1、,THE SCIENCE BEHIND ANAEROBIC DIGESTION,Dr Joseph C. Akunna Reader in Water & Environmental Engineering +44 1382 308141 j.akunnaabertay.ac.uk Dundee, 4th Sept 2013,Biofuels Series: Anaerobic Digestion Industry Engagement Event,CONTENTS,Introduction Factors affecting process efficiency Process monito
2、ring and control Technological options: Examples Conclusions,INTRODUCTION,Process fundamentals,Typical biogas composition: 50-70 %CH4; 30-50%CO2,Main stages - simplified,SOLUBLE ORGANIC MATTER,Organic Acids, CO2, H2,Biogas: CH4 + CO2,Hydrolysis,PARTICULATE ORGANIC MATTER,Acid production Optimum pH:
3、6.0,Methane production Optimum pH: 7.0,Biogas yield of common feedstocks,Manure: Biogas yield = 200-500 m3/t (dry solids) Cow: 200-400 m3/t VS; Pig and Chicken: 350-550 m3/t VS* Wastes: Biogas yield = 400-800 m3/t (dry solids) Household = 100-200 m3/t VS Green (vegetable/garden/parks): 400-500 m3/t
4、(dry solids) Sewage sludge 500 m3/t (dry solids) Farm residuals (e.g. Corn, grass, maize, barely, sorghum, wheat silage): Biogas yield = 500-900 m3/t (dry solids),*VS, Volatile Solids, is a measure of the organic content,FACTORS AFFECTING PROCESS EFFICIENCY,Process conditions,Mixed culture of anaero
5、bic micro-organisms Where possible, use active seed inoculum from operational anaerobic digesters Anaerobic conditions Ensure absence of oxygen/air, nitrogen oxides (i.e., nitrates and nitrites)Moisture contentWet Digestion: less than 16% dry solids; Dry Digestion: 22-40% dry solids Temperature Psyc
6、hrophilic: 5 - 20C Mesophilic: 20 - 45C (optimum: 35C) - most common Thermophilic: 45 - 70C (optimum: 55C),Effect of temperature on anaerobic digestion process (Mata-Alvarez 2003),Process conditions (Contd),Hydraulic Retention Time (HRT) Reaction Time Values depend on feedstock characteristics, proc
7、ess technology, temperature, Solids Retention time (SRT) Time microorganisms remain in a digester Organic loading rate (OLR) Amount of organic matter fed per m3 of digester Values depend on feedstock characteristics, temperature, Consistent operating conditions e.g. HRT, SRT, OLR, TC, ,Process condi
8、tions (contd),Nutrient requirementsMinimum C/N/P ratio: 100/5/1 Trace elements: nickel, cobalt, etc pH: Optimum between 6.5 and 7.5 Alkalinity: High level reduce excessive pH fluctuations Toxic compounds (NH3, H2S, O2, NO2/NO3, antibiotics, disinfectants, etc) Mixing, prevention of pressure build-up
9、, increase in microorganism-substrate contact, etc Feedstock preparation (removal of inorganics, plastics, solid size reduction, pH/nutrient correction, etc),Process conditions (contd),Nature of substrate (type, solid content and size, etc),rate of degradation,retention time,sugars (mono/disaccharid
10、es),starch,proteins,hemicellulose,lignin,waxes,Effect of the nature of organic matter on reaction rates (Eder and Schulz 2006),rate of degradation,retention time,sugars (mono/disaccharides),starch,proteins,hemicellulose,lignin,waxes,Effect of the nature of organic matter on reaction rates (Eder and
11、Schulz 2006),Common methods for enhancing process efficiency,Increasing hydrolysis rates by pre-treatment:Enzymatic treatmentUltra-sound treatmentAcid treatmentAlkaline treatmentHeat treatment, etcCombination of some of the above Phase-separation Multi-stage systems,Digestion phases,PROCESS MONITORI
12、NG & CONTROL,Key parameters,Influent/Feedstock characteristics VS, COD, N, P, pH, sulphates, metals HRT, OLR Temperature Biogas Quantity and Composition (CH4, CO, CO2, H2, H2S) Treated effluent quality Volatile fatty acids (total and constituents) COD, ammonia, total N, pH, VS,Summary of major chall
13、enges of AD process,Delicate process; requires skilled manpowerDifficult to resurrect when process fails Long start-up timesPost aerobic polishing is usually necessary before discharge of treated effluent or digestate into the water or soil environment,TECHNOLOGICAL OPTIONS: EXAMPLES,Options for liq
14、uid organic residues/slurries,Chemical-assistedsolids reduction+ pH and nutrientcorrection,Anaerobic Digester,pH and nutrient correction,Anaerobic Digester,pH and nutrientcorrection,Anaerobic Digester 2,Anaerobic Digester 1,Hydrolytic treatment + pH and nutrientcorrection,Anaerobic Digester,Aerobic
15、polishing,Aerobic polishing,Aerobic polishing,Aerobic polishing,1,2,3,4,Solids for further treatment and/or disposal,Options for solid/semi-solid organic residues,Compost,CONCLUSIONS,1. MOST ANAEROBIC DIGESTIONS SYSTEMS CAN WORK PROPERLY IF WELL OPERATED,2. PROCESS MONITORING AND CONTROL ARE SOME OF THE MOST IMPORTANT COMPONENTS OF SUCCESSFUL DIGESTION APPLICATION3. EARLY DETECTION IS KEY TO EFFECTIVE RESOLUTION OF PROCESS RELATED PROBLEMS,Thank you for listening,Any Questions?,
