ANAEROBIC DIGESTION BASIC CONCEPTS

Transcription

1 ANAEROBIC DIGESTION BASIC CONCEPTS JOAN MATA-ALVAREZ and SANDRA MACE Dpt. Chemical Engineering UNIVERSITY of BARCELONA WAGENINGEN SUMMER SCHOOL AD BIOPROCESS CONCEPTS UNIVERSITAT DE BARCELONA

2 ANAEROBIC DIGESTION PROCESS ANAEROBIC DIGESTION IS A PROCESS WHICH, IN THE ABSENCE OF OXYGEN, DECOMPOSES ORGANIC MATTER. MAIN PRODUCT IS BIOGAS A MIXTURE OF APPROXIMATELY 65% METHANE AND 35% CARBON DIOXIDE, ALONG WITH A REDUCED AMOUNT OF A BACTERIAL BIOMASS. ORGANIC MATTER BIOGAS + MAT. CEL.

3 WAGENINGEN SUMMER SCHOOL AD BIOPROCESS CONCEPTS ANAEROBIC DIGESTION PROCESS APPLICATIONS: TREATMENT OF EFLUENTS WITH ORGANIC MATTER ALTERNATIVE (OR COMPLEMENTARY) PROCESS TO THE AEROBIC WITH THE ADVANTAGE OF: - POSITIVE ENERGY BALANCE - LESS BIOMASS GENERATION

4 BIOMETHANIZATION APPLICATIONS INDUSTRIAL LIQUIDS Food industry, Paper industry SOLIDS SEMI-SOLIDS Food industry Food markets URBAN Domestic wastewaters Sewage-Sludge Organic fraction MSW

5 WAGENINGEN SUMMER SCHOOL AD BIOPROCESS CONCEPTS PROCESS LIMITATIONS - IN SOME CASES ANAEROBIC DIGESTION WILL BE JUST A PRE-TREATMENT - IT CAN BE FOLLOWED BY AN AEROBIC TREATMENT (FOR INSTANCE, AMMONIUM-N REMAINS) - LOW BIOMASS YIELD MAKES START-UP PERIOD QUITE LONG

6 BIOGAS COMPOSITION - BIOGAS IS MAINLY FORMED BY CH 4 AND CO 2 - COMPOSITION DEPENDS ON SUBSTRATE. USUALLY OSCILLATES BETWEEN 60-65% IN CH 4 - THERE ARE STOICHIOMETRIC RELATIONS THAT ALLOW TO ESTIMATE BIOGAS COMPOSTION

7 BIOGAS COMPOSITION IF GREASE THE ORGANIC YIELDS HIGH COMPOUND PERCENTAGES HAS THE FORMULA OF CH 4 AND THE CONTRARY HAPPENS WITH OXALIC OR C x HFORMIC y O z N t ACID. RATIO CH 4 /CO 2 CAN APPROXIMATELY BE GIVEN BY: (4-T)/(4+T) T= (2z + 3t -y)/x

8 DIGESTER COD BALANCE BIOGAS COD 90 INLET COD 100 OUTLET COD 5 DIGESTOR BIOMASS COD 5

9 DIGESTER COD BALANCE BIOGAS COD 45 INLET COD 100 OUTLET COD 50 DIGESTOR BIOMASS COD 5

10 TWO- STEP REPRESENTATION ORGANIC MATTER Fermentation VOLATILE FATTY ACIDS Methanization BIOGAS

11 STOICHIOMETRY OF A.D. OF SEWAGE SLUDGE H Ac At M Proteins Carbohydrates Lipids 21% 39% 40% 5% 34% 66% 46% 35% 12% Acetate Aminoacids Suggars 20% 0% Intermediary products Propionates,butirates,... 23% 11% 8% Fatty acids Hydrogen 70% 30% 34% 11% Methane Gujer and Zehnder, 1983

12 Process Organic polymers Organic monomers Reduced organics - NO3 SO4 = + NH4 H2S Acetic acid Carbon dioxide H2 CH4 Microorganisms Hydrolysis of organic polymers Fermentation of organic monomers Fermentative bacteria Oxidation of reduced organics Acetogenic respiration of bicarbonate Oxidation of reduced organics by SRB and NRB Oxidation of acetate by SRB and NRB Oxidation of Hydrogen by SRB and NRB Aceticlastic methanogenesis Hydrogenotrophic methanogenesis Organic polymers : Carbohydrates, Lipides, Proteins Organic monomers : Sugars, organic acids, aminoacids Reduced organics : Volatile fatty acids (propionic, butiric, valeric) Obligate hydrogen producing bacteria (OHPA) Homoacetogen bacteria Sulfate-reducing (SRB), nitrate-reducing (NRB) bacteria SRB and NRB SRB and NRB Aceticlastic methanogenic bacteria Hydrogenotrophic methanogenic bacteria Pohland, 1992

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15 ENVIRONMENTAL FACTORS NUTRIENTS TOXIC SUBSTANCES TEMPERATURE OTHER (Alkalinity, ph)

16 NUTRIENTS LOW REQUIREMENTS COD / N / P = 600 / 7 / 1 Other micronutrients (Fe, Ni, Mg, Ca, Na, Ba, Tu, Mo, Se, Co, vitamins)

17 TOXIC SUBSTANCES AT LOW CONCENTRATIONS: BENEFIC EFFECTS, AT HIGH CONCENTRATIONS: INHIBITERS OR EVEN TOXIC THERE IS A POSSIBILITY OF ACCLIMATION. SOME DISCREPANCY IN LITERATURE VALUES (SYNERGIC AND ANTAGONIC EFFECTS

18 TOXIC SUBSTANCES Common substances (specially undissociated species) VFA ph SH 2 NH 3 Xenobiotic compounds

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20 TEMPERATURA Operation ranges PSICROPHILIC (< 25ºC) MESOPHILIC (around 35ºC) THERMOPHILIC (around 55ºC)

21 Psichrophilic Mesophilic Thermophilic Rate of the AD process Temperature

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23 OTHER PHSYSICO-CHEMICAL FACTORS ph: Depends on the bacterial group: 6-7,5 ALCALINITY: Is a funtcion of ph and dissolved CO 2. Values over 1.5 g/l at ph 6, are recommended

24 IMPORTANT PARAMETERS IN AD PROCESSES S (substrate concentration) kg substrate/m 3 reactor generally expressed in terms of: TS, TVS, COD X, (microorganisms concentration) kg active biomass/m 3 reactor, generally expressed in terms of TVS or in VSS (volatile suspended solids) although it can be also referred as COD

25 IMPORTANT PARAMETERS IN AD PROCESSES HRT (Hydraulic retention time) It is defined as the ratio of the reactor volume to the of the influent substrate flowrate. Thus, it is a measure of the time that the substrate spends inside the digester: HRT = V Q

26 IMPORTANT PARAMETERS IN AD PROCESSES SRT (Solids retention time). It is a measure of the sludge age and usually expressed in days. V X SRT = = Q. X X HRT e X e

27 IMPORTANT PARAMETERS OLR (Organic loading rate) IN AD PROCESSES It is expressed as kg substrate/m 3 reactor day, is the amount of substrate introduced into the digester volume in a given time: Q S OLR = = V S HRT

28 SOLR = S HRT X IMPORTANT PARAMETERS IN AD PROCESSES SOLR SOLR (Specific organic loading rate). It expresses the OLR per unit of active biomass, It Scan explain why attached biomass systems = HRT X support OLR much higher than suspended growth ones. SOLR = S HRT X

29 YIELD PARAMETERS IN AD PROCESSES Specific gas production (SGP), [m 3 biogas/kgsubstrate fed] Gas production rate (GPR), [m 3 biogas /m 3 reactorday] Substrate removal yield, [%].

30 YIELD PARAMETERS IN AD PROCESSES SGP (Specific gas production) It is the biogas produced per unit of substrate fed or in some cases per unit of substrate converted into biogas. Q biogas is the biogas flowrate, (m 3 /day). It can be set in terms of the total volatile solids in the feed, as m 3 biogas/kg VS fed. SGP = Q Q biogas S

31 YIELD PARAMETERS IN AD PROCESSES GPR (Gas production rate). It is the biogas produced per unit of reactor volume and time: GPR = Q biogas V GPR and SGP are related through the OLR GPR = SGP OLR

32 BIODEGRADABILITY AND DEGREE OF BIODEGRADATION Volatile solids (VS) can be converted to a maximum amount of biogas, provided optimal conditions are prevalent. This conversion can be measured through what has been called ultimate biogas yield (B 0 ). B 0 Initial VS Final non biodegradable VS (After an infinite time )

33 BIODEGRADABILITY AND DEGREE OF BIODEGRADATION In a CSTR: SGP (At a given HRT) Inlet VS Outlet VS (with a fraction of non biodegradable VS) f B = SGP Bo

34 BIODEGRADABILITY AND DEGREE OF BIODEGRADATION Assuming a First Order kinetic model: SGP ds dt = k S Inlet VS Outlet VS (with a fraction of non biodegradable VS) SGP = B 0 / (1 +1/(k HRT))

35 SGP (m3 biogas/kg VS) k=3 k=1 k=0.5 k=0.3 k=0.1 B 0 = 300 m 3 biogas/kg VS HRT (days)

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37 DIGESTER TYPES FLOW-THROUGH DIGESTER (CSTR) (Both soluble and solid wastes) DIGESTERS WITH BIOMASS RETENTION (Soluble wastes/two-phase systems) PLUG-FLOW DIGESTERS (Differents approaches. Solid wastes)

38 CONTINUOUS STIRRED TANK REACTOR Different heating systems Different stirring devices Void volume equipped with heating and stirring systems Cheap and very extended

39 CONTINUOUS STIRRED TANK REACTOR For slurried wastes ex.: piggery, SS, OFMSW * SRT = HRT * HRT d

40 CONTINUOUS STIRRED TANK REACTOR Possible occurrence of shortcircuiting It can impair the proper hygienization of the wastes

41 Digesters with large active biomass concentration Digesters based on keeping suspended biomass inside the reactor Contact Digester UASB Attached growth systems Anaerobic Filter Expanded/Fluidised Bed

42 CONTACT DIGESTER High biomass concentrations (up to 25,000 g/l Problems with settleability Degasifier

43 UASB DIGESTER BIOGAS OUTLET INLET * First in 1976 in a sugar factory located in Breda, in a pilot plant of 6 m 3 (Prof. Lettinga and col.). * It is based on the development of a highly settleable flocs called granules * Diameter up to 5 mm. High density -> in the bottom of the reactor.

44 UASB DIGESTER BIOGAS OUTLET INLET * Settling velocities of 60 m/h * Superficial upflow velocities below 2 m/h. * SRT over 200 days HRT of 6 hours * Biomass concentration up to 30g/L-> High OLR

45 UASB DIGESTER BIOGAS OUTLET INLET * Low loss of solids, * No mechanical mixing is necessary (biogas generation enough to ensure mixing) * Good inlet distribution necessary * New developments

46 ANAEROBIC FILTER (Immobilised biomass) INLET BIOGAS OUTLET * For liquid wastes * Up or Downflow * HRT << SRT * High OLR (20 kg/m 3.d * HRT between 0.5 and 4 days

47 EXPANDED / FLUIDISED BED BIOGAS OUTLET * For liquid wastes * High recirculation ratio * HRT<<SRT INLET * HRT between 0.5 and 4 days

48 EXPANDED / FLUIDISED BED INLET BIOGAS OUTLET Sand and, especially, granular activated carbon (GAC) are the most popular. Usually, particle sizes are between 0.2 and 1 mm.

49 EXPANDED / FLUIDISED BED INLET BIOGAS OUTLET Free movement of the particles, which prevents the digester to clog Drawbacks: Complexity of the system design Cost of the energy required

50 COMBINATION OF TWO DIGESTERS OPTIMISES BIOLOGICAL REACTIONS TWO-PHASE DIGESTERS LARGER OVERALL REACTION RATE AND IN PRACTICE BIOGAS YIELD NOT ALWAYS

51 INCREASED TECHNICAL COMPLEXITY TWO-PHASE DIGESTERS LARGER OVERALL REACTION RATE AND BIOGAS YIELD OPTIMISES BIOLOGICAL REACTIONS The main advantage of two-stage systems is not its putative higher reaction rate, but rather a greater biological reliability for wastes which cause unstable performance in one-phase systems

52 Pre-treated waste OFMSW ZATION Biogas STAGE 1 (hydrolysis) Solid PASTEURI- DEWATE- RING Liquid recycle Liquid Waste and process water COMPOSTING of the solid fraction STAGE 2 ( methanization ) Anaerobic Filter

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54 Capital Cost of Manure Management Technology Source: AgStar Publication: US EPA

55 Digester economics Investment: it has been estimated around $660 to $1,200 per head. Good estimate $1,000 per cow At this capital assumption AD system become economical based on sold energy alone at power sale price of $0.09/kWh. (Mehta, 2002)

56 Plug Flow Digesters Little mixing Few mechanical parts Simple

57 Mixed Digesters Mechanical mixing in tank More gas production Slightly higher capital cost

58 DIGESTERS FOR SOLID WASTE They are also three-phase reactors. An heterogeneous approach is needed to adequately model the system Substrate solubilization, difussion to the cells (several microorganisms involved), gas difussion : It is a complex model.

59 REQUIREMENTS Good mass transfer (contact microorganisms-substrate) Good heat transfer (specially in thermophilic systems) Good yields (depends very much in substrate) Absence of mechanical problems (experience with different types of substrates)

60 TECHNOLOGIES FOR AD OF SOLID WASTES Classical classification Dry (TS contents > 15%) Dranco, Valorga, Linde BRV, Kompogas Wet (TS contents < 15%) Linde KCA, Waasa, RosRoca-Envital, )

61 DIAGRAM OF A WET AD PLANT TRANSPORT Engine Bag of organics Heat BIOGAS URBAN COLLECTION CONDITIONING Electricity BIOMETHANIZATION REACTOR LIQUIDS WASTEWATER (Nutrient flow) Filtration SOLIDS COMPOST

62 TECHNOLOGY DIFFERENCES (Many elements in a plant) Continuos vs. Batch Conditioning Pretreatment Stirring system Recirculation system Heating system Basically: Wet: Perfect mixing Dry: Plug-flow

63 DRY DIGESTION Linde Valladolid Valorga Coruña

64 WET DIGESTION Ros-Roca Envital Boden Waasa Groningen

65 Mainstream technology Energy balance Nutrients FINAL REMARKS ON AD TECHNOLOGY Greenhouse effect Integration AD AD AD AD is minimises presents has a conveys mainstream does its own not the a nutrients place positive increase production technology biological energy to gases of the waste with liquid with treatments balance, sludge great greenhouse stream with and potential, has to be integrated in the overall organic waste the which effect production can specially be an of advantage biogas the solid waste treatment field systems Minimum sludge