As the operator of a 25,000 PE activated sludge plant, every year I see my sludge index go up and filamentous matter appear, always under the same conditions. The critical period in question is the winter, when the biological tank cools (73 °C in summer down to 52° C or lower in winter). Moreover, parasitic water flows in from the roads in large quantities (it is not a true separate system).
Obviously the cooling is one cause, but it is usually a few days after the inflow that the sludge index rises the most.
Do you think the road salt is the culprit?
How can this be measured in the raw water? Chloride, sodium? Hach water analysis products?