Accidental aerobic granules - data evaluation of a full-scale SBR plant

Abstract

This paper describes and evaluates a large-scale SBR with a design capacity of 35,000 p.e. where the activated sludge exhibits excellent settling properties. The sludge volume index (SVI) of all four SBR is mostly below 50 ml g–1 and shows annual fluctuations; the lowest values of 30 ml g–1 are measured during summer. The focus of this study was to identify reasons for this excellent settling behavior. Microscopic images of the sludge showed a compact and dense structure with small granules. Par-ticle size distribution indicates that about 74.4% of the particles had a size above 200 μm, which is a characteristic size of aerobic granules. Approx. 50% of the sludge particles were larger than 320 μm. SV10/SV30 ratio was calculated with 1.21. Based on the existing knowledge of aerobic granular sludge it can be assumed that the long filling during denitrification leads to anaerobic conditions and pro-motes the formation of aerobic granules. Legal requirements for the effluent quality were met the entire year. The average COD and TN removal amounted to 94.2 and 83.3%.