Effects of anaerobic storage on physical, chemical and microbiological parameters in thickened activated sludge from a plant with biological N and P removal were examined. Short-term (0-1 day) as well as long-term (10-14 days) experiments were performed. The dewaterability of unconditioned sludge, measured as specific resistance to filtration (SRF), showed a steady decline and was strongly reduced after 10 days of anaerobic storage. An increase in turbidity in bulk water was observed and correlated well with the decreased dewaterability. Most of the turbidity could be explained by material not passing a 0.45 mu m filter. Presence of free bacteria was observed of which many were motile and came from the flocs. The flee size distribution for flocs larger than 10 mu m did not change during the storage. A significant increase in the conductivity of the sludge was observed as a result-of a release of ions and other products from bacterial degradation processes. This increased conductivity coincided with an increased turbidity, and disintegration of the flocs was discussed in the context of the stability of the flocs under varying ionic strength. A more detailed study of the processes leading to the increased conductivity was performed. On a short term basis a bacterial release of ions (PO43-, Mg2+ and K+) from Poly-P bacteria took place. After a few hours of anaerobic storage no more PO43- was released but the levels of Mg2+, K+ and Ca2+ were continuously observed to increase. Degradation of organic material was observed resulting in a production of dissolved organic carbon (DOC) and NH4+. The amount and the rate of ion release and mineralization were very constant during a year when expressed per amount of volatile solids.
