Effect of the famine phase length on the properties of aerobic granular sludge treating greywater

Abstract

Sequencing batch reactors (SBR) treating high-strength greywater need an aerobic granular sludge (AGS) with good properties, such as a low sludge volume index (SVI) and high settling velocities and substrate uptake rates to yield short settling and aeration stages. To promote the formation of stable granular sludge, the length of the famine phase could be a key factor. In this regard, the effect of the duration of this variable on the AGS properties was assessed by comparing a gradual versus an abrupt reduction of the famine phase in two SBR treating greywater. The initial average famine phase of 3.3 h was gradually reduced to 0.3 h over 20 weeks in one reactor, and abruptly in another one. This condition induced filamentous outgrowth, as well as the deterioration on the properties of the sludge; the effect was more accelerated when the famine periods were abruptly shortened. In both cases the reduction on the famine periods induced increased organic loading rates, which led to degranulation events when it was higher than 2.5 g-chemical oxygen demand g-volatile suspended solids−1 d−1. Afterwards, the biomass adapted to this situation, by forming new small-filamentous aggregates with similar SVI to that of the stable AGS formed with the longest famine period. HIGHLIGHTS The reduction in the famine phase length induces filamentous overgrowth and the deterioration of the properties of AGS.; Overstressed organic loading rates can lead to the biomass degranulation.; Small-size filamentous aggregates can present a similar sludge volume index to that of the stable-AGS.; A famine fraction of 0.55 with respect to the total cycle length is enough to ensure the stability of the AGS.;

Keywords

• aerobic granules
• famine phase
• greywater
• sequencing batch reactor
• wastewater treatment