There are two generic types of process technology used in ballast water treatment:
- solid-liquid separation and
Solid-liquid separation is simply the separation of suspended solid material, including the larger suspended micro-organisms, from the ballast water, either by sedimentation (allowing the solids to settle out by virtue of their own weight), or by surface filtration (removal by straining; i.e. by virtue of the pores in the filtering material being smaller than the size of the particle or organism).
The chemical or physicochemical unit processes used for disinfection are usually preceded by physical solid-liquid separation, by either filtration or hydrocyclone technology.
The filtration processes used in ballast water treatment systems are generally of the automatic back-washing type using either discs or fixed screens. Since the standards relating to treated ballast water are size-based, technologies capable of removing materials above a specific size are most appropriate. Removal of larger organisms such as plankton by filtration requires a filter of equivalent mesh size between 10 and 50µm. Such filters are the most widely used solid-liquid separation process employed in ballast water treatment, and their effective operation relates mainly to the flow capacity attained at a given operating pressure. Maintaining the flow normally requires that the filter is regularly cleaned, and it is the balance between flow, operating pressure and cleaning frequency that determines the efficacy of the filtration process.
In principle, surface filtration can remove sub micron (i.e. less than 1µm in size) micro-organisms. However, such processes are not viable for ballast water treatment due to the relatively low permeability of the membrane material. Hydrocyclone technology is also used as an alternative to filtration, providing enhanced sedimentation by injecting the water at high velocity to impart a rotational motion which creates a centrifugal force which increases the velocity of the particle relative to the water. The effectiveness of the separation depends upon the difference in density of the particle and the surrounding water, the particle size, the speed of rotation and residence time. Since both hydrocylcones and filters are more effective for larger particles, pre-treatment with coagulants to aggregate (or ‘flocculate’) the particles may be used upstream of these processes to increase their efficacy. However, because flocculation is time dependent, the required residence time for the process to be effective demands a relatively large tank. The processes can be advanced, however, by dosing with an ancillary powder of high density (such magnetite or sand) along with the coagulant to generate flocs which settle more rapidly. This is sometimes referred to as ‘ballasted flocculation’, and is used in some municipal water treatment installations where space is at a premium.
A number of different chemicals or chemical processes have been employed in the ballast water treatment systems reviewed including:
• Chlorine dioxide
• Peracetic acid
• Hydrogen peroxide
• Menadione/Vitamin K
The efficacy of these processes varies according to the conditions of the water such as pH, temperature and, most significantly, the type of organism. Chlorine, whilst relatively inexpensive is virtually ineffective against cysts unless concentrations of at least 2 mg/l are used. Chlorine also leads to undesirable chlorinated byproducts, particularly chlorinated hydrocarbons and trihalomethanes.
Ozone yields far fewer harmful byproducts, the most prominent being bromate, but requires relatively complex equipment to both produce and dissolve it into the water.
Chlorine dioxide is normally produced in situ, although this presents a hazard since the reagents used are themselves chemically hazardous.
Peracetic acid and hydrogen peroxide (provided as a blend of the two chemicals in the form of the proprietary product Peraclean) are infinitely soluble in water, produce few harmful byproducts and are relatively stable as Peraclean. However this reagent is relatively expensive, is dosed at quite high levels and requires considerable storage facilities.
For all these chemicals pre-treatment of the water with upstream solid-liquid separation is desirable to reduce the ‘demand’ on the chemical, because the chemical can also react with organic and other materials in the ballast water.
Post-treatment to remove any residual chemical disinfectant, specifically chlorine, prior to discharge using a chemical reducing agent (sodium sulphite or bisulphite) may be appropriate if high concentrations of the disinfectant persist. In potable water treatment this technique is routinely employed. When used in ballast water treatment, dosing to around 2 mg/l of chlorine can take place, leaving a chlorine residual in the ballast water tanks to achieve disinfection. The chlorine level is then reduced to zero (‘quenching’ the chlorine completely) prior to discharge. This technique is used in at least two of the ballast water treatment systems currently reviewed.
Menadione, or Vitamin K, is unusual in that it is a natural product (although produced synthetically for bulk commercial use) and is relatively safe to handle. It is awaiting US FDA and EPA approval which should be granted in 2008. It is marketed for use in ballast water treatment under the proprietary name Seakleen® by Hyde Marine.
Of the physical disinfection options ultraviolet irradiation (UV) is the most well established and is used extensively in municipal and industrial water treatment applications. The process employs amalgam lamps surrounded by a quartz sleeve which can provide UV light at different wavelengths and intensities, depending on the particular application. It is well known to be effective against a wide range of microorganisms, including viruses and cysts, but relies on good UV transmission through the water and hence needs clear water and unfouled clean quartz sleeves to be effective.
The removal of water turbidity (i.e. cloudiness) is therefore essential for effective operation of the system.
UV can be enhanced by combining with another reagent, such as ozone, hydrogen peroxide or titanium dioxide which will provide greater oxidative power than either UV or the supplementary chemical reagent alone. The remaining physical disinfection processes do not inherently require use of pre-treatment. However, the efficacy of both processes is subject to limitations.
Deoxygenation takes a number of days to come into effect due to the length of time it takes the organisms to be asphyxiated. However, most voyages will exceed this time period so this should not be a significant constraint.
Cavitation or ultrasonic treatment processes both act at the surface of the micro-organism and disrupt the cell wall through the collapse of micro bubbles. These processes are currently not as well understood as the other more established disinfection technologies and some systems use these techniques with chemical disinfection to provide the necessary biocidal efficacy.
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