Controlling factors and involved mechanisms on forming alginate like extracellular polymers in flocculent sludge

Alginate like extracellular polymers (ALE) recovered from excess sludge have been evaluated as an eco-friendly, cost effective and sustainable alternative to highly valued materials. However, the ALE extraction from flocculent sludge ranges normally from 90 to 190 mg/g VSS, which is only equivalent to the lowest edge of the ALE production from aerobic granular sludge (AGS). But flocculent sludge is much higher in production than AGS and thus a further investigation was expected on key factors and associated mechanisms controlling ALE formation of flocculent sludge. The investigation was conducted by lab-scale sequencing batch bioreactors. The experiments revealed that flocculent sludge with starch used as an influent substrate contained the highest ALE production (220.3 ± 8.0 mg/g VSS). Low temperature was favorable to enriching ALE, up to 303.3 ± 21.5 mg/g VSS at 12 °C. Moreover, ALE reached up to 137.8 ± 13.2 mg/g VSS at C:N = 5:1 and slightly declined with increased or decreased the C/N ratio. The specific ALE yield was 63.7 mg ALE/(g BOD₅) at a low organic load, which was twice as high as that with high organic loads. However, SRT had a minor effect on ALE formation. Obviously, such scenarios as starch-rich and low temperature could promote the ALE production. Furthermore, the characteristic analysis including alginate equivalent, different fractions and hydrogel forming property among different ALE, confirmed that the ALE extracted from flocculent sludge had a potential in substituting for commercial alginates. However, different working conditions would exert a significant influence on the composition and chemical properties of ALE, which implies that the controlling some parameters could be an approach to directionally cultivating ALE for their unique structures and potential applications.