Microbial community shifts in the oxic-settling-anoxic process in response to changes to sludge interchange ratio

Mechanistic understanding of acclimation and energy metabolism of acetoclastic methanogens under different substrate to microorganism ratios

Mechanistic understanding of acetoclastic methanogenesis is pivotal for optimizing anaerobic digestion for efficient methane production. In this study, two different operational modes, continuous flow reactor (CFR) and sequencing batch reactor (SBR), accompanied with solids retention times (SRT) of 10 days (SBR10d and CFR10d) and 25 days (SBR25d and CFR25d) were implemented to elucidate their impacts on microbial communities and energy metabolism of methanogens in acetate-fed systems. Microbial community analysis revealed that the relative abundance of Methanosarcina (16.0%-46.0%) surpassed Methanothrix (3.7%-22.9%) in each reactor. SBRs had the potential to enrich both Methanothrix and Methanosarcina. Compared to SBRs, CFRs had lower total relative abundance of methanogens. Methanosarcina exhibited a superior enrichment in reactors with 10-day SRT, while Methanothrix preferred to be acclimated in reactors with 25-day SRT. The operational mode and SRT were also observed to affect the distribution of acetate-utilizing bacteria, including Pseudomonas, Desulfocurvus, Mesotoga, and Thauera. Regarding enzymes involved in energy metabolism, Ech and Vho/Vht demonstrated higher relative abundances at 10-day SRT compared to 25-day SRT, whereas Fpo and MtrA-H showed higher relative abundances in SBRs than those in CFRs. The relative abundance of genes encoding ATPase harbored by Methanothrix was higher than Methanosarcina at 25-day SRT. Additionally, the relative abundance of V/A-type ATPase (typically for methanogens) was observed higher in SBRs compared to CFRs, while the F-type ATPase (typically for bacteria) exhibited higher relative abundance in CFRs than that in SBRs.

Evolution and Prospects in Managing Sewage Sludge Resulting from Municipal Wastewater Purification

Municipal sewage sludge is the residual material produced as a waste of municipal wastewater purification. It is a sophisticated multi-component material, hard to handle. For many years, it has been landfilled, incinerated, and widely used in agriculture practice. When unproperly discharged, it is very polluting and unhealthy. The rapidly increasing global amount of municipal sewage sludge produced annually depends on urbanization, degree of development, and lifestyle. Some diffused traditional practices were banned or became economically unfeasible or unacceptable by the communities. In contrast, it has been established that MSS contains valuable resources, which can be utilized as energy and fertilizer. The objective of the review was to prove that resource recovery is beneficially affordable using modern approaches and proper technologies and to estimate the required resources and time. The open sources of information were deeply mined, critically examined, and selected to derive the necessary information regarding each network segment, from the source to the final point, where the municipal sewage sludge is produced and disposed of. We found that developed and some developing countries are involved with ambitious and costly plans for remediation, the modernization of regulations, collecting and purification systems, and beneficial waste management using a modern approach. We also found that the activated sludge process is the leading technology for wastewater purification, and anaerobic digestion is the leading technology for downstream waste. However, biological technologies appear inadequate and hydrothermal carbonization, already applicable at full scale, is the best candidate for playing a significant role in managing municipal sewage sludge produced by big towns and small villages.

Respirometry tests in wastewater treatment: Why and how? A critical review

Respirometry tests are a widely employed method in wastewater treatment field to characterize wastewater streams, assess toxic/inhibitory effects to the biomass, calibrate mathematical models. Respirometry can allow to fractionize the chemical oxygen demand (COD) in biodegradable and inert fractions, but also provide information related to biomass kinetics and stoichiometry through standardized laboratory techniques. Considering the increasing number of emerging contaminants detected in wastewater effluents, such as pharmaceuticals, personal care products and pesticides, respirometry can be a useful tool to promptly assess any toxic or inhibitory effect in wastewater treatment plants (WWTPs) operations. Beside conventional activated sludge (CAS), in recent years respirometric methods have been applied to innovative fields, such as moving-bed bio-reactors (MBBRs), fungi and microalgae, exploiting natural remediation methods. In particular, respirometry application to microalgae, through the so-called photo-respirometry, has been investigated in the latest years in the treatment of high-nutrient loaded streams, allowing resource recovery in biomass form. In this work, respirometric methods are first introduced from a theoretical basis and then critically discussed by considering the experimental apparatus, the available characterization protocols and the fields of application; the most recent literature findings on respirometry are coupled with authors' experience in the field. A comparison between physicochemical methods and respirometry is made, considering common protocols for WWTP modelling and calibration. The future research needed on the topic is finally outlined, including the coupling of respirometry with microbial community analysis, potentially leading to an enhanced process understanding, an extended respirometry utilization to get specific kinetic and stoichiometric parameters for modelling purposes, and a wider respirometry application as diagnosis tool in WWTP operations.

Waste to energy: valorization of spent tea waste by anaerobic digestion

The conversion of renewable resources into value-added products such as bioenergy is one of the growing concerns of bioeconomy strategy. Within this concept, assessing the proper combination of local wastes has major importance. This study set out to assess the feasibility of using spent tea waste as a single and co-substrate on anaerobic digestion and to explore the influence of the amount of microorganisms on the digester performance. For this purpose, biomethane potentials tests were conducted for seven different mixing ratios of spent tea waste and cow manure on a mass basis. The reactors operated under mesophilic conditions for 20 days with two inoculum/substrate ratios. The results revealed that using spent tea waste as a co-substrate did not reveal a significant effect on biomethane production in the reactors. Contrarily, the amount of inoculum had a remarkable effect on biomethane production, resulted in an increase in methane production between 28 and 32%. While the biomethane yields were in the range of 129-138 mL_N CH₄ g_VS^-1 for the co-digesters operated with inoculum/substrate ratio of 1, the range was 165-181 mL_N CH₄ g_VS^-1 for the co-digesters operated with inoculum/substrate ratio of 2. These findings represent the potential usage of the spent tea waste as a co-substrate within the sustainable waste management approach and are relevant to plant operators.

An anaerobic dynamic membrane bioreactor for enhancing sludge digestion: Impact of solids retention time on digestion efficacy

An anaerobic dynamic membrane bioreactor (AnDMBR), which enabled the decoupling of hydraulic retention time (HRT) and solids retention time (SRT), was used for enhancing sludge digestion, with the associated mechanisms elucidated. With the increase of SRT, the biogas production and sludge reduction rate were both enhanced. The specific biogas production and volatile solids (VS) reduction rate were improved to 0.79 L/g VS and 55.9% under SRT 50 d, respectively. Microbial community analysis revealed that Chloroflexi, which is capable of degrading metabolites and dead cells, was enriched at longer SRT. Further analysis showed that both acetoclastic and hydrogenotrophic methanogenesis contributed to the enhanced biogas production under higher SRT compared to the dominance of acetoclastic methanogenesis under lower SRT. The enhanced utilization of organic matter and acetate at longer SRT further confirmed the mechanisms. The results highlighted the potential of AnDMBR for high-efficient sludge digestion.

Bacterial community responses to the redox profile changes of mariculture sediment

Suspended mariculture has significantly influences on the benthic sediment. However, our understanding on how bacterial communities respond to mariculture induced changes in redox profiles is limited. In present study, sediments from two maricultures and reference areas were collected and incubated for 28 day. The results indicated that the dominant pathway of organic matter mineralization in the sediment varied from groups, in the reference, it was the iron reduction, but in the two mariculture groups it was the SO₄^2- reduction. Remarkable changes of bacteria community were recorded in the aerobic zone, where the abundances of 14 OTUs belonging to Gammaproteobacteria and Alphaproteobacteria were significantly higher than that in oxidation and anaerobic zones. However, 4 keystone OTUs were strictly anaerobic and belonging to Desulfobacteraceae (n = 3) and Marinilabiaceae (n = 1). The main environmental drivers determining sediment bacterial distribution were the particle organic carbon, dissolve oxygen, NO₃^-, and moisture content.

Changes in bacterial diversity of activated sludge exposed to titanium dioxide nanoparticles

The rapid growth of the use of nanomaterials in different modern industrial branches makes the study of the impact of nanoparticles on the human health and environment an urgent matter. For instance, it has been reported that titanium dioxide nanoparticles (TiO₂ NPs) can be found in wastewater treatment plants. Previous studies have found contrasting effects of these nanoparticles over the activated sludge process, including negative effects on the oxygen uptake. The non-utilization of oxygen reflects that aerobic bacteria were inhibited or decayed. The aim of this work was to study how TiO₂ NPs affect the bacterial diversity and metabolic processes on an activated sludge. First, respirometry assays of 8 h were carried out at different concentrations of TiO₂ NPs (0.5-2.0 mg/mL) to measure the oxygen uptake by the activated sludge. The bacterial diversity of these assays was determined by sequencing the amplified V3-V4 region of the 16S rRNA gene using Illumina MiSeq. According to the respirometry assays, the aerobic processes were inhibited in a range from 18.5 ± 4.8% to 37.5 ± 2.0% for concentrations of 0.5-2.0 mg/mL TiO₂ NPs. The oxygen uptake rate was affected mainly after 4.5 h for concentrations higher than 1.0 mg/mL of these nanoparticles. Results indicated that, in the presence of TiO₂ NPs, the bacterial community of activated sludge was altered mainly in the genera related to nitrogen removal (nitrogen assimilation, nitrification and denitrification). The metabolic pathways prediction suggested that genes related to biofilm formation were more sensitive than genes directly related to nitrification-denitrification and N-assimilation processes. These results indicated that TiO₂ NPs might modify the bacteria diversity in the activated sludge according to their concentration and time of exposition, which in turn impact in the performance of the wastewater treatment processes.

Effects of interchange ratio on sludge reduction and microbial community structures in an anaerobic/anoxic/oxic process with combined anaerobic side-stream reactor

Oxic-settling-anaerobic (OSA) process is effective in minimizing sludge production, by inserting an anaerobic side-stream reactor (ASSR) in the recycling bypass. Interchange ratio (IR), the quantity ratio of sludge entering the ASSR to the sludge in the main stream reactors, is one of the most important parameters for OSA process. In the present study, a laboratory-scale anaerobic/anoxic/oxic (A²/O) process combined with an ASSR (A²/O-ASSR) was operated for 366 days in parallel with a conventional A²/O process to investigate the effects of IR on sludge reduction. IR was assigned values of 5%, 8%, 10%, and 15%, and the A²/O-ASSR process achieved 14.0%, 16.0%, 24.1%, and 13.7% of sludge reduction, respectively. At the optimum IR of 10%, high through-put sequencing analysis showed that the microbes responsible for pollutant removal and ubiquitous in wastewater treatment remained predominant in the two systems, and slow-growing microbes related to hydrolysis, nitrogen and phosphorus removal increased in the A²/O-ASSR process, which probably played a key role in sludge reduction. 40.6-58.6% of sludge reduction was caused by sludge decay in the ASSR. The tiny amount of extracellular polymeric substance released in the A²/O-ASSR process was subthreshold to cause remarkable sludge reduction.