3.5.1 Designing an Aerobic Digester Tank: An Exercise - Humanitarian Sanitation Hub

3.5.1 Designing an Aerobic Digester Tank: An Exercise

Let us design an aerobic digester with the following assumptions:

ParameterValue
Input Sludge5 m3/d
Total Solids2% (after thickening)
Volatile Solids75% TS
Lowest Temperature during Winter17°C
Oxygen Demand2.3 kgO2/kgVS destroyed
Air Density1.2 kg/m3
Oxygen Concentration in the Air23%
Aerobic Digester Tank Design Assumptions

Step 1: Select Retention Time (RT) and Calculate Digester Volume (VD)


The lowest temperature during winter in the region is 17°C, which will result in a retention time of 31 days, based on the recommendation from the table presented in Topic 3.5 (Table: Design parameters for conventional aerobic sludge digesters).

The following formula can be applied to calculate the digester volume.



Where,

ParameterDescriptionValue in ExerciseUnit
VDDigester Volumem3
VINVolume Input5m3/d
RTRetention Time31d (day)

Let’s calculate the digester volume based on the recommended retention time of 31 days.

Solution:


Step 2: Verify the Organic Loading Rate (OLR) (within the range 1.6 – 4.8 kgVS/m3d)


The following formula can be applied to calculate the organic loading rate.



Where,

ParameterDescriptionValue in ExerciseUnit
OLROrganic Loading RatekgVS/m3/d
VINVolume Input5 m3/d
ρDensity of Water1,000kg/m3
CTSTS Concentration2%TS
CVS(TS)VS Concentration from TS75%VS from TS
VDDigester Volume155 m3

Now, we calculate the organic loading rate and verify if it is within the specified range.

Solution:


The calculated organic loading rate is not within the specified range.


Step 3: Calculate the Total Solids Concentration in the Aerobic Digester (CTS)


Firstly, we calculate the fixed solids (FS), the undigested part which remains constant throughout the process for both the influent and effluent.



Where,

ParameterDescriptionValue in ExerciseUnit
FSFixed Solidskg/d
FSINFixed Solids of Influentkg/d
FSEFFFixed Solids of Effluentkg/d
TSINTotal Solids of Influent100kg/d
VSINVolatile Solids of Influent75kg/d

Solution:


Secondly, we calculate VS of the effluent, VSEFF . We assume a VS removal efficiency of 40% for aerobic digestion.



Where,

ParameterDescriptionValue in ExerciseUnit
VSEFFVolatile Solids of EffluentkgVS/d
REVSRemoval Efficiency40%
VSINVolatile Solids of Influent75kgVS/d

Solution:


Thirdly, we calculate the TS of the effluent, TSEFF



Solution:


Finally, we can determine the TS concentration that is maintained in the digester tank (CTS).


Where,

ParameterDescriptionValue in ExerciseUnit
CTSTS Concentration in digester tank%
ρDensity of Water1,000kg/m3
TSEFFTotal Solids of Effluent70kg/d
VSEFFVolatile Solids of Effluent5m3/d

Solution:


Step 4: Determine the Area Requirements for the Digester (A)


Next, we calculate the area requirements for 4 rounded tanks if one tank has a diameter of 10 m. As a reminder, the formula for the area is A= π r² .


Where,

ParameterDescriptionValue in ExerciseUnit
AArea requiredm2
πConstant3.14
rRadius of the
digester tank
5m
Solution:


Step 5: Determine the Air Requirements for the Digester (VAIR)


Firstly, we need to quantify the VS removed per day, VSREM .


Where,

ParameterDescriptionValue in ExerciseUnit
VSREMVolatile Solids Removedkg/d
REVSRemoval Efficiency40%
VSINVolatile Solids of Influent75kg/d

Solution:


Secondly, we need to quantify the oxygen requirement for the removal of VSREM of 30 kg/d.



Where,

ParameterDescriptionValue in ExerciseUnit
OMMass of OxygenkgO2
VSREMVS Removed30kgVS/d
ODMOxygen Demand2.3kgO2/ kgVS

Solution:


Finally, we can determine the volume of air (VAIR) required per day if the oxygen concentration is assumed to be 30%. As calculated in the previous example, the oxygen required is 69 kgO2/d.

We use the following formula to calculate the volume of air, VAIR , required per day.



Where,

ParameterDescriptionValue in ExerciseUnit
VAIRVolume of Airm3/d
OMMass of Oxygen69kgO2
ρAIRDensity of Oxygen1.2kgO2 /m3
COOxygen Concentration30%

Solution:


We then check the airflow mixing capacity and confirm if it exceeds the minimum requirements as suggested in the table presented in Topic 3.5 (Table: Design parameters for conventional aerobic sludge digesters): The minimum airflow mixing capacity should be between 20 – 40 L/m3/min.



Where,

ParameterDescriptionValue in ExerciseUnit
MIXAirflow Mixing CapacityL/m3/min
VAIRVolume of Air191.7m3/d
VDVolume of the Digester155m3

Solution:


The airflow mixing capacity is lower than the minimum requirements; therefore, the amount of air needs to be increased based on the minimum flow mixing capacity of 20L/m3/min or 29 m3/m3 d.


Let us now calculate the revised air volume.


Solution:

x
News