Question

3) You need to fix your F/M ratio at 0.3 days1. You have a 1MGD plant, running at 450mg/L BOD out of the primary clarifier. Your secondary unit process is 4000m? What MLSS do you need to return in the mixed liquor? 4) For the question above, what mass of return activated sludge (RAS) per hour do you need to recycle to meet the MLSS, using the "standard" wastewater assumption regarding biomass in RAS?

I don't need the work for the first question. For that I got 1420.45 mg/L. Please answer problem 4. Thanks!

Answer 1

Given:

F/ M = 0.3 day-1

MLSS = 1420.45 mg / L

BOD = 450 mg/ L

Secondary process unit volume = 4000 m^3

Solids leaving as WAS/ RAS = WASSS mg/l x WAS flow MGD x 8.34

Solids leaving as WAS/ RAS = 1420.45 mg/l x 0.3 day^-1 x 8.34

Solids leaving as WAS/ RAS = 1420.45 mg/l x 0.3 day^-1 x 8.34

Solids leaving as WAS/ RAS = 3,553.97 mg/ l . day

Solids leaving as WAS/ RAS = 3,553.97 mg/ l . day x 1 hour / 24 hour = 148.08 mg / l . hour