Mesophilic Anaerobic Co-digestion of Olive-Mill Waste With Cattle Manure: Effects of Mixture Ratio

The co-digestion of agri-food by-products and livestock manure is a feasible alternative for waste management and the recovery of biogas provides an option to generate renewable energy. A series of batch experiments were carried out in order to investigate the mesophilic anaerobic co-digestion of two-phase olive-mill waste (2POMW) and cattle manure (CM) in different mixtures (2POMW:CM = 50:50; 60:40; 75:25; 85:15). In addition, the biodegradability of the co-substrates was studied in order to analyze the performance of the co-digestion process. The results obtained in this study indicate that 2POMW has a low biodegradability since a high soluble organic matter concentration and a low accumulated methane production were obtained at the end of the corresponding biodegradability test. However, CM is more easily biodegradable in mesophilic anaerobic conditions. The co-digestion of both wastes produced an enhancement of the hydrolytic-acidogenic phase, increasing the organic matter potentially bioavailable as volatile fatty acids (VFAs) and the biogas productivity, as a consequence of the subsequent degradation of VFAs by methanogens. However, an accumulation of VFA, principally propionic acid, was observed in the reactors with higher proportions of 2POMW. The volatile solids (VS) removal increased with the 2POMW percentage of the mixture up to 75% fresh weight. The increase of 2POMW above 75% led to a decrease in total VS removal. Moreover, a decrease in methane production was observed for the 85:15 mixture, as a consequence of the high concentration of propionic acid, which is a known inhibitor of methanogenesis. The maximum cumulative methane production and methane yield were achieved in the 75:25 mixture with values of 18.70 L and 112.40 LCH4/kgVS(added), respectively. Compared with 2POMW, the co-digestion produced an increase of 264-319% in the volume of accumulated methane (L), 293-351% in the methane yield (LCH4/kgVS(added)) and 312-342% based on the VS consumed (LCH4/kgVS(removed)). These results suggest that the mixture of these agro-industrial by-products could be effective to enhance biogas production and organic matter removal from 2POMW

Evaluación de los gases de efecto invernadero en el compostaje de alperujo y de la inmovilización de carbono en su aplicación al suelo

El compostaje tiene gran interés debido a su potencial para degradar y estabilizar el contenido en materia orgánica (MO) de los residuos, que pueden ser utilizados en agricultura como enmiendas orgánicas. En la actualidad, la aplicación de compost al suelo puede tener un valor añadido como estrategia para reducir la concentración de CO2 atmosférico mediante la estabilización de carbono orgánico en el suelo. Con este fin, son de especial interés los residuos ricos en lignocelulosa, cuyo compostaje dará lugar a un material altamente humificado que minimiza la posterior degradación del carbono a CO2 y favorece la incorporación de la MO del compost al suelo. Esta estrategia es de especial interés en las zonas de clima mediterráneo y semiárido donde los suelos agrícolas sufren además graves problemas degradativos debido a las pérdidas de carbono orgánico. El alperujo, residuo procedente de la extracción del aceite de oliva, tiene un alto contenido en lignocelulosa y compuestos fenólicos, características que favorecen la obtención de compost con un alto grado de madurez y estabilidad y que hacen que tenga una gran resistencia a la degradación. Así, el uso de este residuo como fuente de carbono, tras su compostaje, tiene importantes implicaciones en la inmovilización de carbono orgánico en el suelo. Sin embargo, los beneficios netos del compostaje de residuos orgánicos sobre el secuestro de carbono pueden verse contrarrestados por las emisiones de gases de efecto invernadero (GEI) con efectos potenciales negativos sobre el medio ambiente, tales como son el N2O y el CH4. El objetivo global del presente trabajo es evaluar el impacto ambiental del compostaje de alperujo mediante el estudio de las emisiones de GEI generados durante el proceso y del potencial del compost obtenido para la inmovilización de carbono en suelos enmendados. Los objetivos parciales de la presente memoria han sido: • Determinar el efecto de la composición química de la mezcla inicial de residuos sobre la velocidad de descomposición de la MO (duración del proceso) en el compostaje de alperujo. • Cuantificar la concentración de los gases CO2, CH4 y N2O generados durante el compostaje de alperujo para valorar el efecto de la composición inicial de las pilas y de la fuente de nitrógeno sobre la magnitud de las emisiones. • Evaluar la influencia de la composición y del grado de estabilización de las mezclas de compost en la estabilización de carbono orgánico en suelos enmendados, simulando condiciones ambientales normales. • Cuantificar la cantidad de carbono procedente de las enmiendas de compost de alperujo que puede quedar estabilizado entre las diferentes fracciones de carbono húmico del suelo, bajo condiciones óptimas para la mineralización. Para conseguir estos objetivos, se estudió la evolución de cinco pilas elaboradas a escala industrial empleando distintas proporciones de alperujo, diferentes agentes estructurantes y fuentes de nitrógeno. El sistema de ventilación empleado fue el de pila móvil. Las muestras de los gases CO2, CH4 y N2O se recogieron en la cima de las pilas mediante un sistema de cámara cerrada. Por último se realizaron ensayos con suelos enmendados con las mezclas de compost de alperujo para el estudio de las dinámicas de mineralización y humificación. Los resultados muestran que el uso de compost de alperujo como enmendante orgánico resulta eficaz en la estrategia de estabilizar carbono en el suelo a largo plazo. Hay un beneficio neto al final del proceso dada la resistencia del material a la degradación durante su compostaje y la baja mineralización de carbono tras su aplicación al suelo. A esto se suma el escaso impacto ambiental de su compostaje controlado, evaluado por la baja emisión de CH4 y N2O. Composting is a suitable low-cost strategy for the valorisation of organic wastes as soil amendments. The interest of this technology lies in its potential to degrade and stabilise the organic matter (OM) content of the wastes. At present, the application of compost to the soil could have an added value as a strategy to reduce the atmospheric concentration of CO2 by the stabilisation of organic carbon in the soil. To that end, is of special interest the composting of lignocellulosic wastes due to the quality of the compost obtained: a highly humified material which minimise the subsequent carbon mineralisation and contribute to the stabilisation of waste carbon in soil. This strategy is particularly important in Mediterranean areas, where a large proportion of agricultural lands have been degraded by loss of organic matter and other processes that reduce soil productivity. Two-phase olive mill waste (TPOMW), the main by-product generated by the two-phase centrifugation system, is characterised by a large content of OM, mainly of lignocellulosic nature, and by the presence of polyphenols. These are favourable physicochemical properties to produced a highly humifed compost which can promote long-term stabilisation of waste organic carbon in soil. However, the net benefit of the composting of organic wastes on the carbon sequestration strategy could be counteracted with the emissions of CH4 and N2O generated during composting operations, since these gases have a warming potential higher than CO2. The global purpose of this work is to evaluate the environmental impact of TPOMW composting by studying the emissions of greenhouse gases (GHG) generated during the process and the potential of the obtained compost to sequester carbon in the amended soils. The partial aims have been: • To determine the effect of the initial chemical composition of the composting mixtures on the OM degradation rate (composting time) during TPOMW composting. • To quantify the emissions of CO2, CH4 and N2O during TPOMW composting and to study the relationship between the mechanisms involved in the gas production and the carbon and nitrogen cycles of the composting mixtures. • To evaluate the influence of the chemical composition and the degree of stabilisation of TPOMW composting mixtures on the stabilisation of organic carbon in amended soils under standard conditions. • To quantify the amount of waste carbon stabilised in different soil humic fractions, under optimum conditions for mineralization. To reach these goals, it has been studied the evolution of five composting mixtures prepared from TPOMW and different bulking agents and N sources. Composts were prepared in a pilot-scale plant by windrowing. A static closed chamber technique was used to measure CO2, CH4 and N2O fluxes from the surface of the composting pile. Finally, mineralization and humification dynamics were studied on different agricultural soils amended with TPOMW composting mixtures. Results show the effectiveness of the use of TPOMW compost as organic amendment to promote long-term stabilisation of waste carbon in soil. There is a net benefit at the end of the process due to the low degradation rate of TPOMW during composting and the low mineralization rate in soil. In addition, the environmental impact of a controlled TPOMW composting is scarce due to the low CH4 and N2O surface flux emission observed in the piles

Soil mineralization of two-phase olive mill wastes: Effect of the lignocellulosic composition on soil C dynamics

The low degradation rate of two-phase olive mill wastes (TPOMW) during composting and after soil application is a characteristic feature of these materials. The aim of this work was to evaluate the relationship between the lignocellulosic fraction of TPOMW and the organic matter (OM) degradation rate in three agricultural soils amended with four TPOMW composting mixtures at different degree of stabilisation and prepared with different bulking agents and N sources. The mineralisation kinetics of TPOMW composting mixtures in soil reflected a large amount of slowly mineralisable C even in the starting mixtures (I and T1) where this fraction represented up to 85% of the total potentially mineralisable C pool. The effect of rich lignocellulosic composition was confirmed by the study of the DTS (50% dry TPOMW + 50% sheep manure) mixtures prepared with dry TPOMW, which had undergone partial degradation in a storage pond for one year before composting. These DTS samples showed a more similar kinetic behaviour in soil than the more transformed composting mixtures as reflected in the principal component analysis (PCA) diagram, where they were grouped in the same quadrant dominated by the lignin/holocellulose ratio. Soils amended with mature composts evolved very low amounts of C (between 2 and 6% of the added C) after two months of incubation, which highlights the suitability of these materials as a suitable C source for the soil to promote long term soil C stabilisation.This work was supported by the Spanish Ministry of Science and Innovation, research projects Ref. CTM2005-05324 and CTM2009-1473-C02-02.Peer reviewe

Potential of olive mill wastes for soil C sequestration

The present work deals with the potential of olive mill wastes as a C source for soil C sequestration strategy, which is based on the high lignocellulosic content that makes these wastes to degrade slowly during composting and after land application. A C balance was performed during the whole life cycle of two different two-phase olive mill wastes (TPOMW): C losses were calculated during the composting process and after soil application of the composting mixtures under laboratory conditions. The effect of the degree of stabilization of TPOMW on the overall C waste conservation efficiency was also evaluated. C losses after 34 weeks of TPOMW composting ranged from 40.58% to 45.19% of the initial C, whereas the amount of C evolved as CO2 after 8 months of incubation of soil amended with mature composts only represented between 20.6% and 21.9% of the added C. The total C losses considering the whole life cycle of the TPOMW showed lower losses compared to composts prepared with organic residues of different origin. Conversely to the typical behaviour of other organic wastes, the stabilisation degree of the TPOMW composting mixtures did not show any significant effect on the total C losses measured during composting and later land application. The low rate of degradation of TPOMW both during composting and after soil application makes the use of TPOMW as a C source an attractive strategy for soil C sequestration.The authors wish to thank the Spanish Comisión Interministerial de Ciencia y Tecnología (CICYT) for supporting the research project Ref. CTM2005-05324 under which this work was partially financed. Special thanks to Fundación Séneca (Agencia Regional de Ciencia y Tecnología, Región de Murcia) for financing the postdoctoral grant 02534/PD/05.Peer reviewe

Changes in soil humic pools after soil application of two-phase olive mill waste compost

The use of appropriate amendments derived from two phase olive mill wastes (TPOMW) can represent a suitable option to maintain and restore C levels in agricultural soils under Mediterranean climates. We evaluated soil organic matter stabilisation pathways among different humic pools in a Calcaric Cambisol amended with 2% (40 Mg ha− 1) of TPOMW composting mixtures of different composition and at different degrees of stabilisation: starting mixture, after 14 weeks of composting (thermophilic stage) and after 30 weeks (mature compost). Non-humified soil organic C and two different fractions of humic acids (HA), namely free HA (biochemically stabilised) and bound HA (biochemically and chemically stabilised) were obtained by sequential extraction with NaOH and alkaline Na4P2O7 after 90 and 150-days incubation. HA were characterised by thermal analysis, size exclusion chromatography (HPLC-SEC), FTIR and 13C CPMAS-TOSS NMR. Amendments promoted incorporation of altered lignin structures, carbohydrate moieties and N-containing compounds into free HA and to a lesser extent into the bound HA, and increased the proportion of high MW fractions. There was an average increase of 40% for non-humic C in the free C fraction even after 90 days of incubation under optimum conditions for mineralisation. Augmentation of bound C resulted in an average increase of about 0.7 Mg ha− 1 of humic C in amended soils. This increase is important as it contributes to one of the more inert soil C pools and could represent a useful indicator for soil C stabilisation.This work was supported by the Spanish Ministry of Science and Innovation, research projects Ref. CTM2005-05324 and CTM2009-14073-C02-02.Peer reviewe

Application of compost of two-phase olive mill waste on olive grove: Effects on soil, olive fruit and olive oil quality

Composting is a method for preparing organic fertilizers that represents a suitable management option for the recycling of two-phase olive mill waste (TPOMW) in agriculture. Four different composts were prepared by mixing TPOMW with different agro-industrial by-products (olive pruning, sheep manure and horse manure), which were used either as bulking agents or as N sources. The mature composts were added during six consecutive years to a typical “Picual” olive tree grove in the Jaén province (Spain). The effects of compost addition on soil characteristics, crop yield and nutritional status and also the quality of the olive oil were evaluated at the end of the experiment and compared to a control treated only with mineral fertilization. The most important effects on soil characteristics included a significant increase in the availability of N, P, K and an increase of soil organic matter content. The application of TPOMW compost produced a significant increase in olive oil content in the fruit. The compost amended plots had a 15% higher olive oil content than those treatment with inorganic fertilization. These organics amendments maintained the composition and quality of the olive oil.The authors wish to thank the Spanish Ministry of Science and Innovation for supporting research projects Ref. CTM2005-05324 and CTM2009-1473-C02-02 under which this work was financed. A. Fernández-Hernández wishes to thank for the contract grant funded by the Agricultural Research Institute of Spain (INIA) and the European Social Fund.Peer reviewe

Contribution of the lignocellulosic fraction of two-phase olive-mill wastes to the degradation and humification of the organic matter during composting

One of the main disadvantages in the composting of two-phase olive mill wastes (TPOMW) is the long time required for its transformation (up to 40 weeks). The aim of this work was to evaluate the relationship between the degradation of the lignocellulosic fraction of TPOMW and the organic matter (OM) mineralisation rate in four composting piles prepared with different bulking agents and N-sources used to enhance OM degradation. The kinetics of degradation of the lignocellulosic fraction was compared to conventional maturation and stability indices to evaluate its impact on the duration of the composting process. The composition of bulking agents mainly affected the water-soluble fraction which influenced the OM degradation rate (linear or exponential OM degradation pattern) at early stages of the composting process but it neither modified the duration of the process (between 34 and 36 weeks) nor the total OM degradation underwent by the piles. The high initial mineral N availability was a key factor to significantly enhanced microbial activity. The mixture with urea as N-source registered the most efficient degradation of hemicellulose, cellulose and lignin, reducing the thermophilic phase and the total duration of TPOMW composting.The authors wish to thank the Spanish Ministry of Science and Innovation for supporting the research projects Ref. CTM2005-05324 and CTM2009-1473-C02-02 under which this work was financed.Peer reviewe

Greenhouse gas emissions during composting of two-phase olive mill wastes with different agroindustrial by-products

The evolution of CO2, CH4 and N2O were monitored in five composting mixtures prepared from two-phase olive mill waste (TPOMW) and different agroindustrial by-products in order to assess the effect of the initial composition and the N source on greenhouse gas emission. Surface gas fluxes were measured using a closed static chamber and compared to the changes in different organic matter fractions (organic and watersoluble C) and N forms (NH4+ and NO3-). CH4 emissions depended on the organic matter mineralisation dynamics and the incorporation of manure in the starting mixture. The highest CH4 fluxes were registered during the intense degradation at early stages of the process (up to 100 g C m−2 d−1). The emission of N2O (0–0.9 g N m−2 d−1) occurred from 6th to 10th wk of composting (bioxidative phase), coinciding with an intense nitrification in the pile. The use of urea enhanced the N2O emission up to 3.7 g N m−2 d−1, due to an increase in available mineral N in the pile. Even though well managed TPOMW composting piles only represent a minor source of CH4 and N2O emissions, the addition of urea and easily available C fractions to the starting mixtures can significantly increase the environmental impact of TPOMW composting as far as greenhouse gas emissions are concerned.The authors wish to thank the Spanish Ministry of Science and Innovation for supporting research projects Ref. CTM2005-05324 and CTM2009-1473-C02-02 under which this work was financed.Peer reviewe

Emission of CO2, CH4 and N2O during composting of two-phase olive mill wastes: a four-year study

A four-year monitoring program was performed to quantify the emission of the three most important greenhouse gases (GHG), CO2, CH4 and N2O, emitted during composting of two-phase olive mill wastes (TPOMW). A composting pile was prepared with typical agroindustrial wastes generated from olive oil mills: TPOMW and olive tree prunings, used as bulking agent, enriched with urea as N source. The study was performed during four consecutive years in order to obtain a wide set of experimental data to evaluate the effect of the variability of the raw material characteristics and the performance of the composting plant on GHG emissions. Biogenic CO2 was the main gas generated during TPOMW composting. The range of CO2 emission exhibited low interannual variation regardless the operational conditions at the composting plant. The improvement of the composting operations (better control of pile moisture and improved aeration) reduced the amount of CH4 and N2O emitted from the pile by reducing the formation of anaerobic spots within the pile. The control of the CH4 and N2O emissions reduced the contribution of TPOMW composting to climate change, expressed as kg of CO2 equivalents.The authors wish to thank the Spanish Ministry of Science and Innovation for supporting research project Ref. CTM2009-1473-C02-02 under which this work was financed.Peer reviewe