L10 Flashcards
Anaerobic digestion
- series of processes in which microorganisms break down biodegradable materials in the absence of oxygen
- Different bacteria to process different materials
- Major end product is biogas (methane)
Bacteria in anaerobic digestion
Group 1: Fermentative bacteria
Group 2: Hydrogen producing acetogenic bacteria
Group 3: Homoacetogenic bacteria
Group 4 and 5: methanogenic bacteria
Four stages in anaerobic digestion
Hydrolysis: Reaction where particulates are solubilized and large polymers converted into simple monomers
Acidogenesis: Bio reaction where simple monomers are converted into volatile fatty acids
Acetogenesis: Bio reaction where VFA converted into acetic acid, CO2 and H2
Methanogenesis: Bio reaction where acetates are converted to CH4 and CO2 whilst H2 is consumed
Environmental conditions for anaerobic digestion
Optimal temperatures
- 35 - 40 Celsius for mesophiles (process takes along time)
- 55 Celsius for thermophiles
- Optimal pH = 6.8 - 7.4
- Nutrients and trace metals (iron, nickel) are essential for cell growth
- Toxic substances such as N, heavy metals etc bad
Important factors in anaerobic digestion
Volumetric organic loading rate
- COD
- High COD enhances digestion rate
Mass yield
- measure of cell growth in a system for a given substrate
Specific biological activity
- ability of biomass to utilize substrate
Hydraulic retention time
- time where waste remains in reactor
Solid retention time
- time the waste stays in the reactor and maintains a given degree of stabilised activation
A high SRT/HRT ratio is desirable to washout of slow-growing methanogenic bacteria
Types of anaerobic processes
Based on operation temperature
- mesophilic
- thermophilic (faster but higher capital cost)
Based on feeding wastes
- Wet: Feedstock 5-15% dry solid matter. Water included to allow pumping into digestor
- Dry: Feedstock 20% or higher dry solid matter. More biogas produced per unit volume
Based on operation mode
- Batch: Dry process with multi digestors
- Continuous: better controlled, high efficiency
Sequential batch reactor and operation
Advantages
- Simple operation
- Efficient control of the effluent
- Flexibility of use
- Low in process input and theoretical requirements
- High bio-gas yield
Disadvantages
- Poor self- immobilization
- Biological gas in the sludge may cause insufficient settlement
- Channeling and blocking-up are limitations
Feedstocks for anaerobic digestion
Lignocellulosic wastes
- Crop residues
- Wood logging residues
- Agricultural residues
Organic fraction of municipal solid waste
Sewage sludge
- Composed mainly of microbial flocs
Organic waste from food industry, especially from meat factory
Manure
Targets of pre-treatment to enhance anaerobic digestion
- Reduce particle size
- increasing solubility of feedstock
- Enhancing biodegradability of feedstock
Method of pre-treatment to enhance biodegradability
Thermal
- Heating
- Steam explosion
Mechanical
- Grinding to fine particles
- High pressure treatment
- Ultrasonic heating and vibration
Chemical
- Adding acids of alkalis for hydrolysis
- Adding oxygen for wet oxidation under high pressure
Pre-treatment of lignocellulosic wastes
- Cellulose, hemicellulose and lignin have low bio-degradability.
- Thermal treatment with addition of acid or alkali at 150 - 180 Celsius separates the three components, improving biodegradability
- Addition of enzyme will lead to higher substrate degradation
Pre-treatment of municipal sludge
Biological and thermal phased anaerobic pretreatment
- Intensifying hydrolysis process before the main digestion process
Thermal hydrolysis
- To achieve particle sludge dissolving
- 160 - 180 Celsius for 30 - 60 mins
Ultrasound
- Aims to reduce sizes of flocs or call, releasing extracellular or intracellular organic matters
Chemical pretreatment with oxidation, alkali or co-treatment
