This invention relates to a method for treating ballast water. More particularly it concerns a method for treating ballast water wherein the ballast water is led into a ballast tank via a drop line and wherein a negative pressure is formed at the upper portion of the drop line. The invention also comprises a device for performing the invention.

For marine engineering reasons there has to be some cargo in a ship during its voyage. Non-paying cargo is called ballast as is well known.

Sea water is to a great extent used as ballast due to simple loading and unloading of the sea water. A quantity of species of organisms comes with the ballast water from the place where it is loaded to the place where it is unloaded.

A large part of the living organisms die during the transport and more die when they are released at the unloading site. Under favourable conditions however a reproducing stock may survive. It is known that entry of alien organisms to a new site has had catastrophic consequences for among other things the fisheries in whole oceanic areas.

New directives for treatment of ballast water with a view to render organisms harmless are under preparation in international organs. The directives emphasizes that the treatment of the ballast water must be safe, it must be environmentally acceptable, it must be reasonable to use and it must function.

It is known to expose the ballast water to a relatively short-term negative pressure to kill larger living organisms such as crawfish. A negative pressure has however little or no effect on such as bacteria or virus.

According to prior art ballast water is therefore often treated with chlorine. Chlorine together with the bromide that exists naturally in sea water may however have unfortunate effect on the environment at the site where the ballast water is discharged.

It is also known to utilise ozone for treatment of ballast water. Ozone is according to prior art pumped into the bal- last tank via one or more distribution pipes. This method is however dependent on a relatively large amount of ozone to be used to achieve a satisfactory effect. Bigger organisms are less sensitive to ozone in the relatively low concentrations we speak about here .

The object of the invention is to remedy or reduce at least ^• one of prior art drawbacks .

The object is achieved according to the invention by the features stated in the description below and in the following claims .

There is provided a method for treatment of ballast water wherein the ballast water is led into a ballast tank via a drop line and where a negative pressure is formed at the upper portion of the drop line. The method is characterised in that the ballast water is conducted through a choke posi- tioned upstream of the drop line. The drop line wherein the negative pressure is formed is designed with sufficient height over a vessel deck, typically in the order of 10 metres, to achieve a sufficient negative pressure in the seawater at the upper portion of the drop line.

Under relatively favourable conditions where an absolute pressure of 0.1 bar is achieved at the upper portion of the drop line, the living organisms are exposed to a tenfold pressure drop. Experience shows that some larger organisms may survive such a pressure drop and that a larger pressure drop may be appropriate .

The drop line may be fed with ballast water under a higher pressure as the ballast water flows through the choke, which may be positioned upstream at the upper portion of the drop line. By choosing an absolute pressure upstream of the choke of approx. 2 bar a 20 -fold pressure drop is achieved, while a 40-fold pressure drop is achieved if the absolute inlet pressure is 4 bar. Such increased pressure drops will contribute to ensure that larger, living organisms such as crustaceans and molluscs are killed when they suddenly undergo such a treatment .

A choke body having relatively small openings may constitute the choking. Due to the risk of relatively small openings being blocked; the choke may be designed having one or more smaller extended, tubular bodies. Thereby a desired pressure drop may be achieved without having to reduce the flow area so much that a danger of blocking exists.

The choke may comprise one or more labyrinths that the ballast water has to flow through. Intermixing of ozone in the ballast water may be desirable to contribute to render microorganisms harmless. However, plants for production of ozone require relatively large space.

It has been found that, surprisingly enough, addition of metal ions to the ballast water, particularly in cooperation with a considerable pressure drop, has a favourable effect regarding killing of smaller organisms. Copper ions work for example effectively on algae, while silver ions work effectively on bacteria.

The ions are added to the ballast water by placing a metal electrode containing the relevant metal in a pipe that the ballast water is flowing through. A voltage is applied between the electrode and an earthed body. The earthed body may be the pipe. The voltage depends on the conductivity of the ballast water and is typically chosen to be between 1 and 20 volts. Tests have shown that a voltage of the order of 4-6 volts gives good results in salt water. Higher voltages may be relevant if the ballast water conductivity is poor.

The so-called Froude number is known from channel hydraulics. The Froude number F, which is dimensionless, is defined as a ratio between the inertia force and the gravity force acting on a fluid:

where V = fluid speed in metres per second, g = earths gravi- tation in metres per second ² and h _m = the hydraulic middle depth.

By replacing the hydraulic middle depth h _m in the formula with the diameter D of the drop line, an expression is arrived at which turns out to be appropriate for choice of a suitable drop line diameter. The development work done has shown that a satisfactory pressure drop arises when the value of the expression

is larger than 0.32.

The diameter of the loading column is thus mainly dependent on the speed of the inflowing fluid.

The best pressure drop in the ballast water is achieved for a Froude number greater than 0.35.

The said treatment of ballast water is performed preferably during loading of the ballast water, but may equally well be performed during discharging of the ballast water.

The method and device according to the invention provides a relatively simple and cost effective solution for treatment of ballast water. Larger organisms are rendered harmless by the pressure drop in the drop line while smaller organisms are killed by means of added metal ions.

In the following is described an example of a preferred method and embodiment that is illustrated in the accompanying drawings, wherein:

Fig. 1 shows in section a principle sketch of a vessel provided with a device in accordance with the invention;

Fig. 2 shows an alternative embodiment of the device in fig. 1;

Fig. 3 shows in section and in greater detail a choke; and

Fig. 4 shows in section a device for addition of metal ions to ballast water. In the drawings the reference numeral 1 indicates a drop line positioned onboard a vessel 2. Ballast water is pumped by a ballast pump 4 from the sea 6 at the vessel 2 and via a riser 8 up to the upper portion 10 of the drop line 1.

The riser 8 may be provided with one or more flow regulating devices such as a choke 12. From the upper portion 10 of the drop line 1 ballast water flows down into a ballast tank 14 in the vessel 2.

A metal ion generator 16 is enabled on the riser 8.

In an alternative embodiment, see fig. 2, the drop line 1 flows into a separator 18. Separated material is pumped by means of a sludge pump 20 via a sludge pipe 22 out into the sea 6, as cleaned ballast water flows from the separator 18 into the ballast tank 14 via a connecting pipe 24.

The choke 12, see fig. 3, comprises a choke body 26 in the form of a choke disc having openings 28 therethrough where the choke body is built sealingly into a flange connection 30.

To be able to uphold a satisfactory flow area through the choke body 26 at the same time as a desired choking is achieved, there is in this preferred embodiment arranged a choke pipe in each of the openings 28.

The choke 12 is further designed with a labyrinth 34 in the form of a plate positioned upstream of the openings 28.

The metal ion generator 16, see fig. 4, is by means of flange connections 30 connected into the riser 8. In this exemplary embodiment two metal electrodes 36 protrude into a housing 38. The metal electrodes 36 are electrically insulated from the housing 38 in a cover 40. A voltage is applied to the metal electrodes 36 via respective conductors 42, as the housing 38 constitutes earth relative to the metal electrodes 36.

The metal electrodes 36 may consist of pure metals such as silver or copper, or they may comprise more materials and thereby constitute alloys.

During treatment of ballast water, the ballast water is pumped up via the riser 8 by the ballast pumps 4 in a quantity preferably keeping up the Froude number in the drop line at a value larger than 0.35 and at a pressure causing a desired pressure drop to be achieved over the choke 12.

An electric voltage, which may be in the order of 5 volt, is applied to the metal electrodes 36 whereby metal ions are supplied to the ballast water from the metal electrodes 36.

The pressure drop over the choke 12 kills larger organisms while the metal ions supplied to the ballast water kills smaller organisms.