Enhancement on biodegradation and anaerobic digestion efficiency of activated sludge using a dual irradiation process

Light alters microbiota and electron transport: Evidence for enhanced mesophilic digestion of municipal sludge

Light as an environmental factor can affect the process of anaerobic digestion, but there is no systematic study in municipal wastewater sludge mesophilic digestion. In this study, the effects of light on the performance of the anaerobic digestion system and photo-anaerobic microbiota (PAM) were evaluated in lighted anaerobic batch digesters (LABRs). The methane yield from the reactor under the dark condition (LABR0) was 179.2 mL CH₄/g COD, which was lower than 305.4 mL CH₄/g COD and 223.0 mL CH₄/g COD (n = 3, p < 0.05) from reactors under the light intensity of 3600 lm (LABR1) and 7200 lm (LABR2), respectively. The dominant genera in the bacterial and archaeal communities were Bacillus and Methanosarcina under light conditions, Enterococcus and Methanobacterium under dark conditions. And these two bacteria acted as electroactive bacterial genera, indicating that light changes the combination of direct interspecies electron transfer (DIET) microbial partners and activates the DIET pathway for methane production. The electron conduction pathways analysis further suggests that biological DIET (bDIET) between microbial biomass, rather than DIET via conductive material (cDIET) between microbes and conductive materials, is promoted and behaves as the dominant factor enhancing methane production under light conditions. The morphology of microorganisms and the amount and properties of EPS corroborate these views. Our findings are guided to anaerobic digester constructions under the outdoor environment with light exposure.

Perspective on enhancing the anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) is an important bio-energy strategy that has been hindered by low conversion efficiency. This paper presents a comprehensive review of research on the sludge's property and enhancing AD of WAS, and proposes two perspectives of material structure and microbial activity on improving AD efficiency. In the first part of this review, the key principle problems for hindering AD efficiency are identified based on the concept of AD. Then, the possibility that the complex microstructure and composition of WAS are responsible for poor biodegradability is considered and main methods for enhancing AD are summarized. In the third part, according to the published works, the main knowledge gaps in research are recognized as the identification and specific activity adjustment of functional microbes, the understanding of key constituents of WAS and their interactions, the deciphering of complex structure of sludge organic substance, and the revealing of relationships between complex nature of WAS and biodegradability. Further discussions reveal that to enhance AD more studies should be centered on the sludge's structure and properties in future. However, this review is expected to provide the clear and accurate research directions for enhancing AD efficiency of WAS.

Strategy of mitigating ammonium-rich waste inhibition on anaerobic digestion by using illuminated bio-zeolite fixed-bed process

Intermittent illumination combined with bio-zeolite fixed-bed process was utilized to improve the efficiency of anaerobic digestion with ammonium-rich substrate. The batch experiments were carried out at NH₄⁺-N concentration of 2211mg/L under intermittent illumination and dark (as control) conditions, respectively. The illuminated bioreactor achieved higher methane production (287mL/g-DOC) and ATP value (0.38μmol/L) than that under dark condition. Then the bio-zeolite fixed-bed bioreactor (NH₄⁺-N concentration: 3000mg/L) was used to study the additional efficiency on the illuminated ammonium-rich anaerobic digestion process. The result showed that the illuminated fixed-bed bioreactor presented the greatest methane concentration (70%), methane yield (283mL/g-DOC) and quantity of methanogens comparing with no-bed bioreactor. Furthermore, the illuminated fixed-bed bioreactor achieved better performance during 118-day semi-continuous fermentation. The combination of the intermittent illumination and bio-zeolite fixed-bed strategy contributed to the higher efficiency and stability of the ammonium-rich anaerobic digestion process.

Comparison of sludge digestion under aerobic and anaerobic conditions with a focus on the degradation of proteins at mesophilic temperature

Aerobic and anaerobic digestion are popular methods for the treatment of waste activated sludge. However, the differences in degradation of sludge during aerobic and anaerobic digestion remain unclear. In this study, the sludge degradation during aerobic and anaerobic digestion was investigated at mesophilic temperature, focused on protein based on the degradation efficiency and degree of humification. The duration of aerobic and anaerobic digestion was about 90 days. The final degradation efficiency of volatile solid was 66.1 ± 1.6% and 66.4 ± 2.4% under aerobic and anaerobic conditions, respectively. The final degradation efficiency of protein was 67.5 ± 1.4% and 65.1 ± 2.6% under aerobic and anaerobic conditions, respectively. The degradation models of volatile solids were consistent with those of protein under both aerobic and anaerobic conditions. The solubility of protein under aerobic digestion was greater than that under anaerobic digestion. Moreover, the humification index of dissolved organic matter of aerobic digestion was greater than that during anaerobic digestion.

Improving secondary sludge biodegradability using free nitrous acid treatment

This study presents a novel strategy based on free nitrous acid (FNA) treatment to improve the biodegradability of secondary sludge. Several experiments were conducted to demonstrate the biocidal effect of FNA on activated sludge. The viable fraction as well as the biological activity of the biomass decreased significantly after 8-48 h treatment with FNA. The biodegradability of the FNA treated sludge was compared to that of the same sludge without FNA treatment by aerobically digesting these sludges with a full-scale activated sludge for 14 and 6 days respectively. Ninety percent of the FNA treated biomass was consumed during the 14-day aerobic digestion compared to 41% achieved with the untreated biomass. During the 6-day aerobic digestion, 50% of the FNA-treated sludge was degraded. The results indicate that FNA treatment substantially increases sludge biodegradability.