Factors to consider with filters

Mar 08 2022


Mark Riggio, head of marine with Filtersafe, shares advice about what shipping companies should consider, when deciding whether or not to buy a ballast water filter and what to buy.

In its 2020 report, SGS details its experience of conducting nearly 100 tests of ballast water and observing that 21% of those tests indicated that the discharge sample likely did not meet the IMO D-2 discharge standard.

 

100% of the potentially non-compliant discharges did not meet the IMO standards for organisms greater than 50 μm.

 

The standard is inherently biased against compliance for organisms greater than 50 μm in minimum dimension. There can be 1.5 times as many organisms in one gallon of ballast water between 10 and 50 μm than there can be in an entire Olympic-sized swimming pool of ballast water for the greater than 50 μm size fraction.

 

This highlights the critical nature that the BWMS filter plays in the potential of the system to meet the D-2 discharge standard.

 

Any instances that the ballast water management system has been bypassed may potentially mean a small number of these robust organisms remain viable and create problems during a discharge sample.

 

Ballast water filters have been designed with mesh ratings between 6 and 100 μm with the most common systems being fitted with 20, 40, or 50 μm.

 

Having no filter

The risk is especially high for the BWMS which are approved without a filter.

 

These BWMS allow sediments and larger, hard-bodied, and shelled organisms to pass through the treatment stage and enter the ballast tanks.

 

These complex organisms will remain in the tanks over time from where they could be discharged, hide in dead-ends of piping systems, and develop colonies in the unpumpable residuals and sediments inherent to normal ballasting.

 

As the IMO standards are discharge standards, the ability of these large organisms to persist in the tanks creates significant risks of discharges being non-compliant with the standard.

 

The unpumpable residual sediments building up over the years between vessel drydock periods can harbour many treatment-resistant organisms.

 

Ballast flow rate

The primary reason that ship owners provide for choosing a BWMS that does not include a filter is the inherent risk that the filter will impact the flow rate of the ballast pumps and consequently slow down ballast water flow.

 

This decreased flow occurs for two primary reasons:

 

During heavy loading, the filter will often go into continuous backflush and a significant portion of the water will be diverted through the backflush line; 

 

Filter loading and increased differential pressure across the mesh causes a larger discharge head on the ballast pump, causing the pump to flow less water to the tank.

 

A test was done using water with a sediment mix equivalent to that found in the Yangtze River near the port of Shanghai.

 

It found that for a typical small bulk carrier with an 800 m3/hr ballast pump moving 10,000 m3 of ballast during a typical cargo operation, the time to move that much ballast can range from 12 hours to nearly 90 if the filter is not designed for heavy loading.

 

A filter designed for the loads may be able to move more than 3 times as much water at the extremes.

 

Clogging

Chief among the problems that are encountered by ballast water filters is the potential that they can clog.

 

We define "clogging" here as high organic and sediment deposits that build up on the filter element and reduce water flow to the BWMS and the ballast tanks to less than 5% of its designed rate.

 

Shipboard tests conducted by the Korean Institute of Ocean Science and Technology, using seawater from Shanghai Port (high sediment) and three types of IMO-approved BWMS, found that they all failed to operate properly because of filter clogging.

 

Systems which are unable to be used due to filter clogging must be manually cleaned during the ballasting process, a task that could occupy two or three people to disassemble the filter, manually clean the elements, and reassemble it before restarting ballasting.

 

Clogging might be avoided if a vessel completed all of its ballasting operations prior to entering the challenging waters. For many vessels, this is impractical due to need to navigate up long rivers, over or under restrictions in the waterway, or due to the need to bring on ballast during cargo operations.

 

It would be prudent to explore a filter’s potential for clogging in the regions where it will have to operate before making a final selection.

 

Clogging is primarily related to the filter’s ability to clean itself automatically or remove the deposits on the filter element at a rate faster than, or at least equal to, the rate that the deposits are being formed. So the ability to scale the cleaning mechanism of a ballast water filter is critical to being able to address clogging issues.

 

Sediment

When designing the IMO Type Approval testing requirements, 50 mg/L of TSS (Total Suspended Solids) was the standard chosen by the IMO for the minimum challenge condition.

 

This reflects the average for the world’s waters, it does not represent the maximum challenge condition that ships may face.

 

Two particular locations, the Mississippi River Delta and the Yangtze River, show exceptionally high TSS levels. Both rivers are very long. Both, therefore, have ample opportunity to collect sediment on the way to their estuaries and to the ports located there.

 

Allowing sediments to accumulate onboard the vessel has a further implication to the vessel.

 

First, the build-up between drydocking periods becomes a form of unpumpable, permanent ballast that, if not removed, reduces the vessel’s cargo capacity over time. Second, these unpumpable residual sediments must be handled in the future and remediated under the Convention.

 

This may affect the charter agreement and cause vessels to carry less than the obligatory cargo amounts as per the voyage plan, with implications for a ship’s earning ability. Ferrying around even a few inches of sediment over the entire bottom of ballast tanks can quickly see tonnes of additional weight added to their dead weight.

 

This additional dead weight may offset cargo if vessels, in particular the bulk and oil carriers, are contracted to carry their full load weight.

 

In addition, both the US ballast treatment requirements and the IMO Ballast Water Management Convention require ballast tanks to be regularly cleaned to remove sediments which will add to operational costs.

 

This sediment is considered hazardous because it can allow invasive alien species (IAS) and harmful aquatic organisms and pathogens (HAOP) to accumulate and breed, potentially being released during a subsequent de-ballasting or cleaning.

 

When a vessel goes into dock, many owners are learning that ballast sediments may now be considered a hazardous material and require special remediation and proper disposal under the Convention. This either requires payment at the yard, or bringing a special crew on board and conducting cleaning safely offshore.

 

Filter construction materials

For many years austenitic stainless steel grade AISI 316 has been known simply as ‘marine grade’ stainless steel. But the latest advice from the International Stainless Steel Forum is that AISI 316 and its derivatives "are no longer recommended for permanent contact with seawater."

 

Researchers identify 904L grade as a "super austenitic stainless steel" that is designed "for more aggressive environments with long-term performance." With this in mind, 904L is now recognized as a more appropriate material for filter.

 

 



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