Oily Water Separator

A major source of oil pollution in the past from the operation of ships was the discharge into the sea of tank washing from tankers. This was reduced by the discharge of tank washing to a slop tank for settling, and discharge overboard of the water While retaining the sludge for pumping ashore to the refinery, with the next cargo. Crude oil washing (COW) eliminates the use of water and enables cargo resides to be pumped ashore during discharge because cleaning is carried out simultaneously with the discharge.

Ballast carried in oil cargo and bunker tanks which is therefore contaminated with oil constitutes another pollution source, unless pumped out via an oily water separator. New regulations require tankers of certain sizes to have segregated or clean ballast tanks.

A third pollution source is from machinery space bilges.

The most common type of oily water separators found on ships are of the gravitational type. these rely on the difference in specific gravity of the mix to separate out the oil from the water.

Centrifugal separators

Centrifugal separators have been proposed for the use as oily water separators. The quality of the output is determined by the throughput rate. The slower the flow of oil through the separator the better quality output. A question mark exists over their ability to cope with fine emulsions and chemical pre-treatment is recommended.

Separators capable of emulsion treatment

Gravitational separators are not capable of operation with oil emulsions , or mixtures containing oil of high specific gravity approaching 1 or above. The latter may be improved by the preheating of the mixture before or during the gravitational process. The former is more difficult, current regulation requires the careful control of detergents capable of effecting the operation of the fitted separator.

This means that modern efficient detergents containing surfactants may be only used in restricted quantities or not at all.

Alternatives to gravitational separation are now becoming available capable of dealing with these mixtures. The most common at the moment involves the use of Poly aluminium Chloride. This causes the emulsified oil to join to gather (flock). The emulsion is thus broken and the water and oil separated. Using this process very high quality effluent can be produced with little of no oil or chemical content. The cost is higher than for more conventional gravitational separators.

An alternative method is the use of Electrocoagulation. This relies on the three factors of a stable emulsion
-Ionic Charge
-Droplet or Particle Size
-Droplet or particle density

An electrical charge is passed through a sacrificial anode made of aluminium. The released ions are attracted to the negatively charge fine droplets of contaminants. The overall effect is one of agglomeration with larger and larger droplet sizes being produced. In addition gas bubbles produced by hydrolysis attach to these droplets increasing there buoyancy. The separated droplets rising to the surface may be removed. This is a very efficient process and large volumes can be coped with.

Gravitational Separator Operation

The unit is initially filled with purge water. The discharge from the pump has a sample line take of to a 15 ppm monitor. This is lined up and in used with flushing water used until the pump is running. The unit activation button is pressed, the oil outlet valve is closed, the suction valve is opened and the discharge pump is started. Bilge water is drawn through the unit over a vertical arranged plate stack. The 15 ppm monitor is lined up to the pump discharge sample line

As oil coalesces it is led to the oil discharge chamber. As the oil here builds up the interface drops until the pump cut in probe is activated, the pump is stopped, the suction valve is closed, the oil discharge is opened and the purge water is opened. Oil is forced out of the oil outlet by the purge water.

When the oil water interface reaches the cut out the oil discharge valve and the purge water valve is closed. The suction valve is closed and the pump started.

A complete Oily water separator and filter unit for 15 parts per million purity is shown in figure. The complete unit is first filled with clean sea water, the oily water mixture is then pumped through the separator inlet pipe into coarse separating compartment. Here some oil, as a result of its lower density will separate and rise into the oil collection space. The remaining oil/water mixture now flows down into the fine separating compartment and moves slowly between the catch plates. More oil will separate out on to the underside of these plates and travel outwards until it is free to rise into the oil collecting space. The almost oil free water passes into the central pipe and leaves the separator unit. The purity at this point will be 100 parts per million or less. An automatically controlled valve releases the separated oil to a storage tank. Air is released from the unit by a vent valve. Steam or electric heating coils are provided in the upper and sometimes the lower parts of the separator, depending upon the type of oil to be separated (heating reduces viscous drag of oil and thus makes separation of oil and water easier).

Where greater purity is required, the almost oil free water passes to a filter unit. The water flows in turn through two filter stages and the oil removed passes to oil collecting spaces. The first-stage filter removes physical impurities present and promotes some fins separation. The second-stage filter uses coalesces inserts to achieve the final de-oiling. Coalescence breakdown of surface tension between oil droplets in an oil/water mixtures which causes them to join and increase in size. The oil from the collecting spaces is drained away manually, as required, usually about once a week. The filter inserts will require changing, the period of useful life depending upon the operating conditions.

Oil Content Monitoring

Regulations with respect to the discharge of oily water set limits of concentration upto 15 parts per million. A monitor is required in order to measure these values and provide both continuous records and an alarm where the permitted level is exceeded.

The principle used is that of ultra-violet fluorescence. This is the emission of light by a molecule that has absorbed light. During the short interval between absorption and emission, energy is lost and light of a longer wavelength is emitted. Oil fluoresces more readily than water and this provides the means for its detection.

A sample is drawn off from the overboard discharge and passes through sample cell (Figure ). An ultra-violet light is directed at the sample and the fluorescence is monitored by a photoelectric cell. The measured value is compared with the maximum desired value in the controller/recorder. Where an excessive level of contamination is detected an alarm is sounded and diverting valves are operated. The discharging liquid is then passed to a slop Vault.

Alarms and shutdowns

If the 15 ppm equipment detect discharge with oil content over 15 ppm it shuts the unit down and activates and alarm. But in some cases only alarm is there.

Reasons for improper functioning of an Oily water Separator:

The principle of separation on which this separator function is the gravity differential between oil and water. The force acting on oil globule to move in the water is proportional to the difference in weight between the oil particle and a particle of water of equal volume. The resistance to the movement of the globule depends on its size and the viscosity of the fluid. Thus in general, a high rate of separation is favored by:

Large size of globule.
Elevated temperature of the system (which affects both the specific gravity differential of the oil and water and the viscosity of the water)
The use of seawater.

Pumping consideration. Since the rate of separation depends on the oil globule size it will be appreciated that any disintegration of oil globules in the oily feed to the separator should be avoided and this factor can be seriously affected by the type and rating of the pump used. A large number of bilge pumps are centrifugal and they are often used as the supply pump to separator. They churn the supply and produce small oil droplets dispersed throughout the water, which in turn may seriously affect the separation efficiency. A positive displacement pump e.g. slow running double vane, screw, reciprocating or gear pump enables a much better performance to be achieved from the separator as they do not produce large quantities of small oil droplets. Using pump after the separator may give a discharge having less than 15 ppm. concentration without using second stage filters.

From above two points it is evident that even if the separator is well maintained and correctly operated following factors can cause improper functioning of the separator.

Through put of the separator is excessive.
Excessive rolling and pitching of the ship causing disintegration oil globules.
Pump or and rating is not matching, causing too much of turbulence.

Control of discharge of oil

Subject to regulations regarding discharge of oil mixtures in special areas or in exceptional circumstances listed below, any discharge into the sea of oil or oily mixtures from ships shall be prohibited except when all the following conditions are satisfied:

for an oil tanker, except as provided for in subparagraph (b) of this paragraph:
1. the tanker is not within a special area;
2. the tanker is more than 50 nautical miles from the nearest land;
3. the tanker is proceeding en route;
4. the instantaneous rate of discharge of oil content does not exceed 30 litres per nautical mile;
5. the total quantity of oil discharged into the sea does not exceed for existing tankers 1/15,000 of the total quantity of the particular cargo of which the residue formed a part, and for new tankers 1/30,000 of the total quantity of the particular cargo of which the residue formed a part; and
6. the tanker has in operation an oil discharge monitoring and control system and a slop tank arrangement as required under regulation 15 of Annex 1 of MARPOL 73/78. ( this sets out requirements for approved installations for the handling of tank washing and dirty ballast)

from a ship of 400 tons gross tonnage and above other than an oil tanker and from machinery space bilges excluding cargo pump-room bilges of an oil tanker unless mixed with oil cargo residue:
1. the ship is not within a special area;
2. the ship is proceeding en route;
3. the oil content of the effluent without dilution does not exceed 15 parts per million; and
4. the ship has in operation equipment as required under regulation 16 16 of Annex 1 of MARPOL 73/78.(Oil discharge monitoring and control system and oil filtering equipment)

1. In the case of a ship of less than 400 tons gross tonnage other than an oil tanker whilst outside the special area, the Administration shall ensure that it is equipped as far as practicable and reasonable with installations to ensure the storage of oil residues on board and their discharge to reception facilities or into the sea in compliance with the requirements of paragraph (1)(b) of this regulation.
2. Whenever visible traces of oil are observed on or below the surface of the water in the immediate vicinity of a ship or its wake, Governments of Parties to the Convention should, to the extent they are reasonably able to do so, promptly investigate the facts bearing on the issue of whether there has been a violation of the provisions of this regulation or regulation 10 (discharge in special areas) of this Annex. The investigation should include, in particular, the wind and sea conditions, the track and speed of the ship, other possible sources of the visible traces in the vicinity, and any relevant oil discharge records.
3. The provisions of paragraph (1) of this regulation shall not apply to the discharge of clean or segregated ballast or unprocessed oily mixtures which without dilution have an oil content not exceeding 15 parts per million and which do not originate from cargo pump-room bilges and are not mixed with oil cargo residues.
4. No discharge into the sea shall contain chemicals or other substances in quantities or concentrations which are hazardous to the marine environment or chemicals or other substances introduced for the purpose of circumventing the conditions of discharge specified in this regulation.
5. The oil residues which cannot be discharged into the sea in compliance with this regulation shall be retained on board or discharged to reception facilities.
6. In the case of a ship, referred to in regulation 16(6) of this Annex, not fitted with equipment as required by regulation 16(1) or 16(2) of this Annex, the provisions of paragraph (1)(b) of this regulation will not apply until 6 July 1998 or the date on which the ship is fitted with such equipment, whichever is the earlier. Until this date any discharge from machinery space bilges into the sea of oil or oily mixtures from such a ship shall be prohibited except when all the following conditions are satisfied:
  1. the oily mixture does not originate from the cargo pump-room bilges;
  2. the oily mixture is not mixed with oil cargo residues;
  3. the ship is not within a special area;
  4. the ship is more than 12 nautical miles from the nearest land;
  5. the ship is proceeding en route;
  6. the oil content of the effluent is less than 100 parts per million; and
  7. the ship has in operation oily-water separating equipment of a design approved by the Administration, taking into account the specification recommended by the Organization.

Special Areas

Any discharge into the sea of oil or oily mixture from any oil tanker and any ship of 400 tons gross tonnage and above other than an oil tanker shall be prohibited while in a special area. In respect of the Antarctic area, any discharge into the sea of oil or oily mixture from any ship shall be prohibited.
Except as provided for in respect of the Antarctic area under subparagraph 1(a) of this regulation, any discharge into the sea of oil or oily mixture from a ship of less than 400 tons gross tonnage, other than an oil tanker, shall be prohibited while in a special area, except when the oil content of the effluent without dilution does not exceed 15 parts per million.
- The provisions of paragraph (1) of this regulation shall not apply to the discharge of clean or segregated ballast.
- The provisions of subparagraph (1)(a) of this regulation shall not apply to the discharge of processed bilge water from machinery spaces, provided that all of the following conditions are satisfied:
  1. the bilge water does not originate from cargo pump-room bilges;
  2. the bilge water is not mixed with oil cargo residues;
  3. the ship is proceeding en route;
  4. the oil content of the effluent without dilution does not exceed 15 parts per million;
  5. the ship has in operation 15ppm oil filtering equipment of approved design
  6. the filtering system is equipped with a stopping device which will ensure that the discharge is automatically stopped when the oil content of the effluent exceeds 15 parts per million.

Exceptions

1. the discharge into the sea of oil or oily mixture necessary for the purpose of securing the safety of a ship or saving life at sea; or
2. the discharge into the sea of oil or oily mixture resulting from damage to a ship or its equipment:
  - provided that all reasonable precautions have been taken after the occurrence of the damage or discovery of the discharge for the purpose of preventing or minimizing the discharge; and
  - except if the owner or the master acted either with intent to cause damage, or recklessly and with knowledge that damage would probably result; or
3. the discharge into the sea of substances containing oil, approved by the Administration, when being used for the purpose of combating specific pollution incidents in order to minimize the damage from pollution. Any such discharge shall be subject to the approval of any Government in whose jurisdiction it is contemplated the discharge will occur.

The ‘Oil Record Book’ for cargo/ballast operations which is kept by the deck de~ pertinent on all tankers over 150 gross tonnage is to have chronological entries with date, operational code and item number in appropriate column.

The ‘0il Record Book’ for machinery space operations must have upto date entries of

a) oil fuel tank ballasting; and cleaning.

b) discharge of the (1311′ ballast or cleaning.

c) slugde or fuel oil residue disposal.

d) discharge or disposal of bilges.

e) automatic bilge discharge.

f) oil discharge monitoring/control system failure details.

g) oil discharges (accidental or exceptional).