R. Borja

Influence of different aerobic pretreatments on the kinetics of anaerobic digestion of olive mill wastewater

A kinetic study was carried out on the anaerobic digestion of Olive Mill Wastewater (OMW) and OMW that was previously fermented with Geotrichum candidum, Azotobacter chroococcum and Aspergillus terreus. The bioreactor used was batch fed and contained sepiolite as support for the mediating bacteria. Experimental data observed for methane production (G) against time (t) are described by the following equation: G = GM[1 ? exp( ? Axt/S0), over the COD range studied (3.9–14.5 g/l), where: GM is the maximum methane volume obtained at the end of digestion time; S0 is the initial substrate concentration; X is the microorganism concentration and A is the kinetic constant of the process, which was calculated using a nonlinear regression. This kinetic parameter was found to be influenced by the pretreatment carried out, and was 4.6, 4.1 and 2.3 times higher for Aspergillus, Azotobacter and Geotrichum-pretreated OMWs than that obtained in the anaerobic digestion of untreated OMW. The kinetic constant increased when the phenolic compound content and biotoxicity of the pretreated-OMWs decreased. Finally, the yield coefficient, Tp, was 260 (untreated OMW), 300 (Geotrichum-pretreated OMW), 315 (Azotobacter-pretreated OMW) and 350 (Aspergillus-pretreated OMW) ml CH4 STP/g COD.

Enhancement of the anaerobic digestion of olive mill wastewater by the removal of phenolic inhibitors

Olive mill wasterwater (OMW) has a high organic pollutant load (45–130 g COD/litre) including various phenolic compounds, of which caffeic acid, thyrosol and hydroxythyrosol occur in the highest proportions. These two features make it impossible to purify OMW anaerobically unless a prior dilution is effected. Partial removal of some of the organic matter and phenolic compounds, by aerobic pretreatment with Geotrichum candidum is described. This provides a partially purified effluent that, for the same COD level as that of the original OMW, is degraded anaerobically more rapidly than original OMW as a result of the increase in methanogenic activity by 39 ml CH4 STP /g VSS day for a COD of 7 g /litre. Finally, the presence of a sepiolite support (Pansil) in the digestors is shown to increase the biodegradability of the OMWs and the yield coefficient of the product (Yp).

Influence of different natural zeolite concentrations on the anaerobic digestion of piggery waste

The effect of different natural zeolite concentrations on the anaerobic digestion of piggery waste was studied. Natural zeolite doses in the range 0.2-10 g/l of wastewater were used in batch experiments, which were carried out at temperatures between 27 degrees C and 30 degrees C. Total chemical oxygen demand (COD), total and volatile solids, ammonia and organic nitrogen, pH, total volatile fatty acids (TVFA), alkalinity (Alk) and accumulative methane production were determined during 30 days of digestion. The anaerobic digestion process was favored by the addition of natural zeolite at doses between 2 and 4 g/l and increasingly inhibited at doses beyond 6 g/l. A first-order kinetic model of COD removal was used to determine the apparent kinetic constants of the process. The kinetic constant values increased with the zeolite amount up to a concentration of 4 g/l. The values of the maximum accumulative methane production (Gm) increased until zeolite concentrations of 2-4 g/l. The addition of zeolite reduced the values of the TVFA/ Alk ratio while increasing the pH values, and these facts could contribute to the process failure at zeolite doses of 10 g/l.

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).

Anaerobic digestion of palm oil mill effluent using an up-flow anaerobic sludge blanket reactor

Abstract Anaerobic treatment of palm oil mill effluent (POME) was studied using a 16-litre laboratory scale up-flow anaerobic sludge blanket reactor (UASB) run over a range of influent concentrations from 5.1 to 42.5 g Chemical Oxygen Demand (COD) per litre at a constant hydraulic retention time of four days. Methane production, volatile fatty acid conversion, net sludge growth and Chemical Oxygen Demand reduction were monitored. Over 96% Chemical Oxygen Demand was removed at loadings up to 10.6 g COD l −1 day −1 . At the highest influent concentration reactor instability was observed. Up to this point the results indicated that the UASB could treat POME more effectively than other systems reviewed.

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.

Enhancement of the anaerobic digestion of wine distillery wastewater by the removal of phenolic inhibitors

Wine-distillery wastewater has a high organic pollutant load (40 g COD/litre) which includes various phenolic compounds, the major ones being gallic acid, p-coumaric acid and gentisic acid. This feature makes it difficult to treat this wastewater anaerobically without a prior dilution of the wastewater. Partial removal of some of the organic matter and phenolic compounds by aerobic pretreatment with Geotrichum candidum is described in this study. This method provides a partially purified effluent that is more rapidly anaerobically degraded than the original wine-distillery wastewater for the same COD-loading level. The anaerobic bioreactor contained a suspension of micronized clay (saponite) to which the microorganisms responsible for the process adhered. Assuming that the overall anaerobic digestion process follows first-order kinetics, the specific rate constant, K0, was obtained under various conditions. In contrast to the anaerobic digestion of unmodified wine-distillery wastewater, the kinetic constants of the anaerobic digestion of aerobically pretreated wine-distillery wastewater were virtually constant over the COD range used (1·0–6·7 g/litre).

Comparison of an anaerobic filter and an anaerobic fluidized bed reactor treating palm oil mill effluent

A laborator-scale anaerobic filter (AF) and a fluidized-bed reactor (FBR) were compared for their ability to treat palm oil mill effluent (POME). The reactors were operated at mesophilic temperature (35°C). Reactor performance was assessed by measuring chemical oxygen demand (COD), volatile fatty acids (VFA), gas composition and gas production. Sand of 0·3–0·5 mm diameter was used as a biomass support medium in the FBR, and clay rings of 2–5 mm diameter were used in the AF. Start-up of the AF was achieved with 1·5–4·5-day residence times and FBR residence times were maintained at 6 h. After acclimatization, COD removals higher than 90% were reached in both reactors at 6 h residence time, equivalent to a loading of 10 g COD/litre/day. At higher loadings the FBR gave a better performance; even at 40 g COD/litre/day, with 6-h residence times, 78% COD was degraded. The AF could not be operated above 20 g COD/litre/day without clogging. The AF and FBR performed similar at reactor concentrations up to 1 g COD/litre, while above 2·2 g COD/litre the AF showed a maximum removal rate of 17·0 g COD/litre/day compared to 31·2 g COD/litre/day for the FBR. These differences were probably due to diffusion limitations and a less active biomass in the AF. In both reactors the gas production rate and the levels of organic acids increased in response to a higher feed concentration in less than one day and this could be explained by substrate limitation. Other responses were slower with the microbial culture adapting over periods of 6–10 days; these were apparently growth related. A doubling in loading always resulted in a large increase in organic acids, especially acetic and propionic, as well as increasing the proportion of CO2 in the gas.