Changing dissolved oxygen concentration in your reactor may help you change the settling properties of sludge, if changing you F/M ratio is not possible.
Your SVI values do not indicate a bulking sludge.
Operate with DO > 2.0 mg/L in aeration phase.
Alternative: Chlorinate the returned sludge
what is your aeration method?
by changing DO your F/M ratio (because of changing in kinetic of micro organism and rate of duplication) is changed.
a non oxic (either anaerobic or anoxic) sequence, if possible can act as selector and improve sludge settlability in any activated sludge process including SBR. Low DO in aerobic sequence can lead bulking problem and a DO of no less than 1.5 mg/L is recommended.
There are several reasons for high or low SVI, few details can give idea about the exact cause like, effluent type, design operations, MLSS/MLVSS, DO, pH &Temperature. Which can further troubleshoot by several ways. However, more than 1.5ppm DO surely would be helping to improve settling but it’s depend on design parameter/availability of aerators/diffuser or energy costing. I agree with chlorination or adding peroxide in RAS/clarifier, but if DO is already good than cause would be different.
While as per my real experiences by doing some operational changes and external EPS producing bacterial seeding surely would be improving SVI/ sludge settling. Recently troubleshooting of 15 MLD activated sludge plant (Textile) we face high SVI and TSS carry over problem and analysis showed very less bacterial population/high TDS/very low performance, while DO was almost near to 2ppm. I started external multiplication of EPS producing bacteria (from sludge only) and controlled recycling rate along with systematic draining of old sludge. After 10 days SVI improved almost double and found improved performances by 40% higher.
This can occur when the sludge first forms a blanket and flocculate together before starting to settle. As the particles come together the form particles that have specific gravity greater than water. this can result in sudden drop in sludge volume index.
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