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27 Cards in this Set
- Front
- Back
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Describe Aerobic Composting
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-most common
-some anaerobic zones due to mass transfer limitations -products are CO2, water, biomass and heat -thermophilic temperatures |
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Describe Anaerobic Composting
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-free oxygen is absent
-products are CO2, water, CH4, intermediates, biomass and heat -heat produced is low compared to aerobic -high potential for odours |
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What are the 3 main Aerobic Composting Systems?
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-Windrows
-Static piles -In-vessels |
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Describe "Windrows"
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-composting material is formed into long "windrows"
-aeration by natural convection -regularly mixed and reformed -low cost but land intensive |
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Describe "Static Piles"
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-composting material formed into a pile over an air distribution system
-3 layers -base layer helps provide uniform air distribution -middle layer is composting material -top layer is finished compost which insulates -similar land requirements to windrows |
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Describe "In-vessels"
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-enclosed vessel which either operates as a plug flow or mixed reactor
-air flow supplied -compost moves along vessel -higher capital cost but requires less land |
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What are the phases in compost biology?
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-Phase 1: Mesophilic
-Phase 2: Thermophilic -Phase 3: Maturation |
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Describe "Phase 1" in compost biology
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-mesophilic bacteria dominate at start of process
-mesophilic temperatures (25 to 40 degrees) |
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Describe "Phase 2" in compost biology
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-thermophilic bacteria take over when temp exceeds 40 degrees
-thermophilic temperatures (40 to 65 degrees) -temp continues to rise but at slower rate |
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Describe "Phase 3" in compost biology
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-temp begins to cool
-mesophilic microbes dominate and finish composting process |
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Describe how particle size effects the composting process
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-reduced size of particles increases biochemical reaction rate
-reduced size also increases bulk density and air flow friction -25 to 75mm recommended |
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Describe how blending and seeding effects the composting process
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-adding seed with 1 to 5% by weight reduces start up time
-blending different substrates can help optimize the C:N ratio and moisture content |
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Describe how the C:N ratio effects the composting process
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-too much N can release NH3
-too much C can lead to nutrient limitation -optimum of 25:1 |
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Describe how moisture control can effect the composting process
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-process slows if below 40%
-optimal of 55% -adjusted by blending and water addition/mixing |
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Describe mixing in the composting process
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-important for maintaining aerobic activity and ensuring uniform distribution of bacteria and product
-can also be used for moisture control |
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Describe temperature control in the composting process
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-high heat generation at start due to easily degraded material
-easier to control temp in aerated system -harder for windrows system |
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Describe pathogen treatment in the composting process
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-most destroyed rapidly at 55 degrees
-practically all killed by temp of 70 degrees held for several hours -biological activity drops rapidly above 66 degrees |
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Describe how pH effect the composting process
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-varies with time
-ideal pH between 7 and 7.5, may varie from 5 to 8 -bacteria prefer neutral pH -fungi develop better at acidic pH |
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What are the two main biosolids collected from primary and secondary processes?
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-Pathogens
-Heavy Metals |
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What are the advantages of using anaerobic digestion?
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-no aeration costs
-biogas energy recovery -less biological sludge (yield is 6 to 8 times less than aerobic) |
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What are the disadvantages of using anaerobic digestion?
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-may require alkalinity addition
-normally requires further aerobic treatment -biological N and P removal not possible -sensitive to pH and low temp |
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Describe "Hydrolysis" in anaerobic digestion
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-proteins hydrolyzed to amino acids
-fats and lipids are emulsified to be broken down to fatty acids |
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Describe "Fermentation" in anaerobic digestion
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-Acidogenisis: monomers converted to volatile fatty acids (VFA)
-Acetogenisis: VFA's converted to acetic acid, CO2 and H2 |
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Describe "Methanogenisis" in anaerobic digestion
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-acetic acid, CO2 and H2 converted to CH4
-carried out by methanogens |
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Describe how temperature is used in anaerobic digestion
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-typically carried out in mesophilic range
-can run at low temperature (12 to 15 degrees) but have much slower reaction rates -can run in thermophilic range which have faster reaction rates, can have smaller reactor, but more gas is consumed |
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Describe the different gases produced in anaerobic digestion
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-CH4: explosive, energy source
-VFA: odorous -H2S: corrosive, odorous |
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Describe how biogases are stroed
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-in external bag
-in digestor with floating roof -in external tank with floating roof |
