Over the past decade, a significant amount of research work on extracellular polymeric substances has been done on the “alginate-like exopolysaccharides” (ALE, also called “alginate-like exopolymers”). The term was used based on the FAO (Food and Agriculture Organization) biopolymer identification test. Although various chemical analyses have been conducted to characterize extracted ALE, it remained unclear whether ALE contains the two sugar monomers of alginates. Aiming to obtain a direct answer to the question: are there alginates in the ALE extracted from sludge, activated sludge was collected from two wastewater treatment plants in two different countries, where the ALE was previously extracted, characterized, and reported in the literature. The extracellular polymers were extracted from these sludges and fractionated according to the standard protocol. The sugar monomer composition of each fraction was analyzed, with special attention to the presence of mannuronic acid (M) and guluronic acid (G), which compose alginate biopolymers. None of these monomers were found in the extracted EPS, indicating there are no alginates resembling polymers extracted from the sludges. The possibility of the presence of other glycan components, such as lipopolysaccharides in EPS, was investigated and confirmed.@en

As a significant structure in activated sludge, extracellular polymeric substances (EPS) hold considerable value regarding resource recovery and applications. The present study aimed to elucidate the relationship between the microbial community and the composition and properties of EPS. A biological nutrient removal (BNR) reactor was set up in the laboratory and controlled under different solid retention times (SRT), altering microbial species within the system. Then EPS was extracted from activated and analyzed by chemical and spectroscopic methods. High-throughput sequencing and metagenomic approaches were employed to investigate bacterial community and metabolic pathways. The results showed that lower SRT with a higher abundance of the family-level Proteobacteria (27.7%-53.5%) favored EPS synthesis, while another dominant group Bacteroidetes (20.0%-32.6%) may not significantly affect EPS synthesis. Furthermore, the abundance of alginates-producing bacteria including Pseudomonas spp. and Azotobacter vinelandii was only 2.53%-6.76% and 1.98%-6.34%, respectively. The alginate synthesis pathway genes Alg8 and Alg44 were also present at very low levels (0.05‱-0.11‱, 0.01‱-0.02‱, respectively). Another important gene related to alginates operons, AlgK, was absent across all the SRT-operated reactors. These findings suggest an impossible and incomplete alginate synthesis pathway within sludge. In light of these results, it can be concluded that EPS does not necessarily contain alginate components.

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Entropy is a concept defined by the second law of thermodynamics. Applying this concept to the world we live in, entropy production must be minimized and negentropy (negative entropy production) should be accelerated, in order to produce a healthy and stable ecological system. The present wastewater treatment, however, contributes to entropy production. This means that conventional wastewater treatment, without recovery of resource and energy, will gradually but inevitably contribute to a deteriorating ecological balance. When the self-cleaning ability of the natural ecological system is limited, the need to develop sustainable wastewater treatment in order to delay entropy production and accelerate negentropy becomes urgent. Resource and energy recovery from wastewater should be the first priority, as they can contribute significantly towards minimizing entropy production and accelerating negentropy. Sustainable wastewater treatment must focus on recovering recyclable high value-added organic chemicals from wastewater and/or excess sludge to minimize entropy production caused by methane (CH₄, once combusted, is converted into CO₂ - an even higher substance in entropy) via anaerobic digestion. Instead of CH₄, thermal energy present in the effluent can be utilized for heating/cooling buildings and also for drying excess sludge towards incineration to recover more energy. Overall, this can lead to a carbon-neutral operation and even creating a “carbon sink” could be possible for wastewater treatment.

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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.

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