Validation of the pilot plant
Installation of the pilot plant was completed and validation of BioFoulControl started in mid-May 2011 on board the offshore supply vessel “Far Seeker”.
The validation is aimed at investigating the performance of the treatment system in preventing settlement of marine organisms in the inlets to the sea chest and the heat exchanger at different ozone doses, and monitoring formation of bromates. Ozonation of inlet strainers and the cooling system on board “Far Seeker”, has been running from mid-May until October 13, 2011.
During the validation period, regular visual inspection and photographing of the ozonated locations have been undertaken to monitor settlement of blue mussels that are a common problem in the North Sea where “Far Seeker” is operating.
Along with trials on board “Far Seeker”, conditions of the inlet strainers and cooling system in a sister vessel, “Far Searcher”, which uses a treatment based on a biocide agent called Bioguard, have been inspected occasionally.
The trial period consisted of two main phases driven by the level of ozone dosage. Dosage of the ozone feeding was varied by regulating the capacity of the ozone generator. The capacity was set at 25 % capacity (25 g O3/hr) for the first trial phase. In the second trial phase, the ozone generator was set at 50 % (50 g O3/hr). Trials of shorter duration were also undertaken with 100 % and 75 % capacity to investigate the level of bromates generated at higher levels of ozone dosage.
Ozonated and control water samples for bromate analysis were collected during the vessel’s journeys at constant geographical positions in the sea. The control samples are used to see whether there is background in the water due to other substances than bromate that may disturb the analytical method. The background may vary between different geographical positions and, therefore, it was important to take the control samples in parallel to the ozonated samples.
Validation results
The validation showed that no new settlement of blue mussels or other living organisms has taken place in sections of the sea chest that have been ozonated with the different ozone dosages (Picture 2). On the other hand, seawater treated with the biocide based method was affected by marked settlements as shown in the picture from the “Far Searcher” vessel (Picture 1).
Picture 1: Strainer on board "Far Searcher" applying a biocide agent called Bioguard as a treatment method.
Picture 2: Ozonated strainer on board "Far Seeker".
The mean level of bromate concentration from analysed samples of ozonated seawater is 9,25 and 18,8 µg/ml at 25 % and 50 % generator capacity respectively. The standard deviations are 2,63 and 2,16 for the lowest and highest ozone dosages respectively.
The LC50 value of bromate for Pacific Oyster is 30mg*L-1 which is also the technological objective of the BioFoulControl project. Thus, the bromate level formed under the validation is 0.03 % and 0,05 % of the target level of BioFoulControl at 25 % and 50 % capacity of the ozone generator respectively. No surplus ozone was detected in the cooling system at these levels of ozone dosage, which have effectively eliminated settlement of blue mussels.
