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Wastewater > Biosolids > Flashcards

Biosolids Flashcards

1
Q

Factors that affect anaerobic sludge digestion

A
  • 2% TS sludge has twice the water volume of 4% TS sludge.
    □ Additional water pumped to a digester requires additional heat energy to maintain
    the digester temperature.
  • Additional water lowers the detention time in the digester. Pumping 4% TS sludge
    provides double the detention time compared to sludge pumped at 2% TS.
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2
Q

Mixing of an anaerobic digester has two important functions

A
  • Distribution of food throughout the digester
    □ Aids in maintaining uniform temperature throughout the digester
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3
Q

The most effective way to feed an anaerobic digester.

A

Add raw sludge in small amounts at frequent intervals

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4
Q

Commonly used anaerobic digester process control tests

A

□ pH
* Volatile acid / Alkalinity ratio
* Digester gas percent CO2

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5
Q

The optimum range for anaerobic digester control parameters.

A

□ pH: 6.8 - 7.2
□ Volatile acid / Alkalinity’ ratio: 0.1 - 0.5
* Digester gas percent: 30 - 35%

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6
Q

The overall purpose of anaerobic digestion.

A
  • To reduce the solids content and volume of the sludge
  • To biologically stabilize the sludge to reduce nuisance odors, pathogens, and
    attraction of insects and animals.
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7
Q

Anaerobic digester temperature maximum daily adjustment.

A

No more than 1 °F per day is recommended

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8
Q

Energy savings provided by anaerobic digester co-generation equipment.

A
  • Methane produced by the digester is the fuel for an electrical generator engine that
    supplements the electrical demand of the treatment plant.
  • Exhaust heat from the generator engine is passed through a heat exchanger to help
    maintain the digester temperature.
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9
Q

Symptoms that indicate an anaerobic digester is approaching “sour” condition

A
  • Total alkalinity of the sludge decreases (first indication)
  • Volatile acid content of the sludge increases (first indication)
  • Methane gas production decreases, while CO2 content of gas increases
  • pH drops below 6.0 and possible foaming (final indication of “sour” or upset
    digestion)
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10
Q

Measure of the effectiveness or completeness of anaerobic sludge digestion.

A

Volatile solids reduction, typically at least 38%

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11
Q

Functions of a secondary anaerobic digester

A
  • Provides time for solids to concentrate and a clear supernate to develop
  • Provides source of “seed sludge” for digester start-up or recovery
  • Provides for additional gas storage
  • Provides some additional digestion
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12
Q

Loading to an anaerobic digester is optimum when the ratio is 1:10

A

Pounds of volatile solids added per day per pound of volatile solids under digestion

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13
Q

Gasses that make up anaerobic digester gas.

A
  • Methane (approx. 70%) and carbon dioxide (approx. 30%)
  • Hydrogen, nitrogen and hydrogen sulfide gases are also present in minor amounts
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14
Q

Anaerobic digester control parameter that is the most sensitive to digester conditions and
responds first when the digester is approaching upset conditions.

A

Volatile acid / alkalinity ratio

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15
Q

Anaerobic digester control parameter that is the least sensitive to digester conditions, and
responds last when the digester is approaching upset conditions.

A

Digester pH

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16
Q

Equipment associated with a typical anaerobic sludge digester.
A. Provides fire and explosion protection
B. Removes condensed water vapor from digester gas
C. Handles excess or waste digester gas
D. Protects pipes and tank from over or under pressurization
E. Heat transfer to digester sludge
F. Source of heat for digester

A

A. Flame arrester
B. Water trap
C. Waste gas burner (or perhaps co-generation equipment)
D. Vacuum/pressure relief valve
E. Heat exchanger equipment
F. Boiler (or exhaust gas heat recovery from co-generation equipment)

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17
Q

Actions that may be taken to correct an anaerobic digester that is on the verge of going
“sour.

A
  • Add seed sludge
  • Increase the mixing time or mixing intensity
  • Maintain constant digester temperature control
    □ Add sodium bicarbonate or other form of alkalinity to adjust the pH to the normal
    range (Other forms of supplemental alkalinity used to raise digester pH include
    anhydrous ammonia, caustic soda and soda ash)
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18
Q

Processes used to thicken and condition sludge prior to digestion.

A

Gravity thickening - Air floatation thickening - Centrifuge thickening

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19
Q

Poor anaerobic sludge digestion may be indicated by these symptoms.

A

□ Foul (putrid) odor
* Low pH
□ Sludge attracts flies and rodents

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20
Q

Characteristics of well digested sludge from an anaerobic digester.

A
  • Black color and “musty” or “hot asphalt” odor
    □ pH 6.5 - 7.5
  • Total solids of2- 4%
    □ Volatile solids of 50- 60%
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21
Q

Normal gas pressure in an anaerobic digester.

A

Maintained at 8 - 12 inches of water column pressure (0.35 - 0.43 psi)

22
Q

The source of water that accumulates in anaerobic digester gas piping.

A

Condensation of water vapor from the digester gas

23
Q

Supernate from an anaerobic digester can cause treatment plant problems due to these
characteristics.

A

□ High levels of ammonia and phosphate. Ammonia can add large oxygen demand to an
aeration basin that is nitrifying, and ammonia and phosphate can add to the demand on
nutrient removal treatment processes.
* Zero D.O. and high oxygen demand
* High concentration of total solids (1,000 - 2,000 mg/L) <

24
Q

The explosive danger of anaerobic digester gas is greatly increased when this occurs.

A

When digester gas is mixed with air. Digester gas becomes explosive at5- 20% of the
total volume of air.

25
Q

Operating temperatures of most anaerobic digesters:
Mesophilic range
Thermophilic range

A
  • Mesophilic range: 90 - 100° F (32 - 38° C)
    □ Thermophilic range: 110 - 120° F (43 - 49° C)
26
Q

The pH of an aerobic digester can be lowered by these byproducts of aerobic biological
activity.

A
  • Carbon dioxide, produced during normal aerobic respiration
  • Nitric acid, produced during the process of nitrification
27
Q

Factors that can significantly decrease the effectiveness of an aerobic sludge digester

A
  • Low digester temperature due to cold climate conditions
  • Low pH due to nitrification occurring during digestion process
  • Poor 02 transfer during aerobic digestion, due to excessively high total solids content
    in the digester
28
Q

Aerobic sludge digestion will undergo this change when the temperature of the sludge is
raised 10 °C.

A

The rate of digestionwould approximately double. As a general rule, biological activity
increases 2-fold for every 10°C increase in temperature (within the normal temperature
range of the microorganisms).

29
Q

If an aerobic digester is undergoing nitrification and the pH is depressed below 6, what
will occur if the air to the digester is turned off for 4 - 8 hours each day?

A

The pH of the digester will rise toward neutral, as the result of denitrification.

30
Q

Digested sewage sludge should not be land applied under these conditions.

A
  • Edible root crops are present
  • Close proximity to surface water
  • Bare ground without vegetation (unless sludge is immediately tilled into the soil)
  • Close proximity to a water well
  • Frozen soil
  • Excessive sloped ground surface
31
Q

Characteristics of methane —gas

A

□ Lighter than atmospheric air
* Explosive mixture: 5 - 15% methane to air
* Odorless, colorless gas (The odor associated with anaerobic digester gas is due
primarily to hydrogen sulfide gas.)

32
Q

BTU value of digester gas.

A

500 - 800 BTU / cu. ft. gas

33
Q

Typical anaerobic digester detention time

A

15 to 60 days

34
Q

This is used to heat an anaerobic an digester when a co-generation unit is in operation

A

Heat from the generator engine exhaust is used in conjunction with a heat exchanger unit.

35
Q

Sludge application to a drying bed.

A
  • 8-12 inches is the optimum depth
  • A second layer of sludge (“capping”) should not be applied to the bed until the initial I
    layer has dried completely.
36
Q

Supernate characteristics of anaerobic digesters and aerobic digesters

A
  • Anaerobic digester
    pH 6.5 - 7.5
    TSS 5,000- 10,000 mg/L
    BOD, 1,000- 10,000 mg/L
    Ammonia 400 -1,000 mg/L
    T. Phosphorus 300 -1,000 mg/L

□ Aerobic digester
pH 5.9 - 7.5
TSS 100 - 300 mg/L
BOD, 10- 17,000 mg/L
Ammonia 170 mg/L
T. Phosphorus 100 mg/L

37
Q

Dewatering aid used in sludge conditioning and thickening.

A

Cationic polymer

38
Q

Plugging of filter fabric with solids or grease.

A

Filter blinding

39
Q

“Class A” and “Class B” biosolids

A

“Class A”
□ Pathogens reduced below detectable levels
□ Suitable for public use/exposure

“Class B”
□ Pathogens “significantly reduced”
□ Application restrictions limit immediate contact by the public

40
Q

“Class A” and “Class B” biosolids provide equal public health protection in this way

A
  • “No detectable pathogens” in “Class A”
    “is equivalent to”
  • “Significantly reduced” pathogens in “Class B” + additional “site restrictions” when
    applying the solids
41
Q

Common methods used to meet biosolids vector attraction reduction requirements

A
  • Reduce volatile solids content by 38% (anaerobic and aerobic digestion)
  • Reduce specific oxygen uptake rate to 1.5 mg oxygen/gram/hr (aerobic digestion)
  • Dry biosolids to specific percent solids content
  • Inject biosolids beneath or incorporate biosolids into the soil surface
  • Adjust sludge to pH 12 or higher for at least 30 minutes
42
Q

Pathogens associated with sewage sludge

A

□ Salmonella bacteria
□ Enteric (intestinal) viruses
* Helminth ova (eggs of parasitic worms)

43
Q

Vectors that may be attracted to under processed biosolids.

A

Flies, mosquitos, fleas, rodents, and birds

44
Q

Processes approved to produce “Class B” biosolids.

A
  • Aerobic digestion
    □ Anaerobic digestion
  • Air drying
  • Composting
  • Lime stabilization
45
Q

Directly affects sludge compaction in a gravity thickener.

A

Blanket depth andsludge retention time

46
Q

Paddles used in thickeners to stir and dewater sludge.

A

Pickets

47
Q

Methods used to thicken (dewater) sludge.

A

□ Gravity
* Dissolved air floatation (DAF)
* Gravity belt
* Centrifuge
* Rotary drum
□ Belt filter press

48
Q

Polymer terminology:
Aging
Feeding polymer “neat

A

□ 30-90 minutes required for polymer to unfold and activate in freshly prepared feed
solutions
* Polymer fed directly, without dilution

49
Q

Polymer products and applications.

A

□ Solution dilution recommended although not required (may be fed “neat”)
□ Emulsion dilution, mixing and aging required
□ Dry powder dilution, mixing and aging required

50
Q

Pumps feeding conditioning/thickening equipment:
Recommended
Not recommended

A
  • Recommended: positive displacement pumps thatprovide even flow and low floc
    shear (piston, progressive cavity)
    □ Not recommended: centrifugal pumps (floc shear) or diaphram pumps (pulsing
    uneven flow)
51
Q

Higher percentages cause difficulty in sludge conditioning and dewatering.

A

Volatile solids (organic) content of the sludge

52
Q

Regulations that govern sewage sludge use and disposal.

A
  • 40 CFR Part 503 - often referred to as “Part 503”
  • CFR means Code of Federal Regulations

Wastewater (10 decks)

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