I agree with Nilay. You need to know the starting concentration of biomass (Co). Presume that the growth is in exponential phase, the biomass productivity rate (g/l/unit_of_time) is Co(exp(u*1) - 1)/(one unit_of_time)
You can't measure rate, without measuring growth in some way (as pointed out by Binh). You need to provide some details: What kind of biomass? How are you measuring growth rates?
first you have to define what do you mean by biomass, than set up a protocol for measuring, than get the reference point and from that you can go smoothly ahead