Bilge water treatment: Centrifugal or gravity separation?

Discharge of bilge water with an oil content above 15 ppm into our oceans (less in sensitive waters) is strictly prohibited by international law and subject to heavy fines. 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 onboard ships, certain systems fail to perform as well as they did during the certification testing. Why is this?

Another question. When there are low cost, approved bilge water treatment systems on the market, why do some ship owners invest in higher priced technology? 

First, it is worth looking at the whole picture. 

What exactly is bilge water?

This 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, detergents, oil additives, chemicals, catalytic fines, soot and other solid particles (sludge). Nowadays, 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.

 Bilge water legislation 

Current MARPOL legislation stipulates that 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. In the United States and in the Baltic and North Seas, for instance, 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 water to be reduced to 5 ppm (Great Lakes 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.

Type approval for bilge water separators

Prior to January 1, 2005, International Maritime Organization (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 January 1, 2005, specifies that, in addition to the removal of oil 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 filtration, membrane filtration, conventional coalescers. All are static systems originally designed for “batch” operation, i.e. 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, however, 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. This is because they make use of normal gravitational force, which is easily overcome by a ship’s motion in rough sea, and this adversely affects the treatment process. Oil shocks also cause problems for these systems.

So how can such systems be type approved?

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? This, unfortunately, is a question we must leave unanswered. 

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 most efficient

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 utilised 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 m2.

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.

PureBilge from Alfa Laval

Of the centrifugal bilge water treatment systems on the market today, the most efficient, due to its innovative technical features, is PureBilge from Alfa Laval. 

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. This cleaning performance is unaffected by sea heave, oil shocks or high solids loading, and 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 onboard vessels for fuel and lube oil cleaning the crew is already very familiar working with this type of equipment.

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

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. 

In the PureBilge system a gravitational force of 6,000G generated at 8,000 rpm ensures 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.

Unique XLrator inlet improves separation efficiency

The real key to the system’s superior performance, compared to other centrifugal separation based systems, is the XLrator laminar flow inlet device. 

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.

Suitable for newbuildings or retrofitting

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.

Rigorous testing by a customer

A major ship operator subjected the PureBilge BWPX 307 separator to accelerated testing on board a ship. The test was much more demanding than that 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: 1 m3 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 engine 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 was an important parameter.

The cocktail was stirred by a diaphragm pump for 4 hours. The effect was to emulsify the mixture thoroughly, compounding the separation problem. According to the ship operator, “the PureBilge 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 onboard ships throughout the world.

The answer is simple. When dealing with real life operation on the high seas, conditions are not always as favourable as during onshore type approval testing. Therefore some ship owners, familiar with centrifugal separators for cleaning fuel and lubricating oil, choose to invest in centrifugal based bilge water treatment systems knowing that they are the safest, most efficient and logical approach.