(a)
Explain how the conditions arise which result in
erratic action or total inaction in the holding tanks of biological marine
sanitation devices (sewage systems). (4m)
(b)
Define the problems associated with macerator /
chlorinator units, particularly in vessels engaged in the high density
passenger trade. (4m)
(c)
Identify the advantages of mechanical / chemical
recirculating systems. (2m)
Answer :-
(a)
Conditions leading to erratic action or total
inaction in holding tanks of biological type sewage plants can be:
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.
4)
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.
(b)
Macerator / chlorinator units are simple
inexpensive systems whereby it is similar to retention tanks except that the
sewage is given a certain amount of treatment before discharge. Problems with
these units includes:
1)
minimum contact time for the chlorine to take
effect is approximately 20 minutes, hence affecting the rate of usage.
2)
Also this type of unit is not capable of
discharging an effluent acceptable to the legislation authorities, therefore
they are provided with some soil tank of a considerable size to retain the sewage
during port stay.
3)
Physical separation of sewage involving filters
of some form is normally used and sludge accumulated by this filtration must be
stored or incinerated, if stored measures should be taken to prevent the onset
of septic conditions.
(c)
Advantages of mechanical / chemical
recirculation system are:
1)
complies with even the most stringent
legislation being a ‘zero discharge’ system.
2)
Short retention time allowing for reduced plant
size.
3)
Due to solid separation a minimal quantity of
chemical is required.
4)
Relatively simplicity in operation.
5)
Can handle surges easily.
6)
Will operate in either seawater or fresh water
flush and also handles sewage drains as well as others.
7)
Moderate acquisition and operating costs.
(a)
Draw a line diagram of a zero discharge sewage
system in which the waster is recirculated and the solid processed for
disposal, labeling all principal component and show the direction of all flow
in the system. (4m)
(b)
Describe how such a system operates. (4m)
(c)
Give reason why the system might be considered
superior to that compared to which the sterile water is discharged at sea. List
the disadvantages that might arise from this system. (2m)
Answer :-
(a)
(b) Sewage
Chemical Treatment Plant – Zero Discharge 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 sterilize and further deodorizes 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.
(d)
This system has the distinct advantage of having
the ability to comply with even the most stringent legislations, as there is no
overboard discharge while in restricted waters.
Disadvantages that might arise from
this system includes:
1)
requires handling and storage of corrosive
chemicals onboard.
2)
Solids must be discharged frequently.
3)
Relatively large size, making retrofit
installations difficult.
4)
Regular consumption of chemical adds to incurred
expenses.
Contamination of coastal waters is causing some ship owners
to install equipment to deal with raw sewage in anticipation of maritime
legislation.
Describe with sketches two entirely different methods
currently available for dealing with raw sewage onboard ships.
Answer :-
Biological Treatment Plant (Extended Aeration System)
Unit is divided into three processing chambers, an aeration
compartment, settling compartment and a chlorination compartment.
1)
Aeration compartment
Aerobic bacteria and micro
organism breakdown the human waste into CO2, water and inert organic material
to produce new bacteria cells and organism.
Rotary aeration blower supply air
providing oxygen to the aerobic organism and also keeps the content of the tank
intimately mixed with the incoming sewage and the returned activated sludge.
2)
Settling compartment
Here bacteria is allowed to
propagate under quiet conditions and deposited sludge is air lifted back into
aeration chamber. A surface skimmer provides means of removing floating scum
and simultaneously allow the fluid to be drawn up from it’s purest zone for
treatment in next stage.
3)
Chlorination chamber
Clean liquid passed to chlorinator
at controlled rate for disinfectant whereby remaining bacteria are killed. The
resulting harmless effluent is then ready for discharge overboard.
Chemical Treatment Plant – (Recirculation 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.
With respect to sewage treatment, discuss the following
terms:
(a) Biochemical
oxygen demand. (4m)
(b) Suspended
solids content, (3m)
(c) Coliform
discharge counts. (3m)
Answer :-
(a)
Biochemical oxygen demand
BOD is a measure of the total
amount of oxygen in mg/ltr which will taken up by bacteria and chemicals to
stabilize or oxidize the sewage within a given period of time at a particular
temperature (usually associated with a specific period of time take takes 5
days)
(b)
Suspended solids content
These are solids which are non
soluble matter or inorganic solids present in the raw sewage and will remain in
suspension in the effluent. It is unsightly and over a period of time can give
rise to silting problems. They are usually a sign of a malfunction sewage plant
and when very high will be accompanied by a high BOD.
They are measured by filtering a
sample through a pre weighted asbestos pad, which is then dried and reweigh.
The standard also varies with different countries.
(c)
Coliform discharge counts
Coliforms or e-coliform is the name
given to the group of bacteria normally found in the intestines of man and also
in vegetation. These bacteria are normally not harmful except they contain
pathogenic colonies which cause diseases like typhoid, dysentry etc.
They can be quantified easily in
laboratory test, the result of which is
indicative of the amount of human waste present in a particular sewage sample.
The result of this test is called
the ‘e-coli-count’ expressed per 100mltrs.
(a)
Draw a line diagram of a biological sewage
system in which only sterile water is discharge overboard at sea and labeling
the principle items showing it’s directional flow in all the line. (4m)
(b)
Describe briefly how it operates (4m)
(c)
Define it’s principle disadvantage. (2m)
Answer:-
(a)
(b) Biological
Treatment Plant (Extended Aeration System)
Unit is divided into three
processing chambers, an aeration compartment, settling compartment and a
chlorination compartment.
1)
Aeration compartment
Aerobic bacteria and micro
organism breakdown the human waste into CO2, water and inert organic material
to produce new bacteria cells and organism.
Rotary aeration blower supply air
providing oxygen to the aerobic organism and also keeps the content of the tank
intimately mixed with the incoming sewage and the returned activated sludge.
2)
Settling compartment
Here bacteria is allowed to
propagate under quiet conditions and deposited sludge is air lifted back into
aeration chamber. A surface skimmer provides means of removing floating scum
and simultaneously allow the fluid to be drawn up from it’s purest zone for
treatment in next stage.
3)
Chlorination chamber
Clean liquid passed to chlorinator
at controlled rate for disinfectant whereby remaining bacteria are killed. The
resulting harmless effluent is then ready for discharge overboard.
(d)
Principle disadvantages:
1)
requires continuous operation to maintain the
functioning bacteria.
2)
Use of certain detergents can be harmful to the
functioning bacteria colony.
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