Types of Activated Sludge
Plug Flow - wastewater is routed through a
series of channels constructed in the aeration basin.
- Wastewater Flows through as a plug & is treated as it
winds its way through the tank.
- As the wastewater goes through the system, BOD and
organics concentration are greatly reduced.
- Variations to this method include: adding return sludge
and/or in decreasing amounts at various locations along
length of the tank; wastewater BOD is reduced as it
passes through tank, and air requirements and number of
bacteria required also decrease accordingly.
Complete Mix - wastewater may be immediately
mixed throughout the entire contents of the aeration basin (mixed
with oxygen and bacteria).
- This is the most common method used today.
- Since the wastewater is completely mixed with bacteria
and oxygen, the volatile suspended solids concentration
and oxygen demand are the same throughout the tank.
Contact Stabilization -
- Microorganisms consume organics in the contact tank.
- Raw wastewater flows into the contact tank where it is
aerated and mixed with bacteria.
- Soluble materials pass through bacterial cell walls,
while insoluble materials stick to the outside.
- Solids settle out later and are wasted from the system or
returned to a stabilization tank.
- Microbes digest organics in the stabilization tank, and
are then recycled back to the contact tank, because they
need more food.
- Detention time is minimized, so the size of the contact
tank can be smaller.
- Volume requirements for the stabilization tank are also
smaller because the basin receives only concentrated
return sludge, there is no incoming raw wastewater.
- Often no primary clarifier before the contact tank due to
the rapid uptake of soluble and insoluble food.
Extended Aeration -
- Used to treat industrial wastewater containning soluble
organics that need longer detention times.
- This is the same as complete mix, with just a longer
- Advantage - long detention time in the aeration tank;
provides equalization to absorb sudden/temporary shock
- Less sludge is generally produced because some of the
bacteria are digested in the aeration tank.
- One of the simpler modifications to operate.
Other Modifications (less common)
Oxidation ditch - similar to plug flow but uses
a circulator aeration basin.
Step feed - microbes gradually feed organics in
a step feed mode at multiple points along the tank plug flow.
Tapered aeration - air flow rate to the aeration
basin may be tapered along the length of the basin.
Kraus Process - The anaerobically digested
sludge and digester supernatant may be added to the return
sludge, thus improving settling of the floc.
- A portion of the return sludge flow from the secondary
clarifier is mixed with the anaerobically digested sludge
and the digester supernatant before being combined with
the return sludge stream andrecycled back to the aeration
Denser anaerobically digested sludge settles rapidly and is
added to the lighter secondary sludge to add weight and improve