Aerated Lagoons/Ponds
Typically employed for treating municipal wastewaters, pulp and
paper effluents, etc
Usually designed as CFSTRs without recycle
Typical Process Flowsheet
Mass Balance Equations
Initially, only consider mass balances on viable biomass (X) and
biodegradable COD (S)
Biomass Mass Balance
Substrate Mass Balance
Solving the Equations
The soluble COD in the lagoon effluent will equal the sum of the
residual biodegradable COD + the non-biodegradable soluble
COD in the influent
The total concentration of VSS in the lagoon effluent includes
viable biomass + endogenous decay products + nonbiodegradable VSS that is present in the influent
The TSS concentration in the lagoon effluent includes:
- FSS that are present in the influent
- FSS that are present in the biological solids that are
generated in the lagoon
o Typically assume a volatile fraction of biomass
Sludge Age
A measure of the average time that biomass spends in the system
c 
An example:
mass of biomassin system
rate of biomass disch arg e
Biomass Washout
A condition where biomass is washed out more rapidly that it can
grow
From previous biomass mass balance:
For a fixed reactor volume, as flow increases, substrate (S)
approaches the influent concentration and biomass (X)
approaches 0
Qm is the critical flow when S = So and X = 0
Rearranging the previous equation:
Design of processes should maintain  sufficiently greater than
m to ensure that washout will not occur.
Design
Typical materials of construction:
 concrete
 earthen with liners
Aeration and Mixing:
 none (anaerobic)
 wind (facultative)
 mechanical aerators or diffused aeration (aerobic)
details of design later
Sedimentation of effluent is required
- often an unmixed pond is employed
Some typical design and operating parameters
Parameter
Residence time (days)
BOD Loading (kg BOD/ha-d)
BOD Removal (%)
Effluent -BOD (mg/L)
Depth (m)
Design Hydraulics
Anaerobic
Facultative
Aerated
20-50
<500
<80%
variable
>2
PFR
CFSTR
7-30
<50
80-90%
<30
1-2
PFR
3-10
NA*
85-99%
<30
2-7
CFSTR
A wide range of loadings is possible depending upon level of
aeration
*
Design Process
An iterative approach is required






assume volume and depth
calculate surface area
estimate operating temperature
calculate effluent concentrations
check against required values
adjust volume
1.) Define: Influent characteristics (flow, w/w characterization)
k, Ks, Y, kd - biokinetic coefficients at 20o C
S - treatment efficiency
Volume and depth → Surface Area
2.) Estimate operating temperature
The Mancini and Barnhart equation is an empirical relationship
that has been found to be adequate
3.) Adjust temperature sensitive coefficients
4.) Solve design equations to calculate effluent concentrations
5.) Iterate
6.) Calculate solids production
Required for:
- aeration system design (viable biomass + cell debris)
- settling pond design (TSS)
7.) Calculate oxygen requirements
Oxygen required by biomass to oxidize organics that exert bCOD
A minimum of 2 mg/L is usually required
O2,R = oxygen requirements, kg O2/day
8.) Design aeration system
see later
9.) Design sedimentation pond
An Example: