Does anyone know about any Instrument for BoD testing of waste water at field level? Need to find out the exact BoD of waste water instantly.

BOD, is by definition the amount of oxygen demand over a certain time period (BOD5 is incubated for 5 days) so I can't see that you'll be able to get the exact BOD instantly.

However w-water are advertising BOD analysis within 3-4 hours with their CheckLight Early BOD kit: https://www.w-water.com/uploaded_files/documents/checklight_bod_datasheet_u2727.pdf

I understand the BOD BART system can analyse BOD in 20 hours: https://www.dbi.ca/books/PDFs/BOD-Paper.pdf

And headspace BOD can get values similar to the BOD5 in 2-3 days: https://www.engr.psu.edu/ce/enve/logan/publications/2000-Logan-etal-HBOD.PDF

I have no experience of this, but COD can be measured more rapidly and may be used to estimate an equivalent BOD: https://www.aquadiagnostic.com/downloads/brochures/L100.pdf

Alternatively you could perhaps measure the causative agents of the demand more rapidly and in the field - for example TOC: https://www.geinstruments.com/products-and-services/toc-analyzers-and-sensors/5310-c-portable https://www.hach.com/anatel-a-1000-xp-portable-toc-analyzer/product?id=7640480104

If you just need to know the instantaneous dissolved oxygen concentration (DO), I have had good experience with Hach's LDO probes, otherwise their BOD track apparatus could be more appropriate: https://www.hach.com/bodguide-measurement