Assessing activated sludge morphology and oxygen transfer performance using image analysis

Opportunities in optical and electrical single-cell technologies to study microbial ecosystems

New techniques are revolutionizing single-cell research, allowing us to study microbes at unprecedented scales and in unparalleled depth. This review highlights the state-of-the-art technologies in single-cell analysis in microbial ecology applications, with particular attention to both optical tools, i.e., specialized use of flow cytometry and Raman spectroscopy and emerging electrical techniques. The objectives of this review include showcasing the diversity of single-cell optical approaches for studying microbiological phenomena, highlighting successful applications in understanding microbial systems, discussing emerging techniques, and encouraging the combination of established and novel approaches to address research questions. The review aims to answer key questions such as how single-cell approaches have advanced our understanding of individual and interacting cells, how they have been used to study uncultured microbes, which new analysis tools will become widespread, and how they contribute to our knowledge of ecological interactions.

Single-Cell Techniques in Environmental Microbiology

Environmental microbiology has been an essential part of environmental research because it provides effective solutions to most pollutants. Hence, there is an interest in investigating microorganism behavior, such as observation, identification, isolation of pollutant degraders, and interactions between microbial species. To comprehensively understand cell heterogeneity, diverse approaches at the single-cell level are demanded. Thus far, the traditional bulk biological tools such as petri dishes are technically challenging for single cells, which could mask the heterogeneity. Single-cell technologies can reveal complex and rare cell populations by detecting heterogeneity among individual cells, which offers advantages of higher resolution, higher throughput, more accurate analysis, etc. Here, we overviewed several single-cell techniques on observation, isolation, and identification from aspects of methods and applications. Microscopic observation, sequencing identification, flow cytometric identification and isolation, Raman spectroscopy-based identification and isolation, and their applications are mainly discussed. Further development on multi-technique integrations at the single-cell level may highly advance the research progress of environmental microbiology, thereby giving more indication in the environmental microbial ecology.

Understanding the role of activated sludge in oxygen transfer: Effects of sludge settleability, solids retention time, and nitrification reaction

The current understanding on the oxygen transfer in activated sludge process is primarily developed based on two-phase systems, focusing only on oxygen transfer from air to water. However, this research demonstrates that activated sludge particles significantly impact oxygen transfer from air all the way to the microorganisms. Three bench-scale complete-mix activated sludge reactors, operated under the same influent loading and dissolved oxygen level but different solids retention times (SRTs), were used to develop oxygen transfer performance data as effects of different sludge property parameters. These reactors were also operated under batch modes to further validate the effect of nitrification reaction on oxygen transfer. Results indicate that high overall oxygen transfer efficiency (OTE) is associated with low mixed liquor viscosity, long SRT, and nitrification reaction. Further analyses suggest that low mixed liquor viscosity, which resulted from high sludge settleability or low settled volume of sludge, reduces the thickness of liquid films at all interfaces and the size of air bubbles. Long SRT results in high active nitrifier population and low specific extracellular polymeric substance (EPS). Nitrification reaction, which serves as the rate-limiting step for oxygen transfer, may increase the oxygen transfer driving force. High active nitrifier population also promotes direct air-sludge contact. All of these factors help facilitate oxygen transfer. This research provides a new approach to improve energy efficiency for wastewater treatment, which is to change the activated sludge property by adjusting treatment plant design and operational parameters. PRACTITIONER POINTS: High sludge settleability reduces viscosity therefore liquid film thickness. Long SRT increases active microorganism population and reduces specific EPS content. Nitrification reaction increases oxygen transfer driving force. Direct air-particle contact adds another pathway for oxygen transfer. Nitrification reaction is the rate-limiting step of the oxygen transfer process.

Stratification patterns of anammox granular sludge bed: Linking particle size distribution to microbial activity and community

Anammox granular sludge processes are an attractive and efficient biotechnology in the field of wastewater treatment. In this study, the stratification patterns of anammox granular sludge bed (GSB) at steady states were illustrated and its relationship to microbial activity and community were systematically investigated under different nitrogen loading rates (NLRs). Morphological observation and quantitive particle size distribution analysis demonstrated that the GSB at low NLR was mainly composed of micro and fine granules with a big difference between bottom and top sludge layers. But at high NLR, the volumetric mean diameter (VMD) of GSB increased with the size distribution width (Span) declined forming a more homogeneous and coarse granules population. The particle size distribution parameters of GSB could be fast characterized by the optical lightness (L*) parameter (r = -0.771, p < 0.01, n = 16) and held a significant correlation with the nitrogen removal rate (NRR) of anammox system (r > 0.9, p < 0.05). The microbial spatial distribution patterns of different sludge layers were further investigated by high-throughput sequencing. The microbial community α-diversity index and microbial abundance matrix proved that the community structure tend to coverage at high NLR. Significant difference of the relative abundance of microbial community was detected under different NLRs. The VMD of GSB held a significant correlation with the relative abundance of AnAOB (r = 0.556, p < 0.01, n = 16) and other common accompanying bacteria (Denitratisoma and Chloroflexi). This study proved that the apparent particle size distribution patterns of GSB could be a potential auxiliary indicator to reflect the microbial activity and community, which can facilitate the innovative process monitor of anammox system based on visual features.

Hitchhiking Behavior in Bacteriophages Facilitates Phage Infection and Enhances Carrier Bacteria Colonization

Interactions between bacteriophages (phages) and biofilms remain poorly understood despite the broad implications for microbial ecology, water quality, and microbiome engineering. Here, we demonstrate that lytic coliphage PHH01 can hitchhike on carrier bacteria Bacillus cereus to facilitate its infection of host bacteria, Escherichia coli, in biofilms. Specifically, PHH01 could adsorb onto the flagella of B. cereus, and thus phage motility was increased, resulting in 4.36-fold more effective infection of E. coli in biofilm relative to free PHH01 alone. Moreover, phage infection mitigated interspecies competition and enhanced B. cereus colonization; the fraction of B. cereus in the final biofilm increased from 9% without phages to 43% with phages. The mutualistic relationship between the coliphage and carrier bacteria was substantiated by migration tests on an E. coli lawn: the conjugation of PHH01 and B. cereus enhanced B. cereus colonization by 6.54-fold compared to B. cereus alone (6.15 vs 0.94 cm² in 24 h) and PHH01 migration by 5.15-fold compared to PHH01 alone (10.3 vs 2.0 mm in 24 h). Metagenomic and electron microscopic analysis revealed that the phages of diverse taxonomies and different morphologies could be adsorbed by the flagella of B. cereus, suggesting hitchhiking on flagellated bacteria might be a widespread strategy in aquatic phage populations. Overall, our study highlights that hitchhiking behavior in phages can facilitate phage infection of biofilm bacteria, promote carrier bacteria colonization, and thus significantly influence biofilm composition, which holds promise for mediating biofilm functions and moderating associated risks.

Implementation of long solids retention time activated sludge process for rural residential community

Most rural communities in the United States are facing increasingly rigorous effluent criteria, especially ammonia, for their wastewater treatment facilities. A new baffled bioreactor (BBR) technology, which employs a preanoxic activated sludge process operated with a long solids retention time (SRT), was installed in a small community in Missouri to address the more stringent effluent limits. In a recent full-year normal operation cycle (2018), the average effluent concentrations of BOD₅ , TSS, and ammonia-nitrogen were 3.2, 2.2, and 0.5 mg/L, respectively, with removal efficiencies of 96%, 85%, and 98%, respectively. All these parameters were significantly better than their respective permit limits. The long SRT afforded an enhanced factor of safety for the process, conferring the ability to nitrify at sustained ambient temperatures as low as -22°C. Long SRT also resulted in significant reductions in waste sludge production, resulting in dramatically reduced operational costs for sludge handling. Ultimately, the long SRT activated sludge process afforded the ability to meet stringent effluent quality standards including ammonia and the numerous unique challenges that are inherent to small flows. PRACTITIONER POINTS: Small community hydraulic and mass loadings are highly variable and difficult to quantify during facility design. A long SRT activated sludge process warrants superior performance and enhanced factor of safety. The long SRT process with preanoxic zones generated no excess sludge during the extended operation period, significantly simplifying plant operation. Long SRT process is well suited to accommodate wastewater variability associated with small communities while maintaining superior treatment quality.

Filamentous organisms degrade oxygen transfer efficiency by increasing mixed liquor apparent viscosity: Mechanistic understanding and experimental verification

Recent findings have demonstrated that activated sludge morphology significantly impacts oxygen transfer efficiency (OTE) in the activated sludge process. In this study, we developed a mechanistic understanding of this impact. Mixed liquor samples collected from a domestic wastewater treatment plant (WWTP) were blended with a bulking activated sludge from a bench scale reactor (BSR) cultured on synthetic wastewater to manipulate various morphological parameters such as the settled sludge volume (SV), the sludge volume index (SVI), and the specific filament length (SFL). The filaments that were present in the blended sludges consisted largely of Type 0041 and Type 021N, which are commonly found in WWTPs that treat domestic wastewater. Variations in sludge morphology, as quantified by settled sludge volume after 30 min (SV₃₀), SVI, and SFL, systematically affected the mixed liquor apparent viscosity (μ_app), which consequently impacted OTE. An increase in the SFL from 9.61 × 10⁶ μm g^-1 to 6.88 × 10⁷ μm g^-1 resulted in a 41.4% increase in apparent viscosity and a 24.6% decrease in volumetric mass transfer coefficient (K_La). A new parameter, named the ultimate settleability (SV_ULT), was developed by curve fitting the SV versus time data and found to relate with μ_app through an expanded form of the Einstein Equation for the viscosity. Therefore, SV_ULT is a corollary for the particle volume fraction that incorporates effects of both the sludge morphology and mass concentration on μ_app. Theoretical derivation revealed that an increase in SV_ULT resulted in an increase in μ_app, which reduced oxygen transfer by increasing the air bubble size and reducing refreshment of the liquid at the gas-liquid interface. The K_La was found to be inversely proportional to μ_app^0.75 through fitting the experimental data with the theoretical model. Using a variance-based global sensitivity analysis, three operating parameters that have the most impact on oxygen transfer were identified: the power input per unit volume, the superficial gas flowrate, and the μ_app.

Coupling ammonia nitrogen adsorption and regeneration unit with a high-load anoxic/aerobic process to achieve rapid and efficient pollutants removal for wastewater treatment

In this study, an ammonium nitrogen (NH₄⁺-N) adsorption and regeneration (AAR) was constructed by a zeolite-packed column and NaClO-NaCl regeneration unit, and coupled with an anoxic/aerobic (AO) system to achieve efficient removal of carbon, nitrogen and phosphorus under short hydraulic retention time (HRT) and sludge retention time (SRT). Compared to conventional anaerobic/anoxic/aerobic (AAO) process, the proposed AO-AAR process achieved more efficient and stable nitrogen removal with greatly shorter HRT (5.6 h) and SRT (8 d) at 10.4 °C, with NH₄⁺-N and total nitrogen in the effluent below 1.5 and 8.0 mg/L, respectively. The AO-AAR also obtained efficient phosphorus removal (<0.5 mg/L) by dosing aluminum in aerobic tank. High load and short SRT deteriorated sludge settleability and dewaterability, but enhanced methane production by improving sludge biodegradability. Dosing aluminum made the AO operating module more stable with improved settleability and dewaterability, and further enhanced methane production. Short HRT and SRT also resulted in the thriving of filamentous bacteria (Thiothrix) and heterotrophic nitrifiers (Acinetobacter, Pseudomonas and Rhodobacter) in the AO module, which helped in enhancing denitrification potential and nitrification efficiency under low temperature. Long-term operation showed that exchange capacity and physicochemical properties of zeolite were unchanged under NaClO-NaCl regeneration by introducing the tail gas from aerobic tank into the used regenerant to remove Ca²⁺ and Mg²⁺ exchanged from effluent of the AO module. Techno-economic analysis showed that the AO-AAR process is attractive and sustainable for municipal wastewater treatment by significantly improving nitrogen removal, greatly reducing land occupancy, enhancing methane production and achieving efficient reduction of carbon dioxide emission.