In recently conducted tests, MicroWISE demonstrated that the performance of the BallastWISE Sampler is identical to the standard plankton net filtration setup at the tested flow rates of 22-30L/min. It takes between 30 and 45 minutes to concentrate 1m3 of water at those rates. The measurements were carried out using BallastWISE based on the number of moving organisms, and it can therefore be concluded that organism motility is not compromised relative to the reference filter during filtration. It is very likely that higher flow rates are possible, and this document will be updated as new data becomes available.
The importance and challenges of ballast water filtration methods
It is necessary to filter ballast water to concentrate organisms ≥50µm in size (mainly zooplankton) to test for concentrations as low as 10 organisms per m3 as required for D-2 compliance according to the IMO Ballast Water Management Convention.
The standard method for this filtration process is to filter 1m3 of water through a plankton net with mesh size 35µm (50µm diagonal). When the net is suspended in water, it is obvious that the differential pressure over the net is virtually zero, because the water level on both sides of the net is the same, and stress on organisms contained within the net is therefore minimal.
The downside of this method is that it can be inconvenient to set up in the field. A filter contained within a housing has the advantage that hoses can be run from the sampling point to the unit and from the unit to a discharge point or potentially back into the ballast water discharge stream. A filter housing usually has a built-in flow meter and sample collection port which adds to the convenience. The BallastWISE Sampler provides such a housing and contains a filter cartridge that is made of the same material as a standard nylon plankton net.
In order to compare the performance of this sampler with traditional plankton nets, the two filtration methods were used on the same water, and the number of organisms present in the filtered water was tested and compared.
A field test setup was developed to make use of ambient seawater from a pier at the Ferry Terminal in Ebeltoft, Denmark. A centrifugal pump supplied seawater directly from the harbour through a 25mm ID hose. The ambient zooplankton population can differ significantly from hour to hour and to overcome this variation, the stream from the pump was split into two flows, one to the filter under test (BallastWISE) and the other to the reference plankton filter (KC Denmark, diameter 40cm, length 1m) suspended in a 1m high plastic bucket. In this way, multiple tests could be performed conveniently with virtually unlimited amounts of test water available. It is difficult to achieve identical flow rates to the two filters, so the two flow rates were noted and the final concentration factor for each filter was calculated as F * T / V, where F is the flow rate, T is the total sample time (22 min), and V is the final sample volume in the sample bottle. A schematic is shown in Fig. 1.
Samples were thus collected simultaneously from the two filters and subsequently transferred to 1L sample bottles. Testing was performed using the BallastWISE compliance monitoring device within 1h of sampling.
Figure 1. Experimental setup: Water is drawn from the harbour through the pump and is distributed simultaneously to both the BallastWISE Sampler and the standard plankton net suspended in a bucket of water. The outflow from the filter housing and the overflow from the bucket ran back into the harbour.
Results from the two filtration methods showed they provide the same performance
Filtration tests were made over several non-consecutive days, showing the numbers of organisms on those days. When comparing the number of organisms registered by the two filtration methods, there was no statistically significant difference between the two (Fig. 2). This indicates they filtered the water with the same performance.
A wide variety of sizes and swimming tracks were observed in the filtered samples in general (Fig. 3).
Figure 2. Linear regression for the BallastWISE Sampler vs. the standard plankton net. 22L/min (circles) and 30L/min (triangles).
Figure 3. Swimming tracks and size distributions showing larger organisms (top) and smaller organisms (bottom) all >50µm from one of the samples (approximately 5000 organisms per m3).
Illustration of the typical steps taken for ballast water compliance testing with BallastWISE, including filtration.
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