Sewage treatment plant/Incinerator

Question 1

Sketch an Aerobic Sewage Treatment Plant.

Answer 1

see EOOW IAMI/ORAL Sketch Pack

Question 2

With reference to an aerobic sewage plant:

a) Explain the need for continuous aeration. (6)

b) Describe the hazards that may be present when the plant requires internal maintenance or inspection (10)

Answer 2

a.) aerobic sewage treatment plant uses bacteria for treating waste. these bacteria are aerobic which means they require oxygen to survive. continuous aeration ensures that oxygen is being supplied to the bacteria for treating and stabilising water. without the bacteria the sewage waste cant be removed so it would just accumulate until there is an overflow problem. of course you can direct sewage waste from toilet system into the sewage holding tank but even this has a limited capacity so overflow can still occur. if the bacteria die due to a lack of oxygen it takes approximately 2 weeks for bacteria to recolonize so for that time sewage would just have to accumulate this could cause an overflow problem depending on amount of sewage produced. another reason that continuous aeration is important is it promotes aerobic biodegradation, which is the breakdown of organic pollutants (sewage) by microorganisms (bacteria) when oxygen is present. in other words bacteria can only breakdown the sewage into water and sludge when they are in optimum oxygen conditions. One other reason for continuous aeration done by air blowers is if oxygen supply is interrupted anaerobic bacteria form. this type of bacteria will produce toxic and explosive gases like ammonia, methane and hydrogen sulphide (H2S).

b.) If an entry is required within the plant for maintenance or
inspection has to be considered as an enclosed space entry so all permits and procedures have to be followed with accordance to the ships SMS and enclosed space permit has to be obtained. Additional hazards that can be present when working within the
STP can include gases such as H2S and Methane. These will be identified through the use of a calibrated Atmosphere monitor. Continuous ventilation of the plant must be maintained. Bacteria found within the sewage plant can also be dangerous to
personal if contact is made therefore appropriate full protective
clothing, masks, eye goggles etc listed in the risk assessment shall be worn

Question 3

what are some of the main components in a vacuum sewage treatment plant?

Answer 3

Main components of the vacuum sewage treatment plant on board ship are ejector (for creating vacuum in toilets), pressure switch (to maintain vacuum in the system) and level switch for the operation of treated effluent discharge pump.

Question 4

describe the operation of a air ejector

Answer 4

The air ejector consists of a jet tube of stainless steel, a check valve and a nozzle. The driving water for the ejector is supplied from the collecting tank with the help of a pump and the discharge is fed back to the tank.Sewage is pumped from the tank through thenozzle of the ejector creating a venturi effect whereby air and waste are inducted from the toilet piping, mixed with thesewage jet and passed into the collecting tank.When the pump stops, the rubber flap of the check valve is drawn into the closed position by the vacuum, which isthereby maintained.

Question 5

what maintenance is carried out for air ejector?

Answer 5

Tightness of the ejector flap shall be checked periodically.

Once a year:
Open and clean ejector check valve

Every five years:

Change ejector flap
Clean ejector nozzle
Check and clean ejector from build-up of deposit

Question 6

Describe the Operation of Pressure Switch

Answer 6

The vacuum level in the system is controlled by a pressure switch.The pressure switch or switches and the vacuum gauge should be installed on a separate manifold, to reduce thechance of impurities entering the switch or gauge. The instrument branch must have 5° slope. Water in theinstrument branch will cause malfunctions.
The switch starts a pump when the vacuum level in the system falls below a pre-set value. If the vacuum level stillkeeps declining, another pump will be started (provided that there is more than one pump in the system). As soonas the required vacuum level has been reached, the pump will be stopped.

1.Set the stop pressure by turning the knob. As you turn the knob, the indicator on the scale will move. It should beset between-0.4 bar and -0.6 bar (40-60% vacuum). This is the vacuum level at which the pumps will be turned off.Note that in each system the optimal setting may vary.

2.Set the start pressure by turning the knurled knob, reached by removing the cover of the switch. Turning the knobwill set the starting vacuum level somewhat lower than the “stop “level set by the knob. In effect, whenever the“stop”level is changed by turning the knob, the starting level will also be changed.

3.The plant can be provided with a low level pressure switch for low vacuum alarm. Set the “off” pressure to-0.3 bar.

4.The setting of the switch may vary from system to system depending on the individual requirements.

5.The vacuum gauge installed, in each system enables you to check that pressure switches are functioning properly.

Question 7

what maintenance must be carried out for pressure switch?

Answer 7

Once a year:
Check the operation of both pressure switch and vacuum gauge.

Every five years:
Clean or change piping and components between switch and gauge.

Question 8

Describe the Operation of magnetic Level Switch

Answer 8

The liquid level in the tank is controlled by magnetic level switches, which start and stop the pump.The switches consist of a float, a body, a rubber gaiter, a mounting flange, a switch mechanism in the body, andtwo permanent magnets in repulsion, one in the float, the other in the switch insert.As the float and its magnets move up and down with the liquid level, the magnet in the switch insert movescorrespondingly and either opens or closes the circuit controlling the pump or an alarm function.

Question 9

Describe the maintenance carried out for a magnetic Level Switch

Answer 9

Once a year (or during tank cleaning):Clean the level switch

Before starting to remove a switch, make sure that liquid level in the tank is below it. If necessary, dischargeto required level, but be careful not to let the pumps run dry. Then switch off the electrical supply, remove the securingnuts, and pull out the switch. Clean the float and check that it moves freely.

Under certain conditions an excessive amount of foam may appear in the tank. Should the level switchesreact to the surface of the foam, instead of the actual liquid level, malfunctioning may occur in the system. To avoid oreliminate this problem, see underTrouble Shooting of Biological Vacuum Sewage Treatment Plant.

Question 10

what Precautions must be followed when Operatingaerobic Sewage Treatment Plant

Answer 10

Following precautions in operating biological vacuum sewage treatment plant must be followed to maintain optimal process conditions and effluent quality.

1.If there is no counter pressure for the discharge pump, a bend piping arrangement must be done with a pipeloop (goose neck) over tank’s water level to prevent siphoning.

2.The unit must be incontinuousoperation to maintain biological activity.

3.It will take about one month before effective biomass will overtake the process after first start-up or aftermaintenance break. During this bacteria breeding period unit is not working as designed.

4.Never discharge both aeration chambers totally at same time. Ensure always that there isbacterialpopulationleft in either chamber I or chamber II to avoid collapse of the biological process.

5.Amount of the needed dilution water can be calculated. Needed dilution water is 1 x incoming black water(vacuum system). The amount of black water in vacuum toilet system is15 l/person/day. A total dilution water needcan be calculated:Dilution water per day = (1 x 15 l/person/day) x persons on board.

6.Dilution water can be fresh, brackish or technical water. Dilution water is added into aeration chamber I.

7.Dilution is not needed, if vessel’s grey water is also processed in the treatment plant.

8.Some antifoaming chemicals has a very high BOD5 value. It is recommended to use antifoaming chemicalswhich have low BOD5 value.

9.All substances which might be harmful to the biological process such as large amounts of grease, acids,alkaline, etc. are not allowed to be led to treatment unit.

10.Never let the pump run dry. Mechanical seals may be damaged.

11.Effluent quality is within IMO’s limits after about one month of the start up.

12.Take a sludge content test at least every week or every time when sludge “SLUDGE ALARM” is displayed.

13.Make sure that the unit is well ventilated and there is enough fresh air in the chamber if you have to goinside to avoid inhaling toxic fumes and suffocation. One person must stay outside of the tank and keep eye onperson who is working inside the unit.

14.Make sure that the waste water is lead to a proper holding tank (hull tank or collection tank) during shutdownor maintenance break.

Question 11

what alarms are provided in Sewage Treatment Plant

Answer 11

HIGH LEVEL ALARM
Water level is over high level switch LSH over presetted time, timer setting 30s.

VACUUM FAILURE/COLLAPSE
Led is on (vacuum failure), ejector pump has been running over presetted time, timer setting 15min.
Led is blinking (vacuum collapse), vacuum level below presetted value -0.2bar (Pressure switch alarm) over presetted
time, timer setting 2min.

COMMON ALARM
Emergency stop, emergency stop pressed down.
High level alarm, see above.
Blower overload, circuit breaker tripped.
Discharge pump overload, circuit breaker tripped.
Control voltage failure, circuit breaker tripped.
Chlorination pump overload, circuit breaker tripped.
Antifoaming pump overload, circuit breaker tripped.

SLUDGE ALARM
Presetted number of discharge times reached, counter setting 200 times.

Question 12

with reference to a vacuum sewage treatment plant

provide causes and remedies for each of the following problems

1. Vacuum is not created
   even though pump is
   running.
2. discharge pump will not start
3. discharge pump will not stop
4. Discharge pump is running but not
   pumping.
5. overflow in system.
6. Air blower does not run.
7. Air blower is running, but not creating sufficient pressure (low pressure)
8. Plant smells strongly.
9. There is too much black sludge on
   surface in settling tank compartment III.
10. There is no residual chlorine in the
    discharge water.

Answer 12

1. causes

Tank empty
Foam instead of liquid in tank
Valves are shut
Incorrect rotation of motor
Blockage or leakage in piping system.

remedies

1.If the tank is empty, fill to at least to thelow level switch. Check the condition ofthe pump according to the manufacturer´sinstruction.

2.If the pump is pumping foam instead ofliquid, this will be evident due to severevibration. Add water to the tank and try
again. If adding water does not help,reduce the level of foam by pouringantifoam agent into the tank (1 cup per 2cubic metres of foam and sewage).

3.Check that shut-off valves are fully openand not clogged.

4.If the direction of rotation of the pump iswrong, change wiring accordingly.
Close the valves that isolate the collectingunit from the piping system and start thepump again. If vacuum now builds up, there
must be a leak in the piping system.

2. causes

1.Motor defect
2.No power
3.Automatic fuses off
4.Impeller defect
5. Failure in float switch.

remedies

1.Replace motor.
2.Check the electrical system.
3.Check reason for this before
switching fuses on.
4.Measure clearance of impeller.
5. Check float switch and wiring, repairor replace.

3. causes

1.Failure in float switch.
2. Too much water enters the plant.

remedies

1.Check float switch and wiring, repairor replace.
2. reduce peak flow.

4. causes

1.Wrong rotation direction.
2.Impeller defect.
3.Valve in discharge system closed
or defect.

remedies

1.Change phases on electrical motor.
2.Replace impeller.
3.Check valves in discharge system.

5. causes

1.Pump is clogged.
2.Discharge pump defect.
3.Too much water enters the plant.

remedies

1.Open and clean pump.
2.Open bypass and repair/replace
discharge pump.
3.Allowed in peak periods. Try to
reduce peak flow.

6. causes

Screen in aeration chamber clogged.
* Timer wrongly adjusted.
* No power.
* Automatic fuses off .
* Motor defect.
* Impeller defect.
* Defect non-return valve and water in air blower.

remedies

Clean screen and remove dirt.
* Check and adjust timer .
* Check the electrical system.
* Check reason for this before
switching fuses on.
* Replace or repair.
* Replace impeller.
* Repair and replace valve.
! NOTE: In case of emergency, use
vessel’s air system, but pressure
must be less than 1.5 bar.

7. causes

1.Defect non-return valve.
2.Wrong direction of rotation.

remedies

1.Repair or replace.
2.Change phases on electrical motor.

The direction of a 3 phase motor can be reversed by swapping any two phase connections. This reverses the direction of the rotating magnetic field and causes the rotor to run in the reverse direction.

8. causes

1.Too much mineral sludge.
2.Bacterias killed. Brown sludge
indicates normal bacteria life. Blacksludge indicates that bacteria arekilled.

remedies

1.Check sludge content anddischarge if required.
2.Check air blower function by openingthe man holes. Surface bubblesmust be seen during bloweroperation.

9.causes

1.Insufficient operation of sludge return pump.

remedies

1.Check and adjust return sludge air
valve. Check air hoses.

10. causes

1.Dosing pump is not operating.
2.Disinfectant tank is empty.
3.Dosage rate is too low.
Disinfection chlorine is obsolete.

remedies

1.Check pump operation.
2.Fill it up with approved disinfectant.
3.Increase dosage by adjusting dosingpump. Replace with “fresh” chlorine.

Question 13

what is sludge testing?

Answer 13

Sludge Testing

It is important to keep a sludge content within certain limits i.e. a good biological balance for the treatment process inthe sewage treatment unit.

Sludge content test, aeration chambers I and II

Measure the sludge levels in aeration tanks I and II using test cocks while blowers are running.
Fill the glass funnel with 1000ml and throw the first “sample” water away immediately. This action will “clean” thesample valve and increases test result significantly.
Fill the glass funnel with 1000ml of waste water from the test cock of aeration chamber I or aerationchamber II while the blower is running.
Let the sludge settle for 30 minutes in this glass.
Observe the colour of the sludge and odour. If the biological process is running well, sample colour is grey or brown andsample will not smell significantly. If the sample colour is black and/or it smells, it is a indication that biological process does not work properly.
Read the sludge quantity from the side of the glass.

Interpretation of Results

Sludge content measurementbelow 300 ml/l = sludge content too low for biological process.
Between 300 ml/l and 600 ml/l = normal
Above 600 ml/l = sludge content too high, discharge sludge.

Question 14

with reference to sewage treatment plants;

a,) what is disenfection?

b.) what chemical is used for disinfecting?

c.) why is disenfection and residual testing carried out ?

d.) describe how to carry out residual chlorine testing

Answer 14

Disinfection is a process that reduces the numbers of micro-organisms to a level at which they are not harmful. conductedto ensure compliance with environmental regulations and to assess the effectiveness of the ship’s sewage treatment system.

When sailing in areas where chlorine disinfection of treated waste water is required, the chemical dosing systemshould be prepared as follows:

Check that the chlorine container is filled. Fill it if needed.

Check that dosing pump is on and adjusted correctly.

Chlorine concentration must be monitored regularly. Residualchlorine should be 2-5 ppmin effluent. If chlorine testshows that residual chlorine is higher than 5 ppm or lower than 2 ppm, adjust chlorine pump’s stroking rate.

Disinfection chemical

Recommended disinfection chemical is sodium hypochlorite (NaClO), technical quality, concentration 10% activechlorine.The storage of chlorine solution should be in a dark room, temperature in between 10-15 °C otherwise it willloose free chlorine. The maximum storage time is 180 days at +10°C. The maximum storage time depends on storingtemperature i.e. the maximum storage time is only 50 days at +25°C if redundancy is 50%..

Disinfection

Disinfection chemical (e.g. Sodium hypochlorite (NaClO), solution, active chlorine 10%) is added to the clarified waterin the disinfection chamber to meet IMO’s requirements regarding presence of coliform bacteria in treated water.Residual chlorine must be kept between 2 ppm and 5 ppm. Residual chlorine can be adjusted by the dosing pump’ssettings and/or timer settings. Treated water can be pumped to sea or ashore by the discharge pump.

Residual chlorine test

Take a discharge water sample from a test cock after discharge pump while a discharge pump is running. Measurethe free chlorine content from the effluent with a test kit.

Test procedure:

1.Fill the sample glass and throw the first “sample” water away immediately. This action will increases test accuracy
significantly.

2.Fill the sample glass from the test cock after discharge pump while the discharge pump is running.

3.Immerse test paper in effluent.

4.Read the residual chlorine by comparing test papers color to the reference color map. For detailed information see
the test kit’s instructions.

Question 15

describe the maintenance carried out on aerobic sewage treatment plants

Answer 15

Maintenance

Periodical maintenance of sewage treatment plant is required to ensure treatment process is running as designed and there is nomalfunctions with the risk of anaerobic process resulting formation of methane gas and toxic fumes. The tanks require de-sludging, usually every 3 months, this involves pumping the first
stage out via the discharge pump, the tank should never be totally emptied whilst it is still in operation or all the bacteria will be removed. Air diffusers are changed periodically as the diffuser holes enlarge over time and reduce the effectiveness of the aeration process. Air compressor inlet filters must be cleaned regularly and the compressor vanes must be checked for wear. The level in the chlorinator must be regularly checked and topped up as necessary. Observe that the treatment unit is operating normally and there is no alarms displayed. Check that sludge flows through the sludge return hose (by air lift) when the air blower is running. Check a smell of the unit. If the unit is smelling, it is most probably that aeration is not working and treatmentprocess has changed to anaerobic. These plants run well if not disturbed, it is best to leave them on line even during long passages where they do not have to run legally. If the flow of sewage into the plant is kept constant the size of the colony of bacteria will remain steady at the correct amount to fully digest all the sewage. If the flow is stopped and started the colony will die off and then have to expand instantly to digest the new sewage, this is not possible and the plant will not operate correctly. If the colony of bacteria are allowed to die off it will take between 7 and 14 days for a new colony to be established. Operation of the air lifts and air compressor outlet pressure (0.2 bar) must be checked every watch. Chlorine tablet level or dosing tank level must be maintained in coastal waters, within 12 n.m.

Every week

1.If the chemical dosing system is in use, check the chlorine content in the effluent water regularly.

2.Test sludge content in activation chamber I to ensure that mineral sludge content is within acceptable limits.

Take a sludge content test at least every week or every time when sludge “SLUDGE ALARM” is displayed.

Every month

1.Check that there is no obstruction in the aeration piping and in the air distributors.

2.Check that there is no obstruction in the overflow between the aeration chamber II and the settling chamber.

3.Check that there is no obstruction in the overflow between the settling chamber and the disinfection chamber.

4.Check that there is no obstruction in the venting line.

5.Inspect the tank’s external and internal coatings for corrosion.

6.Check vacuum function.

Every year

1.Empty and clean the unit.

2.Make sure that the unit is well ventilated and there is enough fresh air in the chamber if you have to goinside to avoid inhaling toxic fumes and suffocation. One person must stay outside of the tank and keep eye onperson who is working inside the unit.

3.Make sure that the waste water is lead to a proper holding tank (hull tank or collection tank) during shutdownor maintenance break.

4.Perform the maintenance for the components of the unit according to the component maintenance program.

Question 16

how is vacuum generated what is the vacuum generation unit?

Answer 16

The vacuum is created in sewage piping by means of an ejector. The vacuum generationunit consist of ejector, ejector pumps, pressure switch, vacuum gauge, shut-off valves and antifoamingdosing unit. Vacuum generation is controlled by the vacuum switch.

Question 17

what’s the purpose of sewage holding tank?

Answer 17

A holding tank is required where no discharge of treated or untreated sewage is allowed in a port area. The sewage is pumped out to shore reception facilities or overboard when vessel is proceeding on passage at sea as per the instructions in the MARPOL 73/78, Annex – IV.

Question 18

with the aid of a sketch describe a zero discharge system

Answer 18

see EOOW IAMI/ORAL Sketch Pack

zero discharge system has an initial reception chamber in which separation of liquid and solid sewage takes place. Wastes drop onto a moving rubber belt (driven by an electric motor) which the liquid passes through but solids travel with the belt to fall into a caustic treatment tank. Solids are then transferred by a grinder pump to he sullage or holding tank. The liquid passes via the perforated belt to treatment tanks which contain chlorine and caustic based compounds. These chemicals make the liquid effluent acceptable for use as a flushing fluid. The Pneupress arrangement which supplies liquid for flushing the toilets can deliver re-circulated fluid or, when the vessel is on passage, seawater. Capacity of the holding tank is 2 liters per person per day. The tank is pumped out at sea, or to shore if the ship is in port for a long period. Tank size is small because liquid effluent passes mainly to the flushing system. Excess overflows to the sullage tanks.

Question 19

with the aid of a sketch describe the operation of a biological aerobic sewage treatment plant

Answer 19

see sketch pack for drawing

This plant consists of three main stages.

Stage 1 Aeration

Sewage and toilet water contents enter the first stage either directly or they are first passed over a grid to remove any foreign objects. Air diffusers are fitted at the bottom of the first stage, these are supplied with low pressure compressed air from the plant’s air compressors. The air compressors are usually of the oil free sliding vane type. The air leaves the diffusers through small holes and then bubbles upwards through the sewage, the holes in the diffuser expand over time and this causes the air bubbles to be too large so there is not enough surface area in contact with the sewage. The air bubbling through the sewage and the sewage provide the bacteria with the
nutrients they need to thrive. The bacteria will start to break the sewage down into water, carbon dioxide and an inert sludge

Stage 2 Settling

Liquid from the first stage can pass from this stage to the second “hopper” stage by passing through a course screen. Any solid matter will drop to the bottom of the second stage and any floating particles will move to the top of the second stage. The solids at both the top and bottom of the second stage are removed by air blowers which act to return the solids to the first stage. If the air blowers stopped operating the second stage would eventually block up completely. The liquid near to the surface of the second stage is skimmed off and overflows to the third section.

Stage 3 Chlorine contact tank

The effluent passes through the chlorinator where the liquid is disinfected, due to the chlorine tablets which are added. The tablets dissolve at the correct rate to fully, disinfect the effluent. After disinfection the effluent passes into the third stage this being the chlorine contact tank which is a holding tank. The effluent must stay in the tank for the correct contact time with the chlorine to ensure complete disinfection, this is usually 1 hour. A chlorine dosing pump is connected to the third stage and it operates to dose the stage with chlorine every time the discharge pump has emptied the chamber, this again ensures the correct contact time with the chlorine. After the effluent has been fully disinfected its pumped out of the third stage and into the sea by a discharge pump operated by the high and low level float switches, which control the discharge pump, as well as a high level alarm.

Although the sewage treatment plant runs automatically, without regular maintenance the unit will not function properly and anaerobic bacteria may promote the formation of hydrogen sulphide and methane, both of which are hazardous. The chambers need to be cleaned out occasionally to remove any accumulated matter. The aeration diffusers should be checked to ensure they are clear and that air is bubbling from them. The air lift returns should also be checked to make sure they function correctly. These usually have a clear plastic pipe so that the sludge can be seen returning to the aeration chamber. The internal tank coating should be inspected for any signs of cracking or blistering. When cleaning out a sewage unit rubber gloves and a mask should be worn. After overhaul the external surfaces of the unit and surroundings are to be washed down with disinfectant. Hands should also be thoroughly scrubbed and overalls washed.

Question 20

With the aid of a sketch explain the working principle of a sludge incinerator?

Answer 20

SEE EOOW IAMI/ORAL Sketch Pack for drawing

These are capable of dealing with waste oil, oil and water mixtures of up to 25% water content, rags, galley waste and solid matter from sewage plant if required. Solid matter from sewage system could be incinerated in this unit but a connection has to be made from sewage treatment plant to the pyrolysis chamber of the incinerator. This is used when the excess sludge builds up in the settlement chamber and then has to be discharged at regular intervals. However if the ship is in port this is not possible so instead it pumped to a sullage tank instead. Its possible to instead use the incinerator.

Working principle

Homogeneous oil and water mixture that has been formed by passing them through a comminutor (grinder), macerator (mixer that produces a fine well dispersed emulsion) is supplied to the rotating cup burner. Solid waste from galley and accommodation, etc.. would be collected in bags and placed in the chamber adjacent to the combustion chamber, the loading system of which is self evident in the diagram. The loading arrangement incorporates a locking device that prevents doors (loading and ash pit) from being opened with the burner on. The solid waste goes through a process that is described as pyrolysis (application of heat). hydrocarbon gases are formed, due to low air supply to the compartment, which pass into the main chamber through a series of small holes and burn in the furnace. Dry ash remaining in the chamber is removed periodically through ash pit door.

Question 21

why and how is sewage treated onboard?

Answer 21

The discharge of untreated sewage in controlled or territorial waters is usually banned by legislation. International legislation is in force to cover any sewage discharges within specified distances from land. As a result, and in order to meet certain standards all new ships have sewage treatment plants installed. Untreated sewage as a suspended solid is unsightly. In order to break down naturally, raw sewage must absorb oxygen. In excessive amounts it could reduce the oxygen content of the water to the point where fish and plant life would die. Pungent smells are also associated with sewage as a result of bacteria which produce hydrogen sulphide gas. Particular bacteria present in the human intestine known as E, coli are also to be found in sewage. The E. coli count in a measured sample of water indicates the amount of sewage present. Two particular types of sewage treatment plant are in use, employing either chemical or biological methods. The chemical method is basically a storage tank which collects solid material for disposal in permitted areas or to a shore collection facility. The biological method treats the sewage so that it is acceptable for discharge inshore.

Chemical sewage treatment

This system minimises the collected sewage, treats it and retains it until it can be discharged in a decontrolled area, usually well out to sea. Shore receiving facilities may be available in some ports to take this retained sewage. This system must therefore collect and store sewage produced while the ship is in a controlled area. The liquid content of the system is reduced, where legislation permits, by discharging wash basins, bath and shower drains straight overboard. Any liquid from water closets is treated and used as flushing water for toilets. The liquid must be treated such that it is acceptable in terms of smell and appearance. A treatment plant is shown diagrammatically in Figure 7.9. Various chemicals are added at different points for odour and colour removal and also to assist breakdown and sterilisation. A comminutor is used to physically break up the sewage and assist the chemical breakdown process. Solid material settles out in the tank and is stored prior to discharge into the sullage tank: the liquid is recycled for flushing use. Tests must be performed daily to check the chemical dosage rates. This is to prevent odours developing and also to avoid corrosion as a result of high levels of alkalinity.

Biological sewage treatment

The biological system utilises bacteria to completely break down the sewage into an acceptable substance for discharge into any waters. The extended aeration process provides a climate in which oxygen-loving bacteria multiply and digest the sewage, converting it into a sludge. These oxygen-loving bacteria are known as aerobic. The treatment plant uses a tank which is divided into three watertight compartments: an aeration compartment, settling compartment and a chlorine contact compartment (Figure 7.10). The sewage enters the aeration compartment where it is digested by aerobic bacteria and micro-organisms, whose existence is aided by atmospheric oxygen which is pumped in. The sewage then flows into the settling compartment where the activated sludge is settled out. The clear liquid flows to the chlorinator and after treatment to kill any remaining bacteria it is discharged. Tablets are placed in the chlorinator and require replacement as they are used up. The activated sludge in the settling tank is continuously recycled and builds up, so that every two to three months it must be partially removed. This sludge must be discharged only in a decontrolled area.

Question 22

what does an incinerator do?

Answer 22

Stricter legislation with regard to pollution of the sea, limits and, in some instances, completely bans the discharge of untreated waste water, sewage, waste oil and sludge. The ultimate situation of no discharge can be achieved by the use of a suitable incinerator. When used in conjunction with a sewage plant and with facilities for burning oil sludges, the incinerator forms a complete waste disposal package. The combustion chamber is a vertical cylinder lined with refractory material. An auxiliary oil-fired burner is used to ignite the refuse and oil sludge and is thermostatically controlled to minimise fuel consumption. A sludge burner is used to dispose of oil sludge, water and sewage sludge and works in conjunction with the auxiliary burner. Combustion air is provided by a forced draught fan and swirls upwards from tangential ports in the base. A rotating-arm device accelerates combustion and also clears ash and non-combustible matter into an ash hopper. The loading door is interlocked to stop the fan and burner when opened. Solid material, usually in sacks, is burnt by an automatic cycle of operation. Liquid waste is stored in a tank, heated and then pumped to the sludge burner where it is burnt in an automatic cycle. After use the ash box can be emptied overboard

With ever stricter regulation with regards to marine pollution, sometimes it may not be possible to discharge untreated waste water, sewage, waste oil and sludge. Incinerators are capable of dealing with waste oil, oil and water mixtures up to 25% water content, rags, galley waste, etc., and solid matter from sewage plants if required. After use the ash box can be emptied overboard.

Question 23

What are the requirements for sewage treatment plants can be onboard a vessel?

Answer 23

1) Sewage treatment plant must approved by an Administration.

2) Sewage treatment plant must have facilities for temporary storage of sewage when the ship is less than 3 NM from land.

3) Holding tank of the capacity approved by the administration regarding the operation of the ship and number of persons with a means to visually inspect all contents

Question 24

What substances are prohibited for incineration?

Answer 24

1) Residues of cargoes from Annex I, II and III.

2) Polychlorinated biphenyls.

3) Garbage containing more than traces of heavy metals.

4) Refined petroleum products
containing halogen compounds.

5) Sewage sludge and sludge oil that are not generated on ship.

6) Exhaust gas cleaning system residues.

Question 25

What effluent standards should a sewage treatment plant abide to?

Answer 25

1) Thermotolerant Coliform Standard (100/100ml)
2) Total Suspended Solids (TSS) Standard
3) Biochemical oxygen demand without nitrification and chemical oxygen demand
4) pH (6-8.5)

Question 26

What does the screen filter do in a STP?

Answer 26

Fitted for the first inlet and removed non-sewage items that can clog the system.

Question 27

What does the biofilter do in a STP?

Answer 27

Part of the aeration chamber that treats sewage with the air bubbles from the blower by breaking down organic matter.

Question 28

What does the settling/sedimentation chamber do in a STP?

Answer 28

The mixture is separated to water in the middle, sediment on the top, and heavy waste at the bottom. The untreated items will return into the biofilter reactor.

Question 29

Precautions for efficient operation of STP?

Answer 29

1) Aeration blower is installed to run continuously. Do not stop.
2) Do not throw foreign substances into toilets.
3) Never use unauthorised chemicals to clean toilets.
4) Grey water inlet pipe placed lower than the water level inside STP to decrease foam generation.
5) pH of samples 6-8.5
6) Nitrite content not to exceed 10mg/ltr NO2

Question 30

describe the two types of sewage vacuum systems?

Answer 30

There are two types of vacuum systems:

1. vacuum holding tank
2. The holding tank is at atmospheric pressure.

Vacuum Holding Tank

The tank and the piping are kept under a constant vacuum of about half an atmosphere. The vacuum is created by a vacuum pump connected to the tank. A special vacuum toilet is used in the system. Each toilet is equipped with a discharge valve of rubber membrane type. By pushing a flushing knob the vacuum operated mechanism is activated and the discharge valve is opened. At the same time the water valve is opened and fresh water starts, flushing the bowl. The amount of water used is 1.2 to 1.5 litres per flushing compared to the 8 to 10 litres used by a conventional toilet. The discharge valve is open for about 3 seconds and the water valve for about 7 seconds. When the discharge valve has opened the black water, in the shape of a liquid plug, is sucked into the pipe, due to the difference in pressure between the air in the toilet room and the vacuum in the pipe. Together with the soil and water, 80 to 120 litres of air enters the piping system and pushes the waste at a high speed towards the collecting tank. Due to the friction the air will break through the liquid plugs. To reform the plug, the pipes are fitted with pockets at a certain distance where the soil is gathered under the non transport periods. The pockets are installed at intervals between 40 to 60 metres. When a toilet is flushed all plugs between the toilet and the tank will be transported along the line towards the tank. The transport is therefore intermittent and not continuous. The soil is stored in the tank and the air is evacuated by the vacuum pump. The collected waste is disposed intermittently from the tank either by pumping it overboard or by pumping it into a sewage treatment tank. In order to pump out the tank the vacuum must be broken. This means that the toilets are out of operation whenever the tank is being emptied

Atmospheric Pressure Holding Tank

An atmospheric pressure holding tank vacuum system uses the same principle of operation as previously described except that the tank itself is not under a vacuum only the piping is. A liquid level is maintained in the tank at all times, this liquid is pumped through a pump and an ejector and back into the holding tank. A vacuum is created at the throat of the ejector and this point is connected to the system piping. The ejector pump does not run continuously but is controlled by a vacuum switch in the pipe line.
The holding tank will be arranged to overflow into either a sewage treatment tank or overboard.

This system has several advantages over a vacuum holding tank:

1. The tank is not a pressure vessel, so does not require surveying as a pressure vessel.
2. The toilets do not have to be shut down in order to empty out the holding tank.

Disadvantages

The only major disadvantage is that the ejectors tend to get clogged if there are any foreign objects in the sewage.

Question 31

what are the different types of sewage treatment plants

Answer 31

Sewage treatment plants four types;

1. Chlorination: In this relatively simple system the sewage is kept in storage (such as on cruise ships) or holding tank where it is disinfected (chlorinated) before being pumped out at sea or to a shore facility
2. Physical/Chemical: In this system, the solids and liquids are separated through a course screen filter. The liquid is further clarified by being filtered through fine filters. It is then disinfected before it is discharged overboard. Solids are retained for dumping out at sea or to a shore facility.
3. Electrochemical: In this system, the sewage is through an electro-catalytic cell which oxidises the sewage. The salt in the seawater that is used for flushing the toilets gets converted to sodium hypochloride, a disinfectant, by the cell. No residual sludge is left after the process.
4. Biological: There are two further categories under this heading; aerobic and anaerobic. Both the systems operate with the help of bacterial that consume the sewage. Bacteria that require oxygen for their survival are referred to as aerobic. Aerobic bacteria are normally used in ships’ sewage treatment equipment. Anaerobic bacteria that do not require oxygen are normally used in shore based sewage treatment works and in septic tanks in remote areas.

Question 32

what are the advantages and disadvantages of zero discharge system?

Answer 32

Advantages of Zero Discharge Systems

1. Does not have to rely on a colony of bacteria.
2. Can be started up and shut down at will as the ship enters and leaves regulated
   waters.

Disadvantages of Zero Discharge Systems

1. Requires more attention while in use due to filter and screens which require regular
   cleaning.
2. Solids accumulate at a faster rate than in biological plants.

Question 33

what do the regulations of MARPOL ANNEX IV state?

Answer 33

Every ship of 400 GT and above which is engaged in international voyages, and carrying minimum 15
persons onboard must be equipped with either a sewage holding tank of appropriate capacity or an approved
sewage Treatment Plant (STP) or both

The sewage discharge from the ship is allowed if it has an approved sewage treatment plant, which can treat
the raw sewage and discharge comminated and disinfected sewage. With this arrangement, the discharge is
allowed at a distance of more than 3 nautical miles from the nearest land when the ship is proceeding with a
speed of 4 knots and above

Foreign going cargo ships may be allowed (depending upon the area they are plying) to discharge untreated
sewage only at a distance greater than 12 nautical miles from the closest land only if the ship is proceeding
with a speed of 4 knots and above