BAC1tGROUND ART Industry and mining generate vast quantities of contaminated water. The contaminants may be organic molecules such as phenols, and inorganic contaminants, such as iron, copper, sulphur, sulphuric acid, cadmium and zinc. Volumes of water involved are small to enormous depending on the size of the factory or mine. The problem is that the contaminated water cannot be allowed into river systems or underground water reserves and aquifers. The industries and mines facing these problems face huge costs in cleaning up the water by chemical or physical means. The most attractive options is to use microbes to degrade the organic molecules or immobilize the metal contaminants.

Many bioreactors have been developed for these purposes, using two basic designs : aerobic or anaerobic. These use different microbes and support different chemical reactions. Within the bioreactors, microbes must be provided with a physical medium on which to be immobilized, where a biofilm can form. Once established as a biofilm, the microbes can do their metabolic business of cleaning up the water. So the solid matrix onto which the microbes attach is important. This may vary from solid discs of plastic or polystyrene (up to 5m in diameter), to plastic waste and plastic brushes, or gravels chips.

The characteristics of a good matrix is that it must provide a benevolent surface onto which bacteria may attach (plastics are poor surfaces in this regards), but also allow for a good flow of water through the bioreactor.

It is an object of the present invention to provide such a matrix. It is a further object of the invention to provide a novel bioreactor which has a number of important advantages.

DISCLOSURE OF THE INVENTION According to the invention a medium is provided for a matrix for the bioremediation of water, the medium comprising or including a plant bark which is stable with respect to the bacteria to be encountered and to substances to be encountered in the water, has a high surface area, and is easily colonised by bacteria.

A good example of such a plant bark is pine bark and, in particular, composted pine bark. This is very stable, consisting mainly of lignin which are not degraded by bacteria, has a high surface area, is readily colonised by bacteria and is inexpensive, readily available and is a renewable resource.

The bark of the invention may be used with conventional bioreactors or with a novel bioreactor which will be described below.

BRIEF DESCRIPTION OF THE DRA WINGS Three embodiments of the apparatus are described below with reference to the accompanying drawings in which: Figure 1 is a sectional side view of one form of apparatus; Figure 2 is a plan view of Figure 1; Figure 3 is a sectional side view of a second form of apparatus; and Figure 4 is a sectional side view of a third form of apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION In Figures 1 and 2 a multi-chamber arrangement is provided in a trough 10. The medium comprising composted pine bark is held in grid cages 12 supported on spacers 14.

Effluent is introduced through inlet 16 flowing upwards in the cages 12 and overflowing baffles 18 in the direction of the arrows 20 to exit through outlet 22.

In Figure 3 a drum 30 is mounted for rotation in a tank 32 containing effluent. The drum is of mesh or perforated metal and contains a porous medium of composted pine bark giving a large surface area for the growth of bacteria. Rotation is slow and the mounting provides 40 percent of the drum in the tank 34 at any time. The tank contains the effluent to be treated and this may take the form of a batch treatment or a continuous flow treatment. Thus the effluent may be lead in and out of the drum by appropriate delivery and drainage pipes.

Multiple effluent tanks, each with its own rotating drum, can be located side by side and connected with the effluent stream either in parallel or in series, depending on the desired degree of purity of the water.

In one aspect of the invention, the rotating drum of Figure 3 may use other media such as perlite, plastic shaving, gravel or other material which is light, chemically and physically stable and provides a large surface area for bacteria to attach to, but which does not pack too tightly.

In Figure 4, which is an anaerobic fixed drum, the medium 40 is located in the drum 42 which has a removable lid 44. An inlet 46 and outlet 48 provide for effluent flow and a gas outlet 50 is provided to maintain ambient pressure.