Odour prevention strategies in wastewater treatment plants: A pilot scale study of activated sludge recycling and oxidized nitrogen recycling

Producción CientíficaThe potential of activated sludge recycling (ASR) and oxidized nitrogen recycling (ONR) to prevent the emissions of H2S and acetic acid from the primary settler during domestic wastewater treatment was herein evaluated. The pilot plant consisted of an 8 L primary settler with a 10 L gas-tight headspace coupled to a 11 L nitrification-denitrification activated sludge reactor and an 8 L secondary settler, which were monitored for 175 days. A reduction in the headspace concentrations of H2S and acetic acid by 95 % and 42 %, respectively, was recorded when combining ASR and ONR. Process operation with ASR and ONR supported stable conditions with average organic matter removals of 96 ± 2 %, NO2- concentrations of 24.2 ± 0.4 mg N/L and NO3- concentrations of 9.8 ± 0.4 mg N/L in the effluent, and a biological oxidation of S2- higher than 99 % with average SO42- concentrations of 52 ± 8 mg/L. Interestingly, the sole recirculation of activated sludge to the primary settler without NO3- recycling caused sludge bulking, contributing to increase the concentration of H2S and acetic acid in the primary settler headspace up to 0.99 ± 0.01 ppmv and 2.87 ± 0.12 ppmv, respectively. Sludge bulking also resulted in an increase in the effluent soluble total nitrogen concentration from 5.6 ± 0.1 mg N/L to 50.8 ± 0.2 mg N/L and of NH4+ from 1.3 ± 0.2 mg N/L to 50.7 ± 0.8 mg N/L due to the loss of nitrification under these operational conditions. Overall, the experimental results indicated that ASR and ONR represent cost-effective strategies for the control of malodorous emissions in wastewater treatment plants.Junta de Castilla y León - EU-FEDER (CLU 2017-09, CL-EI-2021-07 y UIC 315)Ministerio de Ciencia e Innovación (FJC2019-039791-I

Optimization of activated sludge recycling and oxidized ammonium recycling as odour control strategies in wastewater treatment plants

Producción CientíficaNew odour prevention strategies in wastewater treatment facilities need to be investigated to find effective and low-cost technologies for the control of malodorous emissions. In this study, the potential of activated sludge (AS) and oxidized nitrogen (N-NOx) recycling as an environmentally-friendly and cost-effective strategy for the prevention and minimization of odour nuisance during wastewater treatment was evaluated and optimized using H2S, acetic acid and α-pinene as model odorants in 2.1 L gas tight bottles. This approach uses by-products from wastewater treatment to mitigate odour annoyance. Preliminary abiotic tests showed headspace concentration losses of 25% for H2S and α-pinene, and 7% for acetic acid due to odorant adsorption in 4 h. The experiments carried out at different concentrations of AS (0, 10, 25, 50, 100 mg VSS/L) and oxidized nitrogen (1.5, 5, 7.5 and 10 mg N-NOx/L) revealed an effective H2S removal at 7.5–10 mg N-NOx/L and 50–100 mg VSS/L. Interestingly, NO3− was more effective than NO2− as electron acceptor during the biodegradation of H2S regardless of the AS concentration and N-NOx concentrations. In the presence of dissolved H2S and AS concentrations of 50–100 mg VSS/L, acetic acid was partially metabolized by microorganisms at the end of experiment (from 27 to 23 ppmv in 4.5 h), while α-pinene concentrations experienced a similar gradual decrease than in the abiotic tests. Finally, the assays carried out at concentrations ≥5 mg NO3−/L and 25 mg VSS/L showed a reduction of NO3− to NO2− correlated with the biological oxidation of H2S, which suggested the need to control NO3− supply under sulphur limiting conditions to prevent toxicity problems during wastewater treatment.Junta de Castilla y León (grants CLU 2017-09 and UIC 315)Ministerio de Ciencia e Innovación (project FJC2019-039791-I)

Influence of organic matter and CO2 supply on bioremediation of heavy metals by Chlorella vulgaris and Scenedesmus almeriensis in a multimetallic matrix

Producción CientíficaThis research evaluated the influence of organic matter (OM) and CO2 addition on the bioremediation potential of two microalgae typically used for wastewater treatment: Chlorella vulgaris (CV) and Scenedesmus almeriensis (SA). The heavy metal (HM) removal efficiencies and biosorption capacities of both microalgae were determined in multimetallic solutions (As, B, Cu, Mn, and Zn) mimicking the highest pollutant conditions found in the Loa river (Northern Chile). The presence of OM decreased the total biosorption capacity, specially in As (from 2.2 to 0.0 mg/g for CV and from 2.3 to 1.7 mg/g for SA) and Cu (from 3.2 to 2.3 mg/g for CV and from 2.1 to 1.6 mg/g for SA), but its influence declined over time. CO2 addition decreased the total HM biosorption capacity for both microalgae species and inhibited CV growth. Finally, metal recovery using different eluents (HCl, NaOH, and CaCl2) was evaluated at two different concentrations. HCl 0.1 M provided the highest recovery efficiencies, which supported values over 85% of As, 92% of Cu, and ≈100% of Mn and Zn from SA. The presence of OM during the loaded stage resulted in a complete recovery of As, Cu, Mn, and Zn when using HCl 0.1 M as eluent.Gobierno regional de Castilla y León (UIC 071, CLU 2017-09 and VA080G18)Ministerio de Ciencia, Innovación y Universidades (project CTQ2017-84006-C3-1-R)Unión Europea - FEDER (CLU 2017-09 and CTQ2017-84006-C3-1-R

Genome scale metabolic modeling reveals the metabolic potential of three Type II methanotrophs of the genus Methylocystis

Producción CientíficaGenome Scale Metabolic Models (GSMMs) of the recently sequenced Methylocystis hirsuta and two other methanotrophs from the genus Methylocystis have been reconstructed. These organisms are Type II methanotrophs with the ability of accumulating Polyhydroxyalkanoates under nutrient limiting conditions. For the first time, GSMMs have been reconstructed for Type II methanotrophs. These models, combined with experimental biomass and PHB yields of Methylocystis hirsuta, allowed elucidating the methane oxidation mechanism by the enzyme pMMO (particulate methane monooxygenase) in these organisms. In contrast to Type I methanotrophs, which use the “direct coupling mechanism”, Type II methanotrophs appear to use the so called “redox arm mechanism”. The utilization of the “redox arm mechanism”, which involves the coupling between methane oxidation and complex I of the respiratory chain, was confirmed by inhibition of complex I with catechol. Utilization of the “redox arm” mechanism leads to lower biomass yields on methane compared to Type I methanotrophs. However, the ability of Type II methanotrophs to redirect high metabolic carbon fluxes towards acetoacetyl-CoA under nitrogen limiting conditions makes these organisms promising platforms for metabolic engineering.Marie Curie grant H2020-MSCA-IF-2016 CH4BioVal (GA nº 750126).Junta de Castilla y León (Ref. Project VA281P18)Ministerio de Ciencia e Innovación (Proyect CLU 2017-09, CTM2015-70442-R

Genome sequence of Methylocystis hirsuta CSC1, a polyhydroxyalkanoate producing methanotroph

Producción CientíficaPolyhydroxyalkanoates (PHAs) are biodegradable plastics that can be produced by some methanotrophic organisms such as those of the genus Methylocystis. This allows the conversion of a detrimental greenhouse gas into an environmentally friendly high added‐value bioproduct. This study presents the genome sequence of Methylocystis hirsuta CSC1 (a high yield PHB producer). The genome comprises 4,213,043 bp in 4 contigs, with the largest contig being 3,776,027 bp long. Two of the other contigs are likely to correspond to large size plasmids. A total of 4,664 coding sequences were annotated, revealing a PHA production cluster, two distinct particulate methane monooxygenases with active catalytic sites, as well as a nitrogen fixation operon and a partial denitrification pathway.Ministerio de Ciencia e Innovación (Proyect CTM2015‐70442‐R)Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. Project UIC 71

Continuous polyhydroxybutyrate production from biogas in an innovative two‐stage bioreactor configuration

Producción CientíficaBiogas biorefineries have opened up new horizons beyond heat and electricityproduction in the anaerobic digestion sector. Added‐value products such aspolyhydroxyalkanoates (PHAs), which are environmentally benign and potentialcandidates to replace conventional plastics, can be generated from biogas. Thiswork investigated the potential of an innovative two‐stage growth‐accumulationsystem for the continuous production of biogas‐based polyhydroxybutyrate (PHB)usingMethylocystis hirsutaCSC1 as cell factory. The system comprised twoturbulent bioreactors in series to enhance methane and oxygen mass transfer: acontinuous stirred tank reactor (CSTR) and a bubble column bioreactor (BCB) withinternal gas recirculation. The CSTR was devoted to methanotrophic growth undernitrogen balanced growth conditions and the BCB targeted PHB productionunder nitrogen limiting conditions. Two different operational approaches underdifferent nitrogen loading rates and dilution rates were investigated. A balancednitrogen loading rate along with a dilution rate (D) of 0.3 day−1resulted in the moststable operating conditions and a PHB productivity of ~53 g PHB m−3day−1.However, higher PHB productivities (~127 g PHB m−3day−1)wereachievedusingnitrogen excess at a D = 0.2 day−1. Overall, the high PHB contents (up to 48% w/w)obtained in the CSTR under theoretically nutrient balanced conditions and the poorprocess stability challenged the hypothetical advantages conferred by multistagevs single‐stage process configurations for long‐term PHB production.Ministerio de Ciencia e Innovación under (BES-2016-077160) contract and (project CTM2015-70442-R)Junta de Castilla y León and EU-FEDER program (UIC 315, CLU 2017-09

Photoautotrophic removal of hydrogen sulfide from biogas using purple and green sulfur bacteria

Producción CientíficaBiogas desulfurization based on anoxygenic photosynthetic processes represents an alternative to physicochemical technologies, decreasing the risk of O2 and N2 contamination. This work aimed at assessing the potential of Allochromatium vinosum and Chlorobium limicola for biogas desulfurization under different light intensities (10 and 25 klx) and H2S concentrations (1, 1.5 and 2%) in batch photobioreactors. In addition, the influence of rising biogas flow rates (2.9, 5.8 and 11.5 L d-1 in stage I, II and III, respectively) on the desulfurization performance in a 2.3 L photobioreactor utilizing C. limicola under continuous mode was assessed. The light intensity of 25 klx negatively influenced the growth of A. vinosum and C. limicola, resulting in decreased H2S removal capacity. An increase in H2S concentrations resulted in higher volumetric H2S removal rates in C. limicola (2.9–5.3 mg L-1 d-1) tests compared to A. vinosum (2.4–4.6 mg L-1 d-1) tests. The continuous photobioreactor completely removed H2S from biogas in stage I and II. The highest flow rate in stage III induced a deterioration in the desulfurization activity of C. limicola. Overall, the high H2S tolerance of A. vinosum and C. limicola supports their use in H2S desulfurization from biogas

Elucidating the role of pH and total solids content in the co-production of biohydrogen and carboxylic acids from food waste via lactate-driven dark fermentation

Producción CientíficaNotwithstanding lactate-driven dark fermentation (LD-DF) can cope with inhibition issues associated with the over-proliferation of lactate producers, there is still a knowledge gap about the role of key operational parameters. In this study, the effect of pH and total solids (TS) content on the co-production of hydrogen and carboxylic acids, including medium-chain carboxylic acids (MCCAs), from food waste (FW) via LD-DF was investigated. A series of batch fermentations was conducted, first, without pH control, and then at fixed pH values of 5.5, 6.0 and 6.5, while maintaining constant the TS content at 5 %. It was observed that the higher the operational pH, the lower the accumulation of lactate and the higher the extent and rate of hydrogen production, sustaining a maximum hydrogen production yield and rate of 81 NmL/g VS fed and 9 NL/L-d, respectively, at pH 6.5. In a second series of batch tests, the TS content was adjusted to 5, 7.5 and 10 % while pH was set at 6.5. The highest hydrogen production performance (103 NmL/g-VS fed and 13.3 NL/L-d) was achieved at 7.5 % TS, which also resulted in the highest accumulation of MCCAs, particularly of caproate, with an associated titer of 8.7 g/L. Hydrogen production plateaued with the exhaustion of lactate regardless of the condition tested. Further assessment through biochemical methane potential tests showed that LD-DF effluents can be alternatively valorised into biogas. Overall, the results obtained confirmed the key role of pH and TS content in the LD-DF of FW and suggested that this non-conventional route may be an alternative approach to cope with lactate flux diverted toward undesirable non-hydrogen-producing metabolic pathways.European Commission-H2020-MSCA-IF-2019 project UP-GRAD (894515)Junta de Castilla y León y EU-FEDER (program CLU 2017-09, CL-EI-2021-07, and UIC 315

A systematic comparison of two empirical gas-liquid mass transfer determination methodologies to characterize methane biodegradation in stirred tank bioreactors

Producción CientíficaThis study aimed at systematically comparing the potential of two empirical methods for the estimation of the volumetric CH4 mass transfer coefficient (klaCH4), namely gassing-out and oxygen transfer rate (OTR), to describe CH4 biodegradation in a fermenter operated with a methanotrophic consortium at 400, 600 and 800 rpm. The klaCH4 estimated from the OTR methodology accurately predicted the CH4 elimination capacity (EC) under CH4 mass transfer limiting conditions regardless of the stirring rate (∼9% of average error between empirical and estimated ECs). Thus, empirical CH4-ECs of 37.8 ± 5.8, 42.5 ± 5.4 and 62.3 ± 5.2 g CH4 m−3 h−1 vs predicted CH4-ECs of 35.6 ± 2.2, 50.1 ± 2.3 and 59.6 ± 3.4 g CH4 m−3 h−1 were recorded at 400, 600 and 800 rpm, respectively. The rapid Co2+-catalyzed reaction of O2 with SO3−2 in the vicinity of the gas-liquid interphase during OTR determinations, mimicking microbial CH4 uptake in the biotic experiments, was central to accurately describe the klaCH4.Ministerio de Economía, Industria y Competitividad - FEDER (Project CTM 2015–70442-R and Red NOVEDAR

Continuous lactate-driven dark fermentation of restaurant food waste: Process characterization and new insights on transient feast/famine perturbations

Producción CientíficaThe effect of hydraulic retention time (HRT) on the continuous lactate-driven dark fermentation (LD-DF) of food waste (FW) was investigated. The robustness of the bioprocess against feast/famine perturbations was also explored. The stepwise HRT decrease from 24 to 16 and 12 h in a continuously stirred tank fermenter fed with simulated restaurant FW impacted on hydrogen production rate (HPR). The optimal HRT of 16 h supported a HPR of 4.2 L H2/L-d. Feast/famine perturbations caused by 12-h feeding interruptions led to a remarkable peak in HPR up to 19.2 L H2/L-d, albeit the process became stable at 4.3 L H2/L-d following perturbation. The occurrence of LD-DF throughout the operation was endorsed by metabolites analysis. Particularly, hydrogen production positively correlated with lactate consumption and butyrate production. Overall, the FW LD-DF process was highly sensitive but resilient against transient feast/famine perturbations, supporting high-rate HPRs under optimal HRTs.European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 894515MCIN/AEI /10.13039/501100011033 - Unión Europea NextGenerationEU/PRTR (Grant RYC2021‐034559‐I)Junta de Castilla y León - EU-FEDER (CLU 2017-09, CL-EI-2021-07 y UIC 315