(a) Describe a
mechanical / chemical recirculation sewage system in which the holding tank is
considerably smaller than the tanks in ‘flow through’ systems. (5m)
(b) State why disposal of tank contents does
not constitute a problem. (3m)
(c) Define the principal advantage of the
system describe in (a) over its counterparts in which the holding tank is of
appreciable size. (2m)
Answer :-
(a)
This system differs from the extend aeration
system in a way that there is no sterile or effluent discharged overboard. Instead
it recirculates for continuous use of the system. Whereas in the extended
aeration system, an appreciable size of holding tank is used and sterile water
are effluent is discharge overboard within limits allowed of its ppm.
In the chemical recirculation
system, water from e.g. water closet is collected in a chemical dosing tank
where it is firstly chemically treated with chlorine and sodium hypochloride.
Sewage is then circulated from dosing tank to comminute and to chemical
treatment tank. A recirculating pump assist this cycle until sewage is
converted into a slurry. This is to help the flocculation process.
It is then allow to enter the
settling tank, whereby the action of chemicals, separation into sludge and
clear liquid takes place.
This clear liquid, now completely
sanitised, is pumped into a pressure tank for recirculation to the toilet tank.
Sludge collected during the process
is either discharged to a sewage tank or incenerated.
(b)
Disposal of contents does not pose a problem
because a solid collected in tank can be transferred to a sewage tank or it can
also be incinerated. If the ship alongside, then a shore facilities can be
arranged to receive the collected sewage.
Also if it is efficiently treated
with chemicals, less solids will result thus, less space occupied in the tank
itself.
The sludge collected in the
treatment and setting tank is later pumped out into sewage tank or incinerated.
(c)
Advantages of this system:
-
No necessity to discharge effluent or sludges in
port or restricted waters.
-
Relatively small, compact plant.
-
Economy, since the treated fluid is reused.
-
Can be started in a very short time.
-
Discharge acceptable effluents almost
immediately.
(a) Draw a line diagram of a circulatory
sewage holding system, labeling the principal items and indicating the
direction of flow in all lines. (5m)
(b) Explain how it operates. (3m)
(c)
State what advantages are claimed for this
system over other sewage systems. (2m)
Answer :-
(a) to sketch
chemical treatment plant (recirculation system).
(b)
This type is the latest for shipboard use and
has a definite advantage over the other types in it’s ability to comply with
even the most stringent legislation in that, there is no discharge overboard
while in restricted waters.
The unit is divided into four main
tanks namely chemical dosing tank, treatment tank, settling tank and a
recirculating pressure tank (hydrophore tank).
1)
Chemical dosing tank
Sewage from closets led to this
tank for mixing up with chemicals, which breaks down the solid matter in the
waste and also to improve the colour of the liquid. From here, it is lead to
the treatment tank via a comminutor.
2)
Treatment tank
Here treatment chemical is added
to sterilise and further deoderises the liquid. A recirculating pump draws from
this tank and discharge it into the dosing tank again, ensuring the incoming
sewage is thoroughly broken down and chemically treated. The sewage remains in
this section for about 5 minutes before passing to the settling tank.
3)
Settling tank
This is designed to give adequate
retention period to allow settlement of suspended solids whereby the
supernatant or treated effluent is then transferred via a mesh filter to the
hydrophore tank.
4)
Hydrophore tank
From here, the clean effluent is
again sent to the various closets for flushing purposes.
(c)
Advantages of this system over other sewage
systems are:
1)
complies with even the most stringent
legislation being a ‘zero discharge’ system.
2)
Short retention period allowing for reduced size
of plant.
3)
Relative simplicity.
(a) Explain how the conditions arise which
result in a rapid action or total in action in the holding tanks of biological
marine sanitation devices (sewage systems). (4m)
(b) Define the problems associated which
macerator / chlorinated units, particularly in vessel engaged in the high
density passenger trade. (4m)
(c) Identify the advantages of mechanical /
chemical recirculating systems. (2m)
Answer :-
(a)
The condition arise which result in erratic
action or total in action in the holding tanks of biological marine sanitation
devices are:
1)
introduction of detergent or chemical not
suitable for the system especially for flocculating process.
2)
Failure of air supply in the aeration
compartment, thus the sewage cannot break down by aerobic activation. Fluid in
this compartment is not continuously agitated by air to keeps the bacteria
logically active sludge in suspension. Hence, supply of necessary oxygen for
purification cannot occur.
3)
The system contaminated with oil or grease which
kills useful bacteria. So, the plant will take more time (5 to 14 days) to
resume its operational because of the prolonged aeration of sewage necessary to
produce bacteria that carry out th purifying process.
4)
Changing over of sea water to fresh water
suddenly which causing bacteria die or in active. So, more time necessary to
resume the plant into operational.
5)
Dirty or clogged filters in the settling tank
which cause difficulty in pumping out into pressure tank, resulting in erratic
action of the system.
(b)
Problems associated with chlorinator / macerator
units are :
1)
contamination of the drain.
2)
Calcium hypochloride tablets are dissolved with
the ????? which leaves undissolved
(c)
Advantages of mechanical / chemical
recirculating systems are :
1)
no necessity of discharge effluent or sludges in
port or restricted waters.
2)
Relatively small, compact plant.
3)
Economy, since the treated fluid is reused.
4)
Can be started in a very short time.
5)
Discharge acceptable effluent almost
immediately.
(a) By comparing the processes involved,
explain why aerobic biological marine sanitation devices (sewage systems) are
considered superior to their anaerobic counterparts. (5m)
(b) Identify those factors tending to reduce
the operational effectiveness of aerobic marine sanitation devices (sewage
systems). (5m)
Answer :-
(a)
(b)
Factors tending to reduce the operational
effectiveness of aerobic marine sanitation devices are:
1)
overloading – every system is designed for a
rated capacity, overloading will give lesser time for the breakdown of sewage
(in aeration chamber) and disinfection (in chlorinator chamber).
2)
Insufficient air supply can be due to defective
air blower / compressor, choked filters and airlines – reduce turbulence
required for the intimate contact between the sewage and the bacteria. Also
result in decrease in oxygen supply needed by the colony for its metabolism.
3)
Oil and grease – destroys the aerobic bacteria
colony and affect plant operation.
Toxicity in the water – water
in presence of unsuitable toilet detergents or high concentration of heavy
metal ion can lead to damage to the activated sludge resulting gin erratic
action
Draw a line diagram of a sewage system in which the water is
recirculated and the solids processed for disposal, labeling the principal
components and showing the direction of flow in all lines.
Explain how the system operates.
Answer :-
To sketch chemical treatment recirculating system.
This type is the latest for shipboard use and has a definite
advantage over the other types in it’s ability to comply with even the most
stringent legislation in that, there is no discharge overboard while in
restricted waters.
The unit is divided into four main tanks namely chemical
dosing tank, treatment tank, settling tank and a recirculating pressure tank
(hydrophore tank).
1)
Chemical dosing tank
Sewage from closets led to this
tank for mixing up with chemicals, which breaks down the solid matter in the
waste and also to improve the colour of the liquid. From here, it is lead to
the treatment tank via a comminutor.
2)
Treatment tank
Here treatment chemical is added
to sterilise and further deoderises the liquid. A recirculating pump draws from
this tank and discharge it into the dosing tank again, ensuring the incoming
sewage is thoroughly broken down and chemically treated. The sewage remains in
this section for about 5 minutes before passing to the settling tank.
3)
Settling tank
This is designed to give adequate
retention period to allow settlement of suspended solids whereby the
supernatant or treated effluent is then transferred via a mesh filter to the
hydrophore tank.
4)
Hydrophore tank
From here, the clean effluent is
again sent to the various closets for flushing purposes.
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