Anaerobic Sludge Treatment Research Articles

Effect, Fate and Remediation of Pharmaceuticals and Personal Care Products (PPCPs) during Anaerobic Sludge Treatment: A Review.

Biomass energy recovery from sewage sludge through anaerobic treatment is vital for environmental sustainability and a circular economy. However, large amounts of pharmaceutical and personal care products (PPCPs) remain in sludge, and their interactions with microbes and enzymes would affect resource recovery. This article reviews the effects and mechanisms of PPCPs on anaerobic sludge treatment. Most PPCPs posed adverse impacts on methane production, while certain low-toxicity PPCPs could stimulate volatile fatty acids and biohydrogen accumulation. Changes in the microbial community structure and functional enzyme bioactivities were also summarized with PPCPs exposure. Notably, PPCPs such as carbamazepine could bind with the active sites of the enzyme and induce microbial stress responses. The fate of various PPCPs during anaerobic sludge treatment indicated that PPCPs featuring electron-donating groups (e.g., ·-NH2 and ·-OH), hydrophilicity, and low molecular weight were more susceptible to microbial utilization. Key biodegrading enzymes (e.g., cytochrome P450 and amidase) were crucial for PPCP degradation, although several PPCPs remain refractory to biotransformation. Therefore, remediation technologies including physical pretreatment, chemicals, bioaugmentation, and their combinations for enhancing PPCPs degradation were outlined. Among these strategies, advanced oxidation processes and combined strategies effectively removed complex and refractory PPCPs mainly by generating free radicals, providing recommendations for improving sludge detoxification.

Study on the Environmental Impact and Benefits of Incorporating Humus Composites in Anaerobic Co-Digestion Treatment.

This study evaluated the environmental impact and overall benefits of incorporating humus composites in the anaerobic co-digestion of kitchen waste and residual sludge. The life cycle assessment method was used to quantitatively analyze the environmental impact of the entire anaerobic co-digestion treatment process of waste, including garbage collection, transportation, and final product utilization. Moreover, the comprehensive assessment of the environmental impact, energy-saving and emission-reduction abilities, and economic cost of using humus composites in the anaerobic co-digestion treatment process was conducted using a benefit analysis method. The results showed that the anaerobic co-digestion of kitchen waste and residual sludge significantly contributed to the mitigation of global warming potential (GWP), reaching -19.76 kgCO2-eq, but had the least impact on the mitigation of acidification potential (AP), reaching -0.10 kgSO2-eq. In addition, the addition of humus composites significantly increased the production of biogas. At a concentration of 5 g/L, the biogas yield of the anaerobic co-digestion process was 70.76 m3, which increased by 50.62% compared with the blank group. This amount of biogas replaces ~50.52 kg of standard coal, reducing CO2 emissions by 13.74 kg compared with burning the same amount of standard coal. Therefore, the anaerobic co-digestion treatment of kitchen waste and residual sludge brings considerable environmental benefits.

Microbiology of post-fermentation leachate

Using post-fermentation leachate as fertilizer is a good alternative and solution to the problem of waste at wastewater treatment plants and contains the principles of sustainable development. Leachate from anaerobic treatment of sewage sludge contains valuable elements such as phosphorus, nitrogen and potassium, which can improve soil properties. The production of liquid fertilizer minimizes the negative impact of leachate on the main biological stream in the WWTP and improves the energy efficiency of the entire wastewater treatment system. This transfers into the operating costs of the facility. When thinking about the agricultural use of digestion leachate and its introduction into the environment, a very important issue is its microbiological contamination. The purpose of the conducted research was to determine the microbiological quality of digestion leachates from municipal wastewater treatment plants and to determine the parameters of their hygienization. The number of bacteria in raw leachate is indicative of a sanitary risk in the case they are used in agriculture as fertiliser. Heating the leachate at 60°C for 15 minutes produces an effect comparable to that achieved by heating the leachate to 70°C. The sonication process itself did not affect the better temperature effect. Heating the leachate for 20 minutes after prior sonication for 20 minutes does not result in the complete elimination of microflora.

Inhibitory effects of long chain fatty acids on anaerobic sludge treatment: Biomass adaptation and microbial community assessment

The study investigated the effects of long-chain fatty acids (LCFA) on anaerobic sludge treating lipid-rich wastewater. It involved batch experiments with three sludge samples: two acclimated to lipids and one non-acclimated. The experiments aimed to observe the degradation of LCFA, specifically oleate and palmitate, by dosing them at concentrations ranging from 50 to 600 mg/L. Measurements of the cumulative methane production and the LCFA concentration, quantified as fat, oil, and grease (FOG) were performed. To ensure the sludge was free from other biodegradable substrates, part of the samples was pre-incubated without feed. The tests were conducted with both pre-incubated and non-incubated inoculum sludge. The findings revealed that oleate was degraded more efficiently than palmitate across all sludge samples, with a greater conversion rate to methane. Sludge samples acclimated to lipids showed a superior capacity to degrade LCFA compared to non-acclimated ones. It was noted that at concentrations above 400 mg/L, the conversion of LCFAs to intermediate compounds was inhibited, although this did not affect the subsequent methane production. The study concludes with a recommendation for sludge adaptation strategies to boost the efficiency of anaerobic wastewater treatment systems dealing with lipid-rich waste. The presence of LCFA-degrading bacteria families like Kosmotogaceae, Petrotogaceae, and Synergistaceae in the acclimated sludge samples underscores the adaptation and potential for improved degradation performance.

Development of a Kinetics Based Onsite Treatment System for Faecal Sludge Treatment and Resource Recovery

The management of faecal sludge in cities without a sewerage system is a challenging problem that requires immediate attention. The high organic and nutrient content, pathogen load, and pharmaceutical residues in faecal sludge necessitate special attention for its treatment to ensure environmental and public health safety. Onsite treatment of faecal sludge with a focus on resource recovery is a sustainable approach to its management. However, studies in these areas are limited, particularly in the Indian context. In this study, the aim is to generate primary data on the current status of faecal sludge management in a model region without a sewerage network. Additionally, kinetic parameters were examined to design an anaerobic bioprocess and an engineered system for resource-oriented faecal sludge treatment. One of the wards in Thiruvananthapuram Corporation, Kerala was selected for the study. Fresh faecal sludge collected from an apartment complex was used for the experimental studies. Anaerobic treatment of fresh faecal sludge was validated in bench-scale experimental bioreactors using different combinations: with mixing without inoculum, with mixing with inoculum, without mixing without inoculum, and without mixing with inoculum. The primary focus was on monitoring the organic removal efficiency as Chemical Oxygen Demand (COD) of these combinations. Among the four combinations tested the units "without mixing with inoculum" and "with mixing with inoculum" demonstrated COD removal efficiencies of 20.89% and 18.91%, respectively, with a first-order rate constant of 0.0008 hr-1 for both combinations. The other combinations exhibited comparatively lower COD removal efficiency and rate constants. Since the combinations with mixing and inoculum showed higher COD removal efficiency, either of them can be adopted for design purposes. Based on these results, a combined anaerobic-aerobic treatment unit is proposed for the treatment of faecal sludge.

Treatment of phenolic wastewater by anaerobic fluidized bed microbial fuel cell using carbon brush as anode: microbial community analysis and m-cresol degradation mechanism.

Anaerobic fluidized bed microbial fuel cell (AFB-MFC) is a technology that combines fluidized bed reactor and microbial fuel cell to treat organic wastewater and generate electricity. The performance and the mechanism of treating m-cresol wastewater in AFB-MFC using carbon brush as biofilm anode were studied. After 48h of operation, the m-cresol removal efficiency of AFB-MFC, MAR-AFB (fluidized bed bioreactor with acclimated anaerobic sludge), MAR-FB (ordinary fluidized bed reactor with only macroporous adsorptive resin) and AST (traditional anaerobic sludge treatment) were 95.29 ± 0.67%, 85.78 ± 1.81%, 71.24 ± 1.86% and 70.41 ± 0.32% respectively. The maximum output voltage and the maximum power density of AFB-MFC using carbon brush as biofilm anode were 679.7mV and 166.6 mW/m2 respectively. The results of high-throughput sequencing analysis indicated the relative abundance of dominant electroactive bacteria, such as Trichococcus, Geobacter, and Pseudomonas, on the anode carbon brushes was higher than that of AST, and also identified such superior m-cresol-degrading bacteria as Bdellovibrio, Thermomonas, Hydrogenophaga, etc. Based on the determination of m-cresol metabolites detected by Gas Chromatography-Mass Spectrometry (GC-MS), the possible biodegradation pathway of m-cresol under anaerobic and aerobic conditions in AFB-MFC was speculated. The results showed that m-cresol was decomposed into formic acid-acetic anhydride and 3-methylpropionic acid under the action of electrochemistry, which is a simple degradation pathway without peripheral metabolism in AFB-MFC.

Impacts of biochar derived from oil sludge on anaerobic digestion of sewage sludge: Performance and associated mechanisms

The addition of biochar is a promising strategy to improve methane production via strengthening direct interspecies electron transfer (DIET) in anaerobic digestion (AD) field. In this study, a novel oil sludge biochar (OSBC) prepared at 500/600/700 °C was used to investigate its dosage effects on performances of sewage sludge AD. The characterization results indicated that OSBC prepared at 600 °C had highest electron accepting and donating capacity, meanwhile, it also contained moderate functional groups such as CO, which were in favor of methane production. Therefore, the highest accumulative methane yield (143.96 mL (g VS)−1) accompanying with the fast consumption of volatile fatty acids (VFAs) was achieved at dosage of 1.2 g. Additionally, the conductivity and enzyme activity (electron transport system activity and coenzyme F420) of AD with OSBC addition were also enhanced. Microbial community structures manifested that the potential syntrophic microbes including archaea Methanothrix, Methanospirillum, and bacteria Comamonas, Petrimonas and Syntrophomonas were enriched by OSBC due to its large surface area and microelement such as iron, and contributed to the improved methane production. The results suggested that OSBC exhibited excellent practical application potential in enhancing anaerobic sewage sludge treatment and biogas productivity.

Evaluation of greenhouse gas emissions from aerobic and anaerobic wastewater treatment plants in Southeast of Italy

An evaluation of the operative functioning data of 183 Wastewater Treatment Plants (WWTPs) in Apulia (Southeast of Italy) has been carried out aimed to assess their Green House Gases (GHGs) emissions and the level for which the use of anaerobic sludge treatment should be more convenient in terms of electricity consumption and of GHGs emissions. Out of the 183 studies WWTPs, 140 are practicing aerobic digestion of sludge, while the remaining 43 are practicing anaerobic digestion of sludge. WWTPs in Apulia are serving about 4,81 million PE (Population Equivalent), yielding approximately 600,000-ton equivalent CO2 per annum. The production of GHGs emissions has been estimated by evaluating the contribution of CO2 deriving from: a) electric energy consumption (fossil CO2), b) biogenic CO2, c) N2O and d) CH4 emissions. The present study investigates a number of technical measures for upgrading the existing WWTPs, so to reduce GHGs emissions through the amelioration of CH4 production and capture in the anaerobic step, and through reducing the production of biogenic N2O and CO2 emissions in the aerated basin. The methodology employees artificial intelligence-based control for upgrading the aerobic oxidation of the organic carbon and the nitrification-denitrification steps. As a result, GHGs emissions are expected to be reduced by approximately: 71% for CH4, 57% for N2O, 20% for biogenic CO2 and 15% for fossil derived CO2.

Pre-Feasibility Study of a Multi-Product Biorefinery for the Production of Essential Oils and Biomethane

Rural areas can benefit from the development of biorefineries for the valorization of endogenous feedstocks. In this study, a pre-feasibility assessment of an integrated multi-product biorefinery to produce essential oils and biomethane is carried out considering current technical and economic conditions. The proposed concept is based on the steam distillation of forestry biomass for the extraction of essential oils (2900 L/y) followed by biomethane production via syngas methanation using the spent biomass as feedstock (30.4 kg/h). In parallel, the anaerobic treatment of WWTP sludge (5.3 kg/h) is used to produce additional biomethane for mobile applications. The results show that the intended multi-product biorefinery delivers attractive benefits for investors as described by the calculated financial indicators: NPV of EUR 4342.6, IRR of 18.1%, and PB of 6 years. Overall, the pre-feasibility analysis performed in this study demonstrates that the proposed biorefinery concept is promising and warrants further investment consideration via cost and benefit analysis, ultimately promoting the implementation of multi-product biorefineries across Europe.

THE INFLUENCE OF THE WAY OF SEWAGE SLUDGE TREATMENT ON AMMONIUM NITROGEN CONTENT IN THE RETURN WASTEWATER FLOW

Wastewater treatment facility receives, in addition to the influent, the return flow after sewage sludge treatment. Thus, the organic load on the facility increases as well as the nitrogen and phosphorus content load. There was studied the effect of different stages of sewage sludge treatment (sludge thickening, mechanical dewatering, storage on the grit bays and in the gathering ponds) on ammonium nitrogen content in the incoming flow. It is established that the most concentrated water by ammonium nitrogen is the return water from the gathering ponds, but the less concentrated is the supernatant water from sludge lagoons. Taking into account volume of the return flow the secondary ammonium nitrogen load from the every stage of sewage sludge treatment is calculated. It is revealed the seasonal and climatic impact on the gathering ponds return flow composition; it is shown that the biggest fluctuations occur in spring-summer period. The dependence of ammonium nitrogen content in the return flow on sludge thickening process time in the sludge lagoons is determined. The load contribution on the treatment facility by ammonium nitrogen in the return flow was 3% in the relation to this in the influent. The conclusion was made that with the existing technology of sewage sludge treatment the isolated scheme of return water treatment is not required. The comparative analysis on the effect of thermophilic and mesophilic conditions of anaerobic sewage sludge treatment on ammonium nitrogen content in the liquor was carried out. It was found that the higher ammonium nitrogen secondary load is created by the liquor of thermophilically treated sewage sludge: it is 16,4% versus 11,8% of mesophilically treated. Hence, the including of anaerobic sewage sludge treatment in the scheme significantly increases the load on treatment facility by ammonium nitrogen. In the conditions of cascade denitrification the distribution of incoming in the denitrification zones water flow is of great importance for providing required content of readily available organic substances. The change of ammonium nitrogen content in this flow will require the adjustment of its volume.

Polystyrene microplastics and nanoplastics distinctively affect anaerobic sludge treatment for hydrogen and methane production

Microplastics and nanoplastics generally accumulated in waste activated sludge (WAS) after biological wastewater treatment. Currently, researches mainly focused on how plastics affected a particular sludge treatment method, without the comparison of different sludge systems. Herein, distinct responses of hydrogen-producing and methane-producing sludge systems were comprehensively evaluated with polystyrene microplastics (PS-MPs) and nanoplastics (PS-NPs) existence. Experimental results showed that PS particles would stimulate inhibition on anaerobic gas production except that PS-MPs were conducive to hydrogen accumulation, which was caused by the enhanced solubilization. Mechanistic investigation demonstrated that severe inhibition of PS-NPs to hydrogen production was derived from the excessively inhibitory hydrolysis despite of improving solubilization. Varying degrees of inhibition to acidification and methanation collectively contributed to reduced methane accumulation with exposure to PS-MPs and PS-NPs. Excessive oxidative stress would be generated in the presence of PS-MPs or PS-NPs, deteriorating microbial activities and richness of species responsible for hydrogen or methane production.

Financial Viability and Environmental Sustainability of Fecal Sludge Treatment with Pyrolysis Omni Processors.

Omni Processors (OPs) are community-scale systems for non-sewered fecal sludge treatment. These systems have demonstrated their capacity to treat excreta from tens of thousands of people using thermal treatment processes (e.g., pyrolysis), but their relative sustainability is unclear. In this study, QSDsan (an open-source Python package) was used to characterize the financial viability and environmental implications of fecal sludge treatment via pyrolysis-based OP technology treating mixed and source-separated human excreta and to elucidate the key drivers of system sustainability. Overall, the daily per capita cost for the treatment of mixed excreta (pit latrines) via the OP was estimated to be 0.05 [0.03–0.08] USD·cap–1·d–1, while the treatment of source-separated excreta (from urine-diverting dry toilets) was estimated to have a per capita cost of 0.09 [0.08–0.14] USD·cap–1·d–1. Operation and maintenance of the OP is a critical driver of total per capita cost, whereas the contribution from capital cost of the OP is much lower because it is distributed over a relatively large number of users (i.e., 12,000 people) for the system lifetime (i.e., 20 yr). The total emissions from the source-separated scenario were estimated to be 11 [8.3–23] kg CO2 eq·cap–1·yr–1, compared to 49 [28–77] kg CO2 eq·cap–1·yr–1 for mixed excreta. Both scenarios fall below the estimates of greenhouse gas (GHG) emissions for anaerobic treatment of fecal sludge collected from pit latrines. Source-separation also creates opportunities for resource recovery to offset costs through nutrient recovery and carbon sequestration with biochar production. For example, when carbon is valued at 150 USD·Mg–1 of CO2, the per capita cost of sanitation can be further reduced by 44 and 40% for the source-separated and mixed excreta scenarios, respectively. Overall, our results demonstrate that pyrolysis-based OP technology can provide low-cost, low-GHG fecal sludge treatment while reducing global sanitation gaps.

Thermophilic anaerobic membrane bioreactor for pulp and paper primary sludge treatment: Effect of solids retention time on the biological performance

Lignocellulosic biomass is a promising renewable feedstock for the production of bioenergy and platform chemicals; however, its use would not be economically viable without adopting effective transformation technologies. In this work, long-term anaerobic treatment (370 days) of pulp and paper primary sludge (PS) was performed by a thermophilic anaerobic membrane bioreactor (AnMBR) for the first time to investigate the effect of various solid retention times (SRTs) on biogas production and solids reduction ratios. The tested STRs (32–55 days) have shown varying biogas productivity, and it can be concluded that the longer the SRT, the higher is the biogas yield. At the optimum solids retention time of 55 d, an organic loading rate of 2.15 ± 0.10 kg-TSS/L.d, and hydraulic retention time of 5 d, average biogas yield of 106.4 ± 8.5 m3 biogas/tonne TSS added (approximately 72.7 m3 CH4/t VSadded) was achieved with an average methane content of 56 ± 4% and the solids reduction ratio ranged between 47% and 54.9%. Besides biogas yield, the quality of the permeates, solids reduction, and digestates characteristic/dewaterability changed by changing the SRT. The mixed liquor suspended solids concentrations and solids reduction increased with increasing SRT, while the effluent COD concentration and sludge dewaterability decreased. The Fourier Transform Infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) findings exhibited that the digestate contains more lignin and cellulose than the other substances, while the nitrogen and carbon concentration decreased with increasing SRT.

Performance and microbial community evaluation of full-scale two-phase anaerobic digestion of waste activated sludge

“Temperature Staging and Biological Phasing” (TSBP) is an improved two-phase anaerobic digestion (AD) technology. This technology hydrolyzes waste activated sludge (WAS) at 45 °C and converts methane at mesophilic temperature (35–38 °C), with hydraulic retention times of 3–5 d and 14–17 d, respectively. In this study, the performance and microbial community dynamics of full-scale TSBP-based sludge anaerobic digestion system were studied, and the technology was evaluated by energy balance and ecological benefit analysis. The stable operation for 390 d showed that the cumulative biogas yield was about 349,041 m3, the maximum biogas yield rate was 563.68 L/kg VS, and the VS degradation rate of organic matters in the sludge was 47.19%. Proteobacteria and Firmicutes were found to be the dominant bacteria in both thermophilic and mesophilic reactors. Methanobacterium and Methanosarcina were the two most abundant methanogenic genera in the AD samples. The aceticlastic methanogenesis was likely the predominant production pathway of methane in AD processes based on metagenomics. The TSBP system operated stably, and the recovered energy could achieve energy self-sufficiency, which provided technical reference for the anaerobic treatment of sludge.

Opportunistic Strategy for Maintenance Interventions Planning: A Case Study in a Wastewater Treatment Plant

Wastewater treatment plants (WWTPs) face two fundamental challenges: on the one hand, they must ensure an efficient application of preventive maintenance plans for their survival under competitive environments; and on the other hand, they must simultaneously comply with the requirements of reliability, maintainability, and safety of their operations, ensuring environmental care and the quality of their effluents for human consumption. In this sense, this article seeks to propose a cost-efficient alternative for the execution of preventive maintenance (PM) plans through the formulation and optimization of the opportunistic grouping strategy with time-window tolerances and non-negligible execution times. The proposed framework is applied to a PM plan for critical high-risk activities, addressing primary treatment and anaerobic sludge treatment process in a wastewater treatment plant. Results show a 26% system inefficiency reduction versus the initial maintenance plan, demonstrating the capacity of the framework to increase the availability of the assets and reduce maintenance interruptions of the WWTP under analysis.

Revealing the mechanism of zinc oxide nanoparticles facilitating hydrogen production in alkaline anaerobic fermentation of waste activated sludge

Zinc oxide nanoparticles (ZnO NPs) has been documented to accumulate in waste activated sludge (WAS). Nevertheless, its potential effect on hydrogen accumulation during anaerobic fermentation has never been reported. Such impact exists in anaerobic sludge treatment systems but is unrecognized before. Therefore, this work aims to explore the effect of ZnO NPs on hydrogen accumulation during WAS fermentation while avoiding hydrogen consumption by maintaining pH around 10. Deep insights into the mechanisms of the enhanced hydrogen production after dosing ZnO NPs can then be provided. The 21.1 to 74.0 mL-H2/g-VS hydrogen yields were obtained after dosing 0–150 mg/g-TS ZnO NPs into the system, which is 2.5 times higher than the control. The zinc ions released from ZnO NPs did not pose obvious cytotoxic effect on organisms based on the mechanism exploration of this study. The reactive oxygen species (ROS) induced by ZnO NPs facilitated sludge disintegration and improved the biodegradability of released organics, but restrained the bio-processes of hydrolysis and acetogenesis simultaneously. This was further verified by the microbial community analysis, as the abundance of key microbes related to hydrolysis and acetogenesis was reduced evidently in the presence of 150 mg/g-TS ZnO NPs. This study would provide a green and economic strategy for the sustainable engineering application of WAS containing ZnO NPs.

Comprehensive investigation of SARS-CoV-2 fate in wastewater and finding the virus transfer and destruction route through conventional activated sludge and sequencing batch reactor

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA transmission route was thoroughly investigated in the hospital wastewater, sewage collection network, and wastewater treatment plants. Samples were taken on four occasions from December 2020 to April 2021. The performance of two different wastewater treatment processes of sequencing batch reactor (SBR) and conventional activated sludge (CAS) was studied for virus destruction. For this purpose, liquid phase, solid phase and bioaerosol samples were taken from different units of the investigated wastewater treatment plants (WWTPs). The results revealed that all untreated hospital wastewater samples were positive for SARS-CoV-2 RNA. The virus detection frequency increased when the number of hospitalized cases increased. Detection of viral RNA in the wastewater collection system exhibited higher load of virus in the generated wastewater in areas with poor socioeconomic conditions. Virus detection in the emitted bioaerosols in WWTPs showed that bioaerosols released from CAS with surface aeration contains SARS-CoV-2 RNA posing a potential threat to the working staff of the WWTPs. However, no viral RNA was detected in the bioaerosols of the SBR with diffused aeration system. Investigation of SARS-CoV-2 RNA in WWTPs showed high affinity of the virus to be accumulated in biosolids rather than transporting via liquid phase. Following the fate of virus in sludge revealed that it is completely destructed in anaerobic sludge treatment process. Therefore, based on the results of the present study, it can be concluded that receiving water resources could not be contaminated with virus, if the wastewater treatment processes work properly.

Unaerated feeding alters the fate of dissolved methane during aerobic wastewater treatment

In municipal wastewater treatment plants, some dissolved methane can enter the aerobic bioreactors. This greenhouse gas originates from sewers and return flows from anaerobic sludge treatment. In well-mixed conventional activated sludge reactors, methane emissions are largely avoided because methane oxidizing bacteria consume a large fraction, even without optimizing for this purpose. In this work, the fate of dissolved methane is studied in aerobic granular sludge reactors, as they become increasingly popular. The influence of the characteristic design and operating conditions of these reactors are studied with a mathematical model with apparent conversion kinetics and stripping: the separation of feeding and aeration in time, a higher substrate transport resistance, a high retention time of granular biomass and a taller water column. Even for a best-case scenario combining an unrealistically low intragranule substrate transport resistance, a high retention time, a tall reactor, an extremely high influent methane concentration and no oxygen limitation, the methane conversion efficiency was only 12% when feeding and aeration were separated in time, which is lower than for continuous activated sludge reactors under typical conditions. A more rigorous model was used to confirm the limited conversion, considering the multi-species and multi-substrate biofilm kinetics, anoxic methane consumers and the high substrate concentration at the bottom during upward plug flow feeding. The observed limited methane conversion is mainly due to the high concentration that accumulates during unaerated feeding phases, which favours stripping more than conversion in the subsequent aeration phase. Based on these findings, strategies were proposed to mitigate methane emissions from wastewater treatment plants with sequentially operated reactors.

Fate of selected organic micropollutants during anaerobic sludge digestion

Organic micropollutants are incompletely removed from wastewater in Water Resource Recovery Facilities using conventional methods and can therefore enter the anaerobic sludge treatment together with primary and secondary sludge. This review compiles literature data on the fate of selected micropollutants (Carbamazepine [CBZ], Diclofenac [DCF], Ibuprofen [IBP], Sulfamethoxazole [SMX], and Triclosan [TCS]) during anaerobic sludge treatment and how the fate is affected by chemical properties, phase distribution and operating conditions. CBZ was found to be persistent to anaerobic degradation in most studies, with some exceptions reporting a degradation efficiency of 60%. Removal efficiencies for DCF, IBP, and TCS varied widely (from no to [very] high removal). For SMX, most studies reported a removal above 80%. A correlation was found between the fate during anaerobic digestion and physicochemical properties (hydrophobicity and molecular structure). Sorption to sludge, affected in some cases by pH changes during digestion, is suggested to reduce bioavailability. IBP and TCS were mainly present in the liquid phase or solid phase, respectively, CBZ and DCF were present in similar proportions in both phases, while statements were contradictory for SMX. Parameters such as temperature and sludge age did not significantly influence the fate of investigated micropollutants during anaerobic digestion. PRACTITIONER POINTS: Most studies report no significant removal of CBZ during anaerobic sludge digestion. Removal efficiencies of DCF, IBP, and TCS vary from study to study between no removal and high or very high removal. Considering such heterogeneous removal efficiencies, it is recommended to conduct digestion trials to find out in which range the values will be for a specific sludge. SMX is very highly removed during anaerobic digestion in most studies. Parameters such as temperature and SRT do not significantly influence the fate of the five investigated micropollutants. Hydrophobicity, which has some effect on the liquid/solid phase distribution of micropollutants, and molecular structure influence the removal efficiencies.

Mechanisms of CuO Nanoparticles at an Environmentally Relevant Level Enhancing Production of Hydrogen from Anaerobic Fermentation of Waste-Activated Sludge

To date, almost all previous studies reported the inhibitory effects of CuO nanoparticles on the production of methane from waste-activated sludge (WAS), whereas their potential effects on hydrogen production remained unknown. This study investigated how CuO nanoparticles affected the production of hydrogen from WAS to fill this knowledge gap. Experimental results surprisingly showed CuO nanoparticles at an environmentally relevant level (5 mg/g of TS) significantly enhanced hydrogen production by 45.2 ± 0.1% compared to the control, while CuO nanoparticles at 50–100 mg/g of TS had little impact on hydrogen production. A mechanistic study revealed that although CuO nanoparticles at 5 mg/g of TS weakly inhibited acetogenesis, they enhanced WAS solubilization significantly, leading to improved hydrogen production. In contrast, although CuO nanoparticles at 50 and 100 mg/g of TS promoted WAS solubilization, the acidogenesis and acetogenesis processes were both suppressed, with the improvement offsetting inhibition. Microbial community analysis confirmed that CuO nanoparticles at high levels reduced the populations of key bacteria (e.g., Rhodobacter sp.) involved in hydrogen generation. Reactive oxygen species-mediated oxidative stress was revealed to be a key factor of CuO nanoparticle toxicity. This work revealed that anaerobic fermentation for hydrogen production rather than for methane production would be a better option for anaerobic treatment of sludge containing CuO nanoparticles.