Microaerobic condition as pretreatment for improving anaerobic digestion: A review.

Pretreatment of waste before anaerobic digestion (AD) has been extensively studied during the last decades. One of the biological pretreatments studied is the microaeration. This review examines this process, including parameters and applications to different substrates at the lab, pilot and industrial scales, to guide further improvement in large-scale applications. The underlying mechanisms of accelerating hydrolysis and its effects on microbial diversity and enzymatic production were reviewed. In addition, modelling of the process and energetic and financial analysis is presented, showing that microaerobic pretreatment is commercially attractive under certain conditions. Finally, challenges and future perspectives were also highlighted to promote the development of microaeration as a pretreatment before AD.

Valorization of oat husk by hydrothermal carbonization: Optimization of process parameters and anaerobic digestion of spent liquors.

The hydrothermal carbonization (HTC) optimization of oat husk was performed using a response surface methodology. Furthermore, anaerobic digestion (AD) of spent liquor and hydrochar addition were evaluated in the biomethane potential (BMP) test. Results found that temperature influences the most in the studied responses (i.e., mass yield (MY) and higher heating value (HHV)). Optimal hydrochar MY (53.8%) and HHV (21.5 MJ/kg) were obtained for 219.2 °C, 30 min, and 0.08 of biomass/water ratio. A successful prediction capability of the optimization approach was observed, archiving an error < 1% between predicted and validated responses. The BMP experiment showed the feasibility of spent liquor as a potential substrate to be treated by AD (144 NmLCH4/gCOD). Hydrochar boosted the methane production of spent liquor increasing up to 17% compared to digestion with no hydrochar addition. These findings provide new insights regarding oat husk valorization by integrating HTC and AD for energy production.

Sulfur-based mixotrophic denitrification with the stoichiometric S0/N ratio and methanol supplementation: effect of the C/N ratio on the process.

The impact of the organic carbon to nitrate ratio (C/N ratio) on mixotrophic denitrification rate has been scarcely studied. Thus, this work aims to investigate the effect of the C/N ratio on the mixotrophic denitrification when methanol is used as a source of organic matter and elemental sulfur as an electron donor for autotrophic denitrification. For this, two initial concentrations of NO3--N (50 and 25 mg/L) at a stoichiometric ratio of S0/N, and four initial C/N ratios (0, 0.6, 1.2, and 1.9 mg CH3OH/mg NO3- -N) were used at 25 (±2) °C. The results showed that when using a C/N ratio of 0.6, the highest total nitrogen removal was obtained and the accumulation of nitrites was reduced, compared to an autotrophic system. The most significant contribution to nitrate consumption was through autotrophic denitrification (AuDeN) for a C/N ratio of 0.6 and 1.2, while for C/N = 1.9 the most significant contribution of nitrate consumption was through heterotrophic denitrification (HD). Finally, organic supplementation (methanol) served to increase the specific nitrate removal rate at high and low initial concentrations of substrate. Therefore, the best C/N ratio was 0.6 since it allowed for increasing the removal efficiency and the denitrification rate.

Modeling of the effect of zeolite concentration on the biological nitrification process in the presence of sulfide and organic matter.

The evaluation of the nitrification kinetics in the simultaneous presence of sulfide and organic matter using zeolite as improver was the main goal of this work. According to the sensitivity and collinearity analyses, five parameters were the most sensitive in the model, whose calibrated values were: µ max, AOB = 0.02642 ± 0.002 h-1; µ max, NOB = 0.3307 ± 0.416 h-1; K S,NOB = 1.65·10-6 ± 2.85·10-6 mgHNO2-N/L; k S2 = 0.8213 ± 0.076 and n = 0.6537 ± 0.030. A good fit between the experimental data and the model's results including the effect of zeolite on the kinetic parameters was obtained, with Theil inequality coefficient values between 0.109 and 0.007 for all the variables studied, with all of these values lower than 0.3. Thus, the model proposed is robust and can simulate the nitrification process in the presence of sulfide and organic matter when zeolite was used as improver.

Valorization of the liquid fraction of co-hydrothermal carbonization of mixed biomass by anaerobic digestion: Effect of the substrate to inoculum ratio and hydrochar addition.

The effect of the substrate to inoculum ratio (SIR) on the anaerobic digestion (AD) of the liquid fractions (LFs) of co-hydrothermally treated mixed biomass (sewage sludge (SS) and the organic fraction of municipal solid waste (MSW)) was evaluated. The impact of SS + MSW-hydrochar was also studied at different hydrochar concentrations (0, 1, 5, 10, 15, 20 and 25 ghydrochar/L), in two of the LFs studied. The SIR had a significant impact on methane yield (YCH4) and organic matter degradation, with low methane production (4-44 NmLCH4/g tCODadded) observed for all LFs at SIR = 1:3. Hydrochar significantly improved YCH4 and specific methane production rate. Compared with the AD without hydrochar, the YCH4 improved up to 1.95 times at 15 g/L. Hydrochar doses from 1 to 10 g/L shortened the lag phase, while higher concentrations (15 to 25 g/L) showed an increment with respect to the control reactor without hydrochar.

Fly ash as stimulant for anaerobic digestion: effect over hydrolytic stage and methane generation rate.

Thermoelectric fly ash was used as a micronutrient source for microorganisms in the anaerobic digestion process of thermally pretreated (1 hour, 120 °C) secondary sludge. The obtained results not only suggest that fly ash improves methane generation in the conversion of volatile fatty acids into methane, but also show a new observation, that the fly ash contributes in the particulate organic solubilization. The maximum methane production rate increased from 6.52 mL/L/d to 22.59 mL/L/d when fly ash was added at a dosage of 150 mg/L in biochemical methane potential tests compared with tests with no added ash. Additionally, the kinetic constants of the hydrolysis of particulate organic matter were obtained in both cases (with and without added ash) in batch reactors using a first-order kinetic model; in the case of no addition, the first-order kinetic parameter was 0.019 ± 0.002 d-1, while with ashes this value increased to 0.045 ± 0.000 d-1. Therefore, the addition of fly ash improves methane generation and hydrolytic kinetics in different orders of magnitude.

Anaerobic digestion of wastewater rich in sulfate and sulfide: effects of metallic waste addition and micro-aeration on process performance and methane production.

This work explores the effect of two metallic wastes (mining wastes, MW; fly ashes, FA) and micro-aeration (MA) on the anaerobic digestion of wastewater which is rich in sulfate and sulfide. Two initial COD concentrations (5,000 and 10,000 mg/L) were studied under both conditions in batch systems at 35 °C, with a fixed COD/SO42- ratio = 10, with 100 mg/L of S2-. It was observed that the use of MW and FA in the assays with an initial COD concentration of 10,000 mg/L resulted in a simultaneous increase in COD removal, sulfate removal, sulfide removal and methane generation, while MA only improved the COD and sulfide removals in comparison with the control system. On the contrary, the use of MW, FA or MA in systems with initial COD concentrations equal to or lower than 5,000 mg/L did not show any improvement with respect to the control system in terms of COD removal, sulfate removal or methane generation, with only sulfide removal being positively affected by MW and FA.

Degradation of ampicillin antibiotic by electrochemical processes: evaluation of antimicrobial activity of treated water.

Ampicillin (AMP) is an antibiotic widely used in hospitals and veterinary clinics around the world for treating infections caused by bacteria. Therefore, it is common to find traces of this antibiotic in wastewater from these entities. In this work, we studied the mineralization of this antibiotic in solution as well as the elimination of its antimicrobial activity by comparing different electrochemical advanced oxidation processes (EAOPs), namely electro-oxidation with hydrogen peroxide (EO-H2O2), electro-Fenton (EF), and photo electro-Fenton (PEF). With PEF process, a high degradation, mineralization, and complete elimination of antimicrobial activity were achieved in 120-min electrolysis with high efficiency. In the PEF process, fast mineralization rate is caused by hydroxyl radicals (·OH) that are generated in the bulk, on the anode surface, by UV radiation, and most importantly, by the direct photolysis of complexes formed between Fe3+ and some organic intermediates. Moreover, some products and intermediates formed during the degradation of the antibiotic Ampicillin, such as inorganic ions, carboxylic acids, and aromatic compounds, were determined by photometric and chromatographic methods. An oxidation pathway is proposed for the complete conversion to CO2.

Effective removal of the antibiotic Nafcillin from water by combining the Photoelectro-Fenton process and Anaerobic Biological Digestion.

The elimination of the antibiotic Nafcillin (NAF), which is usually used in hospitals and veterinary clinics around the world, was assessed through a combination of three advanced electrochemical oxidation processes followed by anaerobic digestion process. In the first stage different electrochemical advanced oxidation processes (EAOPs) were used: electro-oxidation with hydrogen peroxide (EO-H2O2), electro-Fenton (EF) and Photo electro-Fenton (PEF). After PEF, almost complete and highly efficient degradation and elimination of NAF was achieved, with the concomitant elimination of the associated antimicrobial activity. The fast degradation rate produced by PEF is explained by the oxidative action of hydroxyl radicals (•OH) together with the direct UV photolysis of complexes formed between Fe3+ and some organic intermediates. Total removal of NAF occurs after 90min of electrolysis by PEF, with the generation of organic intermediates that remain in solution. However, when this post PEF process solution was treated with an anaerobic biological process, the intermediates generated in the electrochemical degradation of NAF were completely eliminated after 24h. The kinetic degradation of NAF as well as the identification/quantification of products and intermediates formed during the degradation of antibiotic, such as inorganic ions, carboxylic acids and aromatic compounds, were determined by chromatographic and photometric methods. Finally, an oxidation pathway is proposed for the complete conversion to CO2.

A new model for including the effect of fly ash on biochemical methane potential.

The modelling of the effect of trace elements on anaerobic digestion, and specifically the effect of fly ash, has been scarcely studied. Thus, the present work was aimed at the development of a new function that allows accumulated methane models to predict the effect of FA on the volume of methane accumulation. For this, purpose five fly ash concentrations (10, 25, 50, 250 and 500mg/L) using raw and pre-treated sewage sludge were used to calibrate the new function, while three fly ash concentrations were used (40, 150 and 350mg/L) for validation. Three models for accumulated methane volume (the modified Gompertz equation, the logistic function, and the transfer function) were evaluated. The results showed that methane production increased in the presence of FA when the sewage sludge was not pre-treated, while with pretreated sludge there is inhibition of methane production at FA concentrations higher than 50mg/L. In the calibration of the proposed function, it fits well with the experimental data under all the conditions, including the inhibition and stimulating zones, with the values of the parameters of the methane production models falling in the range of those reported in the literature. For validation experiments, the model succeeded in representing the behavior of new experiments in both the stimulating and inhibiting zones, with NRMSE and R2 ranging from 0.3577 to 0.03714 and 0.2209 to 0.9911, respectively. Thus, the proposed model is robust and valid for the studied conditions.

Biochemical methane potential from sewage sludge: Effect of an aerobic pretreatment and fly ash addition as source of trace elements.

The effect of aerobic pretreatment and fly ash addition on the production of methane from mixed sludge is studied. Three assays with pretreated and not pretreated mixed sludge in the presence of fly ash (concentrations of 0, 10, 25, 50, 250 and 500mg/L) were run at mesophilic condition. It was found that the combined use of aerobic pretreatment and fly ash addition increases methane production up to 70% when the fly ash concentrations were lower than 50mg/L, while concentrations higher than 250mg/L cause up to 11% decrease of methane production. For the anaerobic treatment of mixed sludge without pretreatment, the fly ash improved methane generation at all the concentrations studied, with a maximum of 56%. The removal of volatile solids does not show an improvement compared to the separate use of an aerobic pre-treatment and fly ash addition. Therefore, the combined use of the aerobic pre-treatment and fly ash addition improves only the production of methane.

Performance evaluation of micro-aerobic hydrolysis of mixed sludge: Optimum aeration and effect on its biochemical methane potential.

This study evaluated the performance of a micro-aerobic hydrolysis of mixed sludge and its influence as a pretreatment of this waste for its subsequent anaerobic digestion. Three experimental series were carried out to evaluate the optimum micro-aeration levels in the range from 0.1 to 0.5 air volume/min.reactor volume (vvm) and operation times within the range of 24-60 h. The maximum methane yield [35 mL CH4/g volatile suspended solids (VSS) added] was obtained for an aeration level of 0.35 vvm. This methane yield value increased 114% with respect to that obtained with the non-aerated sludge. In the micro-aeration process carried out at an aeration level of 0.35 vvm, increases in soluble proteins and total sugars concentrations of 185% and 192% with respect to their initial values were found, respectively, after 48 h of aeration. At the above micro-aerobic conditions, soluble chemical oxygen demand (CODS) augmented 150%, whereas VSS content decreased until 40% of their initial respective values. Higher COD increases and VSS decreases were found at 60 h of micro-aeration, but the above parameters did not vary significantly with respect to the values found at 48 h.

Autotrophic and heterotrophic denitrification for simultaneous removal of nitrogen, sulfur and organic matter.

The aim of this investigation was to assess the startup and operation of a laboratory-scale hybrid UASB-Anaerobic Filter Reactor (UASFB) of 1 L volume, kept at 30°C, in order to carry out a simultaneous autotrophic and heterotrophic denitrification process. First, the heterotrophic and autotrophic populations were separately enriched, with specific cultures and subsequently the UASFB was inoculated with 2 g L(-1) of volatile suspended solids (VSS), with a ratio of 1.5:1 (autotrophs: heterotrophs). The influent or synthetic wastewater used was composed of: Na2S2O3·5H2O, CH3COOK, NaNO3, NaHCO3, K2HPO4, NH4Cl and saline solution. The concentrations varied depending on the organic loading rate (OLR), nitrogen loading rate (NLR) and sulfur loading rate (SLR) applied. In the UASFB reactor, two experimental conditions were tested and assessed: (i) COD/N ratio of 3.6 and SLR of 0.75 kg S m(-3) d(-1); and (ii) COD/N ratio of 5.8 and SLR of 0.25 kg S m(-3) d(-1). The results obtained demonstrated that an inoculum coming from an anaerobic reactor was able to carry out the process, obtaining a maximum nitrate removal of 85.3% in the first stage of operation and 99.5% in the second stage. The recovery of sulfur in form of sulfate in the effluent did not present a tendency to stabilize during the measured time, with a maximum thiosulfate removal of 32.5%, when the SLR was lowered to 0.25 kg S m(-3) d(-1). The maximum organic matter elimination, measured as COD, was 75.8%, which indicates the relatively good performance and behavior of the heterotrophic microorganisms.

Biodegradability and methane production from secondary paper and pulp sludge: effect of fly ash and modeling.

The effect of fly ash on biodegradability and methane production from secondary paper and pulp sludge, including its modeling, was evaluated. Three tests with fly ash concentrations of 0, 10 and 20 mg/L were evaluated at 32 °C. Methane production was modeled using the modified Gompertz equation. The results show that the doses used produce a statistically significant increase of accumulated methane, giving values greater than 225 mL of CH4 per gram of volatile solids (VS) added, and 135% greater than that obtained in the control assay. Biodegradability of VS increased 143% with respect to the control assays, giving values around 43%. The modified Gompertz model can describe well methane generation from residual sludge of the paper industry water treatment, with parameter values between those reported in the literature. Thus, the addition of fly ash to the process causes a significant increase of accumulated methane and VS removal, improving the biodegradability of paper and pulp sludge.

Simultaneous effect of initial moisture content and airflow rate on biodrying of sewage sludge.

The simultaneous effect of initial moisture content (initial Mc) and air-flow rate (AFR) on biodrying performance was evaluated. For the study, a 3(2) factorial design, whose factors were AFR (1, 2 and 3 L/min kg(TS)) and initial Mc (59, 68 and 78% w.b.), was used. Using energy and water mass balance the main routes of water removal, energy use and efficiencies were determined. The results show that initial Mc has a stronger effect on the biodrying than the AFR, affecting the air outlet temperature and improving the water removal, with higher maximum temperatures obtained around 68% and the lowest maximum matrix temperature obtained at initial Mc = 78%.Through the water mass balance it was found that the main mechanism for water removal was the aeration, with higher water removal at intermediate initial Mc (68%) and high AFR (3 L/min kg(TS)). The energy balance indicated that bioreaction is the main energy source for water evaporation, with higher energy produced at intermediate initial Mc (68%). Finally, it was found that low values of initial Mc (59%) improve biodrying efficiency.

Biodrying of sewage sludge: kinetics of volatile solids degradation under different initial moisture contents and air-flow rates.

This study focuses on the kinetics of the biodegradation of volatile solids (VS) of sewage sludge for biodrying under different initial moisture contents (Mc) and air-flow rates (AFR). For the study, a 3(2) factorial design, whose factors were AFR (1, 2 or 3L/minkgTS) and initial Mc (59%, 68% and 78% w.b.), was used. Using seven kinetic models and a nonlinear regression method, kinetic parameters were estimated and the models were analyzed with two statistical indicators. Initial Mc of around 68% increases the temperature matrix and VS consumption, with higher moisture removal at lower initial Mc values. Lower AFRs gave higher matrix temperatures and VS consumption, while higher AFRs increased water removal. The kinetic models proposed successfully simulate VS biodegradation, with root mean square error (RMSE) between 0.007929 and 0.02744, and they can be used as a tool for satisfactory prediction of VS in biodrying.

Biodrying of pulp and paper secondary sludge: kinetics of volatile solids biodegradation.

This study focuses on the kinetics of volatile solids (VS) biodegradation of the biodrying process using pulp and paper secondary sludge. The experiments were carried out with air-flow rates of 0.51, 1.61, 3.25 and 5.26L/minkgVS) and initial moisture content of 64-66% w.b. Using five kinetic models and a nonlinear regression method, kinetic parameters were estimated and the models were analyzed with two statistical indicators. Higher air-flow rates cause greater moisture content reduction, lower temperature in the matrix, and lower VS reduction. At an air-flow rate as high as 5.26L/minkgVS there is no biodrying but only convective drying. The kinetic models used successfully simulate the VS biodegradation under biodrying conditions, with a root mean square error (RMSE) between 0.007929 and 0.02744. In conclusion, we show for the first time that VS biodegradation in the biodrying process can be successfully modeled with a kinetic model.

Simultaneous C and N removal from saline salmon effluents in filter reactors comprising anoxic-anaerobic-aerobic processes: effect of recycle ratio.

Salmon processing generates saline effluents with high protein load. To treat these effluents, three compact tubular filter reactors were installed and an integrated anoxic/anaerobic/aerobic process was developed with recycling flow from the reactor's exit to the inlet stream in order to save organic matter (OM) for denitrification. The reactors were aerated in the upper section with recycle ratios (RR) of 0, 2, and 10, respectively, at 30°C. A tubular reactor behave as a plug flow reactor when RR = 0, and as a mixed flow reactor when recycle increases, thus, different RR values were used to evaluate how it affects the product distribution and the global performance. Diluted salmon process effluent was prepared as substrate. Using loads of 1.0 kg COD m(-3)d(-1) and 0.15 kg total Kjeldahl nitrogen (TKN) m(-3)d(-1) at HRT of 2 d, 100% removal efficiencies for nitrite and nitrate were achieved in the anoxic-denitrifying section without effect of the dissolved oxygen in the recycled flow on denitrification. Removals >98% for total organic carbon (TOC) was achieved in the three reactors. The RR had no effect on the TOC removal; nevertheless a higher efficiency in total nitrogen removal in the reactor with the highest recycle ratio was observed: 94.3% for RR = 10 and 46.6% for RR = 2. Results showed that the proposed layout with an alternative distribution in a compact reactor can efficiently treat high organic carbon and nitrogen concentrations from a saline fish effluent with OM savings in denitrification.

Behavior of the anaerobic treatment of tannery wastewater at different initial pH values and sulfate concentrations.

The influence of initial pH and sulfate concentration on the anaerobic digestion of tannery wastewater was studied using the specific methanogenic activity (SMA) test. The SMA tests were performed with a chemical oxygen demand (COD) concentration of 6.5 g/L and SO(-2)4 concentrations of 1.95, 3.90, 6.50, 8.45 and 10.4 g/L. The effect of pH was studied at pH 6.8; 6.4 and 6.1. It was found that methane production took place under all conditions, but the use of SO(-2)4 concentrations higher than 8.45 g/L and initial pH levels lower than 6.4 caused a decrease of up to 73% of methane production. The influence of pH was significant only for SO(-2 4 concentrations lower than 8.45 g/L, with up to 50% inhibition. Specific methanogenic activity for COD/SO(-2)4 ratios lower than 0.77 depends basically on this ratio and not on the initial pH value under the conditions of the study.

Modeling of simultaneous denitrification--anaerobic digestion--organic matter aerobic oxidation and nitrification in an anoxic-anaerobic-aerobic compact filter reactor.

A mathematical model was developed for a compact anoxic-anaerobic-aerobic filter reactor with liquid recirculation for the treatment of fishing effluents. The model includes denitrification, anaerobic digestion, aerobic carbon oxidation and nitrification steps, as well as an evaluation of the liquid gas mass transfer and pH. The model was calibrated using one experimental condition at a recycling ratio (R)=10, and was validated with R equal to 2 and 0, with an organic concentration of 554±24 mg TOCL(-1), salinity of 24 g L(-1) and hydraulic retention time (HRT) of 2 d. Carbon total removal is higher than 98%, while maximum nitrogen removal is 62% using total nitrification in the aerobic zone, due to a higher quantity of NO(x) produced which were recirculated to the anoxic zone. In the aerobic zone, simultaneous nitrification and denitrification processes occur, because the diffusion limitations cause a low oxygen penetration in the biofilm. In the anoxic-anaerobic zone, denitrification or methanogenesis inhibition by DO (caused by the recycled oxygen) is not observed.