When wastewater is aerated sufficiently, its organic matter reduces and a flocculant sludge (consisting of various microorganisms) is formed.
In order to improve the process, the flocculant activated sludge is retained in the system as inoculum.
This is achieved by settling the wastewater and recirculating the microbial mass.
A part of this wastewater activated sludge is wasted periodically as synthesis of new cells continues.
* The organisms involved in the activated sludge treatment process are aerobic chemoheterotrophic, i.
e.
, those which utilize organic compounds as source for carbon (for cellular synthesis) and energy (by using oxygen as electron acceptor).
* Phase i: Initially, the macromolecules are hydrolyzed or broken down into their monomer compounds.
These reactions are usually carried out extracellularly in wastewater activated sludge treatment process.
Once their size is reduced they are transported into the cell.
* Phase ii: Later, the small molecules produced in phase i are partially degraded, releasing 1/3rd of their total energy to the cell.
In the process a number of different products are formed which serve as precursors of both anabolic and catabolic routes of phase iii.
* Phase iii: The catabolic route oxidizes the compounds and produces carbon dioxide and energy.
The anabolic route (which requires energy) results in synthesis of new cellular material that participate in activated sludge process control.
Many microorganisms participate in the above reactions during wastewater treatment.
Activated sludge requires that both the lower and higher protists play significant roles.
Generally, the organisms in sludge culture may be divided into four major classes (these are not distinct groups and any particular organism may display more than one such behavior): * Floc-forming organisms: these help to separate the microbial sludge from the treated wastewater.
Zooglea ramigera and a variety of other organisms flocculate in the activated sludge tank.
Flocculation is understood to be caused by the extracellular polyelectrolytes excreted by these microorganisms.
* Saprophytes: the saprophytes are micro-organisms that degrade the organic matter.
These are mostly gram-negative bacilli such as pseudomonas, flavobacterium, alcaligenes and the floc formers.
* Predators: the main predators are protozoa which thrive on bacteria.
It has been found that the protozoa can be upto 5% of the mass of biological solids in the systems.
Ciliates are usually the dominant protozoa.
They are either attached to or crawl over the surface of sludge flocs in this wastewater activated sludge.
Rotifers are the secondary predators.
When rotifers occur in plenty, we can be sure of a well stabilized waste, since rotifiers perish in highly polluted waters.
* Nuisance organisms: nuisance organisms interfere with the smooth functioning of the wastewater activated sludge system, when present in large quantities.
Most problems arise due to sludge settling (due to presence of filamentous forms which reduce the specific gravity of the sludge).
The bacterium sphaerotilus natans and the fungus geotrichium are often responsible for this situation.