Sludge Digestion

Question 1

What is the purpose of sludge digestion?

Answer 1

To lower the volatile and pathogenic content and overall volume of sludge

Question 2

What are the byproducts of anaerobic sludge digestion?

Answer 2

Methane
Water
Stabilized sludge

Question 3

How do you ensure good sludge digestion?

Answer 3

Regular feed rate
Constant temperature
Appropriate nutritional content of food

Question 4

What are the two bacteria types needed for anaerobic digestion?

Answer 4

Acid forming saprophytes and methane fermenters

Question 5

Describe the anaerobic digestion process

Answer 5

Organic materials are converted to volatile acid by the acid fermenters (no waste stabilization occurs during this phase)

The volatile acids are then converted to methane gas by methane fermenters (waste stabilization occurs here)

Question 6

What is the appropriate volatile acid/alkalinity ratio in an anaerobic digester?

Answer 6

.1

Question 7

What conditions are needed for methane fermenters to grow?

Answer 7

Ph 6.6-7.6

Anaerobic

Question 8

What are the purposes of mixing an anaerobic digester?

Answer 8

Distribute raw sludge
Bring microorganisms into contact with food

Distribute alkalinity
Distribute heat
Minimize settling of grit and inert solids

Question 9

What are the different temperature zones in anaerobic digestion?

Answer 9

Psychrophilic (50-68 def F)
Mesophilic (68-113 deg F)
Thermophilic (120-135 deg F)

Question 10

How long does psychrophilic digestion take?

Answer 10

50-180 days

Question 11

How long does mesophilic digestion take?

Answer 11

5-50 days

Question 12

How long does thermophilic digestion take?

Answer 12

5-12 days

Question 13

What is a rule of thumb when changing digester temperature?

Answer 13

No more than one degree per day

Question 14

What is an appropriate amount of volatile solids reduction after digestion?

Answer 14

50%

Question 15

What components of gas will dominate at anaerobic digester startup?

Answer 15

CO2, ammonia, and sulfur

Question 16

At what percentage of methane is digester gas burnable?

Answer 16

56%

Question 17

At what percentage methane is burnable digester gas usable as fuel?

Answer 17

60%

Question 18

At what mixture is oxygen and digester gas explosive?

Answer 18

5-15%

Question 19

What are common causes of foaming in an anaerobic digester?

Answer 19

Acid regression/insufficient digestion
Rapid temperature changes
Thick sludge blanket being broken up
Overfeeding

Question 20

How do you neutralize a sour digester?

Answer 20

Add anhydrous ammonia, lime, soda ash

1lb of lime for every 1000 gallons of sludge
Equivalent mg/l of lime to volatile acids

Question 21

What is struvite?

Answer 21

The scale formed by ammonia, phosphorous, and magnesium. Almost as hard as concrete.

Question 22

What is aerobic sludge digestion?

Answer 22

The conversion of organic sludge solids to odorless end products of carbon dioxide and water by aerobic microorganisms

Question 23

Describe appropriate operating parameters for aerobic digestion

Answer 23

High MLSS (10000-15000)
Long detention time
As thick of sludge as possible