Q1) 10,000 gallons per day of wastewater enters a 1500 gallon activated sludge tank.  The raw wastewater has a SS = 200 ppm and a BOD = 230 ppm.  The MLSS needs to be 2800 ppm and the effluent SS should be less than 10 ppm.  The total wasted sludge has a Q of 5 GPD with a SS concentration of 6000 ppm.

  1. Calculate the recycle flow, Qr, required. (8710 GPD)
  2. Calculate the F/M ratio in lbs BOD/lbs SS – day (0.55  lb BOD/lb SS-day)
  3. Calculate the BOD Loading in lbs BOD/day – 1000 ft3(95.6)
  1. Based on these calculations, is the activated sludge tank over designed or under designed?  What could you do to remedy this situation?

Q2)   Colloidal gold has been released into the water supply from an electronics manufacturing plant.  These particles are 0.1 mm in size with a specific gravity of 19.3.  Your water treatment plant has 3 clarifiers each with a volume of 0.3 million gallons and a height of 6 ft. The total flow going into all 3 clarifiers is 5 MGD.

  1. Will these particles settle in this clarifier or will we be drinking gold? Assume that they are not flocculated.  Defend your answer quantitatively.
  1. What is the minimum detention time needed to settle these gold particles over a height of 6 ft? (214 days)
  1. If you cannot remove these gold particles, should you add a sand filter or a reverse osmosis unit?  Give two reasons for your answer and state your assumptions.
  1. If you decide to flocculate these particles, what is the minimum diameter of a gold particle (with a specific gravity of 19.3) that can be removed in this clarifier? (3.4 mm)

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