Anaerobic Sludge Digestion

The factor that affects the rate of sludge digestion is

( A ) Temperature.

( B ) Mixing or recirculation.

( C ) Manner of feeding raw sludge.

( D ) All of the above.

D

Sludge digestion is a decomposition process whereby waste materials are

( A ) Dried on a drying bed.

( B ) Completely disinfected.

( C ) Filtered.

( D ) Biologically and chemically changed.

D

The contents of a primary digester should be mixed to

( A ) Distribute food in the tank.

( B ) Allow solids separation.

( C ) Keep the temperature below 60° F.

( D ) Warm up the sludge.

A

The most favorable way to maintain a digester fed with primary sludge is to pump

( A ) Once a day.

( B ) In small amounts at frequent intervals.

( C ) In large quantities with rest.

( D ) When the volatile solids drop below 60 percent.

B

Which of the following is less critical in bringing in a digester?

( A ) Proper pH.

( B ) Heat 90° to 100° F.

( C ) A high percentage of solids.

( D ) Seed sludge.

C

Which test is not used to control an anaerobic digester?

( A ) pH.

( B ) Alkalinity.

( C ) Chlorine residual.

( D ) C02 content of gas.

C

The volatile acids level should not be allowed to advance beyond

( A ) 5 mg/l.

( B ) 2.000 mg/l.

( C ) 20,000 mg/l.

( D )

B

Buffer capacity in a digester is measured by

( A ) Total dissolved solids.

( B ) Volatile solids content.

( C ) Chlorine residual.

( D ) Total alkalinity.

D

The purpose of sludge digestion is

( A ) To decompose enough of the organic matter to avoid creating a nuisance.

( B ) To separate the liquid from the solids to facilitate drying.

( C ) To decrease the bulk of the sludge to facilitate handling.

( D ) All of these.

D

The temperature of a digester should not be changed more than 1° F/day to

( A ) Avoid excessive heat losses.

( B ) Avoid overloading the heat exchanger.

( C ) Allow the walls of the digester time to expand and contract.

( D ) Allow the organisms in the digester time to adjust to the temperature change.

D

If the effect of temperature on the rate of sludge digestion is considered. it may be safely assumed that

( A ) A digester operating at 90° F can safely handle more sludge than one operated at 60° F.

( B ) Temperatures of 85° to 95° F will destroy the bacteria present in a digester.

( C ) Digestion will be most satisfactory if the temperature of the digester is maintained at the same temperature as the incoming raw sludge.

( D )

A

The reason for concentrating (thickening) sludge before digestion is to

( A ) Increase detention time.

( B ) Reduce pumping.

( C ) Reduce heat requirements.

( D ) All of the above.

D

The action that occurs in a properly functioning anaerobic sludge digester may best be described as

( A ) A biological and chemical process that breaks down complex organic substances in the absence of atmospheric oxygen into a simpler and more stable state.

( B ) An aerobic process of decomposition of organic solids.

( C ) A purely chemical process of separating the liquid from the solids.

( D )

A

Sludge digestion tank capacity requirements vary with the type of sludge to be handled. With heated tanks, the recommended cubic feet to be allowed per capita are

( A ) 1 to 3

( B ) 2 to 4

( C ) 4 to 8

( D ) 6 to 12

A

The proper pH range for digested sludge is

( A ) 5.8 to 6.2

( B ) 7.8 to 8.2

( C ) 6.8 to 7.2

( D ) None of the above.

C

As a digester approaches a "sour" condition

( A ) Alkalinity of the sludge decreases.

( B ) Concentration of volatile acids increases.

( C ) Amount of methane production decreases.

( D ) All of the above.

D

The volatile acids in a well-operating digester reported as acetic acid should normally be held

( A ) Below 10 mg/l.

( B ) Above 2,500 mg/l.

( C ) Between 50 and 500 mg/l.

( D ) Between 500 and 2,000 mg/l.

C

"Volatile acids" is a good control test for sludge digestion because it

( A ) Measures the pH of the sludge.

( B ) Gives an early indication that the digestion process is upset.

( C ) Measures the alkalinity of the sludge.

( D ) Measures the amount of acid gas produced by the sludge.

B

The purpose of the recirculation of digested sludge may be

( A ) To mix raw sludge with the anaerobic bacteria present.

( B ) To increase the efficiency of the heating of sludge in the digester.

( C ) To aid in scum control.

( D ) All of these.

D

A low hydrogen ion concentration of a digester may be adjusted by the use of

( A ) Chlorine.

( B ) Distilled water.

( C ) Lime.

( D ) C02.

D

In the use of lime in a digester, which of the following takes place?

( A ) The concentration of hydrogen ions decreases.

( B ) The concentration of hydrogen ions increases.

( C ) The concentration of OH ions decreases.

( D ) The calcium carbonate decreases.

A

Digester conditions are most favorable when primary sludge is added

( A ) In small amounts at frequent intervals.

( B ) Preconditioned with lime.

( C ) In large quantities at infrequent intervals.

( D ) Once each day.

A

As a result of the digestion process, the solids are concentrated in such a way that the liquid separates from the solids. The liquid is called

( A ) Supernatant.

( B ) Digested sludge.

( C ) Raw wastewater.

( D ) SS.

A

To prevent the formation of an insulating scale on the outside of digester heating coils, the circulating water inlet temperature should not exceed

( A ) 90° F.

( B ) 1000 F.

( C ) 1400 F.

( D ) 1800 F.

C

The purpose of adding lime to a failing digesters

( A ) To lower the pH.

( B ) To increase calcium content.

( C ) To raise the pH.

( D ) To increase enzyme concentration.

C

Foaming in a digester usually indicates

( A ) That all sludge should be drawn out.

( B ) That the plant should be bypassed.

( C ) That acid conditions are present.

( D ) That the bar screen is not functioning properly.

C

The following would indicate that trouble is brewing in a sludge digestion tank:

( A ) Methane gas production remains steady.

( B ) The pH remains between 6.8 and 7.2.

( C ) The pH falls below 6.8.

( D ) The supernatant is unusually clear.

C

In withdrawing sludge from a digester, it is necessary to allow some sludge to remain in order

( A ) To prevent overloading the drying beds.

( B ) To provide a seed material for raw sludge added to the digester.

( C ) To prevent the tank from caving in.

( D ) To increase the removal of phosphorous.

B

"Coning" in the digesters occurs when

( A ) Production of digester gas increases.

( B ) Sludge withdrawal is too fast.

( C ) Raw sludge pumping is too fast.

( D ) C02 production increases.

B

An accumulation of sand or cinders in a sludge digester reduces the effective capacity of the digester, because such material

( A ) Is not digested but occupies the space that would otherwise be available to sludge in the digestion process.

( B ) Kills the organisms responsible for digestion.

( C ) Prevents the formation of a scum layer.

( D ) Absorbs all of the heat intended for the sludge.

A

The location of a leak in digester gas piping may best be located by

( A ) The odor.

( B ) Soap solution.

( C ) A lighted match.

( D ) Ammonia.

B

Wastewater gas normally obtained from a correctly operated digester contains

( A ) 70 percent methane.

( B ) 70 percent hydrogen sulfide.

( C ) 70 percent oxygen.

( D )

A

Digester gas usually is a mixture of

( A ) Hydrogen, methane, and carbon monoxide.

( B ) Methane, carbon dioxide, and hydrogen.

( C ) Methane, carbon monoxide, and nitrogen.

( D ) Carbon dioxide, nitrogen, and methane.

D

When the content of C02 in digester gas increases to above 30 to 40 percent, the digester is

( A ) Improving its gas production

( B ) Not functioning properly.

( C ) Operating normally.

( D ) Operating very well.

B

Good sludge digestion will yield approximately how much gas per person per day, if it is assumed that there is no industrial waste?

( A ) 0.25 to 0.45 cu ft.

( B ) 0.8 to 1.2 cu ft.

( C ) 2.4 to 5.0 cu ft.

( D ) 6.0 to 8.0 cu ft.

B

The amount of wastewater sludge gas to be expected per pound of volatile solids added to a digester that is functioning properly is about

( A ) 1 cu ft.

( B ) 4 to 6 cu ft.

( C ) 8 to 12 cu ft.

( D ) 16 to 19 cu ft.

B

A pressure relief on the top of a digester should be set

( A ) 5 to 10 psi.

( B ) 10 to 15 in. of water.

( C ) 2 psi above atmospheric.

( D ) 59 in. of water.

B

To burn digester gas, the operating pressure is normally

( A ) Over 2 lb. pressure.

( B ) 2 in. to 7 in. of water.

( C ) 4 in. to 10 in. of mercury.

( D ) Over 20 in. mercury.

B

A flame trap is found on

( A ) A comminuter.

( B ) Plant electrical systems.

( C ) Digester gas piping.

( D ) Aeration tank piping.

C

A well-digested sludge should show on analysis a volatile content, dry basis, in the total solids of

( A ) 10 percent.

( B ) 35 percent.

( C ) 50 percent.

( D ) 70 percent.

C

The main purpose of the secondary digester is to allow

( A ) For more sludge digestion.

( B ) An opportunity for more mixing.

( C ) Storage for digester gas.

( D ) The liquids and solids in digested sludge to separate.

D

If a digestion tank starts foaming at normal temperature, the following control measures should be undertaken at once:

( A ) Increase the temperature 10° F above that normally considered desirable.

( B ) Recirculate the tank contents and add sufficient lime to maintain a pH of about 7.0.

( C ) Maintain the same temperature.

( D ) Shut off all heat.

B

Frequent withdrawing of small quantities of digested sludge is better than occasionally draining the digester because

( A ) It tends to prevent acid conditions.

( B ) It lessens the chance of foaming.

( C ) It helps to maintain overall digester efficiency.

( D ) All of the above.

D

If the percentage of volatile matter in digested sludge removed from a digester indicates trouble, which of the following is least likely to correct the sick or sour digester?

( A ) Stop feeding the unit.

( B ) Pull as much sludge as possible from the bottom of the tank onto drying beds

( C ) Make sure that the tank is mixed and is homogeneous throughout.

( D ) Transfer buffer material [well-digested sludge from another tank] and mix thoroughly.

B

On a dry weight basis, the quantity of fresh solids added daily to the digester should not exceed what percentage of the weight of solids already in the digester?

( A ) 1 to 2 percent.

( B ) 4 to 6 percent.

( C ) 5 to 10 percent.

( D ) 10 to 15 percent.

B

The effective heat value per cubic foot of digester gas is

( A ) 2 to 4 Btu's.

( B ) 500 to 800 Btu's.

( C ) 1,500 to 3.000 Btu's.

( D ) 3,000 to 5,000 Btu's.

B

The Btu value of a cubic foot of gas from a wastewater digester is about the same as the Btu value of

( A ) Manufactured gas.

( B ) Hydrogen.

( C ) Propane.

( D ) Butane.

A

The terms "standard rate" and "high rate" digestion refer to

( A ) The rate of acid-forming reaction.

( B ) The rate of methane-forming reaction.

( C ) The rate of acid- and methane-forming reactions.

( D ) The rate of loading.

D

An Imhoff tank has

( A ) Five compartments.

( B ) Sludge scrapers.

( C ) A piping system that allows the flow in the tank to be reversed from one end to the other.

( D ) A separate sludge digestion compartment under the settling area.

D

The slot or slots in the bottom of the sedimentation chambers of Imhoff tanks must be kept free from obstructions. This job may be done by

( A ) Super chlorinating the particles that cause obstructions.

( B ) Reversing the flow.

( C ) Dragging a chain through the slots.

( D ) Using a scum hoe.

C

The most effective step to inhibit foaming in an overloaded Imhoff tank is to

( A ) Avoid disturbing the surface of the scum in the gas vents.

( B ) Wet down the gas vents frequently.

( C ) Provide for greater velocity of flow through the sludge discharge lines.

( D ) Draw off all the sludge.

B

The best method of maintaining the gas vents of Imhoff tanks in good operating order is to

( A ) Add lime regularly.

( B ) Reverse the flow in the Imhoff tank.

( C ) Break up scum daily with a suitable tool.

( D ) Wash off with chlorine.

C

Excessive quantities of floating sludge through compartment of an Imhoff tank indicate

( A ) Poor digestion.

( B ) Excessive gas production.

( C ) That the slot in the bottom of the flow-through compartment is clogged.

( D )

C

"Short-circuiting" of wastewater through the sedimentation compartment of a Imhoff tank may occur when

( A ) The tank is bypassed.

( B ) The raw wastewater pumps are not functioning.

( C ) The gas vents are covered with a thick layer of scum.

( D ) The influent and effluent baffles are not in place or functioning properly.

D

Sludge withdrawal from a sedimentation or Imhoff tank should be

( A ) At a rapid rate.

( B ) At a slow enough rate so that wastewater will not break through the sludge.

( C ) Forgotten about until such time as the treatment is so fouled up that it has to be done.

( D )

B

An Imhoff tank is designed to serve the same purpose in treating wastewater as

( A ) An activated sludge tank.

( B ) A primary clarifier.

( C ) A primary clarifier and a digestion tank.

( D ) A secondary clarifier.

C