Try this, it is the best that I can find. Hope it helps
There are 1000mg in 1g, 1000g in 1kg, and 1000kg in 1t.
There are 1000L in 1m³.
There are 3600 seconds in an hour and 24 hours in a day (if you run continuously)
Your concentration in mg/L is the same as your grams/m³.
To get the grams per second, multiply by the m³/s, to get the weight in an hour multiply by 3600, to get the weight in a day multiply by 24. (If you run continuously).
For example, if you have a concentration of 76mg/L urea, and a flow rate of 0.145m³/s, the calculation is
76g x 0.145m³ x 3600s x 24h = 952128g, or 952kg, or 0.95 tonnes of urea per day.
I dont know what is COD stand for, but in my opinion Kg/day is a mass flow unit while mg/litre is a density unit so it would not be possible for us to do a conversion between them.