Sewage Treatment PLANT

Biological Vacuum Sewage Treatment Plant on Ships

Introduction to Biological Vacuum Sewage Treatment Plant

The biological vacuum sewage treatment plant consists of an integrated vacuum generator and a biological sewage treatment plant. In the sewage treatment plant an aerobic process of the biopopulation convert organic substances existing in waste water to carbon dioxide and water without danger of methane gas production.

All waste water, both black water (toilets, urinals, hospital) and grey water (galley, showers and sinks) can be treated in the biological vacuum sewage treatment plant. The treatment plant can operate in all vessels which are sailing in salt, brackish or fresh water areas.

The biological vacuum sewage treatment plant is fully automatic in normal operation and they require low maintenance.These plants are designed to fulfill the requirements of IMO MARPOL 73/78 Annex IV and USCG 33 CFR Part 159 for inspected vessels which specify discharged waste water from sewage treatment plants.

The effluent quality is as follow:

• BOD5 < 50 ppm

• Suspended solids < 50 ppm (tested on shore) or 100 ppm (tested aboard ship)

• Faecal coliforms < 250 pcs/100ml M. P. N.

Figure below shows biological vacuum sewage treatment plant normally used on ships.

Operation

Black sewage water is collected by a vacuum ejector system as shown above. Incoming black water must be diluted with technical, grey or sea water. There is a separate dilution/gravity inlet connection built in chamber I. All waste water is led to the aeration chamber I by vacuum from vacuum toilets and by gravity from wash basins. Bacterial growth is stimulated by oxygen of air supplied by the blowers. Required air is produced by the air blowers and led to aeration chambers I and II via air distributors. The aerobic treatment process starts in the aeration chamber I and continues in the aeration chamber II. If treated waste water is coming from a vacuum toilet system, required dilution water must be added into the aeration chamber I.

An activated sludge is separated in the settling chamber by gravity and clarified water floats to the disinfection chamber. The activated sludge is pumped back to the aeration chamber I by an air driven ejector (also called as air lift). A chlorine based disinfection chemical is added by the dosing pump into the clarified water in the disinfection chamber to meet IMO’s requirements regarding presence of coliform bacterias in treated water. Treated water is pumped to sea or ashore by a discharge pump. Disinfection can be also done by UV on some models. A mineralized sludge from activation chambers I and II is pumped at given intervals sea, to on shore facilities or to the vessel’s sludge storing tank. The discharge pump is equipped with an integrated cutting device to prevent blocking.

Vacuum generation

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

Aeration and blowers

Waste water is led to the aeration chamber I by gravity and vacuum. Bacterial growth is stimulated by oxygen of air. Required air is produced by the air blowers and led to the aeration chamber I and II via installed aerators. An aerobic process continues in the aeration chamber II. Inorganic solids (for instance plastic) are stopped in the aeration chambers I and II. Air flow can be adjusted between chambers I and II using air valves.

Settling of Sludge

Activated sludge is separated in the settling chamber by gravity and clarified water flows to the disinfection chamber. Activated sludge is pumped back to the aeration chamber I by an air driven ejector pump (also called as air lift). Air flow for the sludge ejector is adjusted so that the sludge return from the chamber III is about 1/3 of the pipe section (A transparent hose above tank). Rest of the air flow produced by the blower is used for aeration and divided equally between tanks I and II. Sludge has to be removed frequently from the process to maintain a good biological balance in the sewage treatment unit. Sludge content in process (in chamber I) must be kept between 100ml/l and 500 ml/l. Sludge is removed by the discharge pump.

Disinfection

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

Materials of Construction

• Tank material: Steel S355 EN10025

• Ejector body: Acid proof steel EN 1.4404

• Welded flanges, pipe bend: Acid proof steel EN 1.4404

• Loose flange: Al

• Nozzle: PVC

• Flap: Nature Rubber NR

• Gaskets: Rubber EPDM

Additional Components

The biological vacuum sewage treatment plant may require additional components for optimal operation.

Dilution water

Because the combined treatment plant is processing black water from vacuum toilet system, dilution water must be added into incoming sewage. There are several system options to control and monitor dilution water:

• Flow meter and pressurized fresh or technical water

• Grey water buffer/holding tank and constant flow pump

• Grey water buffer/holding tank and dilution water feeding by gravity with flow meter

Screen or Catcher

If there is a risk that a lot of non organic solids obstacles are entering in the treatment plant, a screen or catcher must

be installed before the treatment plant.

Grease separator

Grease is harmful for treatment process. If there is galley water entering to the treatment unit a grease separator must

be installed before treatment plant.

Working and Maintenance of Air Ejector, Pressure Switch and Level Switch

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. Working and maintenance of air ejector, pressure switch and level switch are explained below.

Operation of Air Ejector

The air ejector consists of a jet tube of stainless steel, a check valve and a nozzle of PVC. 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 the nozzle of the ejector creating a venturi effect whereby air and waste are inducted from the toilet piping, mixed with the sewage 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 is thereby maintained.

Maintenance of Air Ejector

The only moving part in the ejector is a rubber flap in the check valve. 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

Operation of Pressure Switch

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 the chance of impurities entering the switch or gauge. The instrument branch must have 5° slope. Water in the instrument branch will cause malfunctions.

The switch starts a pump when the vacuum level in the system falls below a preset value. If the vacuum level still keeps declining, another pump will be started (provided that there is more than one pump in the system). As soon as 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 be set 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 knob will 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.

Maintenance of Pressure Switch

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.

Operation of Level Switch

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, and two 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 moves correspondingly and either opens or closes the circuit controlling the pump or an alarm function.

Maintenance of Level Switch

• 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, discharge to required level, but be careful not to let the pumps run dry. Then switch off the electrical supply, remove the securing nuts, 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 switches react to the surface of the foam, instead of the actual liquid level, malfunctioning may occur in the system. To avoid or eliminate this problem, see under Trouble Shooting of Biological Vacuum Sewage Treatment Plant.

Precautions in Operating Biological Vacuum Sewage Treatment Plant

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 pipe loop (goose neck) over tank’s water level to prevent siphoning.

2. The unit must be in continuous operation to maintain biological activity.

3. It will take about one month before effective biomass will overtake the process after first start-up or after maintenance 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 is bacterial population left 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 is 15 l/person/day. A total dilution water need can 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 chemicals which 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 go inside to avoid inhaling toxic fumes and suffocation. One person must stay outside of the tank and keep eye on person 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 shutdown or maintenance break.

Alarms in Vacuum Sewage Treatment Plant

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.

Troubleshooting of Biological Vacuum Sewage Treatment Plant on Ships

Trouble

Cause

Remedy

Vacuum is not created

even though pump is

running.

1. Tank empty

2. Foam instead of liquid in tank

3. Valves are shut

4. Incorrect rotation of motor

5. Blockage or leakage in piping system.

6. If the tank is empty, fill to at least to the low level switch. Check the condition of the pump according to the manufacturer´s instruction.

7. If the pump is pumping foam instead of liquid, this will be evident due to severe vibration. Add water to the tank and try

8. again. If adding water does not help, reduce the level of foam by pouring antifoam agent into the tank (1 cup per 2 cubic metres of foam and sewage).

9. Check that shut-off valves are fully open and not clogged.

10. If the direction of rotation of the pump is wrong, change wiring accordingly.

11. Close the valves that isolate the collecting unit from the piping system and start the pump again. If vacuum now builds up, there

12. must be a leak in the piping system.

The ejector pumps do not

create enough vacuum.

Sewage plug in the ejector.

1. Make sure that pump is stopped and shutoff valve is closed.

2. Remove plug and clean out ejector. If this procedure is unsuccesful, the following

3. steps should be taken.

4. Dismantle flange connections and remove bend.

5. Dismantle flange connections and remove ejector from tank.

6. Dismantle flange connections and remove connection cone.

7. Withdraw non-return valve from ejector.

8. Clean sealing surfaces and if necessary replace rubber flap.

9. While the ejector is dismantled, it is advisable to check the nozzle and the inside of the ejector housing for sludge and impurities.

10. Refit ejector and non-return valve in reverse order to dismantling, at same time checking the condition of all seals and replacing as

11. necessary.

12. Make sure that non-return valve is correctly positioned.

Discharge pump does not start.

1. Motor defect

2. No power

3. Automatic fuses off

4. Impeller defect

5. Failure in float switch.

6. Replace motor.

7. Check the electrical system.

8. Check reason for this before

9. switching fuses on.

10. Measure clearance of impeller.

11. Check float switch and wiring, repair or replace.

Discharge pump does not stop.

1. Failure in float switch.

2. Too much water enters the plant.

3. Check float switch and wiring, repair or replace.

4. Allowed in peak periods. Try to

5. reduce peak flow.

Discharge pump is running but not

pumping.

1. Wrong rotation direction.

2. Impeller defect.

3. Valve in discharge system closed

4. or defect.

5. Change phases on electrical motor.

6. Replace impeller.

7. Check valves in discharge system.

There is overflow in system.

1. Pump is clogged.

2. Discharge pump defect.

3. Too much water enters the plant.

4. Open and clean pump.

5. Open bypass and repair/replace

6. discharge pump.

7. Allowed in peak periods. Try to

8. reduce peak flow.

Air blower does not run.

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.

•

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.

•

Air blower is running, but not creating

sufficient pressure. Air blower is running, but low pressure.

1. Defect non-return valve.

2. Wrong direction of rotation.

3. Repair or replace.

4. Change phases on electrical motor.

Air blower is running , but no surface

bubbles in the activation tanks.

1. Hose or pipe defect.

2. Air distributors clogged or defect.

3. Repair or replace.

4. Clean or replace air distributors.

Plant smells.

1. Too much mineral sludge.

2. Bacterias killed. Brown sludge

3. indicates normal bacteria life. Black sludge indicates that bacterias are killed.

4. Check sludge content and discharge if required.

5. Check air blower function by opening the man holes. Surface bubbles must be seen during blower operation.

Discharge water is dirty.

1. Too much water enters the plant,

2. thereby creating too high hydraulic loading.

3. From newly started plants, bacterias not yet formed.

4. Bacterias killed.

5. Allowed in peak periods. Try to

6. reduce peak flow.

7. Check start up date. Time required

8. for bacteria formation is approximately 4 (four) weeks.

9. See “plant smells”.

There is too much black sludge on

surface in settling tank III.

1. Insufficient operation of sludge

2. return pump.

3. Check and adjust return sludge air

4. valve. Check air hoses.

There is no residual chlorine in the

discharge water.

1. Dosing pump is not operating.

2. Disinfectant tank is empty.

3. Dosage rate is too low.

4. Disinfection chlorine is obsolete.

5. Check pump operation.

6. Fill it up with approved disinfectant.

7. Increase dosage by adjusting dosing pump.

8. Replace with “fresh” chlorine.

Sewage Testing in Biological Vacuum Sewage Treatment Plant

Sewage testing is necessary to confirm that the biological sewage treatment plant is working as per the requirements of IMO.

Sludge Testing

It is important to keep a sludge content within certain limits i.e. a good biological balance for the treatment process in the 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” the sample valve and increases test result significantly.

• Fill the glass funnel (imhoff glass) with 1000ml of waste water from the test cock of aeration chamber I or aeration chamber II while the blower is running.

• Let the sludge settle for 30 minutes in this glass.

• Observe the color of the sludge and odor. If the biological process is running well, sample color is grey or brown and sample will not smell significantly. If the sample color 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 Result

• Sludge content measurement below 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.

Disinfection and Residual Chlorine Testing

When sailing in areas where chlorine disinfection of treated waste water is required, the chemical dosing system should 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. Residual chlorine should be 2-5 ppm in effluent. If chlorine test shows 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% active chlorine.

The storage of chlorine solution should be in a dark room, temperature in between 10-15 °C otherwise it will loose free chlorine. The maximum storage time is 180 days at +10°C. The maximum storage time depends on storing temperature i.e. the maximum storage time is only 50 days at +25°C if redundancy is 50%. See a chart below.

Residual chlorine test

Take a discharge water sample from a test cock after discharge pump while a discharge pump is running. Measure the 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

2. significantly.

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

4. Immerse test paper in effluent.

5. Read the residual chlorine by comparing test papers color to the reference color map. For detailed information see

6. the test kit’s instructions.

Maintenance of Biological Vacuum Sewage Treatment Plant on Ships

Maintenance

Periodical maintenance of biological vacuum sewage treatment plant is required to ensure biological treatment process is running as designed and there is no malfunctions with the risk of anaerobic process resulting formation of methane gas and toxic fumes (IMO MSC/CIRC. 648 -1994.06.06.).

Every day

1. Observe that the treatment unit is operating normally and there is no alarms displayed.

2. Check that sludge flows through the sludge return hose (by air lift) when the air blower is running.

3. Check a smell of the unit. If the unit is smelling, it is most probably that aeration is not working and treatment process has changed to anaerobic.

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 go inside to avoid inhaling toxic fumes and suffocation. One person must stay outside of the tank and keep eye on person 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 shutdown or maintenance break.

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

Sewage Treatment Plant – Zero Discharge System

A retention and 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.

Straight holding tank for retention of sewage during the period of a ship’s stay in port where of a size large enough to contain not only the actual sewage but also the flushing water. Each flush delivered 5 liters of seawater. Passenger vessels or ferries with automatic flushing for urinals required very large holding tanks. Very little flushing water is required for zero discharge system sewage treatment plants.

Problems resulting from the retention of untreated wastes relate to its breakdown by anaerobic bacteria. Clean breakdown by aerobic organisms occurs when there is amble oxygen. In the conditions of plain retention tank where there is no oxygen, anaerobic bacteria and other organisms thrive. These cause putrefaction, probably with corrosion in the tank and production of toxic and flammable gases.

The Eslan type sewage treatment plant, also known as zero discharge system has an initial reception chamber in which separation of liquid and solid sewage takes place. Wastes drop onto a moving perforated 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.

Biological Sewage Treatment Plant

Raw sewage is passed to an aeration chamber and air is supplied through diffusers. This promotes the action of aerobic bacteria which break down the sewage into carbon di-oxide, water and inorganic waste. Figure below shows a typical biological sewage treatment plant.

Sewage then passes into the clarification or settling chamber. Any solids that settle out are returned via an air lift to the aeration chamber which ensures that they are fully broken down. The sample applies to any surface scum. A small vane type air compressor supplies the air for air diffusers and air lift.

The clear liquid then passes through the chlorinator where the liquid is disinfected, into the chlorination chamber. The chamber has float switches, which control the discharge pump, and 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.