F. Raposo

Evaluation of the hydrolytic–acidogenic step of a two-stage mesophilic anaerobic digestion process of sunflower oil cake

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 degrees 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).

Methylene blue number as useful indicator to evaluate the adsorptive capacity of granular activated carbon in batch mode: Influence of adsorbate/adsorbent mass ratio and particle size

The adsorption of methylene blue (MB) on three commercial granular activated carbons (GACs), 12 x 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 1mm and after 24h 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 anaerobic digestibility of two-phases olive mill solid waste (OMSW) at mesophilic temperature

A study of the anaerobic digestibility of two-phases olive mill solid waste (OMSW) was carried out in a laboratory-scale completely stirred tank reactor at mesophilic temperature (35 °C). The reactor was operated at influent substrate concentrations of 34.5 g COD/l (OMSW 20%), 81.1 g COD/l (OMSW 40%), 113.1 g COD/l (OMSW 60%) and 150.3 g COD/l (OMSW 80%). The hydraulic retention time (HRT) varied between 40.0 and 8.3 days for the first feed used (OMSW 20%) and between 50 and 10 days for the other three influent substrate concentrations. Chemical oxygen demand (COD) and volatile solids (VS) removal efficiencies of 88.4 and 90.9%, respectively, were achieved at an organic loading rate (OLR) of 12.02 g COD/l day for the most concentrated substrate used (OMSW 80%). The maximum methane production rate was found to be 2.12 l CH4/l day for the above-mentioned OLR and a HRT of 12.5 days. The system can tolerate OLRs as high as 15.03 g COD/l day with COD and VS removal efficiencies of 82.9 and 86.6%, respectively, for this feed concentration. The methane yield coefficients were 0.30, 0.27, 0.23 and 0.20 l methane STP/g COD removed for the OMSW concentrations of 20, 40, 60 and 80%, respectively.

Assessment of a modified and optimised method for determining chemical oxygen demand of solid substrates and solutions with high suspended solid content

A modified approach to determine the chemical oxygen demand (COD) of solid substrates based on the DIN 38414-S9 standard method is proposed. The adapted procedure is assessed and compared with standard methods widely used for water and wastewater such as the American Public Health Association-American Water Works Association-Water Pollution Control Federation (APHA-AWWA-WPCF) standard methods 5220 B-open reflux (SM-OR) and 5220 D-closed reflux colorimetric (SM-CR). Solutions with high suspended concentration of solids, as well as digestates from an anaerobic reactor, were used during the comparative test. For solid substrates, the COD recovery was about 100% when the proposed method was used. For solutions with solid content higher than 20 g TS L(-1), the recovery was only completed when the proposed method was used, showing that the methods traditionally employed are not very appropriate for samples with the described characteristics. For instance, percentages of COD recovery in the ranges of 77.3-87.1% and 89.4-94.1% were achieved when the SM-OR and SM-CR methods were used, respectively.

Kinetic study of the anaerobic digestion of vinasse pretreated with ozone, ozone plus ultraviolet light, and ozone plus ultraviolet light in the presence of titanium dioxide

A comparative kinetic study of the anaerobic digestion of vinasse and vinasse following a 2 h treatment with ozone, ozone plus UV light or ozone plus UV light in the presence of titanium dioxide was carried out. All pretreatments decreased both chemical oxygen demand (COD) and total organic content (TOC). Those using no catalyst failed to improve the specific rate of methane production during anaerobic digestion but succeeded in increasing the yield coefficient (YP/S, in ml CH4/mg TOC). Finally, the pretreatment using ozone, UV light and titanium dioxide decreased COD and TOC, and increased the yield coefficient and also the mean specific rate of anaerobic digestion (by 25%) relative to untreated vinasse.

Effect of hydrothermal pretreatment of sunflower oil cake on biomethane potential focusing on fibre composition

The aim of this study was to elucidate the effect of hydrothermal pretreatment at 25, 100, 150 and 200°C on fibre composition and the biomethane potential of sunflower oil cake (SuOC). An increase in pretreatment temperature from 25 to 200°C caused a decrease in hemicellulose content in the solid pretreated fraction from 13 to 6% while the lignin content increased by 16%. Soluble compounds also increased with temperature. Digestion of solid fractions from pretreatments at 25, 100, 150 and 200°C in batch assays at 35±1°C resulted in methane yields of 114±9, 105±7, 82±7 and 53±8mL CH(4) g(-1)COD(added), respectively. The corresponding methane yields for the liquid fractions were 276±6, 310±4, 220±15 and 247±10mL CH(4) g(-1)COD(added), respectively. Therefore the overall methane yield was highest for SuOC pretreated at 100°C; however, this value was only 6.5% higher than that achieved after pretreatment at 25°C.

Performance evaluation of a mesophilic anaerobic fluidized-bed reactor treating wastewater derived from the production of proteins from extracted sunflower flour.

A study of the anaerobic digestion of wastewater derived from the production of protein isolates from extracted sunflower flour was carried out in a laboratory-scale, mesophilic (35 degrees C) fluidized-bed reactor with saponite as bacterial support. Chemical oxygen demand (COD) removal efficiencies in the range of 98.3-80.0% were achieved in the reactor at organic loading rates (OLR) of between 0.6 and 9.3 g COD/I d, hydraulic retention times (HRT) of between 20.0 and 1.1 d and average feed COD concentration of 10.6 g/l. Eighty percent of feed COD could be removed up to OLR of 9.3 g COD/l d. The yield coefficient of methane production was 0.33 l of methane (at STP) per gram of COD removed and was virtually independent of the OLR applied. Because the buffering capacity of the experimental system was maintained at favorable levels with excess total alkalinity present at all loadings, the rate of methanogenesis was not affected by loading. The experimental data indicated that a total alkalinity in the range of 2,000-2,460 mg/l as CaCO3 was sufficient to prevent the pH from dropping to below 7.0 for OLR of up to 9.3 g COD/l d. The volatile fatty acid (VFA) levels and the VFA/alkalinity ratio were lower than the suggested limits for digester failure (0.3-0.4) for OLR and HRT up to 9.3 g COD/l d and 1.1 d, respectively. For a HRT of 0.87 d (OLR of 12.1 g COD/l d) the start of acidification was observed in the reactor.

Thermophilic anaerobic digestion of sewage sludge: focus on the influence of the start-up. A review

The thermophilic anaerobic digestion (TAD) of sewage sludge has often been found to be less stable than mesophilic treatment. In comparison to mesophilic digesters, thermophilic reactors treating sludge are generally characterized by relatively high concentrations of volatile fatty acids (VFA) in the effluent along with poor effluent quality, indicating a lower level of process stability. However, reviewing the literature related to the procedure for obtaining a thermophilic inoculum, it seems that most of the problems associated with the instability and the accumulation of organic intermediates are the result of the manner in which the thermophilic sludge has been obtained. In this paper, the different options available for obtaining an anaerobic digester operating at thermophilic temperature (55°C) have been reviewed. In this light, rapid heating to the target temperature followed by the development of thermophilic microorganisms, which can be determined by VFA dropping to ≤500 mg acetic acid L−1 before increasing the organic loading rate (OLR), has been determined the most suitable means of establishing TAD.

Impact of ultrasonic pretreatment under different operational conditions on the mesophilic anaerobic digestion of sunflower oil cake in batch mode.

In this study ultrasonic (US) pretreatment was investigated with the aim of improving the anaerobic digestion of sunflower oil cake (SuOC), the solid waste derived from the extraction process of sunflower oil. Five ultrasonic pretreatment assays were conducted at specific energy (SE) and sonication times in a range from 24,000 kJ/kg TS and 16.6 min (assay 1: US1) to 597,600 kJ/kg TS and 331.2 min (assay 5: US5), respectively, all operating at a constant sonication frequency (20 kHz) and ultrasonic power (120 W). As regards ultrasonic pretreatment, the working conditions of the first assay (US1) using samples of SuOC at 2% (w/v) showed to be the most appropriate in terms of both lignin and hemicellulose degradation (57.7% and 66.7%, respectively) and cellulose increase (54% increase with respect to its initial concentration). The percentage of COD solubilization increased from only 14% to 21% when SE was 25 times higher. Results obtained in batch anaerobic digestion experiments (biochemical methane potential - BMP - tests) conducted at 35°C of the solid and liquid fractions released from the different ultrasonic conditions tested, indicated that for the first experiment (US1) the average ultimate methane yield obtained was 53.8% higher than that achieved for untreated SuOC. Finally, the kinetic constants of the anaerobic digestion of the solid and liquid fractions released after the ultrasonic pretreatment were virtually independent of the operation conditions assayed.

Treatment of screened dairy manure by upflow anaerobic fixed bed reactors packed with waste tyre rubber and a combination of waste tyre rubber and zeolite: effect of the hydraulic retention time.

Two laboratory-scale anaerobic fixed bed reactors were evaluated while treating dairy manure at upflow mode and semicontinuous feeding. One reactor was packed with a combination of waste tyre rubber and zeolite (R1) while the other had only waste tyre rubber as a microorganism immobilization support (R2). Effluent quality improved when the hydraulic retention time (HRT) increased from 1.0 to 5.5 days. Higher COD, BOD5, total and volatile solids removal efficiencies were always achieved in the reactor R1. No clogging was observed during the operation period. Methane yield was also a function of the HRT and of the type of support used, and was 12.5% and 40% higher in reactor R1 than in R2 for HRTs of 5.5 and 1.0 days, respectively. The results obtained demonstrated that this type of reactor is capable of operating with dairy manure at a HRT 5 times lower than that used in a conventional reactor.