Anaerobic Digestion

Anaerobic digestion

Ngoc Thuan Le ** 김 투안 **

Contents

What is Anaerobic Digestion? Feedstock for AD ? Anaerobic Digestion Metabolic Reactions Parameters in Anaerobic Digestion Types of AD Systems Advantages/Disadvantages of Digestion

What is Anaerobic Digestion? di·ges·tion (Merriam-Webster on-line) Function: noun The process in sewage treatment by which organic matter in sludge is decomposed by anaerobic bacteria with the release of a burnable mixture of gases

Historical Background The Anaerobic Digestion (AD) process is one of the oldest technologies. Biogas was used for heating bath water in Assyria during the 10th century BC and in Persia during the 16th century (www.biogasworks.com) The industrialization of AD began in 1859 with the first digestion plant in Bombay, India 1895- England - Developed a septic tank for Exeter to utilize gas for lighting 1904- England - First dual-purpose tank (sedimentation and sludge treatment) installed in Hampton 1930's- Anaerobic digestion of manure/ agriculture waste for methane gas In recent times, developing AD market because of two significant reasons: High energy prices and stringent environmental regulations,

Feedstock for Anaerobic Digester ? Agricultural Waste Municipal (including Household) Green Waste Food Industry Waste Tourism and Leisure Catering waste Other animal or vegetation wastes and residues Municipal Waste-water sludge Purpose grown vegetation crops Manure Storage Pond

Carrot Processing Wastewater Storage Pond

More Details on the Digestion Process Odor Volatile Solids (VS)

Acid forming bacteria

GAS utilisation

Volatile organic acids Methane forming bacteria

CH4 (55-60%), CO2(35-40%), H2O, trace gases

Anaerobic Digestion Metabolic Reactions insoluble organics Days

1. Hydrolysis Soluble organics

Minutes to days

2. Acidogenesis

Minutes to hours

3. Acetogenesis

Seconds to minutes

4. Methanogenesis

Hydrolysis Degradation of complex substrates such as:  Celluloses  Starch & other sugar polymers  Simple sugars (lactose, sucrose)  Fats, oils

+ H 2O

Monomers + 2H2 (C6H12O6)

Acidogenesis

Converts soluble monomers into simple organic compounds, mostly short chain (volatile) acids (e.g., propionic, formic, lactic, butyric, or succinic acids), ketones (e.g., ethanol, methanol, glycerol, acetone) and alcohols.

C6H12O6 ↔2 CH3CH2OH + 2CO2 C6H12O6 + 2H2 ↔ 2CH3CH2COOH + 2 H2O

Acetagenesis Converts the result of acidogenesis into acetic acid, carbon dioxide, and hydrogen  CH3CH2COO- + 3H2O ↔ CH3COO- + H+ + HCO3- + 3H2  C6H12O6 + 2H2O ↔ 2CH3COOH + 2 CO2 + 4H2  CH3CH2OH + 2H2O ↔ CH3COO- + 2H2 +H+  2HCO3- + 4H2+ H+ ↔ CH3COO- + 4H2O Acetogenic bacteria:  syntrophobacter wolinii, propionate decomposer  sytrophomonos wolfei, a butyrate decomposer.  Other acid formers are clostridium spp., peptococcus anerobus, lactobacillus, and actinomyces  …..

Methanogenesis 2 CH3CH3OH+ CO2 ↔ 2 CH3 COOH + CH4 CH3COOH ↔ CH4 + CO2 CH3OH + H2 ↔CH4 + H2O CO2 + 4H2 ↔CH4 + 2H2O CH3COO- + SO4 2- + H+ CH3COO- + NO- + H2O + H+

2HCO3 + H2S 2HCO3- + NH4+

Methanogenic bacteria: methanobacterium, methanobacillus, methanococcus and methanosarcina.

Parameters in Anaerobic Digestion Acceptable pH for the bacteria: 5.5 -8.5. Most methanogens function in a pH range between 6.7 and 7.4, and optimally between 7.0 and 7.2. Temperature: Anaerobic digestion can occur in two main of temperature ranges: – Mesophilic condition, between: 20-45oC, usually: 35oC – Thermophilic condition, between: 50-65oC, usually: 55oC C/N Ratio: the optimum C/N ratio is between 20-30 This value can be achieved by mixing waste of low and high C/N ratio, such as organic solid waste mixed with sewage or animal manure Retention Time: – Mesophilic condition: 15 – 30 days – thermophilic condition: 12-14 days

Types of AD Systems

• Single Stage Single stage reactors make use of one reactor for both acidogenic phase as well as methanogenic phase.

Types of AD Systems…

Multi Stage Two reactors are used: 1st one for hydrolysis/liquefactionacetogenesis 2nd one for methanogenesis.

Multi-stage AD processes was intended to improve digestion by having separate reactors for the different stages of AD, thus providing flexibility to optimize each of these reactions.

Types of AD Systems… Batch Reactors Batch reactors are loaded with feedstock, subjected to reaction, and then are discharged and loaded with a new batch.

A. The single-stage batch system involves re-circulating the leachate to the top of the same reactor. B. The sequential batch process: the leachate from the first reactor, containing a high level of organic acids, is re-circulated to the second reactor where methanogenesis occurs. C. The hybrid batch-UASB process, which is very similar to the multi-stage process with two reactors. The first reactor is simple batch reactor but the second methanogenic reactor is an upflow anaerobic sludge blanket (UASB) reactor.

Advantages of Digestion

Less farm odor Better fertilizer Source of revenue Heat source Pathogen reduction Weed seeds killed Pollution control Other

Disadvantages of Digestion Is somewhat costly Higher management levels required Startup is sometimes difficult Storage required –Cannot store methane as a liquid!! Some risk of explosion

Thank you for your attention!