Sphaerotilus natans and Type 1701...

Sphaerotilus natans is the flagship of Eikelboom's system of filamentous microorganisms. This type is the most traditional and probably best - described filamentous microorganism found in activated sludge. It is one of those exceptional types with a quite clear and firm taxonomic position in the conventional bacteriological system. Sphaerotilus natans belongs to the Sphaerotilus / Leptothrix group in the order Chlamydobacteriales. This order contains sheath - forming filamentous organisms. Contrary to Leptothrix spp., which oxidize manganase, Sphaerotilus spp. are able to deposit iron in the sheath. Thus some authors classify Sphaerotilus natans as an iron bacterium. However, Sphaerotilus natans is a chemo - organotrophic, not a lithotrophic, organism. the preferred substrates are readily biodegradeable compounds like simple sugars, low fatty acids, alcohols and other low - molecular weight substances.

Metabolically Sphaerotilus natans is reported to be an oxic (aerobic) microorganism which prefers molecular oxygen as an electron acceptor. Sphaerotilus natans can synthesize organic storage products detectable as sudanophilic (presumably PHB) granules. The reactions of Sphaerotilus natans trichomes to the Gram and Neisser stains are negative; neither volutine granules nor sulphur deposits are observed in the cells. The morphology of Sphaerotilus natans filaments is very characteristic, and this type can easily be recognized even in wet mounts examined in phase contrast. The dichotomous trichomes form a tree - like or brush - like structure. The branching is, however, false. The sheath can be seen best when some empty or missing cells are present in the trichome. The trichomes are usually not covered with any epiphytic growth but when the filaments are unhealthy or decaying, the attached growth can be observed. In such a case the heavy attached growth can form the core of a future floc.

An increased occurrence of Sphaerotilus natans filaments in activated sludges leads to bulking by means of the interfloc bridging mechanism. The extended branched filaments or their bunches cause a severe deterioration of activated sludge settling properties. When Sphaerotilus natans dominates the activated sludge, it is always very difficult to operate systems without any remedial actions.

Eikelboom introduced Type 1701 as a filamentous microorganism different from Sphaerotilus natans. Both types exhibit many common features as far as the metabolic properties and nutritional requirements are concerned. However, Type 1701 is capable of using more complex organic carbon and nitrogen sources, so that it can be found in wastewater treatment plants treating a wide variety of municipal and industrial wastes. This could explain a rather high ranking of this type in table shown below.

Dominant filamentous microorganisms in bulking sludges from different countries
Ranking
Germany
The Netherlands
USA
1
Type 0092
M.parvicella
Nocardia spp.
2
M.parvicella
Type 021N
Type 1701
3
Type 0041
H.hydrossis
Type 021N
4
S.natans
Type 0092
Type 0041
5
Nocardia spp.
Type 1701
Thiothrix spp.
6
H.hydrossis
Type 0041
S.natans
7
N.limicola
S.natans
M.parvicella
8
Type 1701
Type 0581
Type 0092
9
Type 0961
Type 0803
H.hydrossis
10
Type 0803
Type 0961
Type 0675

The main difference is in the morphology of Type 1701 because;
  • This type is predominantly found in the floc interior
  • The filaments are not branched
  • The trichomes are covered by usually heavy attached growth
    •

    Contrary to Sphaerotilus natans, when Type 1701 dominates in the activated sludge, the poor settleability is caused by the diffused floc structure. It was observed that Type 1701 growing in a completely mixed activated sludge system continuously fed with glucose was able to form glycogen as a storage product. However, its glycogen storage capacity was not sufficient, so that when the feeding regime was changed from continuous to intermittent, Type 1701 could not compete in the biocenosis with a microorganism with a higher glycogen storage capacity.

    The growth kinetic coefficients for both Sphaerotilus natans and Type 1701, obtained for a glucose - ammonium - mineral salts cultivation medium, were given in table shown below.

    Monod kinetic growth constants for Sphaerotilus natans and Type 1701
    Monod kinetic model constants
    Sphaerotilus natans
    Type 1701
    mumax (1/day)
    6.5
    2.6
    KDO (mg/L)
    0.033
    0.014
    KS (mg/L)
    10
    2
    Y (mg/mg)
    0.53
    0.44
    b (1/day)
    0.065
    0.06
    m (mg/mg.day)
    0.12
    0.14

    The data in table shown above indicate that both filamentous microorganisms exhibit a high affinity to substrates and dissolved oxygen under low concentrations, but this ability is more pronounced for Type 1701. According to literature, Type 1701 may occur when dissolved oxygen defiency is severe, while Sphaerotilus natans may occur under less DO limitation. The limiting value of DO cannot be stated generally, however, because its value is closely linked to the actual substrate loading of activated sludge flocs.