Foaming in membrane bioreactors: identification of the causes

Treatment of volatile organic compounds and other waste gases using membrane biofilm reactors: A review on recent advancements and challenges

This article provides a comprehensive review of membrane biofilm reactors for waste gas (MBRWG) treatment, focusing on studies conducted since 2000. The first section discusses the membrane materials, structure, and mass transfer mechanism employed in MBRWG. The concept of a partial counter-diffusion biofilm in MBRWG is introduced, with identification of the most metabolically active region. Subsequently, the effectiveness of these biofilm reactors in treating single and mixed pollutants is examined. The phenomenon of membrane fouling in MBRWG is characterized, alongside an analysis of contributory factors. Furthermore, a comparison is made between membrane biofilm reactors and conventional biological treatment technologies, highlighting their respective advantages and disadvantages. It is evident that the treatment of hydrophobic gases and their resistance to volatility warrant further investigation. In addition, the emergence of the smart industry and its integration with other processes have opened up new opportunities for the utilization of MBRWG. Overcoming membrane fouling and developing stable and cost-effective membrane materials are essential factors for successful engineering applications of MBRWG. Moreover, it is worth exploring the mechanisms of co-metabolism in MBRWG and the potential for altering biofilm community structures.

Combined recovery of polyhydroxyalkanoates and reclaimed water in the mainstream of a WWTP for agro-food industrial wastewater valorisation by membrane bioreactor technology

The present study investigated the combined production of reclaimed water for reuse purposes and polyhydroxyalkanoates (PHA) from an agro-food industrial wastewater. A pilot plant implementing a two-stage process for PHA production was studied. It consisted of a mainstream sequencing batch membrane bioreactor (SBMBR) in which selection of PHA-accumulating organisms and wastewater treatment were carried out in, and a side-stream fed-batch reactor (FBR) where the excess sludge from the SBMBR was used for PHA accumulation. The performance of the SBMBR was compared with that of a conventional sequencing batch reactor (SBR) treating the same wastewater under different food to microorganisms' ratios (F/M) ranging between 0.125 and 0.650 kgCOD kgTSS-3 d-1. The SBMBR enabled to obtain very high-quality effluent in compliance with the relevant national (Italy) and European regulations (Italian DM 185/03 and EU, 2020/741) in the field of wastewater reclamation, whereas the performances in the SBR collapsed at F/M higher than 0.50 kgCOD kgTSS-1d-1. A maximum intracellular storage of 45% (w/w) and a production yield of 0.63 gPHA L-1h-1 were achieved when the SBMBR system was operated with a F/M ratio close to 0.50 kgCOD kgTSS-1d-1. This resulted approximately 35% higher than those observed in the SBR, since the ultrafiltration membrane avoided the washout of dispersed and filamentous bacteria capable of storing PHA. Furthermore, while maximizing PHA productivity in conventional SBR systems led to process dysfunctions, in the SBMBR system it helped mitigate these issues by reducing membrane fouling behaviour. The results of this study supported the possibility to achieve combined recovery of reclaimed water and high-value added bioproducts using membrane technology, leading the way for agro-food industrial wastewater valorization in the frame of a circular economy model.

Integrated Treatment of Per- and Polyfluoroalkyl Substances in Existing Wastewater Treatment Plants-Scoping the Potential of Foam Partitioning

Foam fractionation is becoming increasingly popular as a treatment technology for water contaminated with per- and polyfluoroalkyl substances (PFAS). At many existing wastewater treatment facilities, particularly in aerated treatment steps, foam formation is frequently observed. This study aimed to investigate if foam fractionation for the removal of PFAS could be integrated with such existing treatment processes. Influent, effluent, water under the foam, and foam were sampled from ten different wastewater treatment facilities where foam formation was observed. These samples were analyzed for the concentration of 29 PFAS, also after the total oxidizable precursor (TOP) assay. Enrichment factors were defined as the PFAS concentration in the foam divided by the PFAS concentration in the influent. Although foam partitioning did not lead to decreased ∑PFAS concentrations from influent to effluent in any of the plants, certain long-chain PFAS were removed with efficiencies up to 76%. Moreover, ∑PFAS enrichment factors in the foam ranged up to 105, and enrichment factors of individual PFAS ranged even up to 106. Moving bed biofilm reactors (MBBRs) were more effective at enriching PFAS in the foam than activated sludge processes. Altogether, these high enrichment factors demonstrate that foam partitioning in existing wastewater treatment plants is a promising option for integrated removal. Promoting foam formation and removing foam from the water surface with skimming devices may improve the removal efficiencies further. These findings have important implications for PFAS removal and sampling strategies at wastewater treatment plants.

Landfill leachate treatment with a full-scale membrane bioreactor: impact of leachate characteristics on filamentous bacteria

Bulking and foaming are extreme filamentous bacterial growths that present serious challenges for the biological leachate treatment process. The current study evaluates the performance of long-term full-scale membrane bioreactor (MBR) treating landfill leachate, specifically focusing on filamentous bacteria overgrowth in the bioreactors. The influence of the variation in leachate structure and operational conditions on floc morphology and filamentous bacteria overgrowth were analyzed for 11 months of operation of the full-scale MBR system. The average chemical oxygen demand (COD) and NH4-N removal efficiencies of the system were 87.8 ± 4% and 99.5 ± 0.7%. However, incomplete denitrification was observed when the F/M ratio was low. The high C/N ratio was observed to enhance the frequency of small flocs. Furthermore, a poor to medium diversity of the microbial community was observed. Haliscomenobacter hydrossis, Microthrix parvicella, and Type 021N were found as the most numerous filamentous organisms. Paramecium spp., Euplotes spp., and Aspidisca spp. were found in small quantities. The limited concentration of PO4-P in the leachate compared to high COD and NH4-N concentrations most probably caused phosphate deprivation and increased abundance of identified filamentous microorganisms. This work is the first study in Türkiye that investigates the bulking and foaming problem in full-scale MBR that treats landfill leachate. Hence, it may provide some pioneering perspectives into landfill leachate remediation by monitoring the hybrid biological system.

Bacteriophages in wastewater treatment: can they be an approach to optimize biological treatment processes?

In this paper, we explore the applications of bacteriophages and the advantages of using these viruses to control undesirable organisms in wastewater treatment plants. Based on this, this paper reviewed the literature on the subject by performing a bibliometric and scientometric analysis of articles published in peer-reviewed journals through 2021. We obtained 806 publications, of which 40% were published in the last 5 years, demonstrating an increase in interest in the subject. These articles analyzed, bacteriophages in treatment plants were strongly linked to bacteria such as Escherichia coli and related to disinfection, inactivation, sewage, and wastewater, in addition, biocontrol studies have gained prominence in recent years, particularly due to the resistance of microorganisms to antibiotics. Studies have shown that bacteriophages have great potential for application in treatment systems to control unwanted processes and act as valuable economic and environmental tools to improve the efficiency of various treatment technologies. Although these viruses have already been studied in various applications to optimize treatment plant processes, technology transfer remains a challenge due to the limitations of the technique-such as physicochemical factors related to the environment-and the complexity of biological systems. The research focusing on application strategies in conjunction with molecular biology techniques can expand this study area, enabling the discovery of new bacteriophages.

A Multi-View Image Feature Fusion Network Applied in Analysis of Aeration Velocity for WWTP

The instability of the aeration system brings a significant challenge to the management of wastewater treatment plants (WWTP). Using image recognition methods to monitor aeration conditions accurately and enhance management efficiency is a promising way to solve this problem. To improve the efficiency of aeration condition identification and provide support for troubleshooting, we propose a method for aeration velocity condition identification based on a multi-view image feature fusion network (MVNN). Firstly, an experimental platform for simulating aeration tanks is established, and two cameras are used to acquire aeration images from different perspectives. Secondly, an image data set with 10 aeration velocity gradients is constructed and applied to the network’s training. Finally, the MVNN is used to extract and fuse the features of aeration images, and the model’s performance is evaluated on the dataset. Experiments show that the average accuracy of the method is over 98.3%, and the AUC of aeration identification is above 0.98, which indicates that the model has the potential for practical application in WWTP.

Evaluation of foaming potential for water treatment: limits and developments

The critical issue generated by foaming in wastewater treatment plants (WWTPs) is a problem that is currently very common and shared, but which to date is treated mainly only at the management level. In this work, an experimental study with foam tests on real and synthetic waters was conducted using a laboratory scale plant and foaming power indices were calculated. To date, the estimation of foaming potential is mainly based on these indices which give information only on height/volume of foams but not on the type of foams, in terms of consistency and therefore stability. Tests showed that foaming power indices were highly variable with the same water: it was not possible to identify a single foaming potential value for each water. Two models were proposed to estimate the percentage increase in height of chemical foams produced following the introduction of air below the surface of a liquid. In terms of determination coefficient, the results obtained from the complex model were better: R² was 0.82 for the simple linear model and 0.90 for the complex one. This approach has allowed to underline some critical aspects of foaming potential as it is determined today and the possible improvements applicable for a more objective evaluation.

Foams in Wastewater Treatment Plants: From Causes to Control Methods

The formation of persistent foams can be a critical problem in wastewater treatment plants (WWTPs) as it could lead to a series of operational problems, especially the reduction of the overall system performance. To date, the effects of foaming in the WWTPs are a problem that is currently very common and shared, but which to date is treated mainly only at the management level and still too little studied through a globally shared scientific method: the complexity of the phenomenon and the systems have led to numerous partially contradictory descriptions and hypotheses over the years. The goal must be to suggest future research directions and indicate promising strategies to prevent or control the formation of foams in WWTPs. This study examines and investigates the problem of foams by a methodological approach of research through a review on the state of the art: the factors influencing the formation of foams are described first (such as surfactants and/or extracellular polymeric substances (EPSs)), then the known methods for the evaluation of foaming, both direct and indirect, are presented, with the aim of identifying the correct and best (from the management point of view) control and/or prevention strategies to be applied in the future in WWTPs.

Application of a novel molecular technique to characterise the effect of settling on microbial community composition of activated sludge

Activated sludge (AS) and return activated sludge (RAS) microbial communities from three full-scale municipal wastewater treatment plants (denoted plant A, B and C) were compared to assess the impact of sludge settling (i.e. gravity thickening in the clarifier) and profile microorganisms responsible for nutrient removal and reactor foaming. The results show that all three plants were dominated with microbes in the phyla of Proteobacteria, Bacteroidetes, Verrucomicrobia, Actinobacteria, Chloroflexi, Firmicutes, Nitrospirae, Spirochaetae, Acidobacteria and Saccharibacteria. AS and RAS shared above 80% similarity in the microbial community composition, indicating that sludge thickening does not significantly alter the microbial composition. Autotrophic and heterotrophic nitrifiers were present in the AS. However, the abundance of autotrophic nitrifiers was significantly lower than that of the heterotrophic nitrifiers. Thus, ammonium removal at these plants was achieved mostly by heterotrophic nitrification. Microbes that can cause foaming were at 3.2% abundance, and this result is well corroborated with occasional aerobic biological reactor foaming. By contrast, these microbes were not abundant (<2.1%) at plant A and C, where aerobic biological reactor foaming has not been reported.

Investigation of foaming causes in three mesophilic food waste digesters: reactor performance and microbial analysis

Foaming negatively affects anaerobic digestion of food waste (FW). To identify the causes of foaming, reactor performance and microbial community dynamics were investigated in three mesophilic digesters treating FW. The digesters were operated under different modes, and foaming was induced with several methods. Proliferation of specific bacteria and accumulation of surface active materials may be the main causes of foaming. Volatile fatty acids (VFAs) and total ammonia nitrogen (TAN) accumulated in these reactors before foaming, which may have contributed to foam formation by decreasing the surface tension of sludge and increasing foam stability. The relative abundance of acid-producing bacteria (Petrimonas, Fastidiosipila, etc.) and ammonia producers (Proteiniphilum, Gelria, Aminobacterium, etc.) significantly increased after foaming, which explained the rapid accumulation of VFAs and NH4+ after foaming. In addition, the proportions of microbial genera known to contribute to foam formation and stabilization significantly increased in foaming samples, including bacteria containing mycolic acid in cell walls (Actinomyces, Corynebacterium, etc.) and those capable of producing biosurfactants (Corynebacterium, Lactobacillus, 060F05-B-SD-P93, etc.). These findings improve the understanding of foaming mechanisms in FW digesters and provide a theoretical basis for further research on effective suppression and early warning of foaming.

Foam-forming bacteria in activated sludge effectively reduced by rotifers in laboratory- and real-scale wastewater treatment plant experiments

Lecane inermis rotifers were shown to diminish sludge bulking due to their ability to ingest the filamentous bacteria in activated sludge. To determine if rotifers are also able to control branched actinomycetes, we investigated three other Lecane species (Monogononta). In a week-long experiment, only Lecane tenuiseta significantly reduced the density of Microthrix parvicella and Type 0092 filaments, but in a 2-week experiment, actinomycetes were significantly reduced by most of the tested monogonont rotifers: L. inermis, Lecane decipiens and Lecane pyriformis. Rotifers L. inermis originating from the mass culture were artificially introduced into real-scale wastewater treatment plant (WWTP) in two series. The WWTP was monitored for 1 year. Rotifer inoculation resulted in diminishing of M. parvicella and actinomycete abundance. The experiments showed that different species of rotifers vary in their effectiveness at limiting various types of filamentous organisms. This is the first report demonstrating that one of the most troublesome bacteria, branched actinomycetes, which cause heavy foaming in bioreactors, can be controlled by rotifers. Knowledge of the consumers of filamentous bacteria that inhabit activated sludge could help WWTP operators overcome bulking and foaming through environmentally friendly methods.

Sewage sludge treatment in a thermophilic membrane reactor (TMR): factors affecting foam formation

Foam formation in the excess sludge treatment facilities of biological wastewater treatment plants (WWTPs) may represent a critical issue as it could lead to several operative problems and reduce the overall plant performance. This trouble also affects a novel technology recently proposed for sludge minimization, the thermophilic membrane reactor (TMR), operating with alternate aeration/non-aeration cycles. This technology, which has proven to be extremely resilient and suitable for treating industrial wastewater of different nature, demonstrated a high potential also as a solution for integrating existing WWTPs, aiming at the "zero sludge production." In this work, an experimental study was conducted with a TMR pilot plant (fed daily with thickened sewage sludge) by adjusting the duration of aeration/non-aeration alternate cycles. Extracellular polymeric substance (EPS) concentration (and its soluble and bound fractions) has been monitored along with foaming power indices. The results highlight that foaming can be correlated to the presence of soluble protein fraction of EPS. Moreover, EPS production seems to be reduced by increasing the duration of the non-aeration cycles: optimal operating conditions resulted 2 h of aeration followed by 6 h of non-aeration. These conditions allow to obtain an EPS concentration of 500 mg L^-1 with respect to 2300 mg L^-1 measured at the beginning of experimental work.

Linking operation parameters and environmental variables to population dynamics of Mycolata in a membrane bioreactor

The community structure and population dynamics of Mycolata were monitored in a full-scale membrane bioreactor during four experimental phases under changing operating and environmental conditions, by means of temperature-gradient gel electrophoresis of partial 16S-rRNA genes amplified from community DNA and RNA templates (total and active populations). Non-metric multidimensional scaling and BIO-ENV analyses demonstrated that population dynamics were mostly explained (30-32%) by changes in the input of nutrients in the influent water and the accumulation of biomass in the bioreactors, while the influence of hydraulic and solid retention times, temperature and F/M ratio was minor. Significant correlations were observed between particular Mycolata phylotypes and one or more variables, contributing information for the prediction of their abundance and activity under changing conditions. Fingerprinting and multivariate analyses demonstrated that two foaming episodes, recorded at temperatures <20°C, were connected to the increase of the relative abundance of Mycolata unrelated to Gordonia amarae.

Effect of chemical and biological surfactants on activated sludge of MBR system: microscopic analysis and foam test

A bench-scale MBR unit was operated, under stressing condition, with the aim of stimulating the onset of foaming in the activated sludge. Possible synergies between synthetic surfactants in the wastewater and biological surfactants (Extra-Cellular Polymeric Substances, EPSs) were investigated by changing C/N ratio. The growth of filamentous bacteria was also discussed. The MBR unit provided satisfactory overall carbon removal overall efficiencies: in particular, synthetic surfactants were removed with efficiency higher than 90% and 95% for non-ionic and ionic surfactants, respectively. Lab investigation suggested also the importance to reduce synthetic surfactants presence entering into mixed liquor: otherwise, their presence can significantly worsen the natural foaming caused by biological surfactants (EPSs) produced by bacteria. Finally, a new analytic method based on "ink test" has been proposed as a useful tool to achieve a valuation of EPSs bound fraction.

Pilot scale experiment with MBR operated in intermittent aeration condition: analysis of biological performance

The effect of intermittent aeration (IA) on a MBR system was investigated. The study was aimed at analyzing different working conditions and the influence of different IA cycles on the biological performance of the MBR pilot plant, in terms of organic carbon and ammonium removal as well as extracellular polymeric substances (EPSs) production. The membrane modules were placed in a separate compartment, continuously aerated. This configuration allowed to disconnect from the filtration stage the biological phenomena occurring into the IA bioreactor. The observed results highlighted good efficiencies, in terms of organic carbon and ammonium removal. It was noticed a significant soluble microbial products (SMPs) release, likely related to the higher metabolic stress that anoxic conditions exerted on the biomass. However, the proposed configuration, with the membranes in a separate compartment, allowed to reduce the EPSs in the membrane tank even during the non-aerated phase, thus lowering fouling development.