Wastewater Treatment 203BOD Suspended solids Phosphorus
Waste (ma) (SS, mgw (ma)Chemical industry 40,OOO^0 0
Pickle cannery 0 300 1
Fertilizer mfg 300 300 200Domestic 200 200 109.8 The success of an activated sludge system depends on settling the solids in
the final settling tank. If the sludge in a system started to bulk and not settle very
well, and the SS concentration of the return activated sludge dropped from 10,000 to
4,000 mgL,
a. What will this do to the MLSS?
b. What will this in turn do to the BOD removal, and why?
9.9 Assume that the volume of settled sludge in a 1-L cylinder after 30mm is
300 mL both before and during a bulking problem, and the SVI is 100 and 250, respec-
tively. What is the MLSS before and after the bulking problem? Does this agree with
your answer to Problem 9.8?
9.10 A percolation test shows that the measured water level drops 5 in. in 30 min.
What size percolation field would you need for a two-bedroom house? What size would
you need if the water level dropped only 0.5 in.?
9.11 A 5-mgd conventional activated sludge plant has an influent BOD5 of
200 mgL. The primary clarifier removes 30% of the BOD. The plant’s three aeration
tanks are 20 x 20 x 10 ft3. What MLSS are necessary to attain 90% BOD removal?
9.12 An aeration system with a hydraulic retention time of 2.5 h receives a flow
of 0.2 mgd at a BOD of 150 mgL. The SS in the aeration tank are 4000 ma. The
effluent BOD is 20 mg/L and the effluent SS are 30 mgL. Calculate the F/M ratio for
this system.
9.13 The MLSS in an aeration tank are 4000mgL. The flow from the primary
settling tank is 0.2m3/s and the return sludge flow is 0.1 m3/s. What must the return
sludge SS concentration be to maintain the 4000 mgL MLSS, assuming no net growth
of biomass?
9.14 An industrial wastewater is composed of SS that are small beads of 0.1 mm
diameter with a specific gravity of 2.65. Assume p = 1.31 cP.
a. For how long would the wastewater have to remain in a settling tank 4 m deep
in order to become clear?
b. If a false floor is put in the settling tank so that it was now only 2m deep,
what fraction of the SS would be removed during the same time as calculated
in (a)?
c. What is the overflow rate for this tank?
9.15 A primary clarifier (settling tank) is 80 ft long, 30 ft wide, and 12 ft deepa. Calculate the overAow rate in gpa/ft2.and receives a flow of 200,000 gallons per day (gpd).