Many European wastewater treatment plants have problems in the stage of biological treatment, which are related to copious growth of filamentous microorganisms.
Reasons for the massive appearance of filamentous microorganisms
- Little sludge load - Lopsided composition of wastewater - Variations in the wastewater
Due to the increasing demands concerning the purification achievement the sludge load, i.e. the ratio of the daily fed BOD load and the dry solid matter in the activated sludge tank, decreases. This leads to copious growth of filamentous organisms, because these organisms attain - unlike the floc-forming bacteria – high growth rates even if substrate and oxygen concentrations are low.
Another reason for the dominance of filamentous microorganisms is the lopsided composition of wastewater, as it occurs in many industrial firms. But also variations in the effluent; e.g. unstable flow, varying temperature and changes in the composition of nutrients in the wastewater, can avail the growth of these organisms, because they are in general much more unassuming than the floc-forming organisms.
Symptoms that are related to copious growth of filamentous microorganisms
When talking about filamentous organisms, usually bacteria are meant, in some cases also filamentous fungi. Having a certain amount of filamentous organisms is advantageous, because in comparison to the floc-forming bacteria they achieve more effective nutrient uptake. Furthermore their prolate build supports their feature to catch floating particles. These advantages front the disadvantages of a lower sludge settleability, which increases the costs of sludge treatment drastically.
An increased development of filamentous microorganisms causes two extremely undesirable phenomena:
- Bulking sludge
- Floating sludge
Abb. 1 : Sickly sludge with high share of filamentous microorganisms
The term bulking sludge refers to sludge with extremely bad settling and thickening characteristics. In most cases bulking sludge accumulates in the clarifier, where it forms a thick layer and has to be removed to prevent that it flows through the outlet and debits the on-site preflooder.
The settleability is described by the parameters of sludge volume (SV) and sludge volume index (SVI). The sludge volume is the specific volume, which sludge takes up in a certain time of settling (mostly 30 min in a barrel) in ml/l. The sludge volume index describes the volume that 1 g sludge (referring to dry solid matter) has after 30 min. of settling. For calculating the VSI the VS in ml/l has to be divided by the dry solid matter (TS) in mg/l.
VSI = VS (ml/l) / TS (g/l) = (ml/g)
The sludge volume index of bulking sludge is more than 150 ml/g.
Floating Sludge This is the second phenomenon that occurs in sickly activated sludge tanks. Floating sludge floats on the surface due to the copious growth of actinomycetes and certain other filamentous organisms, which have a hydrophobic cell surface. The hydrophobic cell surface adsorbs air and nitrogen gas bubbles and causes the sludge to swim upwards. Floating sludge should be removed quickly, because it also leads to the formation of foam in the septic tanks of anaerobic sludge treatment.
There are several methods to contain and prohibit copious growth of filamentous microorganisms. A technical solution for preventing bulking sludge is the application of high-loaded tanks (selectors) as a stage before the activated sludge tank or the installation of cascade-systems. In both cases the occurring substrate gradient compensates the low loading of the wastewater and bulking sludge can be prevented. A similar method of solution is found in the stage of biological phosphate removal in advanced wastewater treatment: Here the effect of the upstream, anaerobic mixing tanks is the same as of the selectors.
Other methods to achieve a better settleability is bypassing the preliminary sedimentation, improving the wastewater characteristics of lopsided effluents or the addition of precipitants and flocculants.
An easier and less dramatic measure than adding constructive selectors or flocculants is supplying LennSludge.
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