Oily Water Conditioning
Oily Water separators
The most common type of oily water separators found on ships are of the gravitational type. these rely on the difference in SG of the mix to separate out the oil from the water
Shown above is a gravitational type seperator capable of very good quality effluent discharge. A safety valve (not shown) is mounted on the unit as are test cocks and a drain valve.
The unit is initially filled with purge water. The discharge from the pump has a sample line take of to a 15ppm 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 15ppm 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.
Alarms and shutdowns
Should the 15ppm equipment detect discharge with oil content over 15ppm it shuts
the unit down and activates and alarm
When the pump cut out probe is activated a timer starts, should the oil interface not reach the pump cut in probe within a set time an alarm is sounded and the unit shutdown.
Should the oil interface reach the alarm and shut down probe and alarm is activated and the unit shut down
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 sg. 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 Polyaluminium Chloride. This causes the emulsified oil to join together (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
-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.
Discharge of Oily Water Regulations
Control of discharge of oil
1) 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:
1a) for an oil tanker, except as provided for in subparagraph (b) of this paragraph:
I) the tanker is not within a special area;
II) the tanker is more than 50 nautical miles from the nearest land;
III) the tanker is proceeding en route;
IV) the instantaneous rate of discharge of oil content does not exceed 30 litres per nautical mile;
V) 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
VI) 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 washings and dirty ballast)
1b) 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:
I) the ship is not within a special area;
II) the ship is proceeding en route;
III) the oil content of the effluent withoput dilution does not exceed 15 parts per million; and
IV) 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)
2) 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.
3) 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.
4) 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.
5) 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.
6) 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.
7) 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:
7a) the oily mixture does not originate from the cargo pump-room bilges;
7b) the oily mixture is not mixed with oil cargo residues;
7c) the ship is not within a special area;
7d) the ship is more than 12 nautical miles from the nearest land;
7e) the ship is proceeding en route;
7f) the oil content of the effluent is less than 100 parts per million; and
7g) 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.
1 a)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.
1b) 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.
2a)The provisions of paragraph (1) of this regulation shall not apply to the discharge of clean or segregated ballast.
2b)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:
I) the bilge water does not originate from cargo pump-room bilges;
II) the bilge water is not mixed with oil cargo residues;
III) the ship is proceeding en route;
IV) the oil content of the effluent without dilution does not exceed 15 parts per million;
V) the ship has in operation 15ppm oil filtering equipment of approved design
VI) 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.
A, 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
B, the discharge into the sea of oil or oily mixture resulting from damage to a ship or its equipment:
I) 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
II) 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
C, 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.