CEE 6560
Prob. Set No. 4
Due: Sept. 30, 2009
1) A groundwater is aerated to remove hydrogen sulfide (H2S). The aeration process
involves spraying the groundwater into the air to form small droplets. The small droplets
provide significant surface area to transfer H2S to the atmosphere. The H2S content of the
groundwater is initially 5 mg/L. The droplets formed by spraying are spherical with an
average radius of 0.2 cm. Each droplet spends an average of 2 seconds in contact with
the air. The droplets are reasonably well mixed so that the H2S concentration is uniform
in each droplet. H2S is a relatively soluble gas and liquid-water transfer is controlled
primarily by the gas film. The gas-film mass transfer coefficient for H2S in the droplets
(kg) is 6 cm/hr. The volumetric flow rate for the aeration process is 1000 liters/min.
Dimensionless Henry's constant for H2S is 27.
a) Determine the rate of H2S transfer in this process in mg/min. Assume that atmospheric
H2S concentration is negligible. Also assume that the groundwater transfers H2S only
during the 2 sec life of the droplet. Once the droplet lands in the small pond that forms
below the spray it will not transfer any H2S.
b) A small temperature inversion has trapped a volume of air just above the aeration pond
so that H2S accumulates in this air pocket. The volume of this air is 0.28 million m3.
H2S is very odorous and at some level is very toxic. It was decided that when the H2S
level reached 0.068 mg/L (in the trapped air) the aeration process was to be halted to
mitigate health and aesthetic problems. Determine how long it will take to reach this
critical H2S level. Assume that the trapped air will be homogeneous with respect to
H2S concentration. Also assume that H2S in the trapped air is negligible compared to
the H2S in the droplet (show that this is a valid assumption).
2) Groundwater often contains reduced iron (Fe2+) that can cause staining of sinks,
laundered clothing, etc. It is therefore desirable to remove the iron if the groundwater is to
be used for domestic water supply. A common iron removal process involves aerating the
groundwater to oxidize iron to Fe3+. Fe3+ is relatively insoluble at neutral pH and will
easily precipitate. Aeration provides oxygen for the oxidation of iron. It was determined
that, for a particular groundwater, 500 kg O2/day needs to be transferred to get effective
oxidation of iron. An aeration tank that uses diffused aeration needs to be designed to
deliver this oxygen. The tank selected is 3 m deep. Diffusers will be located at the
bottom of the tank. A diffused-air aeration system has been selected and the manufacturer
has provided the following performance equation. Air flow to each diffuser will be 10
scfm (at 20oC, 1 atm). Ignore any temperature corrections for air flow rate (Ga).
N  0.002  G 0a.85 H 0.67  (C*l,m  Cl )  1.02 (T20)  
N = lbs/hr of oxygen transferred per diffuser.
Ga = air flow (standard cubic feet/min @ 20oC and 1 atm)
H = depth of diffuser, ft.
C*l,m  mid-depth oxygen saturation, mg/L
Cl = operating dissolved oxygen concentration, mg/L
Since this performance equation represents an empirical correlation the units of the
equation parameters must be as described above. Assuming that the groundwater is
essentially tapwater () and that the aeration tank dissolved oxygen level is to be
maintained at 3 mg/L, determine the required number of diffusers to meet the oxygen
demand. Assume that the aeration tank will be operated at sea level (1 atm pressure) and
that the aeration system has a manufacturer’s efficiency rating of 0.2. Dissolved oxygen
saturation at 25oC is 8.24 mg/L.
3) An aquifer has been contaminated with tetrachloroethane (PCE). The PCE
concentration in the contaminated water is 10 g/L. The contaminated area has been
hydraulically isolated using slurry walls. The contaminated aquifer water within the slurry
walls is pumped out and treated in an air stripping column. This water is pumped for a
time period long enough to draw down the aquifer to approximately 1/3 of its total water
volume. Assume the stripping column attains steady state removal efficiency in a very
short after start up. The liquid effluent from the air stripper is stored in closed tanks. After
the 1/3 volume drawdown is accomplished treated water from the storage tanks is pumped
back into the contaminated aquifer. Withdrawal and recharge of the contaminated aquifer
water is accomplished with multiple wells so that the contaminated aquifer exhibits
completely mixed behavior. Determine the concentration of tetrachloroethane (PCE) in
the aquifer after three (3) treatment cycles.
PCE:
H (Henry's constant)  7 103
atm  m3
mol
R  8.21105
m3  atm
mol  o K
Kla = 0.02 per min
Stripping column:
A = 1.5 m2
Length = 4 m
Air flow rate = 1000 liters/min
Aquifer:
depth = 10 m
Water pump rate = 75 liters/min
T = 293 oK
area = 1500 m2 porosity () = 0.25