REVERSE OSMOSIS DESALINATION

Osmosis:

When fluid of different concentration in a vessel are separated by a membrane, the dilute solution will flow through the membrane into the concentrated solution.

Osmotic pressure:

The level of dilute solution drops and the level of concentrated solution rises until an equilibrium is reached. The pressure difference between the two levels is called osmotic pressure.

Reverse osmosis:

If a pressure excess to a osmotic pressure is applied to the concentrated solution, the flow is reversed from the concentrated solution to  the diluted solution.

Membranes used:

• Hollow fine fibre
• Spirally wound

For sea water application membranes used:

• Polyamide
• Polysulphonate

In reverse osmosis desalination:

Pic: Reverse osmosis desalination

• Sea water feed is pressurized using high pressure pump.
• The high pressure sea water is passed through a chamber containing a permeable membrane.
• The membrane stops the passage for salt and allows pure water to pass.
• The clean water is then sent to a potable water tank.

MARINE SEWAGE TREATMENT PLANT

Principle of sewage treatment

• The breakdown of raw sewage in water is effected by aerobic bacteria if there is a relatively ample presence of oxygen, but by anaerobic bacteria if oxygen has been depleted.
• Aerobic bacteria require free oxygen to survive.They breakdown the organic matter to produce safe products such as water, carbon dioxide. Final discharge has a clean and clear appearance.
• Anaerobic bacteria can only multiply in the absence of free oxygen.They breakdown the organic matter into water, carbon dioxide,methane, hydrogen sulphide and ammonia which are noxious and toxic.

Pic: Sewage treatment plant

Components/Chambers/Tanks

• Wire mesh
• Primary tank
• Aeration chamber
• Settlement tank
• Chlorination and collection tank
• Blowers
• Chlorinator
• Low level, high level alarm

Operation

• Sewage water is introduced into a primary vessel through a wire mesh.
• It is then passed through a wire mesh which grinds the sewage and breakdown into smaller particles.
• A plate is provided which makes the sewage to rise and flow into the aeration chamber.
• Air is forced through the diffuser into the air chamber which helps the aerobic bacteria to grow and attack the sewage.
• Constant pressure of around 0.3-0.4 bar is kept for proper agitation and formation of bubbles.
• The sewage is decomposed into carbon dioxide, water and inorganic sewage.
• The mixture of liquid and sludge is passed to the settling tank from the aeration chamber.
• The sludge settle at the bottom and  clear liquid on the top, sludge is not kept in settling tank as it may produce anaerobic bacteria and foul gases are produced.
• The sludge formed is recycled by sending it back to the aeration chamber.
• The clear liquid produced from the settling tank is overflown and is disinfected with the help of chlorine.
• This is done due to the presence of E-coli present in liquid which has to be eliminated.
• The liquid during chlorination is kept for a period of at least 60 min.
• The collected liquid is discharged overboard when the high level alarm raises and the vessel is in proper geological location. 

Effluent quality standards

• The bio-chemical oxygen demand (BOD) is determined by incubating at 200 degree Celsius, a sample of sewage effluent which has been well-oxygenated. The amount of oxygen absorbed over a five-day period is then measured. The test measures the total amount of oxygen used for complete breakdown of organic matter. This indicates the strength of the sewage. IMO recommends BOD of less than 50 mg/L after treatment through sewage treatment plant.
• Suspended solids - this can give rise to silting problems. Suspended solids are measured by filtering a sample through a pre-weighed pad which is then dried and re-weighed. IMO recommends 50 mg/ litre after treatment.
•  Coliform count - The e-coliform is a family of bacteria which live in the human intestine. The result of this test is called the e-coli count and is expressed per 100ml. Presence of these organisms in water is an indication of pathogen (disease causing bacteria responsible for cholera, dysentery, typhoid). IMO recommends fecal coliform count of less than 250/100 ml. of affluent after treatment.

The rules which has to be followed for the discharge of sewage is given in annex IV as below.

Marpol Annex IV: -

• Vessel should be equipped with a certified sewage treatment system or holding tank.
• Within 3 miles of nearest land, sewage discharges are to be treated by a certified marine sanitation that is a sewage treatment plant device prior discharge.
• Between 3 miles and 12 miles from shore, sewage discharges must be treated by no less than maceration or chlorination.
• Sewage discharge beyond 12 miles from shore is unrestricted.
• The discharge of sewage should be such that the ship is en route and is proceeding not less than 4 nautical miles.

MARINE OILY WATER SEPARATOR (OWS)

OILY WATER SEPARATOR

Engine room bilges in motor ships are frequently contaminated by  leakage of fuel or lubricating oil.

Oily water separator is a machine which is used to separate the oil and  the water mixtures from the bilge tank, the oil is retained on board and the water is discharged overboard.

Principle: Main principle of operation is the gravity differential between oil and water.

Pic: 2-Stage OWS

Components: -

• First stage separator
• Second stage coalesce
• Plates
• Filters
• Oil Probe
• Solenoid
• Vents

Pic: OWS with coalescer

Operation: -

• Clean water is delivered to the separator through the oily water inlet until discharge takes place out of the vent valve which is then closed.
• Oil water is now delivered to the separator and when the pressure inside the separator reaches 2 bar water discharge valve opens automatically.
• The mixture circulates and flow across the coarse separating compartment where some oil as a result of its lower density will separate and rise into the oil collection space.
• The oily mixture passes through corrugated baffle plate where oil sticks and forms globules which when become big will rise above due to up thrust on the globules.
• The almost oil free water passes into a central pipe and leaves the separator unit.
• Purity at this point is 100 ppm or less but more than 15 ppm.
• An oil probe is provided which will sense the oil content and operate a solenoid valve, the oil is then sent into the bilge tank.
• Air is released from the unit by a vent valve.
• Heaters are provided in the top of the unit to heat the oil and maintain a viscosity.
• The first stage filter removes physical impurities and promotes some fine separation.
• The water with small amount of oil is now sent to the second stage coalesce where it is passed through filters.
• The remaining oil separates out and the purity gets down below 15 ppm.
• This water is then discharged overboard by following certain marpol rules. 

Factors affecting separation: -

In general, a high rate of separation is encouraged by 

• a large size of oil globule
• elevated temperature of the system (which increases the specific gravity differential of the oil and water and reduces the viscosity of the oil)  
• use of sea water.  

Turbulence or agitation should be avoided since it causes mixing and  re-entrainment of the oil. Laminar or streamlined flow is beneficial.  

Means to improve separation: -

• Heating coils provided to optimize separation
• The entrance area in oil/water separators is made large so that flow is slow and large slugs of oil can move to the surface quickly. 
• A low capacity pump encourages slow and laminar flow. 
• Alternation of flow path in a vertical direction continually brings oil near to the surface, where separation is enhanced by weirs which reduce liquid depth. 
• Angled surfaces provide areas on which oil can accumulate and form globules, which then float upwards. 
• Fine gauze screens are also used as coalescing or coagulating surfaces. 
• Piping system should be designed to avoid turbulence due to sharp bends or constrictions and to calculate correctly liquid flow and pipe size to guarantee laminar flow.

A brief overview of an oil content monitoring system is given below.

Oil content monitoring system: -

• The oil content monitor consists of a small chamber with two photocells and a light source.
• The intensity of light falling on both the surface are nearly same.
• The oily water is passed through the oil content monitoring system which alters the intensity of light source falling on one of the photocell.
• The comparator compares the intensities of light source falling on both the photocell and checks the ppm with respect to the amount of intensity.
• When oil content increases beyond a certain value, comparator compares the intensity and will raise the alarm.

The rules which has to be followed for the discharge of sewage is given in annex I as below.

Annex I : Regulations for the Prevention of Pollution by Oil (entered into force 2 October 1983)

1. The ship is proceeding en route;
2. oily mixture is processed through an oil filtering equipment meeting the
3. requirements of the Annex
4. oil content of the effluent without dilution should not exceed 15 parts per million
5. The oily mixture does not originate from cargo pump room bilges.
6. oily mixture should not be mixed with oil cargo residues.

MARINE FRESH WATER HYDROPHORE

FRESH WATER HYDROPHORE

Hydrophore is a device which performs actions including pumping, storing and supplying water at a regular controlled pressure in marine ships.

If we use a centrifugal pump directly to the fresh water line, pump will cut in and cut off frequently due to small consumption which drastically drop the system pressure.

Hydrophore serves as a pressure vessel which keeps the system pressure in a particular range with the help of compressed air.

Pic: Hydrophore Tank

Working of a Hydrophore tank: -

• Hydrophore system consist of a hydrophore tank, pressure switches, level gauges, pressure gauges etc.
• Connections from the fresh water tanks are provided to the hydrophore tank.
• Water from the fresh water tank is passed through a filter to remove any dirt if present.
• The fresh water is then pumped into the hydrophore tank.
• The tank comprises of water along with compressed air which keeps the system under pressure.
• If water is consumed, water level will reduce and air pressure will drop.
• A pressure switch is connected to the tank so that whenever air pressure drops below a set pressure, approx. 2.5 bar, it will start the pump.
• The pump builds up water in the tank and when the pressure hits the maximum, approx. 4.5 bar set pressure, it will cut off by the pressure switch.
• The water used is then passed through a mineralizer where the water is purified and then is sent to various receiving unit.

Pic: Fresh water hydrophore system

Procedure to charge the hydrophore system: -

• Close outlet valve of the hydrophore.
• Start the hydrophore pump and keep an eye on sight glass.
• Once the sight glass is filled with 70% of water, then charge the tank with compressed air and put the system in automatic.
• The pump will stop at the desired set pressure and then charging will stop.
• Open the hydrophore outlet valve.
• Monitor the pump’s cut in and cut off pressure at regular intervals.

Reasons for continuous running of hydrophore pump: -

• Insufficient air pressure
• Due to wrong setting of air pressure switches or faulty pressure switches.
• Problem with pump such as loose suction, insufficient capacity, high water consumption etc.

Various mounting in hydrophore system: -

1. Liquid level meter
2. Pressure gauge
3. Pressure controller
4. Safety valve
5. Charge valve
6. Discharging valve
7. Control box

MARINE FRESH WATER GENERATOR

Methods employed to produce fresh water from sea water is mainly as given below: -

• Evaporation of sea water and condensation of vapor being produced.
• Reverse Osmosis 

The most commonly used method is the evaporation method as it is more feasible than the reverse osmosis.

Different types of equipment used for the generation of fresh water from sea water is as follows: -

1. Single and double effect shell type fresh water generator
2. Vacuum type fresh water generator
3. Single and double flash type fresh water generator

The Vacuum type fresh water generator is used on board the ship which is usually manufactured by Alfa-Laval or by Atlas. 

Problem of scale formation: - 

As we all know that scale formation is a major problem in machines which uses sea water as it contains various salts which as corrosive properties thereby damaging the machines.

Proportions of salts in sea water is as follows: -

• Sodium Chloride- 80%
• Magnesium Chloride-10%
• Magnesium Sulphate- 6%
• Calcium Sulphate- 4%   

Major types of scales found in evaporator is as follows: -

1. Calcium Carbonate - Soft scale
2. Magnesium hydroxide - Hard scale
3. Calcium sulphate - Hard scale

These scales are generated mainly due to temperature and density of evaporation.

If the Temperature is less than 80 degree Celsius, calcium carbonate scale is formed and if it exceeds above 80 degree Celsius, magnesium hydroxide scale is formed.

Usual density of evaporator content should be 80,000 ppm, if it exceeds above 96,000 ppm calcium sulphate scale formation takes place.

Method of controlling scale formation: -

• Low pressure evaporator plant - operates below 80 degree Celsius
• Magnetic treatment - alters the charge of salt
• Flexing element - cracks of the scale
• Chemical treatment - Organic polyelectrolyte, Polyphosphate, Ferric chloride 

Low pressure vacuum evaporator is generally used on board the ship as it is more economical and produces soft scales which can be removed very easily.

A complete overview of a fresh water generator used on board a ship is explained as below: -

Low Pressure vacuum evaporator (Alfa-Laval): -

Pic: Plate type FWG

Parts of a fresh water generator: -

• Pressure vessel containing both evaporator and condenser
• Aluminum bronze tube or plate type heater (evaporator)
• Titanium plate type condenser
• Knitted Monel wire demister
• Sea water ejector pump
• Air/brine educator
• Fresh water pump
• Salinometer

Operation: -

• The sea water pump supplies the feed into the condenser unit and then is filled into the evaporator unit.
• The sea water is filled inside the tubes in the condenser unit first and further it is filled in the evaporator unit outside of the tubes and overflows as brine.
• A required vacuum is created inside the chamber using the air educator, a pressure of 0.06 MPa is maintained inside the pressure vessel.
• Brine educator is used to remove the brine from the pressure vessel which overflows from the evaporator unit.
• Due to the generation of vacuum, low pressure generated tends to reduce the boiling point of water to 70-80 degree Celsius.
• Main engine jacket cooling water at a temperature of 80 degree Celsius is introduced into the evaporator which evaporates the sea water present.
• The non-evaporated sea water is discharged over board as brine.
• The vapor being generated passes through the demister which ensures that no droplets of sea water is carried over to the condenser along with the vapors.
• The generated vapors then pass through the condenser and gets condensed and gets collected in the outlet duct.
• A distillate pump is operated to pump the fresh water produced into the fresh water storage tank.
• Salinometer is provided which measures the salt content in the distillate.
• If salinity of water is more it controls a solenoid valve which will divert the flow to bilge or it is returned to evaporator for re-processing.  

Pic: Shell and tube type FWG

The Heating medium used for evaporating the sea water is: -

• Hot steam from boiler, generally in steam ships or conventional oil tankers.
• Hot fresh water from jacket cooling water.

Advantages of using a low pressure vacuum evaporator are as follows: -

1. Control in the type of scale formation due to low pressure.
2. Uses heat energy from main engine cooling water outlet.
3. Due to High temperature differences for lower pressures, heat transfer is improved.

A brief overview of Salinometer: -

• It measures the salinity of fresh water produced by the fresh water generator.
• The salinity is measured in "parts per million" ppm.
• It consists of an electrode unit which measures the conductiveness and this value is calibrated to a certain value of ppm.
• It gives audible and visible warning when salinity increases above a specified value.
• Maximum salinity of a fresh water should be near about 10 ppm.
• According to requirement salinity of fresh water should be 2-2.5 ppm.