Two-phase Olive Mill Solid Waste Research Articles

Integral Valorization of Two-Phase Olive Mill Solid Waste (OMSW) and Related Washing Waters by Anaerobic Co-digestion of OMSW and the Microalga Raphidocelis subcapitata Cultivated in These Effluents.

This study evaluates the comprehensive valorization of the byproducts derived from the two-phase olive oil elaboration process [i.e., olive washing water (OWW), olive oil washing water (OOWW), and olive mill solid waste (OMSW)] in a closed-loop process. Initially, the microalga Raphidocelis subcapitata was grown using a mixture of OWW and OOWW as the culture medium, allowing phosphate, nitrate, sugars, and soluble chemical oxygen demand removal. In a second step, the microalgal biomass grown in the mixture of washing waters was used as a co-substrate together with OMSW for an anaerobic co-digestion process. The anaerobic co-digestion of the combination of 75% OMSW–25% R. subcapitata enhanced the methane yield by 7.0 and 64.5% compared to the anaerobic digestion of the OMSW and R. subcapitata individually. This schedule of operation allowed for integration of all of the byproducts generated from the two-phase olive oil elaboration process in a full valorization system and the establishment of a circular economy concept for the olive oil industry.

Overcoming the bottlenecks of anaerobic digestion of olive mill solid waste by two-stage fermentation

ABSTRACTTwo-stage anaerobic digestion (AD) of two-phase olive mill solid waste (OMSW) was applied for reducing the inhibiting factors by optimizing the acidification stage. Single-stage AD and co-fermentation with chicken manure were conducted coinstantaneous for direct comparison. Degradation of the polyphenols up to 61% was observed during the methanogenic stage. Nevertheless the concentration of phenolic substances was still high; the two-stage fermentation remained stable at OLR 1.5 kgVS/m³day. The buffer capacity of the system was twice as high, compared to the one-stage fermentation, without additives. The two-stage AD was a combined process – thermophilic first stage and mesophilic second stage, which pointed out to be the most profitable for AD of OMSW for the reduced hydraulic retention time (HRT) from 230 to 150 days, and three times faster than the single-stage and the co-fermentation start-up of the fermentation. The optimal HRT and incubation temperature for the first stage were determined to four days and 55°C. The performance of the two-stage AD concerning the stability of the process was followed by the co-digestion of OMSW with chicken manure as a nitrogen-rich co-substrate, which makes them viable options for waste disposal with concomitant energy recovery.

Influence of a steam-explosion pre-treatment on the methane yield and kinetics of anaerobic digestion of two-phase olive mil solid waste or alperujo

This study investigated the effect of a steam-explosion pre-treatment on the anaerobic digestion of the two-phase olive mill solid waste (OMSW) or alperujo. The OMSW was subjected to a steam explosion pre-treatment at 200°C and 1.57MPa during 5min. After this pre-treatment a solid fraction (SF) and a liquid fraction (LF) were generated. These two fractions were used to obtain methane through anaerobic digestion with the aim of evaluating the effect of the pre-treatment on the methane yields obtained. Biochemical methane potential (BMP) tests of both untreated OMSW and steam-explosion pre-treated OMSW, i.e. SF and LF, as described above, revealed that the maximum methane yield (589±42mL CH4/g VSadded) was achieved for the LF generated after the pre-treatment. From a kinetic point of view the BMP tests of untreated OMSW and the SF obtained after the pre-treatment showed a first exponential stage followed by a second sigmoidal stage after a lag period. By contrast for the LF, only a single exponential stage was observed.

Characterization and modeling of pyrolysis of the two-phase olive mill solid waste

In this work, first, the two-phase olive mill solid waste was characterized in terms of its physical–chemical properties, which can be useful for its use in pyrolysis units. Then, its behavior on pyrolysis processes was studied by dynamic thermogravimetry and derivate thermogravimetry. For the simulation of pyrolysis processes a kinetic model including three independent parallel reactions was used. The characterization results showed that the solid presents a size distribution with about 66% of particles >1.00mm, while the percentage of fines (<0.250mm) is only of 1.50%. The moisture content is very high (around 70%), being one of the main characteristics of the solid that makes its direct use as a fuel difficult. However, the low sulfur value (<0.1%) is very suitable from the environmental point of view, reducing SO2 emissions. The results of the proximate analysis are within the range of most biomass waste and the gross calorific value obtained was 4897kcal/kg. The thermal decomposition in an inert atmosphere can be modeled by separate decomposition of each of the three fractions which constitute the material. The results of both, physical–chemical characterization and thermogravimetric study show that the two-phase olive mill solid waste could be treated in energy recovery installations.

Assessment of two-phase olive mill solid waste and microalgae co-digestion to improve methane production and process kinetics

Olive mill solid waste (OMSW) is a pollutant waste coming from olive oil elaboration by the two-phase centrifugation system. OMSW has a high organic matter content and unbalanced carbon to nitrogen (C/N) ratio, 31/1, which avoids obtaining high methane yields in the anaerobic digestion of this waste. In the present study a microalgae, Dunaliella salina, was employed as co-substrate for the OMSW anaerobic digestion in order to decrease the C/N ratio and increase its biodegradability. Different co-digestion mixtures (C/N ratios) were studied. The increase of D. salina from 25% to 50% in the co-digestion mixture clearly increased the biodegradability of the sole substrates. The highest biodegradability was found for the co-digestion mixture 50% OMSW-50% D. salina. Nevertheless, the maximum methane production, 330mLCH4/gVSadded, and the highest methane production rate were obtained for the co-digestion mixture 75% OMSW-25% D. salina, keeping a C/N ratio near to 26.7/1.

Effect of ultrasonic pretreatment on biomethane potential of two-phase olive mill solid waste: kinetic approach and process performance.

The effect of ultrasound (US) pretreatment on two-phase olive mil solid waste (OMSW) composition and subsequent anaerobic biodegradation was evaluated by chemical oxygen demand solubilization and biochemical methane potential (BMP) tests. OMSW was ultrasonically pretreated at a power of 200 W and frequency of 24 kHz for time periods of 20, 40, 60, 90, 120, and 180 minutes, corresponding to specific energies of 11367, 21121, 34072, 51284, 68557, and 106003 kJ/kg total solids, respectively. In order to evaluate the US pretreatment, a low, medium, and high exposure time, that is, 20, 90, and 180 min, were selected for BMP tests. Methane yields of 311 ± 15, 393 ± 14, and 370 ± 20 mL CH4/g VSadded (VS: volatile solids) were obtained for 20, 90, and 180 minutes, respectively, while the untreated OMSW gave 373 ± 4 mL CH4/g VSadded. From a kinetic point of view, the BMP tests showed a first exponential stage and a second sigmoidal stage. In the first stage, the kinetic constant obtained for US pretreated OMSW at 20 minutes was 46% higher than those achieved for the pretreated OMSW at 90 and 180 minutes and 48% higher than that for untreated OMSW. The maximum methane production rate achieved was 12% higher than that obtained for untreated OMSW.

Biochemical methane potential of two-phase olive mill solid waste: Influence of thermal pretreatment on the process kinetics

The effect of thermal pretreatment on two-phase olive mill solid waste was evaluated by chemical oxygen demand solubilisation and biochemical methane potential (BMP) tests. Temperatures of 100, 120, 160 and 180°C were applied during 60, 120 and 180 min for each temperature studied. The highest chemical oxygen demand solubilisation after pretreatment (42%) was found for 120 and 180°C during 180 min in both cases. These two conditions were selected for the BMP tests. BMP tests showed two different stages: a first exponential stage and a sigmoidal zone after a lag period. No influence of the pretreatment was observed on the kinetic constant of the first-stage. Clear difference was observed in the maximum methane production rate of the second stage, 76.8 mL CH4/(g VS day) was achieved after pretreatment at 180°C (180 min), value 22% and 40% higher than that obtained for the untreated and pretreated OMSW at 120°C, respectively.

Batch culture growth of Chlorella zofingiensis on effluent derived from two-stage anaerobic digestion of two-phase olive mill solid waste

This paper presents the use of an effluent derived from two-stage anaerobic digestion of two-phase olive mill solid waste (OMSW) as a substrate for the production of Chlorella zofingiensis in batch mode. Chlorella zofingiensis when grown autotrophycally can accumulate significant quantities of valuable carotenoids which are used as an additive in fish and poultry farming, as colorants in foods and in health care products. It was found that two-phase OMSW previously treated by two-stage anaerobic digestion and further sterilized may be used as a culture medium for the microalgae Chlorella zofingiensis. Typical growth curves were obtained using both the above-mentioned anaerobic effluent and a synthetic medium. Total chemical oxygen demand (TCOD) and soluble chemical oxygen demand (SCOD) removals of 37% and 45% respectively were achieved in batch experiments after 11 days’ operation time. The specific growth rate was lower when the treated effluent was used as the feed substrate (0.02 h-1) in comparison to the synthetic medium (0.03 h-1). The specific growth rates of the exponential phases were determined by using a first-order kinetic model applied to chlorophyll a (Ca) and total chlorophyll (TC) concentrations, as indirect measurements of the microalgae concentration. It was concluded that the effluent from two-stage anaerobic digestion of two-phase OMSW constituted an appropriate culture medium for the growth of Chlorella zofingiensis, providing a simple technology feasible for producing a very useful product for animal feeding.

Batch culture growth of Chlorella zofingiensis on effluent derived from two-stage anaerobic digestion of two-phase olive mill solid waste

Batch culture growth of Chlorella zofingiensis on effluent derived from two-stage anaerobic digestion of two-phase olive mill solid waste

A study of the natural biodegradation of two-phase olive mill solid waste during its storage in an evaporation pond

A study of the natural biodegradation of two-phase olive mill solid waste (OMSW) during its storage in an evaporation pond was carried out. This system is traditionally used as the final disposal of this waste or constitutes a previous stage before being processed by cogeneration or anaerobic digestion processes. A laboratory-scale pond with a total volume of 520 l was used. The experiment was carried out between December 2002 and August 2003 covering a total period of 269 days. During the experiment, the variations of temperature (outside and inside the pond), the effective volume of the OMSW in the pond, moisture content, total solids (TS), volatile solids (VS), the total chemical oxygen demand (TCOD), total volatile acids (TVA), pH and cumulative methane production as a function of the operation time were evaluated. The experimental results obtained showed that the characteristics of the two-phase OMSW in the pond were dependent on the time of operation and temperature. During the experiment, three periods were clearly observed and differentiated: (a) An initial period with a rapid reduction of moisture content and effective volume, with an increase of TS, VS and TCOD removals, a decrease of pH and an increase of TVA and methane production until day 21. (b) A second period of slow decrease of moisture content and effective volume, due to the decrease of the ambient temperature. This period took place between day 21 and days 120–150 of assay, and it was characterised by a slight change in the OMSW properties. (c) A third period where the characteristics of the waste in the pond changed considerably due to the increase of the temperature outside the pond. The methane gas production also showed an increase due to the increase of methanogenic activity with the increase of temperature. An empirical model to describe the changes of two-phase OMSW characteristics and methane production with the operation time was developed. The proposed model, based on the use of polynomial equations, took into account the influence of the temperature and fit adequately into the experimental results obtained. It was found that the pond could probably be used as preliminary treatment of two-phase OMSW before its anaerobic digestion process, because a maximum conversion of organic matter to TVA was obtained after an operation time of around 200 days.

Kinetics of mesophilic anaerobic digestion of the two-phase olive mill solid waste

A kinetic study of the anaerobic digestion of two-phase 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 three different feed substrate concentrations: 34.5, 81.1 and 113.1 g COD/l. The hydraulic retention time (HRT) varied between 40.0 and 8.3 days for the first feed used and between 50 and 10 days for the other two feed substrate concentrations. The results were evaluated using the Chen–Hashimoto methane production model to determine the kinetic constants of the process. The kinetic constants ( μ max and K) were affected by the feed substrate concentration and decreased by 63 and 65%, respectively, when the feed concentration increased from 34.5 to 113.1 g COD/l. This behaviour is believed to be due to the higher levels of phenolic compounds and biotoxicity present in the most concentrated substrate used. Methane production was reproducible within 10% deviations.