By InBallast AS
About us: InBallast is an independent consultancy/ engineering company that delivers services related to ballast water management to ship-owners, vendors and regulators. Visit http://www.inballast.com to learn more about us.
Filters are the Achilles heel of any Ballast Water Management System that applies conventional water treatment technologies – thus, most ballast water management treatment efforts will be at risk of failing unless the filters are up for the task. This article highlights the importance of proper pre-filtration and the necessity to address and to understand filter issues in light of operational limitations and maintenance requirements.
Never better than the weakest link!
Simplistically, the ballast water discharge standard differentiate the target organisms in three size categories; >50um sized organisms (predominately zooplankton), 10-50um sized organisms (predominately algae) and lastly that of bacteria.
Most suppliers choose filtration with a cut-point at around 40 um resulting in an immediate and very efficient removal of zooplankton. For oxidation systems, this will (theoretically) lower the oxidant consumption and further enable compliance with very low or now requirements related to holding time. For UV systems – there are two vectors calling for introducing high efficiency filtration; a) the ability to ensure complete irradiation exposure (removing shades), and b) as zooplankton tend to be more resilient to UV- irradiation, exposure-time is a factor of importance. By eliminating a significant portion of zooplankton, inadequate and prolonged exposure is avoided. Some UV suppliers choose even smaller cut-point to improve over-all performance.
If the filter fails – you’re in trouble!
Filters in BWM applications fail for a number of reasons. Some filter technologies are simply not suited for the challenge – organic matter behave differently from inorganic matter and consequently require a different approach. Zooplankton respond differently from phytoplankton during filtration – there are also differences between freshwater and saline water biomass-response when being filtered.
Over the years, BWMS suppliers have reviewed and tested a number of different filtration concepts. In the early days, standard industrial filters were applied. Nowadays, most suppliers have developed special filters for ballast water filtration and newcomers have entered the market – offering specialised ballast water filters only. Single unit filters have grown in size and are now available matching the ballast water flowrates of very large vessels.
The products in the market today represents a substantial improvement compared with their precursors. However, the complexity of the challenge; limitations in space, vessels in-motion signature, variations in water filterability, variations in pressure, operational requirements and not least the corrosive nature of seawater synergised, unfortunately, by the often-elevated temperatures experienced in many areas of the world and not least in engine-rooms were most ballast water filters are installed – require compromise! We can see this in the different filter designs currently in use. It becomes however, somewhat ironic that still, the most used filter screen material is stainless steel, grade 316. As stated in the introduction in a recent report from one of the more reputable Class societies; Stainless steel is not suited for use in seawater applications. A stainless screen of the quality offered by most vendors will have a very short life unless treated with the utmost care. This include never being stored in saline water for any amount of time! How feasible is that?
Cheapskates, ignorance or couldn’t really be bothered if it works or not…!
So why is this the case? The most important component in the filter seems pretty much overlooked. Throughout the development in ballast water filtration as seen over the last decade or so, we would think that the imperative would have priority – namely that the filter screen material would ensure a practicable functional lifespan. We recently surveyed filters following only three months of operations. We had to take them off the vessel and have them replaced!
The argument for the choice relate of course to cost. Screens in higher quality materials are in use in many industries and are obviously available – they’re just more expensive (initially…) and have a more complex supply-chain. However, nothing that cannot be managed by some level of planning.
Experiences from BWMS’ type approval shipboard sea-trials, shows us that even the slightest contamination, caused by for example a leaking valve or remaining untreated water in the ballast tanks, may result in the ballast water discharge not meeting the ballast water discharge standard. This even goes for systems that treat on both intake and discharge.
Imagine a mere 0,1% valve leakage. A pump delivering 3000 m3/h to a 3000 m3 tank at an intake concentration of 100.000 individuals/m3 of > 50µm organisms will take 360 seconds or six minutes to exceed the IMO D-2 standard. The effect of a damaged filter will also cause a leakage – the consequence is pretty much the same – most likely incompliance!
Feasible solutions in sight?
But can a higher quality screen warrant an improved functional lifespan? And how can its function be assessed? The latter is easy – by sampling and analysis. In fact, the only way of gaining experience with BWMS’ efficiency in operation over time is by sampling and analysis. In order for this to happen simpler methods must be introduced – simple and reasonably accurate indicative methods are available. Regulators need to focus on introducing frameworks allowing these to be used – not for potential violation validation – but to gain knowledge. This is surely an issue to be debated by the IMO in light of the experience-building phase to be considered by MEPC 71. We fear there may not be much experience to gather unless mandatory performance monitoring programs are established. Maybe something similar to the VGP self-monitoring program could be considered?
Back to the screen quality! There are other impacts not assessed during system’s type approval – that of operational wear. Filters are certainly exposed to wear – both due to the debris load experienced during filtration but also as consequence of mechanical wear during backwash. There is good solid documentation available verifying that this will have a serious impact on functional lifespan. The available type approval protocols seem to have completely ignored this.
We took our concerns to InNano AS – a company specializing in smart materials engineering. InNano have developed surfaces applicable to various base materials including steels, stainless steel qualities, titanium, alumina and even some plastics – with some very different capabilities. InNano can provide photo-reactive surfaces, (super) hydrophobic/ hydrophilic surfaces, surfaces with extremely low surface tension, hard surfaces, dielectric surfaces and surfaces combining some of these different characteristics. So, we challenged them and had screens from different suppliers tested under so-called CASS conditions (Copper-Accelerated Acid Salt Spray). The tests used standard grade 316 stainless steel filter screens of the most frequently used sizes (25, 40 and 50 mm) and representing screens as used by different manufacturers. InNano modified the screens using 4 different surface preparation technologies. The reference-screens (as currently installed in ballast water filters) – all suffered corrosion after 6 to 24 hours. The test ran for some 350 hours – and yes, modified screens did survive the entire exposure without signs of corrosion.
So how much cheaper is it to choose a failing material! Well, if you cared about the result, would do chance it? If it affected you commercially, would you?
At the end of the day, it’s the customer who decide on quality! Ask your supplier of choice – how is the screen offered qualified in relation to operational wear and to corrosion? Are there any warranties? And what are the preconditions? The USCG fine for failing to meet the Ballast Water discharge standard is set at maximum US $35.000,00. That per violation per day.
…., so who’s saving!