How can you measure the BOD in real time if it takes 5 days to measure it by definition? The questions does not make sense.
However, you can measure the COD in in real time and compare it with your BOD measurements and thus obtain a reliable BOD/COD ratio.
This will take time to establish a correlation factor and then run the COD and multiply it by factor to determine BOD
There isn't a way to measure BOD or COD in real time, but you can purchase equipment for continuous on-line measurement of Total Organic Carbon (TOC). It is possible to build a correlation factor between TOC and COD or BOD. For more information, check out this handy powerpoint presentation on the topic. http://www.weat.org/Presentations/2014lab_Jackson.pdf
There is no requirement but depending on circumstance it may be beneficial to your operation. A lot depends on your operational requirements, how the plant is currently operating, and what your operation budget is and funds available.
There are methods that would allow you to develop s correlation between TOC, COD, and ORP. Each takes time to make that viable. Each one has s cost in the development for equipment, reagents, maintenance, and labor.
Let me know if you would like to discuss further.