Stable Reactor Performance Research Articles

Microbial population dynamics and meta-proteomic evaluation for functional insight into an anaerobic membrane bioreactor during startup

Startup of an anaerobic membrane reactor (AnMBR) involves a significant concentration of active and functioning microbial communities participating in the process. Here, we monitored the physical, microbial, and functional performance during the first 90 days of AnMBR operation. The results indicate initiation of anaerobic digestion on days 31–60 (transitional stage) and its stabilization on days 61–90 (stable stage), with an average COD removal efficiency of 93.14 ± 1.41% and biogas production of 0.82 ± 0.15 L/d. Moreover, the microbial population and its associated proteome were studied using 16s rDNA-based PCR-DGGE and SDS-PAGE-LC/MS/MS techniques to acquire a detailed understanding of reactor startup conditions. Later, a conventional library was created through the UniprotKB protein database using the observed community of all organisms identified with 16s rDNA sequencing. The results clearly depicted Gammaproteobacteria as the dominant fermentative during the initial and transitional stages. Firmicutes predominated over Gammaproteobacteria during the stabilization stage. During the transitional stage, methanobacteriales and metheanosarcinales coexisted. In the two-step reaction, a combination of syntrophic acetate oxidation and hydrogenotrophic methanogenesis was observed. During the stabilization stage, diverse metheanosarcinales were detected. The methanogenic pathway seemed to shift to acetoclastic methanogenesis, as indicated by detection of acetyl-coenzyme A synthetase, aminomethyltransferase, and methyl-coenzyme M reductase. Thus, by linking microbial community dynamics with the meta-proteomic functions of AnMBR during startup, a stable reactor performance with up-regulated acetoclastic methanogenesis was observed in 90 days.

Impact of operational conditions on methane yield and microbial community composition during biological methanation in in situ and hybrid reactor systems

BackgroundBiogas can be upgraded to methane biologically by adding H2 to biogas reactors. The process is called biological methanation (BM) and can be done in situ in a regular biogas reactor or the biogas can be transferred to a separate ex situ upgrading reactor. The hybrid BM concept, a combination of in situ and ex situ BM, has received little attention, and only a few studies have been reported. The hybrid BM has the advantage of resolving the issue of pH increment during in situ BM, while the size of the ex situ BM reactor could be reduced.ResultsIn this study, the efficiency of in situ and hybrid biological methanation (BM) for upgrading raw biogas was investigated. The hybrid BM system achieved a CH4 yield of 257 mL gVS−1 when degrading a feedstock blend of manure and cheese waste. This represented an increase in methane yield of 76% when compared to the control reactor with no H2 addition. A 2:1 H2:CO2 ratio resulted in stable reactor performance, while a 4:1 ratio resulted in a high accumulation of volatile fatty acids. H2 consumption rate was improved when a low manure–cheese waste ratio (90%:10%) was applied. Furthermore, feeding less frequently (every 48 h) resulted in a higher CH4 production from CO2 and H2. Methanothermobacter was found to dominate the archaeal community in the in situ BM reactor, and its relative abundance increased over the experimental time. Methanosarcina abundance was negatively affected by H2 addition and was nearly non-existent at the end of the experiment.ConclusionsOur results show that hybrid BM outperforms in situ BM in terms of total CH4 production and content of CH4 in the biogas. In comparison to in situ BM, the use of hybrid BM increased CH4 yield by up to 42%. Furthermore, addition of H2 at 2:1 H2:CO2 ratio in in situ BM resulted in stable reactor operation.

The effect of mixing rate and gas recirculation on biological CO2 methanation in two-stage CSTR systems

In-situ biological CO2 methanation (BM) in a biogas reactor by the addition of H2 is an attractive low-cost biogas upgrading process, as it does not require investment in a new reactor and can be incorporated into existing biogas plants. On the other hand, maintenance of stable reactor performance during in-situ BM is challenging due to factors such as high H2 partial pressure and CO2 depletion which may lead to an increase in pH. Thus, BM that uses two-serial connected continuous stirred tank reactors (CSTRs) could be an option. For such a process set-up, the second reactor acts as an upgrading reactor (UR) and a secondary CH4 producer for slow-degrading substrates such as straw and manure. In this study, improvement of the BM process was attempted by varying the mixing speed and gas recirculation to enhance hydrogen transfer to the liquid phase of the UR. The experiments showed that the mixing speed and gas recirculation had a significant effect on BM in CSTRs. CH4 production from BM was highest at 170 rpm, but the total CH4 production fell above 140 rpm due to reduced production of CH4 from the manure substrate. The CH4 production rate from CO2 and H2 conversion was further enhanced as the output gas was recirculated at 12.20 mL min−1. Further gas recirculation above 12.20 mL min−1 did not improve BM.

Cultivation of aerobic granular sludge for the treatment of food-processing wastewater and the impact on membrane filtration properties.

A laboratory-scale sequencing batch reactor was operated for approximately 300 days, divided into four periods based on the feeding strategy, to develop stable aerobic granular sludge (AGS) while treating chocolate processing wastewater. Application of a prolonged mixed anaerobic feeding was not sufficient to develop AGS and reach stable reactor performance. Through the application of a partially non-mixed and a partially mixed feeding strategy, the reactor performance was increased and stable AGS formation was established characterized by low diluted sludge volume index (D)SVI DSVI10,30) values of 78 ± 27 mL·g-1 and 52 ± 17 mL·g-1, respectively, and a capillary suction time/mixed liquor suspended solids value of 0.9 sec·(g·L-1)-1. The membrane bioreactor (MBR) filtration tests showed a reduction of the fouling rate (FR) and an increase of the sustainable flux (SF0.5) for AGS compared to flocs treating the same industrial wastewater. The SF0.5 (FR > 0.5 mbar·min-1) for the flocs was 10 L·(m2·h)-1 while for AGS the SF0.5 is higher than 45 L·(m2·h)-1 because the FR did not exceed 0.1 mbar·min-1. Additionally, the AGS showed reduced irreversible fouling tendencies due to pore blocking. Our results underline the need for an increased substrate gradient during anaerobic feeding for the development and long-term maintenance of AGS under minimum wash-out conditions. The AGS-MBR filtration performance also shows strong advantages compared to a floccular MBR system due to a high increase of the SF0.5 and reduced reversible and irreversible fouling.

Microbial Community Redundancy and Resilience Underpins High-Rate Anaerobic Treatment of Dairy-Processing Wastewater at Ambient Temperatures.

High-rate anaerobic digestion (AD) is a reliable, efficient process to treat wastewaters and is often operated at temperatures exceeding 30°C, involving energy consumption of biogas in temperate regions, where wastewaters are often discharged at variable temperatures generally below 20°C. High-rate ambient temperature AD, without temperature control, is an economically attractive alternative that has been proven to be feasible at laboratory-scale. In this study, an ambient temperature pilot scale anaerobic reactor (2 m3) was employed to treat real dairy wastewater in situ at a milk processing plant, at organic loading rates of 1.3 ± 0.6 to 10.6 ± 3.7 kg COD/m3/day and hydraulic retention times (HRT) ranging from 36 to 6 h. Consistent high levels of COD removal efficiencies, ranging from 50 to 70% for total COD removal and 70 to 84% for soluble COD removal, were achieved during the trial. Within the reactor biomass, stable active archaeal populations were observed, consisting mainly of Methanothrix (previously Methanosaeta) species, which represented up to 47% of the relative abundant active species in the reactor. The decrease in HRT, combined with increases in the loading rate had a clear effect on shaping the structure and composition of the bacterial fraction of the microbial community, however, without affecting reactor performance. On the other hand, perturbances in influent pH had a strong impact, especially when pH went higher than 8.5, inducing shifts in the microbial community composition and, in some cases, affecting negatively the performance of the reactor in terms of COD removal and biogas methane content. For example, the main pH shock led to a drop in the methane content to 15%, COD removals decreased to 0%, while the archaeal population decreased to ~11% both at DNA and cDNA levels. Functional redundancy in the microbial community underpinned stable reactor performance and rapid reactor recovery after perturbations.

Two-stage granular sludge partial nitritation/anammox process for the treatment of digestate from the anaerobic digestion of the organic fraction of municipal solid waste

The increasing amount of source separated organic fraction of municipal solid wastes (OFMSW) treated by anaerobic digestion for energy recovery requires the implementation of cost-efficient processes for the treatment of the produced digestate, especially in terms of nitrogen removal. The autotrophic nitrogen removal process, based on the coupling of two biological processes, partial nitritation (PN) and anammox (A), appears as a suitable solution due to important savings in operational costs compared to conventional treatment processes. However, its application could be hampered by the high salinity and inhibitory potential of this kind of digestate. In this contribution, two lab-scale granular sludge reactors performing the PN and anammox processes, respectively, were used to treat (opportunely diluted) real OFMSW digestate originating from full-scale biogas plants with the aim of assessing their treatment feasibility in a two-stage PN/A configuration. The PN process was implemented in an air-lift granular sludge reactor and was able to treat a nitrogen loading rate of about 1 g N L−1 d−1 at 30 ± 0.5 °C; moreover, its effluent was suitable for the subsequent anammox treatment, with an appropriate effluent NO2−/NH4+ ratio and marginal inhibiting effects. In the anammox granular sludge reactor, the anammox activity was affected by high salinity levels, nonetheless a stable reactor performance at a nitrogen removing rate of 0.83 ± 0.20 and 0.31 ± 0.04 g N L−1 d−1 at 35 ± 0.5 °C, were achieved when treating 50% and 30% diluted real wastewaters at a conductivity in the reactor of 9.1 and 11.2 mS cm−1, respectively.

Enhanced CO2 Conversion to Acetate through Microbial Electrosynthesis (MES) by Continuous Headspace Gas Recirculation

Bioelectrochemical systems (BESs) is a term that encompasses a group of novel technologies able to interconvert electrical energy and chemical energy by means of a bioelectroactive biofilm. Microbial electrosynthesis (MES) systems, which branch off from BESs, are able to convert CO2 into valuable organic chemicals and fuels. This study demonstrates that CO2 reduction in MES systems can be enhanced by enriching the inoculum and improving CO2 availability to the biofilm. The proposed system is proven to be a repetitive, efficient, and selective way of consuming CO2 for the production of acetic acid, showing cathodic efficiencies of over 55% and CO2 conversions of over 80%. Continuous recirculation of the gas headspace through the catholyte allowed for a 44% improvement in performance, achieving CO2 fixation rates of 171 mL CO2 L−1·d−1, a maximum daily acetate production rate of 261 mg HAc·L−1·d−1, and a maximum acetate titer of 1957 mg·L−1. High-throughput sequencing revealed that CO2 reduction was mainly driven by a mixed-culture biocathode, in which Sporomusa and Clostridium, both bioelectrochemical acetogenic bacteria, were identified together with other species such as Desulfovibrio, Pseudomonas, Arcobacter, Acinetobacter or Sulfurospirillum, which are usually found in cathodic biofilms. Moreover, results suggest that these communities are responsible of maintaining a stable reactor performance.

Impact of solids retention time on the biological performance of an AnMBR treating lipid-rich synthetic dairy wastewater

ABSTRACT In this study, the impact of applied solids retention time (SRT) on the biological performance of an anaerobic membrane bioreactor (AnMBR) treating synthetic dairy wastewater with high lipid content was assessed. Two side-stream AnMBR systems were operated at an SRT of 20 and 40 days (R20 and R40, respectively), equipped with an inside-out tubular membrane operated in cross-flow mode under full-scale operational conditions, i.e. crossflow velocity, transmembrane pressure, membrane flux. Successful operation was achieved and removal efficiencies of both reactors were up to 99% applying an organic loading rate (OLR) of 4.7 g COD L−1 d−1. No precipitation of lipids was observed throughout the operational period, keeping the lipids available for the anaerobic degradation. Long chain fatty acid (LCFA) accumulation was very modest and amounted 148 and 115 mg LCFA-COD per gram of volatile suspended solids (VSS) for R20 and R40, respectively. At an SRT of 40 days, a slightly better biological conversion was obtained. Periodically performed specific methanogenic activity (SMA) tests showed stabilization of the SMA for R40 sludge, whereas for R20 sludge the SMA continued to decrease. This study revealed a more stable reactor performance operating the AnMBR at an SRT of 40 days compared to 20 days.

A Novel Mathematical Model for Predicting Performance of the Sequential Batch Anaerobic Digestion Process: Part I

Anaerobic digestion (AD) is a promising technology to produce renewable energy from organic wastes and achieve desirable solids reduction. One critical step for successful and continuous operation of an AD system is the startup phase, when fresh feedstock is added periodically to the digester after a portion of finished material is discharged, until reaching a pseudo-steady state. This process, identified as a sequential batch AD process, describes how many solid-state and liquid AD systems are managed. Currently, the startup and optimization of such a digester is largely determined by trial and error based on several months of continuous monitoring and adjustment by experienced operators. This research proposes a novel mathematical model to predict outputs per unit volume of a sequential batch AD process during the startup phase as well as at steady state. Important model inputs include the feedstock‘s total solids (TS) content, degradable solids content (α), digestion rate (k), batch time (∆t), digestate recycle rate (r), and water supplementation (X2). The model predicts the total methane yield (U), final end product TS and α, and mass quantity (X1, Qin, QCH4) at steady state. The model provides useful information for the design, operation, and optimization of large-scale AD digesters. Equations are used to identify r values for stable reactor performance given k and ∆t, assuming ratios of feedstock to digestate between 1 and 6.

Impact of food-to-microorganisms ratio on the stability of aerobic granular sludge treating high-strength organic wastewater

This work investigated the long-term stability of aerobic granular sludge treating high-strength organic wastewater in a semi-pilot scale sequential batch reactor (SBR). The reactor was operated for 316 days under different operational conditions. It was found that the F/M ratio is an important parameter affecting granules formation and stability. Three selection mechanisms were investigated: (1) cultivation and maturation at moderately high influent COD concentration (2500 mg/L) followed by increase in influent COD concentration to 7500 mg/L; (2) stressed cultivation and operation at high influent COD concentration of 4500 mg/L; and (3) alternate feed loading strategy (variable influent COD concentration across the daily schedule of cycles at 50%, 75%, and 100% of the peak concentration of 5000 mg/L). It was found that adopting high OLR at the reactor start-up accelerated the formation of granules. However, the overgrowth of biomass under high organics concentration negatively affected the stability of granules and led to disintegration due to the presence of methanogens in the granule core. Cultivation at high organics concentration resulted in a rapid loss of microbial diversity and reactor failure. Under alternate feed loading, adequate selection of microbial community was maintained and resulted in stable reactor performance. Moreover, a strong correlation between F/M ratio and the granules settling ability was observed. When F/M ratio exceeded 1.5 gCOD/gSS.d, granules showed poor settleability and under very high sludge loading rates (above 2.5), sludge bulking occurred and led to washout of sludge due to the strong selection pressure of short settling time. Operating the reactor at F/M ratio of 0.5–1.4 gCOD/gSS.d appears to favor stable long-term granule stability.

Simultaneous nitrification, denitrification and phosphorus removal in aerobic granular sequencing batch reactors with high aeration intensity: Impact of aeration time

A new operating approach by reducing the aeration time while keeping high intensity was evaluated for enhanced nutrients removal and maintenance of granular stability. Three aerobic granular sequencing batch reactors (SBR) performing simultaneous nitrification, denitrification and phosphorus removal (SNDPR) were run at different aeration time (120, 90, and 60 min). Aerobic granules could remain their integrity and stability over long-term operation under high aeration intensity and different time, and shorter aeration time favored the retention of biomass, better settleability, and more production of extracellular polymeric substances (EPS). Besides, efficient and stable reactor performance for carbon and phosphorus were achieved, especially, enhanced nitrogen removal was obtained due to reduction of aeration time. Further exploration revealed that the aeration time shaped the bacterial community in terms of diversity, composition, as well as the distribution of functional groups involving carbon, nitrogen and phosphorus removal.

Improving biogas production from anaerobic co-digestion of Thickened Waste Activated Sludge (TWAS) and fat, oil and grease (FOG) using a dual-stage hyper-thermophilic/thermophilic semi-continuous reactor

This paper investigates the feasibility and advantages of using a dual-stage hyper-thermophilic/thermophilic semi-continuous reactor system for the co-digestion of Thickened Waste Activated Sludge (TWAS) and Fat, Oil and Grease (FOG) to produce biogas in high quantity and quality. The performance of the dual-stage hyper-thermophilic (70°C)/thermophilic (55°C) anaerobic co-digestion system is evaluated and compared to the performance of a single-stage thermophilic (55°C) reactor that was used to co-digest the same FOG-TWAS mixtures. Both co-digestion reactors were compared to a control reactor (the control reactor was a single-stage thermophilic reactor that only digested TWAS). The effect of FOG% in the co-digestion mixture (based on total volatile solids) and the reactor hydraulic retention time (HRT) on the biogas/methane production and the reactors' performance were thoroughly investigated. The FOG% that led to the maximum methane yield with a stable reactor performance was determined for both reactors. The maximum FOG% obtained for the single-stage thermophilic reactor at 15 days HRT was found to be 65%. This 65% FOG resulted in 88.3% higher methane yield compared to the control reactor. However, the dual-stage hyper-thermophilic/thermophilic co-digestion reactor proved to be more efficient than the single-stage thermophilic co-digestion reactor, as it was able to digest up to 70% FOG with a stable reactor performance. The 70% FOG in the co-digestion mixture resulted in 148.2% higher methane yield compared to the control at 15 days HRT. 70% FOG (based on total volatile solids) is so far the highest FOG% that has been proved to be useful and safe for semi-continuous reactor application in the open literature. Finally, the dual-stage hyper-thermophilic/thermophilic co-digestion reactor also proved to be efficient and stable in co-digesting 40% FOG mixtures at lower HRTs (i.e., 9 and 12 days) and still produce high methane yields and Class A effluents.

Sample preparation, preservation, and storage for volatile fatty acid quantification in biogas plants.

Volatile fatty acids (VFA) represent short-chain fatty acids consisting of six or fewer carbon atoms that can be distilled at atmospheric pressure. In anaerobic digestion processes VFAs are of central importance for maintaining stable reactor performance and biogas production, are used as indicators for arising problems and are important process monitoring parameters. In the present study, sludge derived form a full-scale anaerobic digester of a wastewater treatment plant was spiked with formate, acetate, propionate, and butyrate in order to evaluate various commonly used techniques for VFA extraction, preservation, and storage. It was shown that VFA extraction after centrifugation warranted the highest recovery rates for spiked VFAs. Moreover, experiments clearly indicated the importance of a fast sample handling, including the necessity of immediate cooling of the samples. Chemical sample preservation within a narrow time frame or deep freezing emerged as an alternative to instant VFA extraction. Short-time storage of extracted VFA samples at + 4°C is an option for up to 7days, for longer periods storage at -20°C was found to be applicable.

Granulation of susceptible sludge under carbon deficient conditions: A case of denitrifying sulfur conversion-associated EBPR process

Sludge granulation has been recognized as a promising biotechnology in wastewater treatment. Whereas the granulation of susceptible sludge in particular with a very low organic loading rate (OLR) (≤0.6 kg COD/m3/day or ≤ 120 mg COD/g VSS/day) is a difficult task that has not been achieved in activated sludge systems yet. This study was aimed at exploring an effective strategy for sludge granulation in the recently developed Denitrifying Sulfur conversion-associated Enhanced Biological Phosphorus Removal (DS-EBPR) process using a sequencing batch pump-lift reactor. Four strategies were studied by manipulating the factors of organic loading rate (OLR), superficial upflow velocity and sludge settling time individually or collectively. Increasing both the OLR and the superficial upflow velocity effectively promoted granule formation but at the same time led to unstable and even deteriorated reactor performance. The development of granules proceeded via several stages: formation, dispersion, reformation and stabilization. Gradually increasing the superficial upflow velocity from 5.1 to 6.8 m/h and keeping the OLR at 112.4 mg COD/g VSS/day proved to be most effective strategy for accelerating granulation while simultaneously achieving stable reactor performance. Under these conditions, the granules became stable with a diameter of 375–400 μm and displayed excellent settleability. The two major microbial groups, sulfate-reducing bacteria and sulfide-oxidizing bacteria, in the microbial community of the DS-EBPR granular sludge were enriched to 17.7% and 15.8% respectively. The newly developed DS-EBPR granular system was able to achieve an almost threefold improvement in phosphorus removal efficiency and 25% reduction in the operating cycle time compared with a flocculent DS-EBPR system.

Understanding the anaerobic BTEX removal in continuous-flow bioreactors for ex situ bioremediation purposes

This work aimed to understand the anaerobic BTEX removal in continuous-flow bioreactors for ex situ bioremediation purposes, evaluating the effect of some operational parameters on efficiency, stability and microbial community structure. The influence of the hydraulic retention time, effluent recirculation and co-substrate (ethanol) concentration was investigated on a mesophilic UASB bioreactor operated under methanogenic conditions. The changes on the bacterial and archaeal communities were evaluated in terms of diversity, evenness and richness. Good BTEX removal (∼63%) and reactor stability could still be reached at a HRT of 24h. The impact of the effluent recirculation on BTEX removal was not evident at high co-substrate (ethanol) concentrations, but it was significant when low concentrations were applied. The reduction of ethanol concentration had a positive impact on BTEX removal (from 80% to 86%), especially for benzene (from 51% to 62%). The optimal degree of evenness likely contributed to the relatively high stability of the system in terms of BTEX removal. Changes observed in bacterial and archaeal richness did not match with the functioning of the system. However, dynamics and evenness parameters seemed to be of importance in maintaining a stable reactor performance.

Diversity and dynamics of dominant and rare bacterial taxa in replicate sequencing batch reactors operated under different solids retention time.

In this study, 16S rRNA gene pyrosequencing was applied in order to provide a better insight on the diversity and dynamics of total, dominant, and rare bacterial taxa in replicate lab-scale sequencing batch reactors (SBRs) operated at different solids retention time (SRT). Rank-abundance curves showed few dominant operational taxonomic units (OTUs) and a long tail of rare OTUs in all reactors. Results revealed that there was no detectable effect of SRT (2 vs. 10days) on Shannon diversity index and OTU richness of both dominant and rare taxa. Nonmetric multidimensional scaling analysis showed that the total, dominant, and rare bacterial taxa were highly dynamic during the entire period of stable reactor performance. Also, the rare taxa were more dynamic than the dominant taxa despite expected low invasion rates because of the use of sterile synthetic media.

Outlining microbial community dynamics during temperature drop and subsequent recovery period in anaerobic co-digestion systems

To improve the stability of anaerobic reactors, more knowledge is required about how the different communities react against operating perturbations and which specific ones respond better. The objective of this work was to monitor the changes in microbial community structure of an anaerobic digester during a temperature drop by applying different complementary molecular techniques. Temperature decrease led to an increase of Bacteroidales order, Porphyromonadaceae family and Bacteroides genus and a decrease in Syntrophomonas and Clostridium genera. Once the temperature was restored, the reactor recovered the steady state performance without requiring any modification in operational conditions or in the microbiome. During the recovery period, Sedimentibacter genus and Porphyromonadaceae family played an important role in the degradation of the accumulated volatile fatty acids. The hydrogenotrophic methanogens appeared to be the keystone archaeal population at low temperatures as well as in the recovery period. This study stands out that the understanding of microbial community dynamics during temperature drop could be utilized to develop strategies for the mitigation of temperature change consequences and speed up the recovery of stable reactor performance.

Reorganization of the bacterial and archaeal populations associated with organic loading conditions in a thermophilic anaerobic digester

Organic loading conditions are an important factor influencing reactor performances in methanogenic bioreactors. Yet the underlying microbiological basis of the process stability, deterioration, and recovery remains to be understood. Here, structural responses of the bacterial and archaeal populations to the change of organic loading conditions in a thermophilic anaerobic digester were investigated by process analyses and 16S rRNA gene-based molecular approaches. The biogas was produced stably without the accumulation of volatile fatty acids (VFAs) at low organic loading rates (OLRs) in the beginning of reactor operation. Increasing OLR in stages disrupted the stable reactor performance, and high OLR conditions continued the deteriorated performance with slight biogas production and high accumulation of VFAs. Thereafter, the gradual decrease of OLR resulted in the recovery from the deterioration, giving rise to the stable performance again. The stable performances before and after the high OLR conditions conducted were associated with compositionally similar but not identical methanogenic consortia. The bacterial and archaeal populations were synchronously changed at both the transient phases toward the deteriorated performance and in recovery process, during which the dynamic shift of aceticlastic and hydrogenotrophic methanogens including the recently identified Methanomassiliicoccus might contribute to the maintenance of the methanogenic activity. The distinctive bacterial population with a high predominance of Methanobacterium formicicum as archaeal member was found for the deteriorated performance. The results in this study indicate the coordinated reorganization of the bacterial and archaeal populations in response to functional states induced by the change of organic loading conditions in the anaerobic digester.

Engineering and microbiological aspects of BTEX removal in bioreactors under sulfate-reducing conditions

Abstract This work assessed the anaerobic BTEX removal performance of a sulfate-reducing bioreactor at different COD/SO42− ratios and co-substrate (ethanol) concentrations. Additionally, the effect of the different operational conditions on the structure (diversity, evenness and richness) of the bacterial and archaeal communities was investigated. The addition of sulfate at different COD/SO42− ratios did not change BTEX removal (60–67%). However, a likely inhibition of COD removal was observed for COD/SO42− ratios below 5 (72–81%). Although, initially, ethanol concentration reduction seemed to enhance BTEX removal (from 60% to 73%), especially for benzene (46%) and toluene (76%), the results about the actual impact of co-substrate shortage were inconclusive, even though the statistical tests indicated that ethanol affected negatively BTEX removal. The optimal degree of evenness likely contributed to the high stability of the system in terms of BTEX removal. Changes observed in bacterial and archaeal richness did not match with the functioning of the system. However, dynamics and evenness parameters seemed to be of importance in maintaining a stable reactor performance.

Alkalinity ratios to identify process imbalances in anaerobic digesters treating source-sorted organic fraction of municipal wastes

Two 5L anaerobic reactors were used to monitor the mesophilic anaerobic digestion of source sorted organic fraction of municipal solid wastes (SS-OFMSW) focusing the attention on the response of alkalinity ratios. Intermediate/partial alkalinity (IA/PA) ratio can be used as a simple and cheaper alternative to VFAs analysis when digester's stability needs to be assessed in full-scale plants treating these organic wastes. However, lab-scale studies in order to establish a specific limit value of IA/PA referred to SS-OFMSW had not been conducted. In this study, a reference reactor (R1) was operated at low organic loading rates (OLR) and high hydraulic retention times (HRT) during 165 days. Besides, severe disturbances were applied to a second reactor (R2) during 281 days by means of increasing both HRT and OLR in order to assess the digester response under continuous overload conditions. The obtained results show that an IA/PA ratio of below 0.3 is recommended to maintain total VFAs between 2.5 and 3.5kgm−3 and achieve a stable reactor performance treating SS-OFMSW in a range of total alkalinity (TA) between 13 and 15kgCaCO3m−3. These results provide a starting point to develop further works in full-scale digesters, in order to improve the monitoring and process control of full-scale anaerobic reactors treating SS-OFMSW.