DESCRIPTION

TOXIC HEAVY METALS REMOVAL SYSTEM FOR SHELLFISH

Technical Field

The invention refers to a closed loop seawater system. It is a technological application that has the ability to remove heavy metals from shellfish by using biological and physicochemical methods.

Background Art

According to the data of the Food and Agriculture Organization of the United Nations, the total annual amount of shellfish production worldwide is approximately 16,7 million tones (2004 data). Shellfish are sea products of great gastronomy interest, with high nutritional and commercial value at national and international level. Due to the fact that they live and are cultured in closed bays or gulfs (coastal ecosystems), which are the final receivers of human pollution (e.g. high concentrations of heavy metals) through rivers, canals, streams, etc., shellfish could be dangerous for the public health after consumption. Therefore, distribution and trading for consumption of live shellfish are ruled by a strict legislation framework in European level (e.g. Regulation EC/1881/2006). The high detection rate of heavy metals hazard limits in shellfish according to the qualified Laboratories in Greece, has lead to the prohibition of fishing either for a total area (e.g. gulf of Geras in Lesvos Island) or for some sensitive species (e.g. the species Modiolus barbatus in Thermaikos gulf). This situation affects an important number of people (shellfish fishermen and shellfish producers) that are occupied in this specific sector. This phenomenon is not limited in Greece but is also observed in other areas as the Black Sea, Adriatic, etc.

The need of technological applications capable of removing or decreasing high concentrations of heavy metals from shellfish is more than obvious and essential. These applications will contribute to local, national and international economy by ensuring food safety and increasing shellfish fisheries industry by allowing the fishing of large amounts of shellfish that until today was prohibited due to high concentrations of heavy metals. Until today, such a technological application has not been recorded. The invention with publication number CN 1736181 (publication date 22.02.2006) from China entitled: "Method for purifying in vivo heavy metal of bivalve shell" is developed in the field (into the sea or coastal ecosystem) and refers to a natural process (not technological). According to the invention CN 1736181, the decrease of heavy metals concentration is

achieved through food addition (microalgae) without water filtration and high level of water clarity is required.

Disclosure of Invention The innovation of the present invention is that through water recirculation and by using multiple methods, heavy metals removal from shellfish is achieved without any food (microalgae) supplementation. The proposed system can be installed almost anywhere and it can be connected with other depuration or water recirculation systems for shellfish. This invention provides the advantage of low demands in space (60-12Om ² , depended on the capacity of the system) and low operating costs due to the avoidance of any food supplementation.

The present invention contributes to the removal of heavy metals from shellfish in a system having a capacity ranging from 1 to 50 tn and consisting of a specific combination and sequence of main and secondary parts. The parts of the system are described below. The main parts of the system are:

• Integrated biosorption filter,

• microfiltration hollow fibre bioreactor,

• suspended material filter,

• macroalgae filter. The secondary parts of the system are:

• isothermal tank for shellfish keeping,

• mechanical filter (e.g. sand filter),

• ultraviolet (UV) lamps,

• appropriate number of water pumps. • appropriate number of adjustable flow splitters

Only the main parts of the system contribute to the removal process of heavy metals from shellfish and are described below with reference (the numbers inside the parenthesis) to the attached drawing:

1. Integrated biosorption filter (3) where the seawater contacts the suspended solids (3A, B), mainly dead biomass, by the addition of air (3C), followed by a flotation step for the separation of the solids from seawater

(3D). There is a connection between the different parts of the integrated biosorption filter in order for the suspended solids to be retained and through the flotation process (3D) to be transported to parts (3A) and (3B), where the mixing of seawater and suspended solids is taking place.

The system can function either in batch or continuous mode of operation.

At the end of the batch cycle, or intermittently for the continuous

operation, the selected biomass (having absorbed the heavy metals) is removed from the system and is replaced with a new one. In independent experiments it was found that within 5 minutes of operation of the integrated biosorption filter, up to 95% of the heavy metals can be removed from the sea water.

2. The microfiltration hollow fibre bioreactor (4) is used to retain suspended particles, in which heavy metals compounds are included. The microfiltration hollow fibre bioreactor can have different types of hollow fibres (differing in size and structure). The hollow fibres are changed when the pressure drop becomes very high due to accumulation of debris/ suspended particles.

3. Suspended material filter (5) includes immobilized microorganisms on spherical parts which is a mixture of at least four size categories (0.4- 0.9cm, 1.1-1.6cm, 1.7-2.0cm, 2.0-3.0cm equivalent diameter). These microorganisms, mainly bacteria, are used as accumulators of chelated heavy metal compounds.

4. Macroalgae filter (6) consists of a tank of appropriate size (depending on the total water volume within the system) with macroalgae composition of at least 90% of the genera Ulva and Gracilaria, in ratio 3:1 and a total loading density 30-40 g/l, situated at the bottom part of the tank. The tank is illuminated with lamps at a light density of about 70 μmol m ^"2 s ^"1 and operates with a photoperiod of 16h light : 8h dark. The function of this macroalgae filter is based on the ability of these macroalgae to absorb high concentrations of specific heavy metals. The secondary parts of the system do not contribute to the removal process of heavy metals from shellfish. They are common in every recirculation system, support the water recirculation process among the parts of the system and their role is to keep water physico-chemical parameters into appropriate levels. The secondary parts of the system consist of a mechanical filter (1), water pumps (2), UV lamps (7), an isothermal tank (8) where the shellfish are stored and adjustable flow splitters (9).

The effectiveness of this invention has already been tested in coordination with the National Reference Laboratory of heavy metal determination of Greek Ministry of Agricultural Development and Food. According to the results of these tests, a decrease (ranging from 40% to 68%) in the concentration of heavy metals (cadmium and lead) was observed, when shellfish were stored for 5 to 15 days in the proposed system, depending on the heavy metals initial concentration.

Brief Description of the Drawing

The drawing illustrates the combination and the sequence of the main and secondary parts of the heavy metals removal system from shellfish. Arrows represent the seawater circulation.

List of reference number

1 Mechanical filter

2 Water pumps

3 Integrated Biosorption filter 3A Mixing system

3B Suspended solids container

3C Air

3D Separation water-solids container

4 Microfiltration hollow fibre bioreactor 5 Suspended material filter

6 Macroalgae filter

7 UV lamps

8 Shellfish isothermal tank

9 Adjustable flow splitter