The whole theory in a nutshell

Influent Flow

Capacity of Wet Well, Aeration Tank Feed flow rate and Retention time are the functions of Influent Flow. Therefore, when combined volume of Aeration Tank and the Wet well becomes less than Influent Flow, the application prompts to take one of the following measures:

a. Increase capacity of the wet well.

b. Increase Aeration Tank feed flow rate.

c. Increase retention time.

Increase capacity of the Wet Well:

Increasing the capacity of the Wet Well would facilitate accommodating additional volume of Influent flow.

Increase Aeration Tank Feed Flow Rate:

Since volume of the Aeration tank is a function of Aeration tank feed flow rate, any change in the feed flow rate would tend to change the volume of Aeration tank as well. The relationship here is directly proportional.

Increase retention time:

Since volume of the Aeration tank is also a function of Aeration tank retention time, any change in the retention time would tend to change the volume of Aeration tank as well. The relationship here is directly proportional.

Pumping hours

Practical experience of working at sewage treatment plant has helped establishing a very significant relationship from design perspective. It has been understood that Aeration tank feed flow rate is inversely proportional to the pumping hours.

Putting it the other way, it means pumping units of lower capacity would be enough if run continuously for prolonged hours. Such arrangement would, in turn, also help preventing shock loading making the system ‘treatment efficient’.

Again, volume of the Aeration tank is also a function of Aeration tank Feed Flow rate, any change in pumping hours would tend to change the volume of Aeration tank. The relationship here is inversely proportional as explained above.

Influent Suspended Solids

Since Percentage Sludge Volume, Return Sludge Rate, System Stabilization duration, Removable Sludge are the functions of Influent Suspended Solids. Any change in Influent Suspended Solids would alter these parameters as under:

Theoretically, at Aeration Tank, approximate Quantity of Sludge, KGS

= (Influent Suspended Solids, mg/L) X (Volume of Aeration Tank, KL)

1000

Theoretically, approximate Volume of Sludge, Cubic Meters

= (Quantity of Sludge, KGS) / Density of Sludge (721 KGS/Cubic Meter)

Therefore, Percentage Sludge volume (%)

= Volume of Sludge X 100

(Volume of Aeration tank – Volume of Sludge)

And, Return – Sludge Rate, Litres / hour

= Percentage Sludge volume X Aeration Tank Feed Flow Rate, KL X 1000

Further, desired Return-Sludge quantity can be worked out as below:

Note: The desirable return flow is approximately equal to the percentage ratio of the volume of settleable solids to the volume of clarified liquid after settling for 30 minutes in a 1000 mL graduated cylinder. This ratio should not be less than 15 percent at any time.

Desired Return-Sludge quantity, Litres / hour

= (150 mL / 850 mL) X 100 X Aeration Tank Feed Flow Rate, KL X 1000.

100

To have approximate estimation of duration of system stabilization, following formula is used:

(Desired Return-Sludge quantity / Return-Sludge Rate) / 24 (duration in day/s).

Finally,

(Return-Sludge Rate, Litres per hour) X 24

will give the quantity of Sludge that can be wasted.

Capacity of Aerators

Capacity of Aerators is a function of Influent BOD and Aeration Tank Feed Flow Rate. The relationship here is directly proportional.

Capacity of Aerators, HP

= (Influent BOD, mg/L) X (Aeration Tank Feed Flow Rate, KL per hour)

Oxygen transfer rate of aerators X 1000

Volume of Aeration Tank

Volume of Aeration Tank is a function of the Feed Flow Rate and Retention time. The relationship here is directly proportional.

Volume of Aeration Tank, KL

= Aeration Tank Feed Flow Rate, KL per hour X Retention time, Hour

Volume of Secondary Clarifier

Volume of Secondary Clarifier is a function of Aeration Tank Feed Flow Rate and Retention time. The relationship here is directly proportional. For sake of convenience Retention time of Secondary Clarifier is taken as 1 hour.

Volume of Secondary Clarifier, KL

= Aeration Tank Feed Flow Rate, KL per hour X Retention time, Hour

Treatability

Treatability of Wastewater is a function of BOD to COD ratio. If the ratio is above 0.5, the wastewater is considered to be Highly Biodegradable. If the ratio is less than 0.3, the wastewater is deemed to undergo a chemical treatment before the routine biological treatment.

Conversions

1000 L = 1 KL = 1 M³ = 0.001 Million Liters