot of pollutants inside, then you can smell that unpleasant odor, right? and this is mainly because the organics are being consumed, right, and then it emits, for example, the hydrogen sulfide or ammonia, so you get these gas into the air and then you can smell it. But during the winter, normally we cannot see that, we cannot get that smell very frequently, and this is mainly because during the winter time the oxygen are consuming at a much slower speed, right. So the rate constant is much smaller at wintertime compared to the summertime. So that's why the BOD or the organic waste are consumed at a smaller speed during the wintertime. It's because of this temperature dependence. And because of that, for the wastewater treatment facility, they also need to consider this temperature dependence. So, for example, during the summer, they can let the... organic waste to stay for a shorter time before they discharge them into the river. But during the winter, they can keep these waste, they should keep these waste for a longer time, to basically remove the organic waste inside before you discharge them into the river, right? So this is how the temperature affects the BOD calculation. So we talked quite a lot about the theoretical BOD, how it depends on the time, depends on the temperature. but we haven't really talked about how we measure it, right? So it is, as we discussed earlier, it is quite straightforward. We just measure dissolved oxygen, okay? So we have a probe that can measure the oxygen concentration in the bottle, and then we just measure it under different time, right? But there are some processes that's going to make it a little bit more complex. So for example, the dilution is going to make it complex. right? And also sometimes we will introduce the control group to estimate what is the oxygen consumed by the microorganisms if there are no organics inside, right? So there are several steps and just pasting it here from your textbook. So the first step is to find this 300 milliliter BOD bottle, right? I would say this is a traditional method. So there are some newer methods that can use different volumes, or they can have very simple probes that directly give you the BOD. But this is, I would say, a standard method to measure the BOD. right? So the first step is to find this bottle here and then fill it with the water sample, all right? To fill it very full here so it doesn't have any gap of the air at the top. And then you should use a cap to enclose it, right? And then also what it requires is that the water has to be properly diluted. So you may have the question why we have to dilute it before we do the measurement. So it is again associated with the maximum dissolved oxygen concentration in water. So let's think about a scenario. Let's say if we have this bottle here directly from the discharge of a waste generating facility. So let's assume that the BOD is 100 milligram per liter. Okay, we get this full bottle of the wastewater and then we'll try to measure what is the actual BOD, what is the real BOD value here. So what we can do again is to introduce some microorganisms inside to let them decompose the organics inside. But what we also know is that the oxygen or the maximum oxygen concentration is just 8 to 8.5 milligram per liter. Okay, so what this means is that we have so much of this organic waste inside. and then the maximum dissolved oxygen or the difference of the dissolved oxygen is just going to be this amount, right? Let's say if we wait for one year or wait for several years, then the dissolved oxygen will always be zero, right? In the beginning, it's maybe around 8 to 8.5, but finally it's always going to be zero, mainly because there are no more oxygen in the water anymore. So that's why we have to dilute this. into an appropriate region so that we can measure what is the dissolved oxygen at the final state, right? So for example, if the final state of dissolved oxygen is one milligram per liter, so then we know that the oxygen demand is just eight minus one, right? It's seven milligram per liter or 8.5 minus one, right? Depends on what is the initial dissolved oxygen concentration. So that's why we have to dilute it. So normally the BOD after dilution should be between 2 and 6 mg per liter, right? So because of that, people also defined a parameter called the sample size or the dilution factor. So the dilution factor is just the volume of the undiluted sample divided by the volume of the diluted sample. So here we know that the volume of the diluted sample is always 300 milliliter, right? and then the volume of undiluted sample. For example, here again from the the waste here, let's say we just take five milliliters of the waste, right, and then put it into this bottle here. Then what we have is five divided by 300 is going to be equal to the sample size or the the dilution ratio. And we'll also call this as p. p is equal to 5 divided by 300. So it depends on what sample volume you really take from the wastewater. So here I have a second quiz to simply show you why this dilution factor matters. So let's say if the BOD in one bottle is five milligram per liter and it is diluted twice from the wastewater. Okay, so what is the BOD of the wastewater or the original wastewater? Okay, five more seconds or we'll stop here. Okay, so I think maybe there are some misconceptions in this problem. So the real sampled water is the wastewater, right? And then what we did is we diluted twice and then put it into the BOD bottle to measure what is a BOD. Okay, so because of that, this dilution factor or the sample size. basically the volume of undiluted sample divided by the volume of the diluted sample is going to be 0.5 or 50 percent. Okay, so then if you know what is the BOD for the diluted sample, then the undiluted sample is going to have a higher BOD. So you're going to multiply that by 2 to get 10 milligram per liter. All right, so let's try to do this quiz again. Okay, I'm just trying to see how many of you are following me since we're at the end of the class. Okay, five more seconds. Okay, the result is quite surprising. All right, so if you have any questions, you should just put me a chat or send me an email. Okay, the correct answer should be the third answer because we're measuring the diluted sample. All right, so it is diluted twice from the wastewater, so you need to multiply that by two to get 10 milligram per liter. Right, this is a very important concept. we're going to use it or carry this away through the entire semester. Okay. So if you don't know what is the reason why this is 10, let me know or let the TA know. All right. So you also get some homework problems regarding this concept. All right. So this is basically the first step to calculate what is the, to basically to dilute the sample to do the measurement. All right. So the second step is to add in the. basically adding the seed into the sample water and then also basically put on the stopper and then to keep it sealed. So we also introduce a blanks which is a control group for the measurement. Okay so the control group is just to offset the oxygen demand just by the microorganisms themselves. Okay, so this is a more accurate result. This is going to be a more accurate method to determine the BOD. And then the third step is just to measure the dissolved oxygen after several days. Normally we will use five days to determine the BOD5. Okay, so for a very simple scenario, let's say we measure the dissolved oxygen at day zero and then measure the dissolved oxygen at day five. So if the dissolved oxygen at day 0 is 8 mg per liter, and then during day 5 is 4 mg per liter, all right, and then further we know that p is equal to 0.1. So what that means is we just took 10% of the sampled water into that 300 ml bottle. Okay, so what we can calculate is that BOD 5 is going to be equal to 8 minus 4 divided by p. Okay, so that's going to be 4 divided by 0.1, which is equal to 40 milligram per liter. Okay, so this is how we can calculate the BOD from the experiments. And this is also considering the condition where we don't have a control. So if we have a control group or the blank group, then it's going to get a little bit complicated. And we're going to talk about that in our next class. Okay, and that's all for this class. Let me know if you have any questions.

So this class we're going to continue our discussion on the BOD or the biochemical oxygen demand. And also we may be able to introduce some new contents regarding the water quality in rivers. Right.

So so basically. So regarding the reading assignment, it's still going to be 9-2 and 9-3 of them. of your textbook.

So if we do a quick recap, so last class we learned about the final contents about the hydrology. So we discussed basically what is the Darcy velocity and also what is the actual or linear velocity. So we need to consider when the water flows by a certain area, the soil that the underground water is going by, it's not an open structure, right? So it's always, there's going to be rocks or sands.

So the actual velocity of the water is going to be higher compared to the Darcy's velocity, right? So we need to consider what is the porosity of the soil that's in the, basically for the water to flow by. So regarding that, I have a quick quiz.

So maybe we can do a... really quick calculation here. all right i'll give it 20 more seconds Okay, we'll stop here. So the final answer or the actual, the correct answer should be two millimeter per hour. Okay, so we mentioned that, so for the porosity, and when you divide the the Darcy velocity by the porosity to get the actual linear velocity, right?

So for example, this is the soil, basically the water is going this way, right? So the soil is composed of all the sands or the rocks here. So the water velocity that's going through the void is going to be higher compared to the average velocity going through this cross section here, right?

So what we do is we need to divide the Darcy velocity by the porosity, let's say eta here, all right? So if the porosity is 50 percent, then what that means is that the actual linear velocity is going to be twice of the Darcy's velocity. So this is how we consider this problem. So we also mentioned or discussed what is the major sources and types of the water pollutants. So in terms of the sources, we have the point sources and non-point sources.

Point sources are just like the sewage discharge. We directly put it. put them into the river and then for non-point source it can be fertilizers or some chemicals from the farmlands right so they infiltrate into the soil and then further get emitted into the river or into the ocean right so we also discussed what is oxygen demand and dissolved oxygen right so for the for these types of the water pollutants we mentioned that Actually, the most important one is the oxygen demanding materials. So we said when we see some water is heavily polluted and then there are no fish inside, it's not because those trash are toxic.

It's because those trash demands oxygen to decompose them into carbon dioxide and water that requires oxygen. And if there are not enough oxygen in the water, then the fish will not be able to survive in the water system. right?

So because of that, we need to discuss what is the oxygen demand and what is the dissolved oxygen, right? So the oxygen demand, you can treat it as the required oxygen, basically the demand for oxygen to oxidize those organics, right? So you can treat it as a reactant.

So basically, it requires a certain amount of oxygen. But the dissolved oxygen is basically the concentration of oxygen in water, right? So we said that the dissolved oxygen under normal condition, normal temperature, and normal pressure, the dissolved oxygen has to be below 8 or 8.5 milligram per liter, right?

This is the maximum amount of oxygen that's available in the water. So because of that, if there are too much of the organic waste in the water, then it's going to consume far more of the oxygen. or is going to have a far larger oxygen demand than the dissolved oxygen, then we may have some trouble.

So if we just pick up the discussion from there, so basically if the oxygen demand is larger than the dissolved oxygen, then we know for sure that there are no fish inside. And if the oxygen demand is smaller than dissolved oxygen, then maybe there are fish, but definitely there will be some lives. for example, microorganisms being survived in the water. So in terms of the oxygen demand, there are different types, right?

So last class, we also talked that there are theoretical oxygen demand, which is basically, if you know what is the composition of the wastewater, then you can calculate what is the oxygen demand. For example, if there are glucose, right? And then we know that the glucose is going to react with six number of oxygen. So this is the oxygen demanded by this chemical here right to form carbon dioxide, six number carbon dioxide and six number of water right. And then if we know what is the compost what is the concentration of the glucose in the water, then we can calculate what is the corresponding amount of oxygen that's required to oxidize these organics, right?

But we also mentioned that to determine this theoretical oxygen demand is almost impossible, mainly because there's no such a wastewater that you know what is the composition of the waste is in there and also you know what is the concentration of the waste. It's going to be very difficult for us to tell what is the composition of wastewater, okay? So it contains all different types of organics.

So basically the composition is so complex. that we cannot do this calculation to determine the theoretical oxygen demand. So that's why people come up with other methods, for example the chemical oxygen demand, to kind of find a tracer or a substitute for the oxygen demand.

For example, the chemical oxygen demand, what that means is if we collect a wastewater sample, what we do is we introduce some highly oxidizing species. For example, here the chromic acid. So the chromic acid can oxidize anything that's either organic or that's living organisms.

So anything that meets with this oxidizing species, they will get oxidized into carbon dioxide and water. So what we can do is we can measure, we can know what is the concentration of the chromic acid in the beginning. and know what is the concentration of the chromic acid at the end after we mix this solution here.

Then basically the amount, let's say, the change of the concentration of the chromic acid would correspond to the oxygen demand, right? But there is another problem associated with this, it's because there are many species in the nature that are not biodegradable. for example, the plastic bag, okay, plastics. So if we just throw plastic bottles into the water, although it's still polluting the environment, but these plastic bottles are not going to consume any oxygen, mainly because they're so stable, right? They're not being consumed by any of the microorganisms.

So because of that, it's not going to reduce the dissolved oxygen. So it's not affecting the oxygen demand. But what happens is that if you have these plastics in this solution here, and then you add in this. chromic acid, they're still getting oxidized into carbon dioxide and water. So by using this chemical oxygen demand, it is kind of like an overkill or like an overestimation of the oxygen demand because there are many things that theoretically are not reducing the dissolved oxygen in the wastewater.

So that's why people came up with this final approach, which is the biochemical oxygen demand. or the BOD. So I think you guys did the BOD lab last week, right?

Hopefully you got some some basic ideas about the BOD, but we're going to talk more about how to measure these BOD later this class, okay? So what this means is that we're trying to replicate what's happening in the nature. So after we collect these wastewater bottles, wastewater samples here, So instead of introducing these highly oxidizing species, we're introducing the microorganisms, which are bacteria, right?

All different types of bacteria as an assay or as a seed into the water sample. Okay. So theoretically, if there are waste, organic waste inside, right?

So these seeds are going to consume oxygen. So similar to what's happening in the... river or the lake, right? And then what we can do is we can just measure the oxygen or the DO, dissolved oxygen, at the beginning when we collect the sample and at the end, let's say after a few days, after five days or seven days, right?

So we have the initial DO and the final DO. So the BOD is just going to be equal to the DO initial minus DO final, right? So theoretically, if there is no dilution of this sample here, so this is very simple.

You just measure the oxygen concentration before and after to determine what is a BOD. But for a lot of the times, the BOD for the sample is too high. So there's no way we can get this accurately. So that's why the equation will also get quite complex if you consider the dilution and also if you're adding controls.

for these experiments. So the advantage for this method is that it reflects biodegradability. So we know that these seed or these microorganisms, they're not so dumb that they're going to decompose the plastics, right?

They don't have that capability to do so. So this reflects what's happening in the nature, right? And also the bile acids or the seed we use, also it's quite similar. They're quite similar to the natural water, right? So in this way, the BOD is a more appropriate way to describe what is the level of pollutants, what is the amount of organic pollutants that's in the water, right?

So we introduce three types of oxygen demand. We have the theoretical oxygen demand, the chemical oxygen demand, and the... biochemical oxygen demand. So what are their relationships? So first, regarding the theoretical oxygen demand and chemical oxygen demand, what we can know is that under rare circumstances, these two are going to be equal to each other.

So for example, just like the case or the example we gave earlier, if we know that the wastewater is composed of glucose, which is very simple sugar, So if it's composed of simple organics, right, then these two are going to be equal to each other, right? So first thing is that these sugars are very easy to decompose and also we know exactly what is the composition of the wastewater. But if the wastewater is getting very complicated, which is the normal case, let's say from a wastewater facility, wastewater treatment facility, then the wastewater composition is so complex that there's no way we can do this theoretical oxygen demand calculation, then these two are definitely not going to be equal to each other, right? So under rare circumstances, these two are going to be equal to each other.

And in terms of the BOD, the BOD never equals theoretical oxygen demand or chemical oxygen demand. So typically the BOD is smaller than these two. So the reasons are two ways, okay?

So if you consider The theoretical oxygen demand, or the COD, what it assumes is that all the carbon are getting consumed into carbon dioxide. But for the microorganisms, it's not the same thing. So we can think of human, right? So we eat a lot of things every day.

We eat a lot of organic carbon, right? For example, apple or cake. So not all of them are converted into carbon dioxide. So some of them are being converted into, let's say, muscle, right?

And some of them are converted into, let's say, fat. So they can basically convert it into a part of their body. So in that case, not all of the carbon are getting converted into carbon dioxide.

And because of that, the BOD is going to be smaller than COD or theoretical oxygen demand. And at the same time, not all of the carbon are getting converted into part of their body, mainly because these organisms are going to generate... waste, right? These wastes are not pure carbon dioxide.

So there can be some other soluble organic matter that are just generated by these microorganisms, right? So in this way, the BOD should be smaller than the theoretical oxygen demand or the COD. But because the BOD represents what's happening in the nature, so the BOD is still the most widely used. parameter to describe how polluted this water is. So now how do we measure the BOD?

So a simple way, as I have introduced earlier, is just to measure the dissolved oxygen concentration, the DO, before and after we add in these microorganisms. So basically we measure the dissolved oxygen as a function of time. right?

So we will also add in the microorganisms into the body, into the bottle, right? So that they can consume the organic matter inside. So now the thing is, if we just measure these dissolved oxygen as a function of time, what people are really interested in is the total amount of BOD, right?

So for example if you can measure what we do in our lab is we either measure five days or or 10 days or seven days or 10 days right but you don't know when does it reach to the final bod right so we name it bod5 bod7 and bod10 but what we are really caring about is the ultimate bod So this ultimate BOD can happen, let's say, after several days, after one month, or after one year. So it's not reasonable to do these measurements like day by day after the entire year, right? So that's why we need to understand what is happening in the kinetics. So whether we can just use the BOD, let's say BOD5, to calculate what is the ultimate BOD. So is there a way to do so, right?

So then we need to understand what is a kinetic scene. So as we have introduced earlier in this class, we assume that most of the reactions are first order reaction. So it also applies for these organics. OK, so what happens is, let's say if you if you plot the concentration of the organics in the wastewater, let's say this is L.

So. what you know is that at the time equal to zero, when you take out this wastewater sample out, then organic concentration is going to be the highest, right? And then after some time, these microorganisms inside are going to compose these organics, right? Then its concentration is going to decrease, right?

And then we assume that it's going to decrease according to the first-order reaction. So what that means is that the dL is going to be negative k multiplied by L. So k here is the rate constant, right, reaction rate constant. So because of that, we can calculate that the concentration of these organics are just going to be L0 multiplied by exponential of minus kT, right, L equal to L0.

exponential minus kT. So basically the concentration of these organics are going to decrease exponentially. All right.

So what about the BOD, right? So this is the concentration of the organics. What about the BOD? So the BOD is quite different from the concentration of organics, right? So basically the definition of the BOD is the oxygen that's consumed, right?

So ultimate BOD is the final amount of oxygen that's being consumed. So that's why the BOD at the ultimate condition that's also going to be the highest, right? So if we just draw the L here again, right, this is the concentration of the organics.

Then the BOD is going to be reverse of it. So just think about it. At the beginning, when we introduce the microorganisms, the oxygen that's being consumed is just zero, right? We haven't consumed any oxygen yet.

But as time goes on, first the Oxygen are being consumed at a fast rate, and then it's going to get slower and slower, mainly because the organics are being consumed, right? And when all the organics are gone, they reach to the highest BOD value, right? So this is the BOD, right? And this is the organics, okay?

So basically, the BOD is going to follow a trend that's reverse of this concentration of the organics, right? So we can write it out as BOD equal to L0 minus L0 exponential of minus kT, right? So you can think about this basically.

What this means is that when T is equal to zero, then BOD is equal to zero, right? When T is equal to infinity, then BOD is equal to L0, right? So this is going to basically the BOD and the organics, they're reciprocal to each other, right? So if they add up together, they're going to be equal to the ultimate BOD, right? So because of this, then we know that now we have the kinetics of the BOD.

And normally people will also name this as BODT. as a subscript because this is valid for any time, right? So for example, if you know what is a BOD at three days, right?

And then you can just plug in this value, L0 minus L0 exponential of minus k multiplied by three, right? So let's say if you know what is this value, let's say this is equal to a, right? And if you further know what is a k here, then you can calculate.

what is the L0. So in this way, even if you just did the BOD measurement for five days, seven days, or ten days, you can calculate what is the ultimate BOD. You don't need to wait for an entire year to determine the BOD value of the wastewater. So here I'm just showing the figure on your textbook. So basically it's talking about the same thing.

Again, the concentration of the organics, the L here, is going to reduce. It's very rapid at the beginning, mainly because the organics concentration are the highest. And also, you just introduced these microorganisms into the water, right? So it's going to reduce very rapidly and then finally go to zero because there are no more organics in the water anymore.

And then in terms of the BOD, it's going to increase. from zero right at the beginning there is no oxygen consumption and then increase very rapidly at the beginning and then at the end the BOD is going to stop since there's no more organics it's going to it cannot consume any more of the oxygen from the water right and then regarding the quantitative behavior of the BOD you can see that can be calculated by L0 minus L, right? It's going to be just L0 minus L0 exponential of minus kT, right? And then the ultimate BOD is going to be equal to this L0 here, right? So this is how we can calculate the ultimate BOD from the measurement at a single day.

Right, so here I have a short example. It's also the example from your textbook. Maybe we can use a minute or two to do this calculation here. Okay, you can use this time to read this question here.

Okay, so let's try to solve this problem. Okay, so the problem says that if the three-day BOD of a wastewater is 75 mg per liter, and we know what is a BOD decay constant, which is a rate constant, k equal to 0.345, what is the ultimate BOD? It is quite straightforward, right? So we have the BOD3 is equal to L0.

minus L0 exponential of minus kT, right? And that's going to be L0 1 minus exponential of what? So k we know is 0.345, right? And then the time, let's see, and the time is 3, right?

And this equation turns out to be equal to 75 milligram per liter, right? So you can use your calculator and then find out is that what you can find out is L0 is equal to 75 divided by 0.645, which is equal to 116 milligram per liter, okay? So this is going to be the ultimate BOD. This is the total amount of the oxygen demand that's required by these organic waste. So you can use this simple equation here to calculate or to convert from one another.

So the same thing, if you know what is the ultimate BOD, you can convert to see what is the BOD after three days or after five days, seven days, and so on. So here you may notice that in this set of equations here, so we can measure what is a BOD for different time, right? We know what is time.

So the ultimate BOD is also a kind of a parameter that describes how polluted the water is. So this factor k here is quite important, right? So let's see, if you have a polluted water, but you have a very large k, a very large BOD decay rate, basically a very large decomposing rate, right?

So what that means is that all of these organic waste are going to get consumed very fast and then it's easier for the water to resume back into the the clean state, right? And if this rate here is so slow then what that means is that the organics are going to just stay in that way, the concentration barely moves after a long time, right? So basically this K here is going to determine how quick this water can resume to the to the unpolluted state. Right.

So there are several factors that can determine this K values. So basically this K, the rate constant determines how fast the oxygen is consumed. Right.

So it depends on the following conditions. The first is the nature of the waste. Right. So if you have simple sugars, for example, the glucose, they're very simple. They can be directly.

consumed by the microorganisms, then the K is going to be larger. But if you have very complex organics, right? So for example, the cell walls, what they call the lignins of the bowel mass, right? So it's very difficult for the microorganisms to use them, then the K value will be smaller, right? So also depends on the ability of the microorganisms to use the waste.

It depends on the type. and also depends on the amount. So you have a lot of the microorganisms in the water, then they can consume these organics very fast, right?

And also it depends, it will affect how fast the oxygen is consumed. So it also depends on temperature. So if you recall, when we discussed the reaction kinetics, right?

So the k here, if you remember the equation, k is equal to k2 is equal to k1 multiplied by theta t2 minus t1, right? So what this equation means is that the rate constant is really dependent on temperature, right? So if you know the rate constant at one temperature, you can use this equation to calculate the constant at another temperature.

And specifically for this class, the theta here, they have fixed values. So for example, typically the k in this equation here, let's just use this equation, and typically people will determine the rate constant at the temperature of 20 Celsius, okay? But we know that in the nature, we have different seasons, right? For example, a few weeks ago, the temperature in the Midwest is very low, right?

So under that situation, the rate constant is going to be definitely different compared to the lab determined rate constant at 20 Celsius. So we can still use this equation, but we need to convert the rate constant to a new temperature. Right. So generally we will use this equation and then plug in the theta values. So, for example, if the theta is between 4 and 20 Celsius.

then we can use a theta value of 1.135, okay? And if the temperature is between 20 and 30 Celsius, then we can use the theta value of 1.056, okay? So what that means is, let's say, under the lab condition, under 20 Celsius, we found that this k value here is one per day, okay? And then Let's try to determine what is the rate constant at 30 Celsius.

Okay, so what we can do is basically k 30 Celsius is equal to 1 multiplied by theta 30 minus 20. Okay, so the theta here you can find that if it's between 20 and 30 then the theta is 1.056 to the power of 30 minus 20. Okay, so that's basically 1.056 to the power of 10, right? So basically you can use some simple methods. You can either use your calculator or use the Taylor series expansion, right? This term is close to around 10.10. It's around 1.106, okay?

So this is how we can determine the rate constant at a different temperature. So here I have another example to let you practice. How does the temperature affect the calculation for the BOD? Let's use another minute to read through the problem and think about how to solve this. Okay, so let's try to look at this problem, right?

So it's saying that a waste is being discharged into a river, and we're looking at this problem at a different temperature. It's not 20 Celsius anymore, it's 10 Celsius. So it's asking what fraction of the maximum oxygen consumption has occurred in four days if the BOD rate constant k has certain value, okay? So basically, the fraction is asking for what is the fraction of the maximum BOD has been consumed. It's basically it's asking for what is the BOD for divided by the ultimate BOD, right?

The fraction of the maximum oxygen consumption, right? So there are already four days passing by, right? So basically it's BOD4 divide by L0. And we know that for BOD of any time, that can be solved by L0, 1 minus exponential of minus kT, right? Divide by L0.

This is basically 1 minus exponential of minus k multiplied by 4, right? We know that the time is equal to 4 days. So now it's just a matter of...

calculating what is the k value here. So we cannot directly use the 0.115 per day, mainly because this is determined at 20 Celsius. We're trying to look for 10 Celsius.

So now we have to use this equation here. So kT is equal to k20 theta to the power of T minus 20. So basically is going to be equal to k20 theta to the power of 10 minus 20, right? So that turns out to be 0.115 multiplied by theta is 1.135 times minus 20, right?

So if you see, if the temperature is between 4 and 20, then we should use 1.135, right? So this is how we can... plug in these values and then calculate and find out that the rate constant is 0.032 per day.

Right, so this is the new rate constant at 10 Celsius. So now since we already have k10, we can just plug it in and then we can find out that this fraction here is 0.12. Okay, so basically this tells us how the temperature is going to affect how fast these organics are consumed, right? So let's think of two scenarios.

Let's think of the condition under hot summer, right? If you go to a nearby river that has a lot of pollutants inside, then you can smell that unpleasant odor, right? and this is mainly because the organics are being consumed, right, and then it emits, for example, the hydrogen sulfide or ammonia, so you get these gas into the air and then you can smell it. But during the winter, normally we cannot see that, we cannot get that smell very frequently, and this is mainly because during the winter time the oxygen are consuming at a much slower speed, right.

So the rate constant is much smaller at wintertime compared to the summertime. So that's why the BOD or the organic waste are consumed at a smaller speed during the wintertime. It's because of this temperature dependence.

And because of that, for the wastewater treatment facility, they also need to consider this temperature dependence. So, for example, during the summer, they can let the... organic waste to stay for a shorter time before they discharge them into the river.

But during the winter, they can keep these waste, they should keep these waste for a longer time, to basically remove the organic waste inside before you discharge them into the river, right? So this is how the temperature affects the BOD calculation. So we talked quite a lot about the theoretical BOD, how it depends on the time, depends on the temperature.

but we haven't really talked about how we measure it, right? So it is, as we discussed earlier, it is quite straightforward. We just measure dissolved oxygen, okay? So we have a probe that can measure the oxygen concentration in the bottle, and then we just measure it under different time, right? But there are some processes that's going to make it a little bit more complex.

So for example, the dilution is going to make it complex. right? And also sometimes we will introduce the control group to estimate what is the oxygen consumed by the microorganisms if there are no organics inside, right?

So there are several steps and just pasting it here from your textbook. So the first step is to find this 300 milliliter BOD bottle, right? I would say this is a traditional method.

So there are some newer methods that can use different volumes, or they can have very simple probes that directly give you the BOD. But this is, I would say, a standard method to measure the BOD. right?

So the first step is to find this bottle here and then fill it with the water sample, all right? To fill it very full here so it doesn't have any gap of the air at the top. And then you should use a cap to enclose it, right?

And then also what it requires is that the water has to be properly diluted. So you may have the question why we have to dilute it before we do the measurement. So it is again associated with the maximum dissolved oxygen concentration in water.

So let's think about a scenario. Let's say if we have this bottle here directly from the discharge of a waste generating facility. So let's assume that the BOD is 100 milligram per liter. Okay, we get this full bottle of the wastewater and then we'll try to measure what is the actual BOD, what is the real BOD value here. So what we can do again is to introduce some microorganisms inside to let them decompose the organics inside.

But what we also know is that the oxygen or the maximum oxygen concentration is just 8 to 8.5 milligram per liter. Okay, so what this means is that we have so much of this organic waste inside. and then the maximum dissolved oxygen or the difference of the dissolved oxygen is just going to be this amount, right?

Let's say if we wait for one year or wait for several years, then the dissolved oxygen will always be zero, right? In the beginning, it's maybe around 8 to 8.5, but finally it's always going to be zero, mainly because there are no more oxygen in the water anymore. So that's why we have to dilute this.

into an appropriate region so that we can measure what is the dissolved oxygen at the final state, right? So for example, if the final state of dissolved oxygen is one milligram per liter, so then we know that the oxygen demand is just eight minus one, right? It's seven milligram per liter or 8.5 minus one, right? Depends on what is the initial dissolved oxygen concentration.

So that's why we have to dilute it. So normally the BOD after dilution should be between 2 and 6 mg per liter, right? So because of that, people also defined a parameter called the sample size or the dilution factor. So the dilution factor is just the volume of the undiluted sample divided by the volume of the diluted sample. So here we know that the volume of the diluted sample is always 300 milliliter, right?

and then the volume of undiluted sample. For example, here again from the the waste here, let's say we just take five milliliters of the waste, right, and then put it into this bottle here. Then what we have is five divided by 300 is going to be equal to the sample size or the the dilution ratio. And we'll also call this as p. p is equal to 5 divided by 300. So it depends on what sample volume you really take from the wastewater. So here I have a second quiz to simply show you why this dilution factor matters.

So let's say if the BOD in one bottle is five milligram per liter and it is diluted twice from the wastewater. Okay, so what is the BOD of the wastewater or the original wastewater? Okay, five more seconds or we'll stop here.

Okay, so I think maybe there are some misconceptions in this problem. So the real sampled water is the wastewater, right? And then what we did is we diluted twice and then put it into the BOD bottle to measure what is a BOD.

Okay, so because of that, this dilution factor or the sample size. basically the volume of undiluted sample divided by the volume of the diluted sample is going to be 0.5 or 50 percent. Okay, so then if you know what is the BOD for the diluted sample, then the undiluted sample is going to have a higher BOD.

So you're going to multiply that by 2 to get 10 milligram per liter. All right, so let's try to do this quiz again. Okay, I'm just trying to see how many of you are following me since we're at the end of the class.

Okay, five more seconds. Okay, the result is quite surprising. All right, so if you have any questions, you should just put me a chat or send me an email. Okay, the correct answer should be the third answer because we're measuring the diluted sample.

All right, so it is diluted twice from the wastewater, so you need to multiply that by two to get 10 milligram per liter. Right, this is a very important concept. we're going to use it or carry this away through the entire semester.

Okay. So if you don't know what is the reason why this is 10, let me know or let the TA know. All right. So you also get some homework problems regarding this concept. All right.

So this is basically the first step to calculate what is the, to basically to dilute the sample to do the measurement. All right. So the second step is to add in the.

basically adding the seed into the sample water and then also basically put on the stopper and then to keep it sealed. So we also introduce a blanks which is a control group for the measurement. Okay so the control group is just to offset the oxygen demand just by the microorganisms themselves. Okay, so this is a more accurate result. This is going to be a more accurate method to determine the BOD.

And then the third step is just to measure the dissolved oxygen after several days. Normally we will use five days to determine the BOD5. Okay, so for a very simple scenario, let's say we measure the dissolved oxygen at day zero and then measure the dissolved oxygen at day five. So if the dissolved oxygen at day 0 is 8 mg per liter, and then during day 5 is 4 mg per liter, all right, and then further we know that p is equal to 0.1.

So what that means is we just took 10% of the sampled water into that 300 ml bottle. Okay, so what we can calculate is that BOD 5 is going to be equal to 8 minus 4 divided by p. Okay, so that's going to be 4 divided by 0.1, which is equal to 40 milligram per liter.

Okay, so this is how we can calculate the BOD from the experiments. And this is also considering the condition where we don't have a control. So if we have a control group or the blank group, then it's going to get a little bit complicated. And we're going to talk about that in our next class. Okay, and that's all for this class.

Let me know if you have any questions.

Transcript for:Understanding Biochemical Oxygen Demand (BOD)

Transcript for:
Understanding Biochemical Oxygen Demand (BOD)