NC State DVDs - error I think in Wastewater Systems

[elarocque]

i think i found an error on slide 124 (pg. 62) of the NC State DVD: Wastewater Systems

they use BOD_ult = 5.8 mg/L to solve for D_t@1 day downstream...this 5.8 mg/L occurs at the same point D_t@1 day occurs, which is wrong i think

bottom line: Deficit at downstream point corresponding to travel time t, D_t = fn(BOD_ult at point upstream defining time zero), correct?

if this is an error, than my frustrations with this DVD Wastewater program are justified...if i'm wrong, no sweat, i'll learn from it

thx

-Eric

 

[Dleg]

I'm not exactly sure what your question is, but I wonder if you have misunderstood what BOD "ultimate" means. BODu is simply the maximum BOD reading that would occur if you ran the BOD test forever on the wastewater sample (with corrections to account for nitrification, etc.). BODu is not a measure of anything in the stream, and it is not a measure of oxygen deficit, like the term Dt is. BODu is usually something you will need to calculate based on BOD5, using something like equation 25.29. Or, the problem statement will give you BODu (which is a cheat - you would never get BODu from any lab test in real life - you would need to estimate it as in 25.29).

So remember: - BOD is a measure of the wastewater characteristics, not the stream.

- Dt is a measure of the oxygen deficit it is not a measure of the dissolved oxygen in the stream, nor the BOD of the stream/effluent combination - which means you need to subtract Dt from the natural dissolved oxygen of the stream (based on temperature - from a chart in the appendices) to get the true dissolved oxygen caused by the impact of the wastewater.

 

[elarocque]

thanks for the two replies - regarding point of wastage, i suppose Lindeburg just tries to keep us honest by giving an example problem with wastage directly from reactor and showing with Figures, wastage prior to secondary clarification

i think i understand BOD is a WW characteristic, oxygen deficit a receiving stream/river characteristic, etc...my confusion is with the BOD_ult value used by the NCS solution - i think they should have used 6.7 mg/L, not 5.8 mg/L

when computing D_t=1 day downstream, they used the BOD_ult at the point 1 day downstream (incorrectly, i think) instead of BOD_ult at point t zero (upstream), gaining BOD_ult at 1 day downstream with 1st order decay of BOD_ult at point t zero

thanks for your continued help...much appreciated