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.
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.
Please click here for further information on LennSludge.