TECHNOLOGY – TANK CLEANING/WASTE TREATMENT

detergents, oil additives, chemicals, catalytic

fines, soot and other solid particles (sludge).

Today, bilge water is a three-phase separation

task with sludge as the third component. 

The marine sector uses large amounts of

chemicals for cleaning, service and maintenance

activities in the engine room and many of these

products are surfactant-based. As such, these

chemicals contribute to the formation of

suspensions and emulsions that are difficult to

break in a ship’s bilge water system.

An emulsion is a mixture of oil and water,

where small oil droplets are dispersed in the

continuous water phase. The formation of

stable emulsions can compromise separation

efficiency and this becomes a challenge 

when an emulsion is stabilised by surfactants

and particles.

Current MARPOL legislation stipulates tha

separated bilge water containing 15 ppm or

below oil in water can be discharged into

international waters. 

Some national, regional and local

authorities have more stringent regulations.

For instance, in the US and in the Baltic and

North Seas, disposal of treated bilge water 

is only permitted at least 12 nautical miles

from shore. 

Future legislation is expected to be even

more stringent, requiring levels of oil in wate

to be reduced to 5 ppm (Great Lakes is

already 5 ppm) for discharge at sea and to

zero-discharge in sensitive waters. 

Detection methods used by government

agencies and other authorities are becoming

more effective and now include both aerial 

as well as satellite detection of oil spills in

the oceans.

Separator type approval 

Prior to 1st January , 2005, IMO resolution

MEPC.60 (33), which specified how to type

approve equipment used on board ships,

required bilge water separators to be tested

with a mixture of only oil and water. 

Today’s IMO regulation, resolution

MEPC.107 (49), effective from that date,

specifies that, in addition to the removal of oi

from bilge water, bilge water separators must

be tested with a stable emulsion (including

fine particles and a surfactant chemical). Also

bilge water treatment systems must include an

oil-in-water monitor with a recording function

for date, time, oil ppm alarm and operating

status. The recording of the operation must be

stored for 18 months.

Traditional technologies

There are a number of less effective

technologies in use for treating bilge water,

including chemical treatment, absorption

As a result, all ships must have

systems to treat bilge water and,

to eliminate inefficient solutions,

the equipment must be tested and

type approved according to MEPC 107(49). 

“Strangely, once installed on board, certain

systems fail to perform as well as they did

during the certification testing. Why is this?

Also when there are low cost, approved bilge

water treatment systems on the market, why

do some ship owners invest in higher priced

technology?” Asks Alfa Laval.

First, it is worth looking at the whole

picture. 

Bilge Water is difficult to define. Fifty years

ago, bilge water was mainly composed of

diesel oil and water. Now it can be a mixture

of water, fuel oil, lube oil, hydraulic oil,

20 TANKEROperator May 200

Bilge water treatment - centrifugal or gravity separation?

Discharge of bilge water with an oil content of above 15 ppm into the sea, less in

sensitive waters, is strictly prohibited by international law and subject to heavy fines. 

EPC 60 Bilge controller.
filtration, membrane filtration, conventional

coalescers. All are static systems originally

designed for ‘batch’ operation, that is,

processing large volumes of bilge water

during a short period of time. 

Large waste and back flush volumes are

normally generated by these systems. Filter

elements, active carbon, and coalescence

elements require replacement when saturated.

In systems using flocculation chemicals, up to

25% of the treated bilge will become reject

and must be landed at great expense to the

operator. Maintenance is man-hour intensive

and operation of these systems requires

frequent supervision.

Possibly the most serious drawback of all

is that these systems (except membranes)

lose their efficiency at sea in rough weather

conditions and when difficult emulsions are

present in the bilge water, Alfa Laval said.

This is because they make use of normal

gravitational force, which is easily overcome

by a ship’s motion in rough sea. This

adversely affects the treatment process. 

Oil shocks also cause problems for these

systems.

Static conventional systems originally

designed for batch operation often fail to do

the job properly in real life conditions. 

So how is it possible that such systems can

be type-approved according to IMO

regulations? The answer lies in the testing

process. It’s hard to believe, but the tests are

not performed under real life sea-conditions,

during a more extended period of time. Why

is this the case? 

The equipment is tested using just one

chemical while, as discussed earlier, bilge

water in reality is composed of a complete

cocktail of oil, different chemicals and

particles in emulsified form. The duration of

the emulsion test is only 2.5 hours, hardly

giving the filters time to clog up or become

saturated with oil and particles as they would

in real life conditions. 

But most surprising of all, the test is

conducted ashore in stable conditions, without

the pitching and rolling that occurs much of

the time when a ship is at sea.

According to an Alfa Laval spokesman: “If

traditional static systems were to be tested

with a realistic bilge water ‘cocktail’ under

conditions simulating a rough sea state 24/7

for 20 days, they would immediately be

eliminated.”

Centrifugal separation 

By far the most efficient technology for this

application are dynamic bilge water treatment

systems, utilising high speed, disc-type

centrifugal separation technology. 

The gravitational force of 1G used in static

separation systems is multiplied many

thousands of times in centrifugal systems. One

typical centrifugal disc-type separator equals a

conventional gravity system with a settling

area of 20,000 sq m.

At the same time, the gyroscopic effect of

the liquid circulating at high speed inside the

separator bowl offsets the pitching and rolling

motion. The result is sustained high separation

efficiency. Centrifugal disc type separators

have since decades demonstrated their

undisputed performance in removing water

and solids from diesel engine fuel and lube

oils in marine industry.

Alfa Laval’s answer

Of the centrifugal bilge water treatment

systems on the market today, the most

efficient, due to its innovative technical

features, is Alfa Laval’s PureBilge, the

company claimed. 

PureBilge is the only system on the market

that provides a cleaning performance in real

life conditions of 0-5 ppm oil content in the

water without chemicals, adsorption filter or

membranes, the company said. This cleaning

performance is unaffected by sea heave, oil

shocks or high solids loading while no back

flushing is required. 

With PureBilge there is no reject to pump

ashore, no need to land wastes such as filter

elements, coalescence elements, active carbon

or flocculation deposits and no man-hours

required for operation or supervision. As

centrifugal separators are standard on board

vessels for fuel and lube oil cleaning the crew

is already very familiar working with this typ

of equipment.

The company said that PureBilge offers

premium separation efficiency of large

volumes of oily water with the capacity to

handle large amounts of solids through

intermittent discharge at variable intervals.

Certified according to IMO resolutions,

MEPC.107 (49) and USCG, the system is

designed for unmanned 24/7 operation

May 2009 TANKEROperator 21

TECHNOLOGY – TANK CLEANING/WASTE TREATMENT

The patented Alfa Laval XLrator gently accelerates bilge water into the separator bowl with a minimum of shearing and foaming. This

greatly improves separation efficiency by preventing the splitting of oil drops and the formation of further emulsions. This is the real key

to PureBilge’s superior performance, compared to other centrifugal separation based systems.
stipulated in the regulations to obtain a type

approval certificate. The aim was to achieve a

true picture of the system’s efficiency in real

life compared with conventional bilge water

treatment systems. 

The process fluid was an impressive

cocktail, simulating bilge water, comprising: 

cu m sea water, 1 litre compressor oil, 10

litres DO, 10 litres HFO, 1 litre hydraulic oil,

1 litre corrosion inhibitor, 1 litre carbon

remover, 1 litre solvent based oil cleaner, 20

litres ‘mud’, 5 litres rust, 50 litres main engin

air cooler condensate, and 5 litres soot. 

Instead of just 2.5 hours of testing on

emulsions the operating time here was over a

period of weeks. Also, rough seas instead of

onshore conditions were an important parameter

This cocktail was stirred by using a

diaphragm pump for four hours. The effect

was to emulsify the mixture thoroughly,

compounding the separation problem.

According to the ship operator, “the PureBilg

at this very extreme test proved its capability

to process bilge water down to less than 10

ppm and down to 0 ppm at normal real life

operating conditions.” 

Questions answered

This result effectively addresses the second

question posed at the beginning of this article

When there are low cost, IMO approved bilge

water treatment systems on the market, why

do some ship owners invest in price leading

technology? There are, for instance, already

more than 300 BWPX 307 centrifugal disc

type bilge systems installed and operating on

board vessels throughout the world.

The answer is simple. When dealing with

real life operation on the high seas, condition

are not always as favourable as during

onshore type approval testing. Therefore som

shipowners, familiar with centrifugal

separators for cleaning fuel and lubricating

oil, choose to invest in centrifugal based bilge

water treatment systems knowing that they ar

the safest, most efficient and logical approach

PureBilge will be the formerly launched at

Nor-Shipping in June, while certification

approval is expected during the summer,

which is being conducted by DNV. The first

systems are due to roll off the production line

in September and the first shipboard deliverie

are due for January, 2010.

Four versions will be offered costing

between Eur70,000- Eur90,000. Alfa Laval

claimed that these prices were 20% less than

the other systems on the market. 

PureBilge is part of the Pure range of

equipment now on offer from Alfa Laval.

More will follow, the company said. 

22 TANKEROperator May 200

TECHNOLOGY – TANK CLEANING/WASTE TREATMENT

An all-in-one system

PureBilge is a fully automatic, all-in-one

system with a pumping stage, a preheating

stage and a centrifugal separation stage, with

full process control and monitoring. It

comprises a BWPX 307 high speed separator,

a control cabinet housing an EPC 60 Bilge

process controller, a valve and pipe rack and a

feed pump module. A Rivertrace monitoring

system has been installed within PureBilge.

In the PureBilge system a gravitational

force of 6,000G generated at 8,000 rev/min is

claimed to ensure an extremely particle and

oil droplet separation efficiency. Furthermore,

normal coalescence of oil droplets and

flocculation of particles takes place in the

separation channels in the disc stack, which

also enhances the efficiency, Alfa Laval said.

XLrator inlet 

The company said that the real key to the

system’s superior performance, compared to

other centrifugal separation based systems, is

the XLrator laminar flow inlet device, or

‘magic screw’. 

The patented Alfa Laval XLrator gently

accelerates bilge water into the separator bowl

with a minimum of shearing and foaming.

This greatly improves separation efficiency by

preventing the splitting of oil drops and the

formation of further emulsions.

Newbuildings and retrofits

PureBilge is an attractive solution for

newbuildings with unmanned engine rooms.

The system is easy to install and saves space

and costs. Fully automatic, continuous

operation considerably reduces the need for

large holding tank volumes. 

Its compact, modular design also makes

PureBilge a viable solution for existing

vessels where it can operate as a stand-alone

system, or as back-up for an existing bilge

water treatment system. The equipment only

needs a small footprint, making retrofits

simple, Alfa Laval claimed. 

Maintenance is also claimed to be an easy

operation and should be carried out around

three times per year. The cost will be around

Eur1,500 per annum, Alfa Laval said. 

Rigorous testing 

A major ship operator – Teekay - subjected the

PureBilge BWPX 307 separator to accelerated

testing on board the Aframax Falster Spirit.

The test was much more demanding than that

PureBilge from Alfa Laval is the only system on the market that provides a cleaning

performance in real life conditions of 0-5 ppm oil content in the water without chemicals,

absorption filter or membranes. Performance: 2.5 or 5.0 m3/hr, 5.0 or 15 ppm

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