Anaerobic digestion (AD) transform organic compounds (biomass wastes) to methane biogas by microbes.

 Large amount of water  Large amount of degradable organic substances such as starch and cellulose  High content of nitrogen (N) and phosphorus (P) or nutrients

Potential feedstocks Municipal sludge Animal manures Organic fraction of municipal solid wastes (OFMSW) Food and food- processing waste Agricultural residues and energy crop

 Includes primary and waste activated sludge derived from centralized waste water treatment plants  High water content, little readily fermentable substrate but plenty of nutrients (nitrogen and phosphorus), high density of bacterial cells  6.2 million dry tons of sludge = 6 million m 3 of methane biogas

 High water content  Have very little readily degradable substances (Carbohydrates and proteins) low biochemical methane potentials (BMP) & slow AD process  High concentration of nitrogen and large pH buffering capacity against acids  High concentration of ammonia cause toxicity to methanogens  Contains large amounts of microbial biomass (does not need external digested sludge as starter culture)

 High water content and volatile solid (VS) contents  Large amount of readily fermentable substrates  348 m3 of methane can be produced per dry ton of food wastes within 10 days of AD.  Most food processing waste are poor in nitrogen except meat processors  Food processing wastes have been codigested with nitrogen- rich feedstocks (municipal sludge & animal manures) to enhance system stability and methane production

 Includes paper, grass mowings, food scraps  Little moisture and readily fermentable substrates  Deficient in nitrogen and phosphorus but large methane potential if digested adequately  Pretreatment such as grinding is required  Codigestion with other nutrient-rich biomass

 Low moisture content, high VS content and variable content of readily fermentable substrates  Nonleguminous crops have little nitrogen  Codigestion with animal manures or municiple sludge  Biomass rich in starch/proteins is easier to digest than cellulosic biomass  Reduced particle size of insoluble feedstock can enhance AD  N and P are important as nutrients for microbes to grow

Anaerobic Digestion

 Biotechnologies - biomass is converted by microbe to produce methane (CH 4 ) biogas.  Anaerobic process – through a series of complex microbiological process in the absence of oxygen.  Process is applied universally in hot anaerobic digester – using heterotrophic bacteria.  Advantage – can generates energy in the form of methane by produce small amount of sludge (10%).

 Eq 1: By Acid forming bacterias  Organic matter  Acid intermediate+ CO 2 + H 2 S + H 2 0  Eq: 2 By Methane forming bacterias  Organic acids  CH 4 + CO 2

 The anaerobic treatment is characterized by the production of “biogas” consisting mainly of methane (60-80%) & carbon dioxide (40-20%).  Used as fuel for generating thermal energy / electric.  Used only small amount of COD (5-10%) to form new bacteria.  Used complex process involving several groups of bacteria – strictly anaerobic & facultative.  Every stage of process is used different group of bacteria.

1) Hydrolysis 2) Acidogenesis 3) Acetogenesis 4) Methanologies

 Bacteria hydrolyze the biomass polymers (feedstock) to monomers/oligomers.  It catalyzed by the extracellular hydrolytic enzymes that secreted by the hydrolytic bacteria. Acidogenesis  Resulting hydrolytic products are immediately fermented to short chain fatty acid (SCFA), CO 2 & H 2.

 Product of acidogenesis are converted into the final precursors for methane generation; acetate, hydrogen & carbon dioxide. Methanogenesis  Methane is produced from:- i. Acetate - acetotrophic ii. Reduction of carbon dioxide by hydrogen – hydrogentrophic

Mixed Plug Flow Loop Reactor (MPFLR)

 Consists of an U shaped tank reactor.  Influent enters the reactor at one end and flows forward and loops back, exiting at the other end. Influent Effluent Settling compartment

 Digester content is mixed by gas or water jets in direction perpendicular to the plug flow of the reactor.  Solids separated from effluent can be recycled if increased microbial biomass is needed.  Biogas

 MPFLR has been built at several dairy farms in US.  Examples: √ Herrema Dairy located at Fair Oaks, Indiana operates a MPFLR that digests more than 400 m 3 of manure slurry (8% solids generated daily by 3800 cattle). √ The biogas produced fuels two Hess engine-generators (375 kWh)

√ Separated solids are being dried and subsequently used as bedding in the barns. √ Heat from the engine-generators is recovered and used to heat the digester, barns and alleyways.

H 2 S, and moisture 40% CO 2 60% CH 4 trace amounts of ammonia Methane biogas

 Solid Particles( dust etc.) filtered –dust collectors  Sludge & foam- Cyclones  Traces of gas- scrubbing, adsorption, absorption & drying  Most concerns H 2 S hydrogen sulfide and CO 2 Carbon dioxide

 Corrosive  Limit for CHP combined heat and power generator mg/Nm 3 or 0.05% by volume  Target 20mg/Nm3  Difficult to achieve thus needs combination of biological and physichochemical procedures.  Biological desulfurization-Thiobacillus and Sulfolobus  Immobilisaion-Needs air (aerobic) and surface (contact)- air 4-6% of biogas to prevent explosion & 1m 2 for 20M 3 d -1 at 20 o C  Trickling filters can remove up to 99% <75md/Nm3  Bioscrubber

 Physicochemical removal  Sulfide precipitation- FeCl 2, FeCl 3 use  Absorption- solutions of Fe 3+ reduced to Fe 2+  2Fe 3+ + H 2 S -  2Fe 2+ +S (precipitate)+ 2H +  The Cat. Fe is oxidized back to Fe 3+ by oxygen and water  Adsorption FeO 3 +3H 2 S  Fe 2 S 3 + 3H 2 O  Cat. is regenerated by air Fe 2 S 3 +3O 2 +6H 2 O  4Fe(OH) 3 + 6S  Activated charcoal adsorption  Binding to Zn  Surfactants foam

 Absorption  Diaphragm-membranes  Mineralization

 Oxygen removal- use desulfurization procedure  Water  Ammonia-acidic solution  Siloxanes-activated charcoal

 Natural gas -energy value of 5.8–7.8 kWhm 3 whereas  the calorific value of typical methane biogas (60% CH 4 and 40% CO 2 ) ranges from 5.5 to 6.5 kWhm 3.  biogas -as a substitute for natural gas.  Biogas produced from nearly all large-scale AD reactors - used to power CHP systems to generate heat and electricity.  heat and electricity generated - used to operate AD reactors and associated facilities with excess heat and electricity perhaps being provided to nearby communities or utility companies.  CHP systems -very efficient in utilizing methane biogas.

 Direct electricity generation from biogas using fuel cells - appealing alternative.  conventional fuel cells - based on precious metal catalysts can only use pure H 2 or H 2 -rich gas as fuel  Example: biogas with 60% methane can be processed to H 2 And CO in a steam reformer, -such that the resultantH 2- rich gas (H 2 >50% of the total gases) is sufficient for efficient and stable operation of a polymer electrolyte membrane (PEM) fuel cell stack.

 the solid oxide fuel cell (SOFC) – that uses solid oxide catalysts can use biogas directly without prior reformation  The SOFC uses a hard, ceramic compound of metal (e.g., calcium or zirconium) oxides as an electrolyte, and operates at temperatures ranging from 900 to 1000 o C (Singhal and Kendall, 2003).  high temperatures - a reformer is not required to extract H 2 from biogas, and the waste heat can be recycled to produce additional electricity. Pilot-scale SOFCs have achieved an efficiency of approximately 60%.  Because SOFCs operate at high temperatures, they have the greatest fuel flexibility and can use biogas without prior cleansing.  In theory, the ammonia and H 2 S in biogas can be used by the SOFC as fuels

Vertex, A., Qureshi, N., Blaschek, H. P., Yukawa, H. (2010). Biomass to Biomass to Biofuels: Strategies for Global Industries. References