Energy and Environment 5

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Outline anaerobic digestion
Anaerobic degradation of biodegradeable materials releases CH4 which can be utilised as energy, is landfill/sewage sludge treatment process. Can treat food waste/livestock manure. Stabilised residual material used as fertiliser
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How much sewage production is there per year and how much is treated anaerobically?
UK total sewage production is 1.4 million tonnes of dissolved solids (DS) annually. 70% sewage treated by anaerobic digestion
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What are benefits of sewage sludge treatment?
Increases stability of biodegradeable matter, reduces odour and vector (fly, rat) attraction, reduces/eliminates pathogens, improves physical properties
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What's first stage of anaerobic digester process?
Pretreatment can be used, pasteurisation for 5 hrs at 57 degrees. Raw sludge enters primary digester. Retention for 12 dats. Digested sludge heated to 35 degrees and methane released.
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What is second stage of anaerobic digester process?
Dewatering and storage of cake is alternative to secondary state. No heating and mixing in secondary digester, residual gas captured, increased pathogen removal, increased sludge consolidation/ Effluent released and digested sludge out of other path
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What is first step of anaerobic digestion? (Hydrolysis)
Hydrolysis is breaking down lipases, proteases, hydrolases (large polymers), to monomers such as fatty acids and amino acids. Done by extracellular enzymes?
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What is second step of anaerobic digestion? (Fermentation)
Microorganisms break down biomass. Acidogenic - produces acetate and volatile acids, as well as alcohols, CO2 and H2O. Acetate complex substrate converted to CH4 by methanogenic bacteria.
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What is third step of anaerobic digestion? (Acetogenesis). (CH4 also produced by symbiotic association between 2 obligate anaerobic bacteria)
Further fermentation of volatile acids to acetate, CO2 and hydrogen. CH3 CH2 OH (ethanol)+ H2O (water)-> 2H2 + CH3COOH (acetate) done by obligatory hydrogen producing bacteria
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What is methanogenesis ? (final step of anaerobic digestion)
CH4 relieved from acetate by specialised bacteria. CH3COOH-> CH4+CO2 4H2+CO2 -> CH4 + 2H2O (acetate reduction produces 2 times the methane)
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What is composting?
Composting is autothermophilic aerobic digestion. Uses stackable solid substrates which are organic and putrescible (able to decay), sewage, sludge, municipal solid waste and green waste used, degradation occurs over weeks by microbial succession.
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How is heat generated in composting and what product is released?
Heat is generated by mesophilic activity. Produces stabilised dark brown residue - soil conditioner. Generation of compost has increased in UK
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What stages are there in composting?
Substrate (to be broken down), bulking agent such as wood, and amendment added. Pre-processed, then in high rate phase mesophilic bacteria and thermophilic bacteria break down substrate. In curing phase cooling and maturation happens. Then postproces
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What is the post-processing stage of composting?
Grinding, screening, blending of compost
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What do hydrolytic bacteria produce?
Proteins produce NH3 and amino acids, fats produce fatty acids and glycerol, and polysaccarides make sugars
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What do acid forming fermentative bacteria make?
Volatile acids, alcohol, ketones, and formate acetate, CO2, and H2O
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What do obligatory hydrogen producing bacteria make?
Acetate and H2
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What do methanogenic bacteria make?
CH4, CO2, H2O. All bacteria become biomass
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What are the ideal C/N ratio, particle size, moisture content, free air space and temp for composting?
C/N ratio of 25/1-30/1. Particle size: 12.5 mm for vessels, 50mm windrows. Moisture content: 50-60%, free air space: 30%, temp- 55 degrees
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What agitation required for fermentation and heap size?
No agitation to periodic turning in simple systems, small vigorous bursts in mechanised systems (more O2 availability). Heap size 1.5m high and 2.5 m for windrows using natural aeration
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What microorganisms are involved in composting?
Mesophiles multiply. Temperature rises in 30 hours to around 50 degrees. Mesophilic activity falls at 40 degrees and thermophilic funghi take over. 40-60 degrees maintained with fluctuation. Mesophilic bacteria, actinomyces and funghi return in cool
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How long does multiplication and cooling take?
Bacteria multiplication: 14-21 days. Maturation/cooling - 2-3 months
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What are benefits of treating waste?
Treatment of industrial/municipal/food waste and biodegradeable sludges reduces odour, pathogens and CH4 can be channelled, recovers organic matter, nutrients, and energy
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Other cards in this set

Card 2

Front

How much sewage production is there per year and how much is treated anaerobically?

Back

UK total sewage production is 1.4 million tonnes of dissolved solids (DS) annually. 70% sewage treated by anaerobic digestion

Card 3

Front

What are benefits of sewage sludge treatment?

Back

Preview of the front of card 3

Card 4

Front

What's first stage of anaerobic digester process?

Back

Preview of the front of card 4

Card 5

Front

What is second stage of anaerobic digester process?

Back

Preview of the front of card 5
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