Technical report | In-water Treatment of Biofouling in Internal Systems: Field Validation of Quaternary Ammonium Compound (QAC) Chemical Treatment Protocols
Mussel growth in the internal sea water systems of Navy vessels can result in significant performance issues and biosecurity concerns for affected vessels. The primary in-water treatment method for mussel fouling of the internals of Navy vessels is to flush with a 1% detergent solution containing quaternary ammonium compounds (QAC). Parameters for the application of this treatment are based on previous research; however, much of the research has been conducted at small-scales under laboratory conditions. This study examined the efficacy of two commercial QAC solutions for treating mussel biofouling under realistic field conditions using experimental sea water piping systems. The efficacy of the QAC solutions was found to be highly dependent on the size of the mussels present. All treatment solutions were effective at killing large sized mussels in the pipework and sea chest of the system following a 24 h dosing period. In contrast, small mussels appeared resilient to the majority of treatment regimes tested. Changes in water temperature and increased exposure time to treatment chemicals did not enhance efficacy of treatment.
Ship internal sea water systems have long been recognised as high risk mechanisms for non-indigenous species (NIS) transfer due to their propensity to readily accumulate and shelter sessile and mobile marine species. Fouling in sea chests and sea water pipework is also an operational issue for marine engineers, as it restricts and changes water flow regimes to essential vessel systems and may enhance biocorrosion. In the past five years, there have been ten instances of unwanted mussel biofouling on RAN vessels, five of which involved sea chests and/or internal sea water system fouling.
While dry docking vessels is the surest method of effectively minimising biosecurity risks, the process is expensive, time consuming and has the potential to impact operational availability of vessels. In contrast, in-water treatment of vessel biofouling is significantly more cost effective. Non-oxidising disinfectant/sanitiser solutions containing quaternary ammonium compounds (QACs) are a recognised method for treating biofouling in sea water pipework systems.
The overall aim of the present study was to field-validate previous DSTO research assessing the usage parameters of QAC solutions for the control of mussels occurring in vessel sea water systems, using a replica experimental piping system. The study examined the effectiveness of two commercially available QAC disinfectants formulations,
Conquest TGA' and 'Quatsan', in killing the southern Australian blue mussel Mytilus galloprovincialis planulatus. Conquest is currently recommended to the RAN for emergency biosecurity response treatment and the use of Quatsan as a biosecurity response agent has been previously studied by DSTO, NT Fisheries and Neil and Stafford [1, 2]. These agents were therefore chosen for further examination during this study.This study showed that the efficacy of two commercially available QAC formulations (Conquest and Quatsan) in treating mussel biofouling of sea water systems was highly dependent on the size of the mussels present. Treatment solutions of both Quatsan and Conquest appeared very effective at killing large sized (50 – 90 mm) mussels in the pipework and sea chest environments following a 24 h dosing period. 100% mortality of large mussels was achieved in all treatment groups, with the exception of the 1% Conquest treatment group. In contrast, small (0 – 30 mm) sized mussels appeared quite resilient to the majority of treatment regimes tested, with 100 % mortality throughout the entire test system only recorded for one of the treatment regimes (5% Quatsan). Changes in water temperature and increased exposure time to treatment chemicals did not enhance efficacy of treatment.Despite this study showing that the efficacy of QAC treatment varies with respect to mussel size, its use as an effective biosecurity emergency response tool should not be discouraged. Rather, QAC treatment should be used in conjunction with other management strategies to ensure its effectiveness against unwanted mussel species. Despite the additional intervention steps required of this approach, the cost of treatment and disruption to operational availability would still be significantly less than if the vessel were to be placed in dry dock and treated.
Based on the findings of the current study assessing the efficacy and usage parameters of QAC solutions for the control and eradication of mussels in sea water systems under field conditions, we recommend the following:
- Revise the recommended RAN QAC emergency response dosing protocols for controlling mussel biofouling to 5% v/v disinfectant solution for 24 h, rather than the currently recommended protocol of 1% v/v for 14h.
- Mandatory follow-up inspections and monitoring of vessels found to contain unwanted mussel species to ensure resistant organisms have not survived and grown after initial QAC treatment.
- Procurement and storage of sufficient quantities of a selected commercial QAC disinfectant across all RAN bases where vessels are berthed.
- QAC treatment of mussel fouling should be viewed as an emergency response option, not an on-going management strategy.