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Aerobic digestion operates with a free oxygen supply (dissolved oxygen), other factors that effect its operation are:
1. Detention time, temperature and oxygen transfer efficiency
2. Nitrification, anaerobic decomposition and CBOD₅ uptake
3. Denitrification, grit removal and primary treatment
4. Phosphorus removal, screenings removal and scaling
The fundamental concept of aerobic digestion is that:
1. Methane gas is a by-product
2. A net increase in cell mass is achieved
3. The process must operate in the log growth phase
4. Endogenous decay occurs at a faster rate than microbial growth
A properly operated aerobic digester should have a D.O. range of about:
1. 0.01 to 0.05 ppm
2. Minimum of 0.5 to 1.0 ppm
3. Minimum of 5.0 ppm
4. 4 to 6 ppm
Adequate mixing in an aerobic digester is:
1. Not necessary only dissolved oxygen is important
2. Accomplished with mechanical aerators only.
3. Accomplished with diffused air systems only.
4. Essential to ensure contact between the organisms, their food supply and uniform D.O. throughout the tank.
Carbon dioxide is a common by-product in an aerobic digester; if carbon dioxide is trapped it will tend to:
1. Increase pH
2. Decrease pH
3. Neutralize the pH
4. It has no effect at all on pH
The first stage of anaerobic digestion is with methane formers.
1. True
2. False
A properly operated anaerobic digester will have a volatile solids reduction of about 20% to 30%.
1. True
2. False
Anaerobic digestion is complete when the acid forming bacteria are done working.
1. True
2. False
Acetic acid, fatty acid and volatile acid are all organic acids.
1. True
2. False
Methane forming bacteria convert carbon dioxide to methane gas.
1. True
2. False
Which anaerobic digester is normally NOT mixed and/or heated?
1. Primary
2. Secondary
Improper operation of the primary digester may result in which of the following potential problems? select only one answer.
1. Foaming from the cover vent ports
2. Low volatile solids reduction
3. Poor quality dewatering
4. Poor digester gas quality (low methane, high CO₂₎
5. All of the Above
The following elements must be present for an explosion to occur: (1) Fuel, (2) Oxygen, and (3) Heat Source. What is the major fuel source in the anaerobic digester system?
1. Methane
2. Feed Sludge
3. Tank Walls
4. Carbon Dioxide
What device is designed to protect the gas holder tank and cover from damage?
1. Hot water system
2. Pressure/vacuum relief
3. Gas recirculation compressor
What does a high concentration of H₂S in the digested sludge (or gas) usually indicate?
1. That the digester is being mixed and/or heated properly
2. That the digester feed rate is very low
3. That the digester is being mixed and/or heated inadequately
4. That the digester volatile solids loading rate is very low
H₂S gas smells like rotten eggs at low concentrations. What does H₂S gas smell like at high concentrations?
1. Sulfur
2. No odor perception because it deadens the senses
3. Ammonia
4. Chlorine
What color should the digester gas be (in the waste burner) when the methane quality is high?
1. Yellow
2. Amber
3. Green
4. Blue
Which device should be used to adjust the gas pressure that affects the entire low pressure gas network? select only one answer.
1. Weighted disks on top of the digesters
2. Weighted disks at the waste gas burners
3. Pressure regulating valve at the waste gas burners
4. All of the above
What does grey or brown colored sludge mean when it is being applied to the belt filter press from the gas holder tank?
1. Improper digestion
2. Excellent digestion
3. Digester detention time too long
4. Adequate mixing and temperature
What can cause a rise in the volatile acids/alkalinity ratio?
1. Underfeeding the primary digester
2. Hydraulically or organically overloading the primary digester
3. Low volatile solids loading rate
4. Proper mixing of the primary digester
How much gas will typically be generated in a properly operated anaerobic digestion process?
1. 1 to 2 ft³/lb VS reduced
2. 5 to 7 ft³/lb VS reduced
3. 11 to 20 ft³/lb VS reduced
4. 40 to 60 ft³/lb VS reduced
A phenomenon in which Magnesium Ammonium Phosphate causes precipitation of solid mineral deposits on the belts and rollers of a belt filter press that is receiving anaerobically digested sludge is known as:
1. Struvite
2. Vivianite
3. Alum
4. Phosphorus
What does the term anaerobic mean, in respect to oxygen?
1. Free oxygen and combined oxygen are present.
2. Only free oxygen is present.
3. No free oxygen or combined oxygen are present.
4. Only combined oxygen is present.
Which anaerobic digestion temperature range is considered the Mesophilic range?
1. 75 to 90°F
2. 120 to 135°F
3. 85 to 100°F
4. 110 to 120°F
Which is considered the Thermophilic temperature range?
1. 75 to 90°F
2. 120 to 135°F
3. 85 to 100°F
4. 110 to 120°F
What are the two major gases in a properly operated anaerobic digester?
1. Oxygen and nitrogen.
2. Hydrogen sulfide and carbon dioxide.
3. Methane and carbon dioxide.
4. Oxygen and methane.
What is the wrong parameter to use as a control for anaerobic digestion?
1. pH
2. Temperature
3. Acid/Alkalinity ratio
4. Organic loading
Anaerobic digestion is a two stage process. What are these two stages called? (pick two answers).
1. Acid forming and methane
2. Carbon dioxide and ammonia forming
3. CBOD₅ and Alkalinity forming
4. Oxygen and sulfur dioxide forming
Other than gas production, what is one other by-product produced in a healthy anaerobic digester?
1. Gray septage
2. Chlorine
3. Scum
4. Green sludge
What does the chemical formula H₂S represent?
1. Hydrogen Sulfide
2. Methane
3. Carbon Dioxide
Why would you add sodium bicarbonate an anaerobic digester?
1. To increase acid production
2. To decrease alkalinity
3. To increase pH
4. To decrease pH
What should digested sludge look like in a properly operated anaerobic digestion process?
1. Light brown
2. Black
3. Green
4. Gray
What is the maximum temperature change in a 24 hour period that should occur in an anaerobic digester?
1. 5° F
2. 10° F
3. 1° F
4. 0.1° F
What is the formula to calculate the volatile solids reduction through the anaerobic digestion process?
1. In - Out divided by In x 100 = percent.
2. Lbs/day sludge feed x percent volatile content divided by digester volume = percent.
3. Digester volume, gallons divided by Inlet sludge feed, gpd.
4. In - Out divided by In - (In x Out) x 100 = percent.
Why should the anaerobic digester be recirculated? select only one answer.
1. The break-up the scum layer.
2. To bring food into contact with bacteria.
3. To equalize temperatures throughout the tank.
4. All of the above.
What is the pH range of a properly operated anaerobic digester when operating in the Mesophilic temperature range?
1. 5.2 to 6.2
2. 6.8 to 7.2
3. 6.3 to 6.5
4. 7.3 to 7.9
What is the typical volatile solids reduction in a properly operated anaerobic digestion process?
1. About 25 percent
2. About 50 percent
3. About 75 percent
4. About 90 percent
What two parameters are used to make-up the ratio that is typically used as the main process control parameter for the anaerobic digestion process?
1. Gas production and detention time.
2. Volatile acids and alkalinity.
3. Organic loading and volatile solids production.
4. pH and gas production.
What is the most sensitive phase of digestion?
1. Acid formation
2. Phase one
3. Methane formation
4. Dewatered sludge
What is the feed sludge initially converted to?
1. Volatile/Acetic acids
2. Methane gas
3. Hydrogen sulfide
4. Carbon dioxide
What is a common ratio of methane gas to carbon dioxide gas in a properly operated digester?
1. 10% methane 90% carbon dioxide
2. 70% CO₂ 30% CH4
3. 25% CH₄ 75% CO₂
4. 70% CH₄ 30% CO₂
In order to minimize an overloaded condition from occurring in the aeration system, when should supernatant be removed from the digestion process?
1. During high flow periods
2. During low flow periods
3. When aeration D.O. is low
4. After lunch
Which constituent in digester gas can cause the most severe corrosion in the gas network?
1. Hydrogen sulfide
2. Methane
3. Carbon dioxide
4. Argon
Which form of nitrogen is usually the highest in digester supernatant?
1. Organic
2. Nitrate
3. Nitrite
4. Ammonia

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Sludge Digestion Module ... Practice Quiz ... Test Your KnowledgeReturn to the Quiz Home Page by Clicking the Upper Right "x" on Your Browser

Sludge Digestion Module ... Practice Quiz ... Test Your Knowledge
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