Cranfield University Cranfield Water Science Institute School of Applied Sciences PhD 2014 Yadira Bajón Fernández Carbon dioxide utilisation in anaerobic digesters as an on-site carbon revalorisation strategy Supervisors: Prof. Elise Cartmell Dr. Ana Soares Peter Vale November 2014 This thesis is submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy Cranfield University Cranfield Water Science Institute School of Applied Sciences PhD 2014 Yadira Bajón Fernández Carbon dioxide utilisation in anaerobic digesters as an on-site carbon revalorisation strategy Supervisors: Prof. Elise Cartmell Dr. Ana Soares Peter Vale November 2014 This thesis is submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy ABSTRACT The increasing carbon footprint of the water and organic waste sectors has led to water utilities to voluntarily include carbon mitigation approaches within their strategic plans and to an increase in research aimed at mitigating carbon dioxide (CO ) emissions. Injection 2 of CO in anaerobic digesters (ADs) for its bioconversion into methane (CH ) has been 2 4 identified as a potential solution. However, previous literature provided limited knowledge of the carbon benefits obtainable and presented conflicting information regarding the mechanisms of CO utilisation. 2 This thesis investigated the potential of injecting exogenous CO into ADs for its 2 bioconversion into CH to reduce CO emissions from water and organic waste facilities. 4 2 Batch laboratory scale and continuous pilot-scale ADs enriched with CO were operated. A 2 substrate dependant response to exogenous CO was reported for the first time and potential 2 CO savings of up to 34% and 11% were estimated for sewage sludge and food waste batch 2 ADs, respectively, injected with CO before the digestion process. Higher benefits in CH 2 4 production were observed in sewage sludge ADs than in food waste units. An up to 2.4 fold increase in CH production during the 24 hours following saturation with CO was obtained in 4 2 sewage sludge units, while benefit was limited to 1.16 fold in food waste ADs. Microbial community analyses were performed to elucidate CO fate mechanisms. An increase of up to 2 80% in the activity of Methanosaetaceae (obligate acetoclastic methanogen) was observed in sewage sludge ADs periodically enriched with CO . Methanosaetaceae was scarce 2 (4.3±1.7%) in food waste units, which was attributed to an inhibitory concentration of ammonia (4 gL-1 NH -N). Based on Archaea analyses and on monitoring hydrogen (H ) and 4 2 volatile fatty acids (VFA) speciation dynamics in a pilot-scale AD, it was proposed that exogenous CO is reduced by homoacetogenesis (Wood-Ljungdahl mechanism) and the 2 acetate generated by this route is converted to CH by acetoclastic methanogenesis. 4 Gas to liquid mass transfer was identified as limiting of the amount of dissolved CO 2 loaded to an AD and the complex rheology of anaerobically digested media as detrimental for transfer performance. An increase of apparent viscosity (µ ) from 130 to 340 cPo (typical a variability of sewage sludge) reduced gas transfer efficiency (GTE) by 6 percentage points. The use of bubble columns was identified as suitable for further scaled-up units. Injection of CO could be performed in the digestate recirculation loop of single phase ADs or in the first 2 phase of two phase ADs (TPADs), with CO sourced from off-gas of biogas upgrading 2 technologies. It has been demonstrated that bioconversion of CO in ADs can reduce carbon 2 footprint and increase CH production, with the possibility of becoming an on-site carbon 4 revalorisation strategy. RESUMEN (TRANSLATION OF THE SECTION ABSTRACT) La creciente huella de carbono de los sectores del agua y residuos orgánicos ha llevado a las empresas del sector a incluir voluntariamente estrategias de mitigación de carbono en sus planes estratégicos y a un aumento de la investigación enfocada a reducir la emisión de dióxido de carbono (CO ). La inyección de CO en digestores anaeróbicos (DAs) 2 2 para su bioconversión a metano (CH ) ha sido identificada como una posible solución. Sin 4 embrago, investigaciones anteriores ofrecen escasa información de los beneficios obtenibles y presentan información contradictoria en relación a los mecanismos de utilización de CO . 2 Esta tesis ha investigado el potencial de inyectar CO exógeno en DAs para su 2 bioconversión a CH parar reducir la huella de carbono del tratamiento de aguas residuales y 4 residuos orgánicos. Se han operado DAs enriquecidos con CO a escala de laboratorio y de 2 planta piloto. Se ha obtenido una respuesta dependiente del substrato tratado y se ha estimado una reducción de CO de hasta un 34% y 11% en DAs discontinuos saturados con CO que 2 2 tratan lodos de depuradora y residuos alimentarios, respectivamente. Se ha observado un mayor beneficio en la producción de CH en DAs de lodos de depuradora que con residuos 4 alimentarios. Se ha obtenido un aumento de hasta 2.4 veces en la producción de CH durante 4 las 24 horas posteriores a saturar con CO DAs de lodos de depuradora, mientras que el 2 beneficio máximo en residuos alimentarios ha sido de 1.16 veces. Se han desarrollado análisis microbianos para elucidar los mecanismos de utilización de CO , obteniéndose un incremento 2 de hasta un 80% en la actividad de Methanosaetaceae (arquea metanogénica acetoclástica) en DAs tratando lodos de depuradora enriquecidos periódicamente con CO . Methanosaetaceae 2 se ha encontrado en escasa proporción en residuos alimentarios (4.3±1.7%) debido a su inhibición por amoníaco (4 gL-1 NH -N). En base al análisis de arqueas y a la concentración 4 de hidrógeno (H ) y ácidos volátiles grasos en un DA a escala piloto, se ha propuesto que el 2 CO exógeno es reducido por homoacetogénesis (ruta Wood-Ljungdahl) y el acetato generado 2 es convertido a CH por metanogénesis acetoclástica. La transferencia de masa gas-líquido es 4 un factor limitante de la cantidad de CO disuelto introducida en un DA y la reología de 2 substratos digeridos anaeróbicamente dificulta dicha transferencia. Un incremento de viscosidad aparente (µ ) de 130 a 340 cPo (típico en lodos de depuradora) puede reducir en 6 a puntos de porcentaje la eficiencia de transferencia de gas. Se ha considerado adecuado el uso de columnas de burbujas para disolver CO en unidades a mayor escala. Ha sido concluido 2 que la inyección de CO puede realizarse en la recirculación de material digerido en un DA de 2 fase única o en la primera fase de un DA de dos fases. El CO inyectado podría capturarse de 2 los gases de salida de varias tecnologías de enriquecimiento de biogás. Se ha demostrado que la bioconversión de CO en DAs puede reducir la huella de carbono y aumentar la producción 2 de CH , teniendo así potencial para ser una estrategia de revalorización de carbono in-situ. 4 TABLE OF CONTENTS ACKNOWLEDGEMENTS .............................................................................................. i LIST OF FIGURES .......................................................................................................... ii LIST OF TABLES ......................................................................................................... vii ABBREVIATIONS ......................................................................................................... ix NOTATIONS .................................................................................................................. xi 1. INTRODUCTION ....................................................................................................... 3 1.1. BACKGROUND................................................................................................ 3 1.2. AIMS AND OBJECTIVES................................................................................ 4 1.3. THESIS PLAN ................................................................................................... 5 1.4. REFERENCES ................................................................................................. 10 1.5. INTRODUCCIÓN (translation of the section Introduction) ........................... 11 1.5.1. Contexto .................................................................................................... 11 1.5.2. Metas y objetivos ...................................................................................... 12 1.5.3. Estructura de la tesis ................................................................................. 13 2. A REVIEW OF RE-USE AND VALORISATION OF BY-PRODUCT CO GAS 2 STREAMS FROM ANAEROBIC DIGESTION SITES .......................................... 19 2.1. MOTIVATION AND TARGETS FOR GHG REDUCTION ......................... 19 2.2. IDENTIFICATION AND QUANTIFICATION OF CO EMISSIONS FROM 2 THE WATER AND ORGANIC WASTE SECTORS..................................... 23 2.3. OPTIONS FOR IMPLEMENTATION OF CARBON CAPTURE AND STORAGE OR REVALORISATION STRATEGIES IN THE WATER AND ORGANIC WASTE SECTORS ...................................................................... 27 2.4. STATE OF THE ART OF BIOCONVERSION OF CO IN ADs AS AN ON- 2 SITE CARBON MANAGEMENT STRATEGY ............................................ 29 2.4.1. Previous evidence ..................................................................................... 29 2.4.2. Possible mechanisms of CO utilisation in the digestion process ............. 33 2 2.4.3. Other potential impacts of CO injection in ADs ..................................... 39 2 2.4.4. Potential implications of a full-scale application ...................................... 42 2.5. REQUIREMENT FOR FURTHER WORK .................................................... 43 2.6. CONCLUDING REMARKS ........................................................................... 44 2.7. REFERENCES ................................................................................................. 45 3. CARBON CAPTURE AND BIOGAS ENHANCEMENT BY CARBON DIOXIDE ENRICHMENT OF ANAEROBIC DIGESTERS TREATING SEWAGE SLUDGE OR FOOD WASTE ................................................................................................... 53 3.1. INTRODUCTION............................................................................................ 53 3.2. MATERIALS AND METHODS ..................................................................... 55 3.2.1. Description of the anaerobic digester equipment ...................................... 55 3.2.2. Absorption tests methodology .................................................................. 55 3.2.3. Methodology for enriching the digesters with CO .................................. 57 2 3.2.4. Analytical methods ................................................................................... 58 3.3. RESULTS AND DISCUSSION ...................................................................... 59 3.3.1. Estimation of gas-liquid contact time to achieve CO equilibrium during 2 enrichment ................................................................................................. 59 3.3.2. Assessment of digestion performance: renewable energy enhancement and digestate quality ........................................................................................ 60 3.3.3. CO utilisation in the batch digesters ........................................................ 64 2 3.4. CONCLUSIONS .............................................................................................. 66 3.5. REFERENCES ................................................................................................. 66 4. ENHANCING THE ANAEROBIC DIGESTION PROCESS THROUGH CARBON DIOXIDE ENRICHMENT: MECHANISMS OF UTILISATION .......................... 73 4.1. INTRODUCTION............................................................................................ 73 4.2. MATERIALS AND METHODS ..................................................................... 75 4.2.1. Description and operation of anaerobic digesters ..................................... 75 4.2.2. Ammonium or calcium carbonated precipitates detection ........................ 76 4.2.3. Fluorescence in situ hybridisation (FISH) ................................................ 76 4.2.4. Analytical methods ................................................................................... 79 4.3. RESULTS AND DISCUSSION ...................................................................... 79 4.3.1. Biogas production following CO enrichment and digestate 2 characterisation ......................................................................................... 79 4.3.2. Potential formation of ammonium and calcium carbonated precipitates when enriching ADs with CO .................................................................. 82 2 4.3.3. Microbial populations diversity by FISH analysis .................................... 85 4.4. CONCLUSIONS .............................................................................................. 92 4.5. REFERENCES ................................................................................................. 93 5. BIOLOGICAL CARBON DIOXIDE UTILISATION IN FOOD WASTE ANAEROBIC DIGESTERS ..................................................................................... 99 5.1. INTRODUCTION.......................................................................................... 100 5.2. MATERIALS AND METHODS ................................................................... 101 5.2.1. Description and operation of the AD rig retrofitted with CO 2 enrichment..... .......................................................................................... 101 5.2.2. Analytical methods ................................................................................. 103 5.3. RESULTS ...................................................................................................... 104 5.3.1. Substrate characterisation ....................................................................... 104 5.3.2. Assessment of digestion performance: digestate quality and renewable energy enhancement ................................................................................ 106 5.3.3. Impact of CO enrichment on dissolved CO and digestate‘s ammonia 2 2 concentration ........................................................................................... 111 5.4. DISCUSSION ................................................................................................ 113 5.4.1. Suitability of injecting CO into ADs with an external bubble column .. 113 2 5.4.2. Impact of CO injection in AD performance and mechanisms of utilisation 2 based on VFA and H dynamics ............................................................. 114 2 5.5. CONCLUSIONS ............................................................................................ 118 5.6. REFERENCES ............................................................................................... 119 6. GAS TO LIQUID MASS TRANSFER IN VISCOUS FLUIDS ............................ 125 6.1. INTRODUCTION.......................................................................................... 125 6.2. MATERIALS AND METHODS ................................................................... 128 6.2.1. Fluids selection and solutions preparation for the absorption tests ........ 128 6.2.2. Absorption experimental rig description ................................................. 128 6.2.3. Considerations for the calculation of volumetric mass transfer coefficients ...............................................................................................................130 6.3. RESULTS AND DISCUSSION .................................................................... 132 6.3.1. Impact of viscosity and superficial gas velocity on mass transfer .......... 132 6.3.2. Impact of viscosity and superficial gas velocity on hydrodynamics ...... 137 6.3.3. Mechanistic understanding ..................................................................... 141 6.3.4. Practical implications for gas to liquid mass transfer in sludge .............. 143 6.4. CONCLUSIONS ............................................................................................ 145 6.5. REFERENCES ............................................................................................... 145 7. CARBON DIOXIDE BIOCONVERSION IN ANAEROBIC DIGESTERS AS AN ON-SITE CARBON REVALORISATION STRATEGY: CONSIDERATIONS AND REQUIREMENTS FOR IMPLEMENTATION ........................................... 151 7.1. DRIVERS FOR GREENHOUSE GAS MITIGATION AND POTENTIAL BENEFITS OF CARBON DIOXIDE BIOCONVERSION IN ANAEROBIC DIGESTERS .................................................................................................. 151 7.2. IMPLEMENTATION FEASIBILITY ........................................................... 153 7.2.1. Systems where CO bioconversion could be implemented .................... 153 2 7.2.2. Possible gas streams to be used for CO bioconversion in ADs ............. 154 2 7.2.3. Possible points of CO injection in an AD .............................................. 159 2 7.2.4. Means of injection ................................................................................... 167 7.3. CONCLUSIONS: RECOMMENDED ALTERNATIVE AND POTENTIAL BENEFITS ..................................................................................................... 168 7.4. REFERENCES ............................................................................................... 168 8. THESIS DISCUSSION ........................................................................................... 175 8.1. POTENTIAL OF BIOCONVERSION OF CO IN ADs TO CONTRIBUTE 2 TO GHG MITIGATION ................................................................................ 176 8.2. MECHANISMS OF UTILISATION OF EXOGENOUS CO IN ADs ........ 177 2