Synergic role of ferrate and nitrite for triggering waste activated sludge solubilisation and acidogenic fermentation: Effectiveness evaluation and mechanism elucidation.

Enhancing anaerobic treatment efficiency of waste activated sludge (WAS) toward preferable resource recovery would be an important requirement for achieving carbon-emission reduction, biosolids minimization, stabilization and security concurrently. This study demonstrated the synergic effect of potassium ferrate (PF) and nitrite on prompting WAS solubilisation and acidogenic fermentation toward harvesting volatile fatty acids (VFAs). The results indicated the PF+NaNO2 co-pretreatment boosted 7.44 times and 1.32 times higher WAS solubilisation [peak soluble chemical oxygen demand (SCOD) of 2680 ± 52 mg/L] than that by the single nitrite- and PF-pretreatment, respectively, while about 2.77 times and 2.11 times higher VFAs production were achieved (maximum VFAs accumulation of 3536.25 ± 115.24 mg COD/L) as compared with the single pretreatment (nitrite and PF)-fermentations. Afterwards the WAS dewaterability was improved simultaneously after acidogenic fermentation. Moreover, a schematic diagram was established for illustrating mechanisms of the co-pretreatment of PF and nitrite for enhancing the VFAs generation via increasing key hydrolytic enzymes, metabolic functional genes expression, shifting microbial biotransformation pathways and elevating abundances of key microbes in acidogenic fermentation. Furthermore, the mechanistic investigations suggested that the PF addition was conducive to form a relatively conductive fermentation environment for enhancing electron transfer (ET) efficiency, which contributed to the VFAs biotransformation positively. This study provided an effective strategy for enhancing the biodegradation/bioconversion efficiency of WAS organic matters with potential profitable economic returns.

Polysorbate-80 pretreatment contributing to volatile fatty acids production associated microbial interactions via acidogenic fermentation of waste activated sludge.

Polyoxyethylene dehydration sorbitol monooleate (polysorbate-80) pretreatment enhanced volatile fatty acids (VFAs) production of waste activated sludge (WAS) in acidogenic fermentation. The results showed that polysorbate-80 ameliorated WAS solubilization obviously with a soluble chemical oxygen demand (SCOD) increasing to 1536 mg/L within 4 h. Within 2 days of acidogenic fermentation, the maximal VFAs arrived to 2958.35 mg COD/L via polysorbate-80-pretreatment. The polysorbate-80 pretreatment boosted microbial diversity and richness in fermentation process. The Clostridium, Macellibacteroides and Acidocella strengthened microbial cooperation for the metabolic functions enhancement (e.g. amino acid metabolism and carbohydrate metabolism) for VFAs generation from WAS organics. Overall, the polysorbate-80 could play positive roles on the transformation of organic matter from sludge solid matters to VFAs, which was turned out to become an effective enhancing strategy for future WAS treatment / bioresource recovery with relatively low cost.

Synergistic pretreatment of CaO and freezing/thawing to enhance volatile fatty acids recycling and dewaterability of waste activated sludge via anaerobic fermentation.

To avoid the generally deteriorated dewaterability of sludge in waste activated sludge (WAS) anaerobic acidogenesis, the combination of varied calcium oxide (CaO) dosage (i.e., 0.01-0.07 g/g TS) and freezing/thawing pretreatment (5 F/T cycles) was investigated for concurrently improving the production of volatile fatty acids (VFAs) and dewatering performance of sludge. The highest release of soluble chemical oxygen demand (SCOD) (1836 ± 96 mg/L) and accumulation of VFAs (448.0 mg COD/g VS) were reached through the co-pretreatment of CaO (0.07 g/g TS) and F/T (50 h at -24 °C) (i.e., 0.07 CaO-F/T). Meanwhile, optimal dewaterability of sludge was also achieved in 0.07 CaO-F/T co-pretreated WAS fermentation, which was reflected by large particle size (98.32 µm), low capillary suction time (41.6 s), decreased specific resistance to filtration (SRF, reduced 47.5% against blank) and high zeta potential (-9.59 mV). Co-pretreatment of CaO and F/T reduced the species number of total microbial population, but improved the abundance of acid-producing bacteria. Increased abundance of Bacteroides, Macellibacteroides, Petrimonas, Prevotella, Clostridium, and Megasphaera was positively relevant to the high yields of VFAs. The economic analysis indicated that CaO-F/T was economically acceptable with considerable estimated net profits, which provided a feasible and efficient alternative for further WAS treatment at large scale.