All marine vessels will tend to accumulate some bilge water, and with a mixture of oils and other hydrocarbons on board, discharging directly to the sea is irresponsible, and indeed illegal in many situations. Waste oily water is a by-product created by ocean going vessels globally which runs into millions of tonnes annually. A mixture of water, oily fluids, lubricants and greases accumulate in the lowest part of each vessel known as the bilges.
While larger vessels above 400 gross tonnes are well regulated, smaller vessels have differing requirements. AtZ can offer a range of filtration systems which can selectively remove hydrocarbons from bilge water as well as particulate matter. In cases where older oily water processing equipment is fitted – performance can be improved by the addition of bilge water filtration.

Bilge Water Filtration

Ships of 400gt and above will have an oily water separator fitted which allows processing of bilge water for overboard discharge. In many cases however, we see older separators from the MEPC 60(33) standard struggling to meet the 15ppm requirements for discharge, perhaps as they have been fitted with a later MEPC 107(49) monitor which is more sensitive. In this case, fitting of a supplemental bilge water filtration system after the separator can further reduce water oil content to allow overboard discharge.

Smaller Vessels

In the case of smaller vessels under the 400gt MARPOL threshold, while there is no requirement for separators or monitoring, it remains best practise not to discharge any contaminated bilge water. Some sensitive environmental areas, marinas, rivers or lakes will specific have restrictions preventing any discharge.

For these smaller vessels, fitting a bilge filtration system between the bilge pump and discharge will capture any hydrocarbons within the filter material, leaving just clean water to be discharged.

Filament Wound Filters

The filters are of a filament wound construction with materials specifically chosen to chemically attract hydrocarbons to their surface and retain them. Water passes through the filter leaving oils and other hydrocarbons bound to the filter material. 

Over time, the filters will gradually become saturated with oil at which point they are removed, changed for a new cartridge and disposed of. The length of time between changes will vary depending on the concentration of oil within passed water, as well as the total water volume. The larger filters can hold several litres of oil before saturation, and so many months lapse between filter changes in a typical application.

AtZ are official representatives for the Wave International product lines and can supply new units, spares, service and system design and upgrade services to our customers.


We are operating a static separator and are at times not able to discharge as output remains over 15PPM. What can be done?

A filtration system can be fitted between the output of the separator and the oily water monitor. This additional filtration stage will reduce oil content from the separator output still further and bring discharge within the 15ppm limit.
Over time as the filter becomes saturated and output PPM readings are seen to increase again, the cartridge can be exchanged for a new one.

Does installation of a system affect Classification Society approvals?

As oily bilge systems for any vessel over 400gt are of interest to Class, we would recommend that any intended modifications are discussed and approved as required. The system components supplied are Type Approved, and installations we have conducted to date have all been to the satisfaction of Classification Societies who appreciate efforts to make bilge management and any discharge more environmentally sound.

How long does installation take, would it need to be done in a dry dock?

Typically, installation in a medium to large commercial vessel would be completed within 1-2 days. There is no requirement for the ship to be docked, only the bilge system needs to be isolated and then run at completion to conduct a system test.

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