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Biodiesel is a renewable and environmentally friendly energy source. The use of biological wastewater treatment systems for excess sludge in the biodiesel production can achieve both sludge reduction and resource recovery. In this study, an in situ one-step method for producing biodiesel from excess secondary sludge was investigated; then the differences in both biodiesel yield and composition produced from the five industrial wastewater sludges and two municipal wastewater sludges were studied. The results showed that ultrasonic pretreatment of sludge was not necessary for biodiesel production by in situ acid-catalyzed transesterification. The addition of n-hexane during transesterification was significant to determine the performance of biodiesel production. Significant differences were found in yield and composition of the sludge lipid from seven different types of wastewater. The sludge oil of starch wastewater and restaurant wastewater was significantly higher than those of tannery wastewater, landfill leachate, and livestock wastewater, which was mainly due to the significant differences on wastewater quality and sludge microbial structure. The higher organic content (especially sugar), COD/TN and COD/TP ratios and better biodegradability in wastewater led to the dominance of Saprospiraceae and Anaerolineaceae, finally benefited the lipid accumulation in excess secondary sludge for biodiesel production.

Granulation of nitrifying bacteria could achieve high biomass concentration in treatment system and result in the higher rate of ammonia oxidation and removal even under short hydraulic retention time. However, it is a major challenge to achieve granular sludge in nitrification stage. The addition of chemically synthetic N -acyl-homoserine lactones (AHLs) into bioreactors could enhance the attached-growth of nitrifying microorganisms from sludge and followed by sludge granulation, but it is not suitable for full-scale operations due to the high cost. In this study, cellular extract of AHLs from nitrifying granular sludge (NGS) was added into the nitrifying activated sludge to investigate the effect of the NGS cellular extract on the adhesion and granulation of autotrophic nitrifying sludge. The results indicated that NGS contained AHL molecules and the exogenous addition of the NGS cellular extract could improve the cell adhesion and aggregation. Moreover, addition of the NGS cellular extract increased the microbial activity, the concentration of extracellular proteins (PN) and the biomass of autotrophic nitrifying bacteria, which eventually contributed to the nitrifying sludge granulation. Addition of exogenous cellular-based AHL from NGS in the early start-up stage is a potentially promising approach for full-scale application of nitrifying sludge granulation with reduced cost.

Utilizing excess sludge from a wastewater treatment plant to produce biodiesel has become an increasingly popular topic. However, the low lipid content in activated sludge has hindered the application of biodiesel production from excess sludge. This study was designed to enhance biodiesel production from granular sludge fed with synthetic sugar-containing wastewater. Granules cultivated at different initial sludge loading rates had different morphologies and microbial structure. Low seed biomass and high initial sludge loading resulted in the dominance of the white filamentous fungi Dipodascaceae in granular sludge, thereby enhancing the lipid accumulation of granular sludge and changing the biodiesel constituent produced from the sludge. However, overgrowth of filamentous fungi deteriorated the settling and compression ability of granular sludge. Thus, controlling the filamentous fungi at a moderate level through an effective operating strategy is of great importance to biodiesel production from a granular sludge system fed with sugar-containing wastewater.

With the goal of reutilizing excess sludge, and decreasing the cost of lipid feedstocks, the effect of pH on enhancing biodiesel production via culturing special microbial communities from municipal activated sludge was investigated. The results indicated that controlling pH around 7.5 made biomass production and lipid accumulation more quickly under high initial glucose loading (40 g COD L−1) and COD:N ratio (90:1). Compared with the seed sludge (biodiesel yield of 5.2 ± 0.3 mg g−1 dry sludge), the cultivated sludge achieved higher biodiesel yields both with pH control and without. However, the biodiesel yield of the cultured sludge with pH control (69.3 ± 1.2 mg g−1 dry sludge) was higher than that of the sludge without (63.3 ± 0.4 mg g−1). More unsaturated fatty acid esters (64%, w/w total FAMEs) were obtained from the cultured sludge without pH control, which could improve the cold flow properties of biodiesel. The pH control did not make an essential difference in the microbial community at the end of cultivation. However, it improved the accumulation of Enterobacteriaceae and Shewanella in the cultured sludge. The dominant bacteria of Shewanella, Raoultella, Enterobacter, Kluyvera and two unclassified genera clustered with γ-proteobacteria within the cultured sludge possibly contributed to enhancing biodiesel production.

The effect of biodelignification of rice straw by two different ligninolytic organisms, Phanerochaete chrysosporium (white-rot fungus) and Streptomyces badius (actinomycetes), on humus quality was investigated during a 56-day incubation at 30 °C. Lignin degradation, the release of humic extract (HE), humic acid (HA) and fulvic acid (FA), E4/E6 ratio of HA, and humification index (HI, HA/FA) were measured during the incubation. Lignin was degraded by both organisms, but to different extents. Lignin was degraded to 41% and 31% by P. chrysosporium and S. badius, respectively. HE released by P. chrysosporium and S. badius were, respectively, 2.10 and 2.13 times larger than that in the control at the maximum values. A significant correlation between lignin degradation and humus-related parameters involving HA fraction showed that both organisms are converting lignin to humic substances.