4) a) Waste activated sludge (WAS) is removed from the reactor at a flow rate of 54 m3/d. Sludge concentration (VSS) in the reactor is 6000 mg/l. With this WAS flow rate the sludge concentration remains constant over time. Influent water contains 500 mg/l of COD and effluent 50 mg/l of COD. Influent flow rate is 100 m3/h. Calculate the biomass yield Yobs in kgVSS/kgCOD. b) Another wastewater contains 200 mg/l of COD and 40 mg/l of total nitrogen. Effluent concentrations are 30 mg/l and 0 mg/l respectively. Calculate how much heterotrophs and how much autotrophs (nitrifying bacteria) there is in the process when yield factors are Yobs,H = 0,5 kgCODCr/kgCODCr and Yobs,A = 0,2 kgCODCr/kgN. Influent flow rate is 1 m3/d. Calculate also what is the percentage of autotrophs of the total bacteria population. The process is at steady state. It can be asssumed that all the influent nitrogen is ammonium.