Operating the Extended Aeration Activated Sludge Process

[UncleD]

Hi,
Could anyone tell me where I could find a process description/process schematic for an extended aeration plant. Preferably a plant or designs that has been tried before. A manufacturer's catalogue may also be useful.
UncleD

 

[AquaDoc]

Extended aeration activated sludge process is the most common form of activated sludge treatment. Activated sludge treatment can involve primary clarification (settling basin) to remove coarse solids, followed by an aerated basin. Bacteria which grow in the aerated basin are settled out in a secondary clarifier (settling basin), with the bacteria (biosolids or sludge) settling to the bottom and the clarified (clear) liquid at the top is discharged (often disinfected first). The bacteria (activated - hungry) are pumped back into the beginning of the aerated basin to be combined with the incoming wastewater.

Eventually the population (concentration) of bacteria increase to a point where some must be wasted (taken out). If the amount of new bacteria grown in one day is removed each day, we say it is a one day sludge age, often referred to as a high rate activated sludge process. If only 1/20th of the bacteria grown in a day is removed, it would be said to have a 20 day sludge age, and would be referred to as an extended aeration activated sludge process.

Extended aeration sludge is typically held for a long time (10 to 30 days, or longer) and operates in the endogenous respiration (growth) phase, which requires low organic loading (higher sludge concentrations in the aerated tank) and long aeration times. The higher concentration of sludge and longer aeration times makes it well suited for small household treatment plant applications. Small applications do not usually have primary clarifiers.

Now here's where it gets a bit complicated .. as described above, extended aeration activated sludge processes have an older sludge population (held longer within the system). This population is made up of a large number of different types of bacteria. As it turns out, bacteria responsible for nutrient removal (phosphorus and nitrogen) tend to take longer to grow. Therefore, biological nutrient removal (BNR) treatment plants are also extended aeration activated sludge processes. But BNR systems have a wide variety of possible tank configurations (i.e. not just a simple aerated tank followed by a secondary clarifier), which may include internal pumping, and complex control strategies.

Now back to your question regarding process diagrams. Here are a couple of URL addresses which have some good examples:

Site with a free A/S Process Simulator and pdf manual with process illustrations:

http://www.syscon.uu.se/~psa/

http://www.syscon.uu.se/Research/WWT/JASS/JASSNY.html

Online Activated Sludge Journal

http://www.scitrav.com/wwater/asp1/

http://wwwsti.htm