Question

An activated sludge system has three square reactor tanks in
series (CFSTRs) that are each 60 ft wide, 60 ft long, and 18 ft
deep. The engineers have decided to add a new reactor in parallel.
It will be a plug flow reactor (PFR) with a depth of 18 feet and a
width of 18 feet. The length is to be calculated. The flowrate of
wastewater is 2 MGD and the BOD is 165 mg/L. This will be split in
half so that each side (3 square tanks in series and the new PFR)
will treat one half the flow. The reaction rate is first order and
the constant (k) = 4.0 d^{-1}.

What is the steady-state effluent BOD concentration from the last of the three existing tanks at flow = 1.0 MGD (mg/L)?

What is the required length of the new PFR so that is will have the same steady-state effluent concentration as the existing CFST reactors (ft)?

What is the BOD concentration at the middle length of the PFR (mg/L)?

Answer #1

Two identical sedimentation tanks are to be designed for an
activated sludge plant with a flow of 2 MGD (flow is split evenly
between the two clarifiers). The MLSS will be 1950 mg/L. The
underflow (return sludge) concentration will be 12,000 mg/L. A
settling curve was developed (using a column with Ho = 2.13 ft) and
is shown below. For clarification area use a safety factor of 2.0
and for thickening area use a safety factor of 1.5. Tanks are...

A completely mixed activated-sludge process is being designed
for a wastewater flow of (2.64 mgd) using the kinetics equations.
The influent BOD of 120 mg/Lis essentially all soluble and the
design effluent soluble BOD is 7 mg/L, which is based on a total
effluent BOD of 20 mg/L. For sizing the aeration tank, the mean
cell residence time is selected to be10 days and the MLVSS 2000
mg/L. The kinetic constants from a bench-scale treatability study
are as follows: Y...

Two identical sedimentation tanks are to be designed for an
activated sludge plant with a flow of 2 MGD (flow is split evenly
between the two clarifiers). The MLSS will be 1950 mg/L. The
underflow (return sludge) concentration will be 12,000 mg/L. A
settling curve was developed (using a column with Ho =
2.13 ft) and is shown below. For clarification area use a safety
factor of 2.0 and for thickening area use a safety factor of 5.
Tanks are...

Two identical sedimentation tanks are to be designed for an
activated sludge plant with a flow of 2 MGD (flow is split evenly
between the two clarifiers). The MLSS will be 1950 mg/L. The
underflow (return sludge) concentration will be 12,000 mg/L. A
settling curve was developed (using a column with Ho = 2.13 ft) and
is shown below. For clarification area use a safety factor of 2.0
and for thickening area use a safety factor of 1.5. Tanks are...

A conventional activated sludge plant treating a domestic flow
of 150 ML/d is operated at a SRT of 10 d with a MLVSS of 3500 ppm.
The activated sludge plant is required to reduce the influent BOD
from 200 ppm to less than 10 ppm prior to discharge. The discharge
from the aeration basin is clarified, such that the clarifier
overflow contains no particulate material and the MLVSS of the
recycle stream is 8000 ppm. You are required to determine...

ANSWER as much as possible:
Design an activated sludge process consisting of a complete mix
flow through aeration tank followed by final settling tanks. The
sizing of the reactor and all appurtenances should be based on a
solids retention time, (SRT), of 4 days and the particulates given
below.
Biological characteristics: Ks = 20
mg/L COD, kd = 0.1 per day, mumax = 3
gVSS/gVSS-day,
Y = 0.4 gVSS/gCOD
Primary settling Tank Effluent
Characteristics:
Q= 20MGD, biodegradable COD, bCOD...

Mass of solids Generated from the activated sludge tanks.
13. Assume: The total design flow is 3.75 MGD (15,000
m3/day). The NPDES limit is 25/30. Assume that the waste
strength is 170 mg/L BOD after primary
clarification. Y = 0.55 kg/kg.
What is the mass of solids generated each day (Kg/day or
lbs/day) in the activated sludge tankage?

A 1.0 � 106 gallon reactor is used in a sewage-treatment plant.
The influent biochemical oxygen demand (BOD) concentration is 100
mg/L, the effluent BOD concentration is 25 mg/L, and the flow rate
through the reactor is 500 gal/min. (a) What is the first-order
rate constant for decay of BOD in the reactor? Assume that the
reactor can be modeled as a comply mixed flow reactor (CMFR).
Report your answer in units of per hour. (b) Assume that the
reactor...

You have been retained to design a wastewater treatment plant using
a conventional activated sludge process for a town of 40,000
people. The industrial flow contribution to the sewer system is
300,000 gallons per day. Effluent discharge standards require
effluent BOD5 and total suspended solids concentration of <10
mg/l.
Assume the following design criteria are applicable:
Domestic sewage flow = 100 gal/cap/day (including I/I)
BOD loading = 0.18 lbs/cap/day
TSS loading = 0.20 lbs/cap/day
BOD removal in primary = 33%...

Problem #3: Biochemical Oxygen Demand (BOD) and Stream
Quality
A wastewater
treatment plant designed for a community of 45,000 people has a
flow of 100 gal/person/day and a BOD5 loading of 0.2
lb/person/day. The upstream characteristics of the receiving waters
are:
stream flow rate =
20 cfs and BOD5 = 2 mg/L
The water quality
standards require the in-stream BOD5 to be less than 7
mg/L in this stretch of the river.
The minimum BOD
removal efficiency that the treatment...

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