Fluorescence ballast water testing tools come in two varieties: STAF (Single Turnover Active Fluorometry) and PAM (multiple turnover Pulse Amplitude Modulated). In this article, we’re taking a look at some of the key differences that set these two techniques apart, and how these differences impact the results of ballast water compliance testing.
Fluorescence ballast water testing tools come in two varieties: STAF (Single Turnover Active Fluorometry) and PAM (Multiple Turnover Pulse Amplitude Modulated). In this article, we’re taking a look at some of the key differences that set these two techniques apart, and how these differences impact the results of ballast water compliance testing.
Ballast water testing: why?
Ballast water is defined as water taken on board a ship to control trim, list, draught, stability or stresses of the ship. Ballast water operations are essential for the safe operation of ships and a perfectly standard part of shipping – ballast water provides stability and maneuverability both during voyages and during loading and unloading operations. Ships are designed and built to move through water carrying cargo (ie weight), but if the ship is empty of cargo, or has discharged some cargo in one port and is on route to its next port of call, ballast water may be loaded on board to keep the ship deep enough in the water to ensure safe and efficient rudder and propellor operation and to ensure that safe operating conditions are met.
That said, contaminated ballast water is a biohazard, as a ship’s sailing can involve taking in ballast water at a location 1000s of miles away from its ballast water discharge point, thus potentially transferring harmful species contained in the ballast water into new environments. A step has been taken by international community, through the International Maritime Organisation (IMO), which has drafted guidelines in the pursuit of controlling this menace to all the nations. The Ballast Water Management Convention has set forth these guidelines to be adopted by the member states of IMO. The International Convention for the Control and Management of Ships’ Ballast Water and Sediments, 2004 (BWM Convention), entered into force globally on 8 September 2017.
From 8 September 2017, ships have had to manage their ballast water so that aquatic organisms and pathogens are removed or rendered harmless before the ballast water is released into a new location, which of course requires to check the ballast water. In order to comply to the the International Convention for the Control and Management of Ships’ Ballast Water and Sediments, ships now carry apparatus to test their ballast water, including fluorescence ballast water testing tools such as Chelsea Technologies’ FastBallast, which employs STAF technology (single turnover active fluorometry).
Fluorescence ballast water testing
Fluorescence ballast water testing is a relatively new technique, much more immediate and convenient for shipowners than the previous modus operandi which involved taking a sample of the ballast water out to a land-based laboratory for microscopy and staining and waiting for the hand-counted result to come back. Fluorescence ballast water testing gives the result here and now and has been a popular innovation for shipowners around the world since the BWM Convention took force. Microscopy and staining, the conventional technique, suffers from a number of disadvantages:
- Time consuming – delays ships
- Samples risk being compromised in transit to lab
- Limited number of labs globally
- Restricted to small sample sizes
- Non-objective cell counts
- Need skilled laboratory staff
Ballast water testing involving fluorescence revolves around a relatively simple idea: namely shine blue light at a water sample, and if the water contains phytoplankton, they emit red light from chlorophyll, which can be measured there and then and give a reliable indication of their presence or otherwise.
Fluorescence ballast water testing – two approaches
At its core, STAF (Single Turnover Active Fluorometry) and PAM (multiple turnover Pulse Amplitude Modulated) are both variants of fluoroscopy, but whereas STAF (Single Turnover Active Fluorometry) revolves around interpreting and aggregating data that comes in from numerous standardised short-duration measurements, PAM (multiple turnover Pulse Amplitude Modulated) revolves around data that comes in from far fewer long-duration measurements.
“Ultimately, this unfortunately means that the standard procedures employed in PAM testing require another visit to the laboratory, whereas STAF technology and its distribution approach in Fastballast gives reliable stage 2 compliance tests as well as stage 1 indicative tests”
PAM – slow to test
Because of the fluorescence mechanisms being interrogated by PAM, it takes a minimum of 10 seconds between data samples to achieve reliable measurements. In contract STAF returns data every few hundred microseconds so numerous measurements can be taken each second. This matters, because any form of testing becomes more accurate the more samples you take, and whereas STAF gives 10s of samples per second, PAM is limited to 6 a minute or so.
PAM – fewer samples
Sample size is always a key consideration for accuracy. More samples (ie frequency) provide more accurate mean values and identifies outliers that could skew the data in a smaller sample. This in turn creates a much smaller margin of error, which is what we’ve found at Chelsea Technologies when comparing STAF and PAM. Sample size is directly related to a statistic’s margin of error, or how accurate a statistic can be calculated to be. For a yes-or-no question, such as whether a test is passed or not, we can determine the margin of error for a statistic by dividing 1 by the square root of the sample size and multiplying by 100.
At its core, reliable data depends on signal vs. noise (S/N). The signal is the meaningful information that we’re actually trying to detect, whereas noise is the random, unwanted variation or fluctuation that interferes with the signal. This attention to detail to ensure accurate data is why the level 2 test on our FastBallast ballast water testing unit includes inbuilt noise analysis and compensation.
PAM – static sample only
One thing that PAM cannot do is reliably measure in moving water. The reason is perfectly simple – with moving water, PAM measurements are so slow that the sample detected at the start of a measurement cycle will have moved out of the interrogated sample volume by the end, thus leading to inaccuracies; whereas with the STAF short snapshot pulse, in the order of 100 microseconds, ensures that the same sample is interrogated throughout the measurement cycle giving a much more accurate result. This means that FastBallast can be considered for continuously monitoring ballast water discharge.
PAM: Fv data problems
PAM ballast testing apparatus such as the Hach BW680 handheld fluorometer, Turner Ballast-Check 2 and the BBE Moldaenke 10 cells ballast water testing device all rely on measuring just Fv (variable fluorescence), as that’s an inbuilt limitation of PAM ballast testing apparatus.
The problem is that the amount of variable fluorescence generated by a cell is related to its size. This means that a high density of small cells can give the same Fv value as a low density of large cells, which can lead to wildly inaccurate cell counts. As an example, the graph “FastBallast – single turnover approach” shows the same variable fluorescence recorded from a low concentration of large cells (blue) compared to a low density of small cells. As PAM techniques need to assume the cell size to derive a cell count result and can, therefore, be inaccurate by orders of magnitude. In contrast, FastBallast assesses the cell count not on the magnitude of Fv but on its statistical distribution, which is uniquely achievable because of the high sampling rate possible using STAF.
The chart titled “Problem with measuring just Fv” further illustrate the inaccuracies that simply using an Fv value alone can give. Here, the red & green plots correspond to dose response from large and small cells respectively, assessed using Fv alone, which are inaccurate by factors of 10 to 100. By contrast the FastBallast’s statistical approach (in grey), correlates very well with the cell count assessed using microscopy.
Ultimately, the inaccuracies associated with PAM testing will require samples to be sent to a laboratory for results close to the regulatory curt-off, whereas STAF technology and its distribution approach in FastBallast gives reliable stage 2 compliance tests as well as stage 1 indicative tests.
FastBallast ballast water testing
The Chelsea Technologies range for ocean shipping includes the FastBallast, which allows for rapid on-board testing of treated ballast water to ensure compliance with the IMO D-2 & USCG Discharge Standards. All ships must now meet the IMO D-2 Performance Standard which states that discharged ballast water must not contain more than 10 viable cells/mL in the 10-50 μm size range (smallest dimension).