Nostocoida limicola...

Contrary to Type 0092, Nostocoida limicola is not so difficult to observe in activated sludges. Nostocoida limicola is described in three forms which differ in the diameter of trichomes:

• Nostocoida limicola I : 0.6 - 0.8 micro - m
• Nostocoida limicola II : 1.2 - 1.4 micro - m
• Nostocoida limicola III : 1.6 - 2.0 micro - m

All three forms can be found as long, bent or coiled filaments both within the flocs and in the bulk liquid. The trichomes are composed of oval or discoid cells, so that the Nostocoida limicola filaments resemble sometimes the cyanophyceae - like filaments of Type 021N from which Nostocoida limicola is easy to distinguish by means of the Neisser stain.

The reaction of Nostocoida limicola filaments to the Gram stain is not certain. In most cases, the reaction is positive but Nostocoida limicola II can also be Gram negative. The variability of the reaction to the Gram stain results from the composition of cultivation media. As Nostocoida limicola is often found in activited sludge systems treating wastewaters with a higher portion of industrial wastes, some components from industrial wastewaters may change the properties of cell walls upon which the result of the Gram stain depends. One should always be aware that the staining reactions described in manuals were mostly obtained from the samples of activated sludges growing on municipal wastewaters.

Fortunately, the reaction to the Neisser stain is not so variable, and all three forms of Nostocoida limicola are supposed to be Neisser positive. The Neisser stain result is very characteristic. The entire trichome is light blue or blue violet, and the individual cells are brighter than the septa. An author described the appearance of a Nostocoida limicola filament resulting from the Neisser stain as a "stocking ladder". It is very difficult to misidentify Nostocoida limicola after the Neisser stain when the reaction is so clear.

The characteristic properties of Nostocoida limicola were studied in detail by Novak and Brown on isolates from activated sludge in the Gold Bar wastewater treatment plant, Edmonton, Canada. The Nostocoida limicola isolates grew poorly on organic acids (acetate, citrate, succinate) but good growth response was observed to carbohydrates and alcohols. The utilization of these single carbon substrates was enhanced by the addition of complex substances like peptone. The Nostocoida limicola filaments exhibited a fermentation capability and behaved as facultative anaerobes. PHB granules were frequently observed in the individual cells of Nostocoida limicola. The kinetic parameters of the Nostocoida limicola isolates were not estimated.

The authors also performed a full - scale study at the Gold Bar wastewater treatment plant. They compared three parallel sections of activated sludge system which differed in feeding and hydraulic regime and in cultivation conditions in these ways:

• Step - feed aeration tank, high degree of mixing
• Plug - flow aeration tank, fully aerobic
• Plug - flow system with anoxic zone

The BOD loading of the sections was in a range of 0.21 - 0.25 kg BOD₅/ kg MLVSS. day and sludge age of 6 - 8 days. An abundant growth of Nostocoida limicola was observed in the step - feed section and in the fully aerated plug - flow sections. In the section with the anoxic zone prior to the aerated part, the activity of Nostocoida limicola was significantly reduced. A decrease in Nostocoida limicola abundance was also observed in the fully oxic sections when the sludge age was reduced to 4 days.

Two researchers also studied the effect of Nostocoida limicola presence on settling properties. As Nostocoida limicola grew mostly in the flocs, there was not a good correlation between the filament length and sludge volume indices. Nostocoida limicola affected the SSVI values when the floc structure became loose and irregular with lower settling velocities. In general, Nostocoida limicola did not deteriorate the settling properties to the extent that could be expected from the observed abundance of filaments.

In two researchers' experiments they operated five sequencing batch reactors with sludge ages of 3, 8, 16, 25, and 50 days. All systems were fed with the same synthetic wastewater, the composition of which was (in COD units):

• Peptone : 500 mg / L
• Ethanol : 250 mg / L
• Glucose : 125 mg / L
• Acetic acid : 125 mg / L

The volumetric loading (on a COD basis) was 1 kg / m³ .day in all SBRs. Although the main aim of the experiments with SBRs under different sludge ages was to study the effect of sludge retention time on microfloc formation, the experiments also provided an oportunity to observe the growth of Nostocoida limicola. Surprisingly enough, all SBRs inoculated with the activated sludge from the Prague Central wastewater treatment plant suffered from bulking in the start - up period regardless of the plug - flow hydraulic regime in the SBRs. The dominating filamentous microorganism was Nostocoida limicola present in the inoculum (see above). After one month of operation, the settleability of activated sludges improved to such an extent that the activated sludge settled below one - half of the SBR volume after 1 h sedimentation period, so that recirculation ratio (1) could be maintained. However, a significant occurrence of Nostocoida limicola in biocenoses persisted for the rest of a 4 - month experiment in all activated sludges. The only remarkable difference between activated sludges with different sludge ages was in the appearance of the Nostocoida limicola trichomes. The long coiled filaments without any strange structures, were observed in the activated sludges cultivated under 3, 8, and 16 days. In the activated sludge with a sludge age of 25 days the Nostocoida limicola filaments formed swollen cells and knots. The long filaments also tended to fragment. Nostocoida limicola in the activated sludge with a sludge age of 50 days was present only in a form of short fragments mostly incorporated in flocs.

Nostocoida limicola filaments are found in biological foams in activated sludge plants. However, the foaming problems are not so severe and persistent when Nostocoida limicola is the causative microorganism as they are when nocadioform or Microthrix parvicella cause foams. Some authors experimentally verified that the Nostocoida limicola foams are not so stable as the foams caused by nocardioforms. The possible explanation is in a lesser degree of hydrophobicity of the Nostocoida limicola cells.