(87h) Environmental Risk Analysis of Application of Electro-Disinfection on Ballast Water Treatment

Ballasting and de-ballasting are essential in balancing stability of ship during loading and unloading operation. As a result, biological invasion becomes a serious global issue to the fishing industry as well as eco-environmental system. It is there important to reduce the organisms prior to the water discharge to sea. It is observed that electro-disinfection is a cost-effective technology in the ballast water treatment. Due to the operation, a series of disinfection by-products (DBPs) is created, which may cause risk to the waters.
This study addresses the formation and speciation of DBPs, including four trihalomethanes (THMs) and nine haloacetic acids (HAAs), resulting from the chlorination in ballast water treatment. The impact of several variables that influence DBPs formation, such as chlorine dosage, water quality, organic content, temperature, and chlorine contact time or residence time was investigated. The chlorine decay in the chlorination process was monitored and modeled. The potential risk of the formed DBPs to human being and marine environment was evaluated.
Higher water temperature and/or total organic carbon (TOC) in the water increased the total residual chlorine decay rate. The rate of decay is different in the samples that had different represent organic compounds. The effect of salinity however less was obvious.
Water quality and operation conditions affected the different classes of DBPs at different magnitudes. Four compounds, namely Ethanol, acetic acid, glucose and humic acid were used to represent the organic matters in seawater. More DBPs were produced as the TOC concentration was increased. In the predication of the DBPs, both TOC concentration and the distribution of organic species should be involved. Higher chlorine dose caused a higher DBPs formation. The dominant constitutes of THMs and HAAs were the bromo-substitutions, namely tribromomethane (TBM), dibromoacetic acid (DBAA) and tribromoacetic acid (TBAA). The concentrations of chloro-dervatives were at much lower levels in comparison with bromo-derivatives. The rate and total yield of THMs increased as the temperature was increased. For HAAs, more HAAs were generated as the temperature was increased from 4 to 25oC. When the temperature reached above 35 oC, HAAs concentration initially increased and then decreased after reaching its peak. DBAA and TBAA were the predominant HAAs species at the temperature ranging from 4 to 25oC, which accounted for more than 94% of total HAAs. Less TBAA content was observed when the temperature was increased. At 45 oC, no TBAA could be detected at all after 5 days reaction.
The formation and speciation of THMs and HAAs were further investigated under various water quality and operational conditions typically used in land-based and shipboard tests. Higher salinity led to more THMs generation; more THMs species with chlorine components could be found at low salinity. Conversely, at same experimental temperature, it was observed that more HAAs were found in lower salinity seawater. Five separate seawaters were collected from different locations in Singapore to simulate ballast water taken from different sources/locations in tropical marine environment. The THMFP and HAAFP of the five tested natural seawater samples were different from each other. Among the five samples, the predominant THMs compound formed was TBM. The next of the THMs found was due to the DBCM generation; other two THMs compounds, namely BDCM and TCM, were not formed during the testing. TBAA was the major HAAs species found in all the natural water samples studied. DBAA appeared in all samples but its concentrations were at lower levels than TBAA.
Generally speaking, the levels of DBPs in the chlorinated seawater at the points of discharges were found to be far higher than the drinking water regulated values set by USEPA and WHO. However, they would be well diluted when they are released to the sea. Modeling simulation by MAM-PEC was conducted based on various situations, with the consideration of several physico-chemical and biological processes and interactions. It was demonstrated that the PEC values calculated by the MAM-PEC model were at a safe level in most cases. It indicates that there is no risk to the marine environment caused by the discharged chlorinated ballast water.