CEE 371

Fall 2009

Homework #9 Wastewater Treatment

Consider the design of a wastewater treatment plant (WWTP) for a community with average daily and peak hourly wastewater design flows of 2.0 MGD and 5.0 MGD. The raw sewage has an average of 230 mg/L BOD₅ and 260 mg/L of suspended solids.

1. After screening and grit removal, the WW is to be treated by primary sedimentation in two parallel treatment trains of circular clarifiers (so 2 sedimentation tanks, each gets ½ the flow). Determine the diameter of each tank and the nominal side water depth for design criteria of an overflow rate of 700 gpd/ft² and detention time of 3 hours at average flow. Also, determine the overflow weir loading rate at peak hourly flow if the weir is placed all around the circumference of the tank. Compare to design standards.

2. Assume that the primary sedimentation process removes 60% of the suspended solids and 40% of the BOD₅ of the raw sewage. Determine the SS and BOD₅ concentrations in the primary sedimentation effluent flow. Also determine the mass of primary sludge produced per day at average flow conditions, as both dry solids and as wet sludge assuming a sludge concentration of 6% solids and a specific gravity of 1.03.

3. The primary effluent is to be treated by two parallel trains of the complete mix activated sludge process. Assume average flow conditions, and the primary sedimentation performance as described in part 2 above. Assume the following for the activated sludge process:

· Plant effluent BOD₅ of 8 mg/L

· Biomass yield of 0.55 kg biomass / kg BOD

· Endogenous decay rate (k_d) = 0.04 day^-1

· Solids Retention Time (q_C) = 8 days

· MLVSS concentration in the aeration tank of 3000 mg/L

· Waste and recycle solids concentration of 12,000 mg/L

a) Determine the aeration tank volume in cubic meters. b) Determine the mass and volumetric flow rates (kg/day and cubic meters per day) of wasted sludge. c) Determine the return (recycle) flow rate in cubic meters per day (and in MGD). d) Determine the volumetric BOD loading to the aeration tank in lb BOD per 1000ft³. e) Determine the food to microorganism ratio (F/M) for the aeration tank in kg BOD per day per kg MLVSS. f) Determine the design hydraulic detention time (q) in hours.

4. Suppose that the primary effluent is to be treated by two parallel high-rate single stage tricking filters with the following characteristics:

· 6 feet deep rock trickling filter media

· Design BOD loading of 50 lb/day/1000 ft³

· Design hydraulic loading of 0.30 gpm/ft²

· Recirculation ratio of 1.5

a) Determine the volume (ft³) and diameter (ft) of each trickling filter. b) Determine the effluent BOD concentration at 20 ºC and at 10 ºC and comment on the results.

Assigned: 4 Dec 09

Due: 11 Dec 09