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The anaerobic thermophilic co-digestion of mixtures of the organic fraction of municipal solid waste (OFMSW) and the liquid fraction from the hydrothermal carbonization (LFHTC) of dewatered secondary sludge was studied. Mixtures with a low OFMSW to LFHTC ratio (50, 75, and 100% LFHTC) exhibited accumulation of volatile fatty acids (VFA) as well as low degradation of organic matter and methane production. However, the mixture containing 25% LFHTC performed quite well in terms of methane production: (179 ± 3) mL CH STP g COD, which was only slightly lower than the value obtained with 100% OFMSW. The experimental results fitted the modified Gompertz model reasonably well and the maximum methane production rate for the mixture containing 25% LFHTC (11.96 mL CH g COD d) was 29.3% higher than that obtained with the substrate with 100% OFMSW. Therefore, centralized co-digestion of OFMSW in mixtures with 25% LFHTC seemingly provides an effective method for valorizing the latter substrate.

This paper reviews the most relevant uses and applications of zeolites in anaerobic digestion processes. The feasibility of using natural zeolites as support media for the immobilization of microorganisms in different high-rate reactor configurations (fixed bed, fluidized bed, etc.) is also reviewed. Zeolite, with its favorable characteristics for microorganism adhesion, has also been widely used as an ion exchanger for the removal of ammonium in anaerobic digestion due to the presence of Na +, Ca 2+ and Mg 2+ cations in its crystalline structure. This property is also useful for improving the anaerobic process performances in the treatment of wastewaters with high concentrations of nitrogen compounds, such as cattle, pig and chicken wastes, as it prevents process inhibition. The influence of zeolite particle size and doses in batch mesophilic and thermophilic processes when referring to synthetic or different wastewaters is also reviewed. Finally, the role of zeolite in granulation processes, in anaerobic oxidation processes (Anammox) for promoting the retention of the biomass involved (given its low growth rate), and in hybrid and sequencing reactors such as the moving bed biofilm reactors (MBBR) are also discussed. © 2012 Elsevier B.V.

Anaerobic digestion is considered a competitive source for the production of renewable energy as far as efficiency and cost are concerned. To evaluate the anaerobic biodegradability of an organic substrate such as feedstocks, a test known as biochemical methane potential (BMP) has been commonly used. Current worldwide interest in using different organic substrates for anaerobic bioconversion is growing but there is a lack of clear references and comparability as a result of multiple factors that affect BMP determination. Several batch methods have been used to determine the methane potential. However, these technical approaches vary significantly from one reported method to the next another. In this review, the research works on the influence of different parameters of BMP determination have been discussed for critical and comparative evaluation. In addition, the extensive literature previously published dealing with BMP assays has been compiled and summarized focusing on two main subjects: firstly, methane yields of substrates, and secondly, the description of the various experimental procedures used to achieve the reported data. © 2011 Elsevier Ltd. All rights reserved.

The disposal of excess sludge from wastewater treatment plants is a serious problem that needs to be addressed. Temperature-phased anaerobic digestion (TPAD) which combines thermophilic and mesophilic processes in one, brings together the advantages of both systems. The aim of the present work was to develop a simple kinetic model to describe the TPAD of sewage sludge in batch completely stirred tank reactors (CSTRs) and to determine the kinetic parameters of both thermophilic and mesophilic stages. A zero-order kinetic equation described the thermophilic step after 2, 4 and 6 days of digestion time (experiment 1, 2 and 3, respectively), yet a first-order equation was found to be adequate to correlate the methane gas accumulated with time in the mesophilic step, the kinetic constant being 0.21 days 1. The methane yield coefficient obtained was found to be almost proportional to the digestion time used in the thermophilic step with values of 0.067, 0.132 and 0.193 L CH4 STP/g VSadded for experiments 1, 2 and 3, respectively. By contrast, the kinetic constant of the mesophilic stage was not influenced by the digestion time used in the thermophilic phase. © 2012 Copyright Taylor and Francis Group, LLC.

Biochemical methane potential (BMP) assays of sunfloweroilcake (SuOC) were carried out to research the effect of different particlesizes and their chemicalcomposition on methane yields and kinetics. Particlesize ranges of (1) 0.355–0.55 mm, (2) 0.710–1.0 mm and (3) 1.4–2.0 mm in diameter were evaluated. The highest methane yield 213 ± 8 mL CH4 g−1 VSadded was obtained for the largest particlesize analyzed (3), against 186 ± 6 mL CH4 g−1 VSadded obtained for particles 1 and 2. This may be attributed to the different lignocellulose compositions of the various particlesize ranges studied and to organic matter removals (47.2% for 3, against ∼41.5% for 1 and 2). The evolution of propionic acid concentration was found to be fundamental for explaining the lowest rate of biogas production for the smallest (1) particlesize studied, with a specific rate constant k of 0.45 ± 0.02 d−1, while values of 0.61 ± 0.02 d−1 and 0.50 ± 0.01 d−1 were obtained for particles 2 and 3, respectively.

Background: This paper describes results obtained for different participating research groups in an interlaboratory study related to biochemical methane potential (BMP). In this research work, all experimental conditions influencing the test such as inoculum, substrate characteristics and experimental conditions were investigated. The study was performed using four substrates: three positive control substrates (starch, cellulose and gelatine), and one raw biomass material (mung bean) at two different inoculum to substrate ratios (ISR). Results: The average methane yields for starch, cellulose, gelatine and mung bean at ISR of 2 and 1 were 350 ± 33, 350 ± 29, 380 ± 42, 370 ± 36 and 370 ± 35 mL CH4 g-1 VSadded, respectively. The percentages of biotransformation of these substrates into methane were 85 ± 8, 85 ± 7, 88 ± 9, 85 ± 8 and 85 ± 8%, respectively. On the other hand, the first-order rate constants obtained from the experimental data were 0.24 ± 0.14, 0.23 ± 0.15, 0.27 ± 0.13, 0.31 ± 0.17 and 0.23 ± 0.13 d-1, respectively. Conclusion: The influence of inocula and experimental factors was nearly insignificant with respect to the extents of the anaerobic biodegradation, while the rates differed significantly according to the experimental approaches. © 2011 Society of Chemical Industry.

Sunflower oil cake (SuOC) is the solid by-product from the sunflower oil extraction process and an important pollutant waste because of its high organic content. For the anaerobic digestion of SuOC three different industrial reactors were compared as inoculum sources. This was done using a biochemical methane production (BMP) test. Inoculum I was a granular biomass from an industrial reactor treating soft-drink wastewaters. Inoculum II was a flocculent biomass from a full-scale reactor treating biosolids generated in an urban wastewater treatment plant. Inoculum III was a granular biomass from an industrial reactor treating brewery wastes. The highest kinetic constant for methane production was achieved using inoculum II. The inoculum sources were analyzed through PCR amplification of 16S rRNA genes and fingerprinting before (t = 0) and after the BMP test (t = 12 days). No significant differences were found in the bacterial community fingerprints between the beginning and the end of the experiments. The bacterial and archaeal communities of inoculum II were further analyzed. The main bacteria found in this inoculum belong to Alphaproteobacteria and Chloroflexi. Of the Archaea detected, Methanomicrobiales and Methanosarcinales made up practically the whole archaeal community. The results showed the importance of selecting an appropriate inoculum in short term processes due to the fact that the major microbial constituents in the initial consortia remained stable throughout anaerobic digestion.

Thermophilic–mesophilic-phase anaerobic digestion assays were carried out in three discontinuous reactors: R1, R2 and R3, which were in thermophilic conditions (55 °C) during 2, 4 and 6 days, respectively. After the first stage, the temperature was reduced to 35 °C, and at the end of the two-stages, the R2 and R3 systems achieved volatile solid reductions of 45% and 52%, respectively. Faecal coliforms and Salmonella spp. densities below 1000 MPN/g TS and 3 MPN/4 g TS, respectively, were also observed. The highest methane yield and the best dewaterability were obtained for R3: 193 mL CH4 g−1 VSadded and 3 min of time-to filter. For R2 and R1, 133 and 97 mL CH4 g−1 VSadded, and 4 and 9 min, respectively, were obtained. Although the best results were achieved maintaining thermophilic conditions during the 6 days; however, to take Class A-cataloguing product, for agrarian ends, 4 days operating under thermophilic conditions was sufficient.

Batch experiments to remove ammonia from food waste digestate were set up, and preliminary runs undertaken. These experiments were based on gas stripping in a heated column reactor; the effects of different temperatures (35, 55 and 70 °C) and gas flow rates (0.125, 0.250 and 0.375 min−1) were considered using biogas as the stripping gas. At 35 °C, an increase in the ammonia removal rate by approximately 4.5 times was observed when the flow rate increased from 0.125 to 0.375 Lbiogas L-1 digestate min−1. At 55 °C, and flow rates of 0.250 and 0.375 Lbiogas L-1 digestate min−1, ammonia removal of 3.46 and 9.38% day−1, respectively, were achieved. The highest values of removal of ammonia were reached at 70 °C: 18.4 and 10.4% day−1, for 0.250 and 0.375 Lbiogas L-1 digestate min−1 flow rates, respectively.

A study of the anaerobic digestion process of synthetic domestic sewage (total COD: 705 mg/L) was carried out. The digestion was conducted in an upflow anaerobic filter with corrugated plastic rings as packing media at psychrophilic temperature (15–17 °C). For HRTs of between 10.0 and 17.1 h, the total COD removal efficiency was almost constant and independent on the HRT, achieving an average value of around 80%. However, when the HRT decreased from 7.0 to 3.2 h the efficiency diminished from 77% to 65%. This decrease in removal efficiency was parallel to the increase in the VFA/Alkalinity ratio for this HRT range. The flow pattern observed in the reactor studied was intermediate between plug-flow and CSTR systems, although the plug-flow was predominant. It can also be observed that Young and McCarty’s model almost coincided with the CSTR model, when the biodegradable COD was used for fitting the data. The Manariotis equation allowed a better fit of the experimental data (total COD removal efficiency with influent substrate concentration and HRT) than the Young model. The methane yield coefficient obtained was 0.15 L CH4/g COD consumed.

Chemical oxygen demand (COD) is a critical analytical parameter in waste and wastewater treatment, more specifically in anaerobic digestion, although little is known about the quality of measuring COD of anaerobic digestion samples. Proficiency testing (PT) is a powerful tool that can be used to test the performance achievable in the participants’ laboratories, so we carried out a second PT of COD determination in samples considered “difficult” to analyze (i.e. solid samples and liquid samples with high concentrations of suspended solids). The results obtained (based on acceptable z-score values) may be considered satisfactory. When compared with the results of a previous similar scheme, the overall performance improved by around 30%, again demonstrating that analytical performance can be improved by regular participation in PT.

In 2008, the first Proficiency Testing Scheme of Chemical Oxygen Demand (1stCOD-PTADG) was conducted to assess the results obtained for different research groups whose field work is mainly anaerobic digestion. This study was performed using four samples, two solid samples as raw materials and two solid samples to prepare high concentration suspended solid solutions. Invitations were sent to a large number of laboratories, mainly to anaerobic digestion research groups. Finally, thirty labs from sixteen countries agreed to participate, but for different reasons four participants could not send any data. In total, twenty-six results were reported to the COD-PT coordinator. This study showed the importance of continuous participation in proficiency testing (PT) schemes in order to compare the results obtained. Taking into account the lack of a general standard method and high quality certified reference materials (CRMs), the traceability of COD determination is not currently easy to check. In addition, the spread of participants’ results obtained was high and pointed to the advisability of using consensus values due to their unreliability. Therefore, the theoretical oxygen demand (ThOD) values were considered as assigned values for all the samples analysed. On the other hand, in this PT the established standard deviation (ESD) has been determined by the Horwitz modified function. Participants of this 1stCOD-PTADG were asked to give a short report on the analytical method used. Although all the participants used potassium dichromate as their oxidant reagent, their experimental procedures were very different. With the purpose of comparing the results obtained, the different experimental conditions used were classified into five methods, corresponding to two main categories, open and closed reflux. The performance of laboratories was expressed by the z-score, whose value is considered satisfactory when z-score ≤±2. The overall analytical data evaluation showed that 64% of

The adsorption of methylene blue (MB) on three commercial granular activated carbons (GACs), 12 × 40 mesh size, namely Filtrasorb 400, Norit and Picacarb has been researched. A comparative study of adsorptive capacity using the proposed single-point test and the traditional multi-point isotherm test was carried out. For the single-point test, the influence of some parameters such as MB/GACs mass ratio and contact time were evaluated. For this test the adsorptive capacities of the three GACs studied were 319 ± 14, 280 ± 7 and 260 ± 6 mg g−1 for Filtrasorb 400, Norit and Picacarb, respectively. For multi-point isotherm adsorption test the Langmuir model was used. The parameters involved were obtained by linear and non-linear regression methods. The maximum adsorptive capacity values obtained for both methods were similar and statistically not different than those obtained with the single-point tests. This experimental work also aimed at establishing a relationship between the adsorbent particle size and the adsorptive capacity which could be used complementarily to evaluate the quality of GACs as adsorbents. For a mean particle diameter of 1 mm and after 24 h of contact time the adsorptive capacity values were 255 ± 7, 222 ± 7 and 160 ± 7 mg g−1 for Filtrasorb 400, Norit and Picacarb, respectively.

A study of the anaerobic digestion of the solid waste generated in the extraction process of sunflower oil (sunflower oil cake, SuOC) was conducted at mesophilic temperature (35 °C) in batch mode. A laboratory-scale multi-reactor system was used to compare the volatile solids (VS) degradation and methane production (G) at inoculum–substrate ratios (ISRs) of 3.0, 2.0, 1.5, 1.0, 0.8 and 0.5 (expressed as VS basis). All tests were carried out against controls of inoculum without substrate. The stability and progress of the reaction from solid substrate to methane as an end product was monitored by measuring the pH, the soluble chemical oxygen demand, and the total volatile fatty acids-total alkalinity (TVFA/TA) ratio. The results obtained demonstrated that in the ISR range from 3.0 to 0.8, the pH ranged from 7.1 to 7.6 and this parameter was always stable during the anaerobic digestion process. In addition, within the above ISR range the TVFA/TA ratios were always lower than the failure limit values (0.3–0.4), which demonstrated the high stability of the anaerobic digestion process of this substrate at mesophilic temperature. Two kinetic models for substrate (VS) degradation and methane production were proposed and evaluated. The apparent kinetic constants for volatile solids degradation (K1) and methane production (K2) decreased from 0.54 ± 0.09 to 0.32 ± 0.03 d−1 and from 0.36 ± 0.04 to 0.16 ± 0.03 d−1, respectively, when the ISR decreased from 3.0 to 0.5, showing the occurrence of an inhibition phenomenon by substrate concentration. The kinetic equations obtained were used to simulate the anaerobic digestion process of SuOC and to obtain the theoretical VS and methane production values. The low deviations obtained (equal to or lower than 10%) between the theoretical and experimental values suggest that the proposed models predict the behaviour of the reactors very accurately.

The influence of the hydraulic retention time (HRT) and organic loading rate (OLR) on the performance of the hydrolytic–acidogenic step of a two-stage anaerobic digestion process of sunflower oil cake (SuOC) were assessed. The experiments were performed in laboratory-scale completely stirred tank reactors at mesophilic (35 °C) temperature. Six OLR (ranging from 4 to 9 g VS L−1 d−1) for four HRTs (8, 10, 12 and 15 days) were tested to check the effect of each operational variable. Based on the results obtained, it can be concluded that the hydrolysis yields obtained for all HRTs and OLRs assayed were in the range of 20.5–30.1%. In addition, the acidification degree of the substrate was mainly influenced by the OLR but not by the HRTs, the highest value (83.8%) being achieved for an HRT of 10 days and an OLR of 6 g VS L−1 d−1.

Micellar-enhanced ultrafiltration (MEUF) has been used to remove dissolved metals including Cd2+, Cu2+, Ni2+, Pb2+, and Zn2+ from synthetic water using two anionic surfactants: sodium dodecyl sulfate (SDS), and linear alkylbenzene sulfonate (LAS) in a lab-scale membrane system. The effect of surfactant concentration, pH and conductivity on metal retention was studied. The molar concentration ratio of the surfactant to metal (S/M ratio) is higher than 5 in all experiments. When the initial SDS concentration was below the critical micelle concentration (CMC), metal retention higher than 90% was unexpectedly obtained (at conditions used in these experiments), except for Ni2+. Moreover, it was shown that complete removal of metal ions, except for Ni2+, could be achieved at an LAS concentration below CMC. The addition of NaCl provokes the complexation of metal cations with chloride ions and the adsorption competition of sodium cations with metal ions, therefore increasing conductivity decreases metal retention. The presence of the NaCl in the feed surfactant solution had a drastic adverse effect on metal retention by SDS and LAS.

Two 50 kDa polyacrilonitrile ultrafiltration membranes, one cationic (CM) and the other anionic (AM), have been tested in order to study the removal of humic acid or natural organic matter and to examine their applicability for water purification. Experiments were carried out using synthetic water formulated from humic acids, and two natural source waters having different physical and chemical characteristics. The effect of the pressure (ranging between 100 kPa and 400 kPa), ionic strength (pI) (1.4 × 10−3 mol L−1, 2.5 × 10−2 mol L−1, and 4.1 × 10−2 mol L−1; expressed as KCl) and pH (2.7, 5.8, 6.8, 7.8 and 8.8), on synthetic water ultrafiltration, were investigated. The membranes tested produce high permeate flows, higher in the case of AM. Pressure increases do not greatly enhance permeate flows. In general, a slight decrease in flux results for an increase in pI (8% and 6% in the case of CM and AM, respectively if the pI increases from 1.4 × 10−3 M to 4.1 × 10−2 M). At pH 2.7, the flux reductions are in excess of 42% for both membranes, whereas at pH 8.8, the flux reduction is a minimum; 11% with CM and 4% with AM. AM undergoes a greater permeate flux reduction than CM in natural water ultrafiltration experiments, in contrast to assays with humic acid. This is probably due to the high hydrophilicity of the anionic membrane, and to the different nature (hydrophilic character in these waters) and molecular weight distributions of the organic matter.

A two-phase anaerobic digestion process of synthetic domestic wastewater was studied at ambient temperature in mild to cold climates. The hydrolytic stage was carried out in a continuous stirred tank reactor with an effective volume of 1.2 L. The hydrolytic reactor operated at hydraulic retention times (HRTs) in the range of 1.3 to 2.7 h, which allowed for optimum HRT to be obtained in order to achieve a maximum amount of soluble COD. For the methanogenic stage, an up-flow anaerobic filter with a volume of 1.35 L and corrugated plastic rings as biomass immobilization support were used. During the investigation, the ambient temperature ranged between 21°C and 24°C. Synthetic domestic wastewater with a COD of 700 mg/L was used as substrate. The study was performed at total organic loading rates (OLR T ) of 2.0–4.3 g COD/L· d, with a global HRT (including both hydrolytic and methanogenic stages) of 2.8–5.8 hours. A maximum percentage of organic matter removed of 88% was achieved at a global HRT of 5.8 hours. Under these operating conditions, the production of biogas was 97% higher than that obtained in the one-phase anaerobic digestion process. Additionally, the kinetics involved in the hydrolytic stage was determined using the Contois kinetic model, which adequately predicted the experimental results.