The Anaerobic Digestion Process Andrew Gabriel and Tidasate Success

 What exactly is biogas (composition)?  What is the biological process by which it is created?  What type of substance is used in this process?  What are the three stages of the biogas production process?

 Biogas is a gas composed of methane and carbon dioxide  It is desired for its high methane content for use as a fuel

 The commercial recovery of biogas hasn’t been a main concern in industry until the mid 1970’s the fuel’s potential was realized. Today, farm based anaerobic digestion facilities are the most common in biogas production industry. Most of these facilities use the biogas as an energy source for their own usage.

 The process by which biogas is produced  Uses carbonaceous organic matter  In the absence of oxygen  C 6 H 12 O 6 → 3CO 2 + 3CH 4.  (Organic Material)(Carbon Dioxide)(Methane)

 sewage sludge  solids refuse  food waste  biomass including plants  Crops  plant and crop wastes  industrial liquids and solids  anything else organic

The three main stages are:  Hydrolysis/fermentation  Acid phase digestion  Acetogenesis  Methane phase digestion

Legend 1. Feeding header 2. Gas header 3. Gas collecting system Methanisation stage Hydrolyse stage Methanisation stage Feeding piping Evacuated into the substrate storage

 Bioreactor type: fixed dome and floating drum ◦ Other types include balloon, horizontal, earth-pit ferrocement plant

 Parts of the plant: ◦ Mixing pit ◦ Digester ◦ Strirring system ◦ Gas holder ◦ Slurry storage ◦ Heating system ◦ Pumps ◦ Rings

 Solids content ◦ High or low ◦ Wet or dry  Temperature: ◦ Psychrophilic ◦ Mesophilic ◦ Thermophilic

 C:N ratio  15:1-25:1  Excess Carbon ◦ organic acids: lower pH  Excess Nitrogen ◦ Ammonia: higher pH

 Acidity:  Carbon Nitrogen ratio ◦ 30:1 ◦ Excess Carbon  organic acids: lower pH ◦ Excess Nitrogen  Ammonia: higher pH

 Daily ◦ Gas pressure, substrate temperature, gas production, strong sludge odor  Weekly ◦ Clean gas appliances, lubricate movable parts, control gas appliances  Monthly ◦ Maintenance of slurry agitator  Annually ◦ Check for corrosion

 Main producers of biogas come from:  Farm based facilities  Waste water treatment plants  Sewage treatment plants

 In 1977 Pacific Gas and Electric Co. and Southern California Gas Co. contracted Bio Gas of Colorado  Project used waste from 30,000 head of cattle  Two 21,300 gallon digesters  1.5 million standard cubic feet per day of biogas (60% methane composition)  average-year gas cost was $3.44/million Btu  Average retail cost that year was $10.99

 contribute to a number of health hazards to humans.  These emissions are regulated by The United States Clean Air Act and Title 40 Part 59 of the Code of Federal Regulations (CFR).

 For employers to ensure employee safety dealing with flammable or explosive materials: ◦ Dangerous substances ◦ Control measures to remove risks. ◦ Controls to reduce the effects of any incidents. ◦ Plans and procedures to deal with accidents, incidents and emergencies. ◦ Employees are trained to deal with the risks. ◦ Classify areas where explosive atmospheres may occur and avoid ignition sources in those areas.

 Number of Reactors  Single Stage ◦ Entire process in one step  the two main phases can conflict with each other  Multi-Stage ◦ hydrolysis and acid phase steps are combined, and the methane phase digestion proceeds separately.  Better pH regulation

 The size of the digester ◦ Vd = Sd x RT [cubic meter/days x number of days]  Daily gas production ◦ Based on volatile solids content G = VS x Gy ◦ Weight of the moist mass B: G = B x Gy ◦ STD gas yield values per livestock unit  Digester loading ◦ Daily total solids input  The size of the gas holder ◦ depends on the relative rates of gas generation/consumptions

 Biogas is made up of methane and carbon dioxide  Biogas is produced through the process of anaerobic digestion  The substance used in this process is carbonaceous organic matter The three stages of biogas production are ◦ Hydrolysis/fermentation ◦ Acid phase digestion ◦ Methane phase digestion

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