01. A :
* Metabolic selection is an effective practical method of controlling the growth of filamentous microorganisms.
* Metabolic selection is the competition between floc - formers FF2 and filamentous microorganisms from Groups S and C.
* Eikelboom expressed exactly this feature when he described the metabolic selection as "selection by cutting out filamentous bacteria".
* Metabolic selection can affect the composition of activated sludge biocenoses whenever the cells can obtain energy by means of more than one metabolic process. For metabolic selection, the most important processes are :
• Nitrate respiration and dissimilative nitrate reduction (denitrification).
• Enhanced biological phosphorus removal (EBPR).
* Metabolic selection can result in a very rapid improvement of settling properties. The growth of filamentous microorganisms can be completely stopped in this way.
* Metabolic selection can be expected in all nutrient removal systems in which the activated sludge is alternately exposed to anaerobic / anoxic and oxic conditions.
02. Q : What are the advantages and disadvantages of a selector operated under oxic conditions ?
02. A :
Advantages...
* More than 90 % of readily biodegradable substrate (as COD) was removed in the oxic contact zone.
* The decrease in filament abundance can be achieved by a slow decrease in SVIs.
Disadvantages...
* The oxic contact zone will fail to control the growth of filamentous microorganisms if there is no substrate gradient between the zone and the main reactor. If not compartmentalized, the oxic contact zone in this case acts as a small completely mixed reactor, that favourize the growth of filaments, simply placed ahead of the main reactor that favourize the growth of filaments.
* The alternation of anoxic and oxic cultivation conditions creates some selective advantage for all zones growers over floc - formers.
* The internally formed nitrite by nitrification inhibited oxic respiration, which could not continue until the nitrite had been removed by oxic denitrification.
* Metabolic selection in anaerobic / oxic cultivation conditions, however, does not seem to be effective in suppressing, all known filamentous microorganisms in activated sludges.
03. Q : What are the advantages and disadvantages of a selector operated under anoxic conditions ?
03. A :
Advantages...
* Metabolic selection under anoxic conditions does not necessarily require alternation between anoxic and oxic conditions. For balanced growth, the substrate is taken up and metabolized directly in anoxic conditions. In actual activated sludge plants, the alternation between anoxic and oxic conditions is unavoidable when nitrification and denitrification are required. In addition, most denitrifiers exhibit accumulation / storage capacity during unbalanced growth under anoxic conditions, so that the subsequent oxic conditions are necessary for restoring this capacity.
* Indications that the removal of available substrate under anoxic conditions may support the growth of floc - forming microorganisms.
* In terms of correlation between phosphorus content in activated sludge and SVIs, the positive effect of the anoxic premixing tank is explained by an increased content of phosphorus. The reason for this increase is supposed to be a local increase in pH due to denitrification, which supports the production of calcium phosphate in activated sludge flocs.
* A substantial proportion of the nitrate in the recycled sludge was utilized as an oxygen source by denitrifying organisms.
* Extremely fast response.
* Complete elimination of filaments from activated sludge.
* The higher the fraction of substrate removed by anoxic processes, the better settling properties can be expected.
* The low SVIs can be achieved in an anoxic zone without substrate concentration gradient.
Disadvantages...
* When the anoxic zone was partially aerated, the SVIs immediately started to deteriorate.
* The filaments resistant to the metabolic selection in activated sludge systems with anoxic zones can be classified as all zones growers (group A) or as filamentous organisms FO2.
* Filamentous microorganisms denitrify nitrate only to nitrite and thus NO cannot be accumulated in their cells. Floc - formers are inhibited under oxic conditions by nitric oxide or to a lesser extent by nitrite as denitrification intermediates accumulated in their cells during the preceding anoxic conditions.
04. Q : What are the advantages and disadvantages of a selector operated under anaerobic conditions ?
04. A :
Advantages...
* Substrate removal under anaerobic conditions has a positive effect on activated sludge settleability.
* No substrate gradient was necessary in anaerobic conditions for suppressing the growth of such filamentous microorganisms as Sphaerotilus natans and Type 021N.
* The anaerobic selector also promotes biological phosphorus removal from wastewater.
Disadvantages...
* Metabolic selection in anaerobic / oxic cultivation conditions, however, does not seem to be effective in suppressing, all known filamentous microorganisms in activated sludges.
* Anaerobic zones fail to control the growth of Microthrix parvicella.
Anaerobic selector followed by multiple oxic reactors operating in series ( A / O Process ).
