Phase separation as a strategy to prevent sulfide-related drawbacks in methanogenesis: performance and energetic aspects.

The establishment of sulfate (SO42-) reduction during methanogenesis may considerably hinder the efficient energetic exploitation of methane, once removing sulfide from biogas is obligate and can be costly. In addition, sulfide generation can negatively impact the performance of methanogens by triggering substrate competition and sulfide inhibition. This study investigated the impacts of removing SO42- during fermentation on the performance of a second-stage methanogenic continuous reactor (R2), comparing the results with those obtained in a single-stage system (R1) fed with SO42--rich wastewater (SO42- of up to 400 mg L-1, COD/SO42- of 3.12-12.50). The organic load (OL) was progressively increased to 5.0 g COD d-1 in both reactors, showing completely discrepant performances. Sulfate-reducing bacteria outperformed methanogens in the consumption for organic matter during the start-up phase (OL = 2.5 g COD d-1) in R1, directing up to 73% of the electron flow to SO42- reduction. An efficient methanogenic activity was established in R1 only after decreasing the OL to 0.625 g COD d-1, after which methanogenesis prevailed by consuming ca. 90% of the removed COD. Nevertheless, high sulfide proportions (up to 3.1%) were measured in biogas. Conversely, methanogenesis was promptly established in R2, resulting in a methane-rich (> 80%) and sulfide-free biogas regardless of the operating condition. From an economic perspective, processing the biogas evolved from R2 would be cheaper, although the techno-economic impacts of managing the sulfur pollution in the fermentative reactor still need to be understood.

Efficient reactivation of anammox sludge after prolonged storage using a combination of batch and continuous reactors.

Due to the slow growth rate of anammox bacteria, enriched sludge is required for the rapid start-up of anammox-based reactors. However, it is still unclear if long-term stored anammox sludge (SAS) is an effective source of inoculum to accelerate reactor start-up. This study explored the reactivation of long-term SAS and developed an efficient protocol to reduce the start-up period of an anammox reactor. Although stored for 13 months, a low level of the specific anammox activity of 28 mg N/g VSS/d was still detected. Experimental Phase 1 involved the direct application of SAS to an upflow sludge bed reactor (USB) operated for 90 d under varying conditions of hydraulic retention time and nitrogen concentrations. In Phase 2, batch runs were executed prior to the continuous operation of the USB reactor. The biomass reactivation in the continuous flow reactor was unsuccessful. However, the SAS was effectively reactivated through a combination of batch runs and continuous flow feed. Within 75 days, the anammox process achieved a stable rate of nitrogen removal of 1.3 g N/L/day and a high nitrogen removal efficiency of 84.1 ± 0.2%. Anammox bacteria (Ca. Brocadia) abundance was 37.8% after reactivation. These overall results indicate that SAS is a feasible seed sludge for faster start-up of high-rate mainstream anammox reactors.

Removal of organic matter and nitrogen from dairy effluents in a structured bed reactor operated with intermittent aeration.

The objective of this research was to evaluate the performance of a structured bed reactor (SBRIA), carried out with intermittent aeration (IA), in the removal of organic matter and nitrogen from dairy effluent, when run with different organic loading rates (OLR). The SBRIA was operated for 227 days, with 2:1 AI cycles (2 h with aeration on and 1 h off) and Hydraulic Retention Time (HRT) of 16 h. Three phases, with different OLR, were evaluated: phases A (1000 gCOD m-3 day-1 - 63 days), B (1400 gCOD m-3 day-1 - 94 days), and C (1800 gCOD m-3 day-1 - 70 days). The percentage of COD, NH4+-N removal, and nitrogen removal, respectively, were above 85 ± 7%, 73 ± 27%, and 83 ± 5, in all phases. There was no accumulation of the oxidized forms of nitrogen in the reactor. The kinetic test, performed to evaluate the nitrification and denitrification in the system, indicated that even in dissolved oxygen concentrations of 4.5 mg L-1, it was possible to obtain the denitrification process in the system. The results demonstrate that the reactor under study has positive characteristics to be used as an alternative for removing the removal of organic material and nitrogen in the biological treatment of dairy effluents.

Strategies to control pH in the dark fermentation of sugarcane vinasse: Impacts on sulfate reduction, biohydrogen production and metabolite distribution.

pH is notably known as the main variable defining distinct metabolic pathways during sugarcane vinasse dark fermentation. However, different alkalinizing (e.g. sodium bicarbonate; NaHCO3) and/or neutralizing (e.g. sodium hydroxide; NaOH) approaches were never directly compared to understand the associated impacts on metabolite profiles. Three anaerobic structured-bed reactors (AnSTBR) were operated in parallel and subjected to equivalent operational parameters, except for the pH control: an acidogenic-sulfidogenic (R1; NaOH + NaHCO3) designed to remove sulfur compounds (sulfate and sulfide), a hydrogenogenic (R2; NaOH) aimed to optimize biohydrogen (bioH2) production, and a strictly fermentative system without pH adjustment (R3) to mainly evaluate lactic acid (HLa) production and other soluble metabolites. NaHCO3 dosing triggered advantages not only for sulfate reduction (up to 56%), but also to enhance the stripping of sulfide to the gas phase (75-96% of the theoretical sulfide produced) by the high and constant biogas flow resulting from the CO2 released during NaHCO3 dissociation. Meanwhile, molasses-based vinasse presented higher potential for bioH2 (up to 4545 mL-H2 L-1 d-1) and HLa (up to 4800 mg L-1) production by butyric-type and capnophilic lactic fermentation pathways. Finally, heterolactic fermentation was the main metabolic route established when no pH control was provided (R3), as indicated by the high production of both HLa (up to 4315 mg L-1) and ethanol (1987 mg L-1). Hence, one single substrate (from which one single source of inoculum was originated) offers a wide range of metabolic possibilities to be exploited, providing substantial versatility to the application of anaerobic digestion in sugarcane biorefineries.

Enhanced biological nitrogen and phosphorus removal from sewage driven by fermented glycerol: comparative assessment between sequencing batch- and continuously fed-structured fixed bed reactor.

The nutrient biological removal from sewage, especially from anaerobic reactor effluents, still represents a major challenge in conventional sewage treatment plants. In this work, the nitrogen and phosphorus removal from anaerobic pre-treated domestic sewage in an up-flow anaerobic sludge blanket (UASB) reactor was assessed in a structured fixed bed reactor (SFBR) operated in a continuous and in a batch mode using polyurethane foam as material support for biomass and fermented glycerol as the exogenous carbon source. The SFBR was operated as a sequencing batch reactor with cycles of 90, 120, and 150 min under anaerobic, oxic, and anoxic conditions, respectively, reaching average efficiencies for total nitrogen and phosphorus removal of 88% and 56%, respectively. Fermented glycerol was added during the non-aerated periods. Under continuous feeding, the SFBR was operated with aeration/non-aeration periods of 2/1 (h) and 3/1 (h), hydraulic retention time of 12 h, and a recirculation ratio of 3. Without fermented glycerol addition, the maximum removal of total nitrogen (TN) reached 42%, while adding glycerol in the non-aerated period improved TN removal to 64.9% (2/1 h) and 69.5% (3/1 h). During continuous operation, no phosphorus removal was observed, which was released during the non-aerated period, remaining in the effluent. Optical microscopy analyses confirmed the presence of polyphosphate granules and of the phosphorus accumulating organisms in the reactor biofilm. It was concluded that the batch feeding method was determinant for phosphorus removal. The structured fixed bed reactor with polyurethane foam proved to be feasible in the removal of organic matter and nutrients remaining in the UASB reactor effluent.

Co-digestion of biofuel by-products: Enhanced biofilm formation maintains high organic matter removal when methanogenesis fails.

Ethanol and biodiesel industries generate large volumes of by-products, such as vinasse and glycerol, which are suitable for biogas exploitation. This paper assessed the applicability and process performance of the anaerobic structured-bed reactor (AnSTBR) for the mesophilic (30 °C) continuous (105 days) anaerobic co-digestion of sugarcane vinasse and distilled glycerol under increasing organic loading rates (OLR) (0.5-5.0 kgCOD m-3d-1). The highest methane yield (211 NmL g-1CODrem.) and volumetric production (668 NmL L-1d-1) occurred at an OLR of 3.5 kgCOD m-3d-1. The performance of the AnSTBR showed high removal efficiencies of total COD (77.1%), carbohydrates (81.9%), and glycerol content (99.7%). Biofilm growth enhancement within the reactor offset the impairment of methanogenesis activity at high organic loads. The prompt biodegradability of glycerol reinforced the importance of gradually increase the organic load to prevent the buildup of volatile acids and maintain a stable long-term co-digestion system.

Two-stage partial nitrification-Anammox process for nitrogen removal from slaughterhouse wastewater: Evaluation of the nitrogen loading rate and microbial community analysis.

The production of inputs for animal feed using slaughterhouse byproducts is a predominant waste valorization route of the meat industry. This practice generates complex effluents containing high concentrations of organic matter and nutrients. The partial nitrification process followed by the Anammox process (PN/A) has been shown to be a viable technology for nitrogen removal from wastewaters with high concentrations of ammonia and low COD/N ratios, as found in Upflow Anaerobic Sludge Blanket (UASB) effluent from animal feed inputs industries. However, its application has not been assessed for slaughterhouse byproducts processing wastewaters. This work aimed at evaluating the influence of the nitrogen loading rate (NLR) on the removal of total nitrogen (TN) of a PN/A process treating real animal feed industry wastewater. The NLR in the Anammox reactor varied from 1.3 to 6.3 g N L-1.d-1, with a constant COD/N ratio of 0.5 ± 0.1 mg COD.mg N-1. An average removal efficiency of TN of 84.2 ± 9.8% was observed throughout 440 days of operation. Microbiological analyses of the granular Anammox sludge performed before and after the operation revealed an increase in the population of heterotrophic denitrifying bacteria, while the relative abundance of Anammox species decreased. It was demonstrated that although both microbial groups can coexist synergistically, the presence of organic matter contributed to the growth of heterotrophic denitrifying species and impaired the growth of Anammox bacteria, without affecting system performance.

Increasing salinity concentrations determine the long-term participation of methanogenesis and sulfidogenesis in the biodigestion of sulfate-rich wastewater.

The competition between sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) depends on several factors, such as the COD/SO42- ratio, sensitivity to inhibitors and even the length of the operating period in reactors. Among the inhibitors, salinity, a characteristic common to diverse types of industrial effluents, can act as an important factor. This work aimed to evaluate the long-term participation of sulfidogenesis and methanogenesis in the sulfate-rich wastewater process (COD/SO42- = 1.6) in an anaerobic structured-bed reactor (AnSTBR) using sludge not adapted to salinity. The AnSTBR was operated for 580 d under mesophilic temperature (30 °C). Salinity levels were gradually increased from 1.7 to 50 g-NaCl L-1. Up to 35 g-NaCl L-1, MA and SRB equally participated in COD conversion, with a slight predominance of the latter (53 ± 11%). A decrease in COD removal efficiency associated with acetate accumulation was further observed when applying 50 g-NaCl L-1. The sulfidogenic pathway corresponded to 62 ± 17% in this case, indicating the inhibition of MA. Overall, sulfidogenic activity was less sensitive (25%-inhibition) to high salinity levels compared to methanogenesis (100%-inhibition considering the methane yield). The wide spectrum of SRB populations at different salinity levels, namely, the prevalence of Desulfovibrio sp. up to 35 g-NaCl L-1 and the additional participation of the genera Desulfobacca, Desulfatirhabdium, and Desulfotomaculum at 50 g-NaCl-1 explain such patterns. Conversely, the persistence of Methanosaeta genus was not sufficient to sustain methane production. Hence, exploiting SRB populations is imperative to anaerobically remediating saline wastewaters.

Hacking biofilm developed in a structured-bed reactor (SBRRIA) with integrated processes of nitrogen and organic matter removal.

Biomass samples from a structured-bed reactor subjected to recirculation and intermittent aeration (SBRRIA) were analyzed to investigate the bacterial community shift along with the changes in the C/N ratio. The C/N ratios tested were 7.6 ± 1.0 (LNC) and 2.9 ± 0.4 (HNC). The massive sequencing analyses revealed that the microbial community adjusted itself to different organic and nitrogenous applied loads, with no harm to reactor performance regarding COD and Total-N removal. Under LNC, conventional nitrification and heterotrophic denitrification steered the process, as indicated by the detection of microorganisms affiliated with Nitrosomonadaceae, Nitrospiraceae, and Rhodocyclaceae families. However, under HNC, the C/N ratio strongly affected the microbial community, resulting in the prevalence of members of Saprospiraceae, Chitinophagaceae, Xanthomonadaceae, Comamonadaceae, Bacillaceae, and Planctomycetaceae. These families include bacteria capable of using organic matter derived from cell lysis, ammonia-oxidizers under low DO, heterotrophic nitrifiers-aerobic denitrifiers, and non-isolated strains of Anammox. The DO profile confirmed that the stratification in aerobic, anoxic, and anaerobic zones enabled the establishment of different nitrogen degradation pathways, including the Anammox.

Effect of operating conditions on N2O production in an anammox sequencing batch reactor containing granular sludge.

Nitrous oxide (N2O) is one of the gases with the greatest impact in the atmosphere due to its persistence and significant contribution to the greenhouse effect. This study provides an insight into the dynamics of N2O production in wastewater nitrogen removal systems. A 10 L sequencing batch reactor containing enriched anammox biomass was subjected to different operational conditions, i.e., temperature, feed time, NO2 -/NH4 + ratio and the initial concentrations of NH4 + and NO2 -. Tests showed no significant differences in maximum N2O production when the system was operated with a shorter feed time and no increase in the operating temperature. A higher N2O production was observed when the initial NO2 -/NH4 + ratio increased from 1.3 to 1.7 and 1.9. The highest initial concentration of NO2 - was linked to an increase in residual N2O at the end of the batch cycle, probably due to heterotrophic denitrifying metabolism.

Characterizing phenol-removing consortia under methanogenic and sulfate-reducing conditions: potential metabolic pathways.

Phenol removal was investigated in anaerobic fixed-structured bed reactors, namely R1 and R2, treating synthetic wastewater simulating the soluble fraction of vinasse under strictly methanogenic (R1) and simultaneous methanogenic/sulfidogenic conditions (R2). Next-generation sequencing (Illumina MiSeq System) was used to further characterize the microbial communities in both systems. Phenol was completely and stably removed in R1 after a short operating period (≈55 days). Conversely, phenol removal in R2 required a longer period for biomass acclimation (≈125 days) to reach levels equivalent to R1. Volatile fatty acids (VFA) accumulation in R2, mainly due to the inhibition of the acetoclastic methanogenesis by sulfide, may have limited phenol removal in the initial operating phases, as intermediate steps from phenol degradation are thermodynamically dependent on the removal of acetate, hydrogen and bicarbonate. Overall, the potential for anaerobically removing phenol from complex wastewaters was confirmed, even at low phenol/COD ratios. 16S rRNA gene sequencing analysis showed a high correlation of taxonomic profile between R1 and the inoculum, whereas a lower correlation was observed between R2 and the inoculum samples. Functional inference further indicated that Syntrophus and Bacillus genera in R1 and Clostridium genus in both reactors possibly played a key-role in phenol degradation.

Determining the distribution of granule diameter from biological sludge.

Anaerobic granule sizes from various types of anaerobic biological wastewater treatments were investigated in order to understand the influence of this characteristic on the performance of the treatment system. To date, there is no standardised methodology in the current literature, which provides details of a process to obtain data, such as a suitable sample volume, a description of the precision and limitations of the techniques used. Therefore, the aim of this protocol is to standardise the granulometry assay that can measure granule sizes accurately and quickly. In addition, the proposed methodology comprises about 1500-3000 granules in a single sample, a representative number compared to the currently applied methodologies.

Effects of intermittent aeration periods on a structured-bed reactor continuously fed on the post-treatment of sewage anaerobic effluent.

This study assessed the simultaneous nitrification and denitrification processes and remaining organic matter removal from anaerobic reactor effluent treating wastewater in a single reactor. A structured-bed reactor, with polyurethane foam as support media, was subjected to intermittent aeration and effluent recirculation. Aerated/non-aerated periods varied in the range of 2/1-1/3 h. The chemical oxygen demand (COD) in the effluent remained between 26 and 42 mg L-1 throughout all the aeration conditions. Aeration periods of 1/2 h removed 80 and 26% of Total Kjeldahl Nitrogen and Total Nitrogen, respectively. A low solid production was observed during the 300 days of operation, resulting in a solid retention time of 139 days. The results indicate that the non-aerated periods generated alkalinity that favored nitrification, maintaining low COD concentrations in the effluent. The structured bed reactor presented a low solid production and effluent loss below 20 mgSSV L-1, similar to concentrations obtained in secondary decanters.

Performance improvement of a thermophilic sulfate-reducing bioreactor under acidogenic conditions: Effects of diversified operating strategies.

The establishment of a sulfidogenic environment under thermophilic (55 °C) acidogenic conditions was assessed in an innovative structured-bed bioreactor to enhance sulfate removal and acetate production prior to methanogenesis. Diversified operating strategies, i.e., variations in the hydraulic retention time (HRT; 6-12 h), sulfate loading rate (SLR; 8-16 kg SO42- m-3 day-1) and liquid phase recirculation ratio (0.0-57.0) were assessed to both enable the establishment of sulfate-reducing conditions and remove H2S from the liquid phase. Ethanol was used as the only carbon source. Applying a low HRT (6 h) as the initial operating strategy severely hindered the establishment of sulfate-reducing bacteria (SRB) populations within the system (sulfate removal < 27.5%). In turn, applying effluent recirculation had a positive impact on the system (sulfate removal âˆ¼ 60%) by providing an adequate buffer control along the entire height of the system, as well by displacing over 70% of the H2S to the gaseous phase. The maintenance of pH values above 6.1 proved to be adequate for the sulfidogenic activity, whereas enhanced acidic conditions (pH < 6.0) at the basal portion of the reactor comprised a determining factor to hinder sulfate reduction. SRB were able to handle H2S and acetate concentrations as high as 232 mg L-1 and 3111 mg L-1, respectively, after establishing an effective acidogenic/sulfidogenic environment, indicating that the proposed system has the potential to be used as the first stage in the anaerobic processing of sulfate-rich wastewater streams.

Down-flow fixed-structured bed reactor: An innovative reactor configuration applied to acid mine drainage treatment and metal recovery.

A down-flow fixed-structured bed reactor (DFSBR) was operated for 277 days treating a mixture of synthetic substrates simulating an iron-rich acid mine drainage (AMD) and the soluble fraction of a sugarcane vinasse. The synthetic sugarcane vinasse was used as electron donor for biological sulfate-reduction, resulting in influent chemical oxygen demand (COD) close to 4000 mg L-1 and volumetric organic loading rate of 4.8 g L-1d-1. The influent sulfate concentration was kept close to 2000 mg L-1 (volumetric sulfate loading rate of 2.5 g L-1d-1) while a gradual increase of iron concentration (2-400 mg L-1) was applied. COD removal efficiencies were higher than 93% and the sulfate removal efficiencies were close to 100%. With the highest iron concentration (400 mg L-1) applied, the DFSBR achieved 95% of iron removal efficiency. The precipitate collected at the reactor bottom showed increasing concentrations of fixed suspended solids (FSS), as well as an increasing proportion of iron, indicating the possibility of metal recovery from the system. The association between sulfidogenic and methanogenic processes also enables energy recovery from the methane-rich biogas produced.

Removing organic matter from sulfate-rich wastewater via sulfidogenic and methanogenic pathways.

The simultaneous organic matter removal and sulfate reduction in synthetic sulfate-rich wastewater was evaluated for various chemical oxygen demand (COD)/sulfate ratios applied in a horizontal-flow anaerobic immobilized sludge (HAIS) reactor. At higher COD/sulfate ratios (12.5 and 7.5), the removal of organic matter was stable, likely due to methanogenesis. A combination of sulfate reduction and methanogenesis was clearly established at COD/sulfate ratios of 3.0 and 1.9. At a COD/sulfate ratio of 1.0, the organic matter removal was likely influenced by methanogenesis inhibition. The quantity of sulfate removed at a COD/sulfate ratio of 1.0 was identical to that obtained at a ratio of 1.9, indicating a lack of available electron donors for sulfidogenesis. The sulfate reduction and organic matter removal were not maximized at the same COD/sulfate ratio; therefore, competitive inhibition must be the predominant mechanism in establishing an electron flow.