CVEN 5434

BioWin Exeercises

1. Calculating and fixing SRT ().

Recall SRT = total mass in reactors/mass wasting rate

In the “Project” tab, click on “Active SRT.”

A menu appears; for now, make sure the box “Active SRT” is checked.

Click on “Select elements for total mass.” A list of all your unit processes will appear in a window.

Select all process elements (bioreactors) where reactions are occurring and use the arrow to put

them in the calculation window, “Selected elements.” Click “OK” and return to the previous

menu. Then click on “Select wastage elements.” A similar menu appears. Now select the elements

where solids leave the system. Typically this includes the waste activated sludge removed from

the secondary clarifier underflow, NOT the sludge wasted from the primary clarifier. You may

also include the solids in the treated effluent. When you have selected these elements, return to

the main menu and click OK. BioWin will now calculate the SRT during a simulation and report

the value in a window at the bottom of the output display.

You may also set the SRT to a desired value and BioWin will adjust the wasting rate to obtain that

value. To do that, go to the Active SRT menu and check the box for Control SRT. Then select the

elements for total mass and wastage as before. Enter the desired SRT value and select the splitter

to be used to control SRT. It will be the splitter that divides the return activated sludge from the

waste activated activated sludge in the secondary settler underflow. A new splitter with a small

“valve” will replace the old splitter, at the “sidestream” leg of the splitter. Make sure that the

wastage pipe comes off the sidestream leg of the splitter. Now you may run the simulation under

the constraint of a fixed SRT, which is a good way to test the effect of the important activated

sludge process operation of controlled wastage.

Using your Metro simulation, try a range of SRT values from 4 to 10 days. What is the effect on

effluent water quality? MLSS? Wasted sludge production? Oxygen consumption?

2. Using Tapered aeration in a multi-basin process line.

From the last class, you configured the Metro WWTP as a single-tank CSTR. As you heard on the

field trip, the aeration basins are best described as a dispersed plug flow reactor with three

passes. Adjust you model to simulate this aeration basin design. You may want to try between 3

and 5 CSTRs of equal volume in series, with the same total volume as your first simulation. Keep

the DO setpoints and wasting rate the same.

What was the impact of sub-dividing the aeration basin on effluent water quality?

Now try using tapered aeration. Check your oxygen consumption rate in each CSTR in series

from the last simulation. Change the DO setpoints in individual basins to as to reduce the total

oxygen consumption. What is the effect on effluent water quality as DO setpoints are reduced to

low levels? How low can you go and maintain water quality? How much oxygen can you save?