AboutDavid L. Russell, PE Expertise I`m a Chemical,Civil and Environmental Engineer and have a number of projects in all phases of the environment.
I have worked in the chemical industry and am active in professional societies, and am currently on an industrial wastes committee for the Water Environment Federation, and have taught courses in remediation in the US and abroad.
I have written one book on Remediation of petroleum Contaminated Sites, and have a second book on PRACTICAL WASTEWATER TREATMENT to be published by John Wiley in September, 2006. I've also written over 30 articles on various elements of environmental problems and cleanup. Most Recently, I have addressed a NATO Scientific and Techical Conference on Ecoterrorism, and have worked with the same group on remediation of sites contaminated with Chemical Warfare Agent materials and othe materials as well. . I can answer q`s about Chemical and Environmental Engineering, land development, air pollution, water pollution, soil and water cleanup, combustion, international environmental problems, industrial processes chemical processes. Civil and Environmental and Chemical Engineering. Overall, I have over 35 years of experience in this area. Note: I do not answer homework questions
Experience I love work in the third world and developing areas
because it is challenging and one can get a sense of accomplishment.
Question Dear Mr. Russell, Do you have any sense of the carbon emissions from treatment of municipal wastewater? Of course different treatment technologies will have different indirect carbon footprints (due to their relative use of electricity, etc), but I'm talking about the direct emissions from destruction of BOD, say from a typical influent value of 220 mg/L BOD5 down to a typical final effluent discharge value of 10 mg/L BOD5. What carbon emissions are likely to come from that process? I realize also that carbon footprint would be different from aerobic treatment than from anaerobic digestion, and whether the resulting biogas is burned, etc. Please just comment regarding whatever you can share about this topic, which will undoubtedly gain increasing importance. Thanks!
Answer There are several studies on this topic. Some of the best information is probably from NYSERDA (NYSERDA.ORG). However that study deals with the direct energy consumption from POTW's.
Let's consider your problem. According to a couple of sources, the average composition of wastewater is in it's simplest formula: C10H19O3N Now the efficiency of the conversion should be considered as well, and cell yield will be on the order of 0.12 lb of cells per lb of BOD destroyed. That means that the reaction will go to completion if you supply a lot of oxygen to take it to CO2, H2O & NO3. So it's a lot more straight forward than it looks.
If I've done the math correctly, and that maybe a big IF.. the yield is 10CO2+8.5 H20 +N03, and the O2 required is 12.125 O2 to complete the reaction. Now the CO2 yield is 10 CO2 less the about 0.12 that goes into the cell formation. So you have an effective CO2 yield of about 8.8 mol of CO2 per Lb of waste destroyed. Considering that this same set of reactions takes place in a BOD bottle, I believe that what you are looking at is a straight ratio. Don't forget that we are measuring the BOD by the DO drop. I believe it's fair to say that in the bottle we use 12.125 O2 to get about 8.8 CO2 + the amount incorporated into the cells. That's what happens in the plant as well. The difference between the bottle and the plant is that you have an massive XS of O2 and turbulence, so it's a stripper which removes CO2. (I'm ignoring the carbonate chemistry for the time being and the pH effects).
Given your premesis of 220mg/l BOD down to 10 mg/l BOD, that's basically about 210 mg/l of BOD reduced. 210 mg of Oxygen consumed or 210/32 = 6.5625 m mol of O2. divide again by 12.125 and multiply by 8.8 gives you 4.763 mmol of CO2 for the reaction.
Again, If Ive done the math right, that should be somewhere close to the right answer.
