Contamination profile of wastewaters associated with in-water hull cleaning and their environmental risks

Abstract

In certain countries (e.g. Australia and New Zealand), cleaning of the ship hull is required before entering local ports to prevent the introduction of non-indigenous species into the marine environment. This procedure is carried out using an in-water cleaning and capture (IWCC) system. However, not all IWCC systems are designed to effectively treat the wastes before releasing the effluent back to the water column, thus possibly posing risks to the marine environment. The current study focused on the cleanings by divers and ROVs, and particles and metals in wastewaters were quantified and characterized for environmental risks. 23-80% of total particles were observed in 8-10 µm size fraction, which suggests that by using 8 µm of filter pore size, up to 93.7% of particles can be removed from the wastewater. Total suspended solids (TSS) generated during manual and ROV cleanings were at similar levels, with average concentrations of 173.4 and 189.0 mg/L, respectively. By utilizing treatment systems in ROV cleaning, TSS was reduced from 197.6 to 55.7 mg/L, and average Cu and Zn concentrations were reduced from 161.8 to 24.5 µg/L, and from 1140 to 776.5 µg/L, respectively. On the other hand, average Cu and Zn concentrations in the wastewaters from manual cleaning were 209.2 and 1513 µg/L, respectively. These metal concentrations were then converted to release rate and the release rates were used for predicting environmental risk caused by these cleaning procedures. Manual cleaning showed a higher risk compared to ROV cleaning. The results also proved that the risk brought by in-water cleanings is less when a capture system is applied. However, to maintain a port with low risk, the number of ships to be cleaned should be controlled based on the port size and hydrodynamic factors, and solid waste produced during the cleanings should be collected and disposed on-shore.