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Development of Blue-green Algae Removal Equipment
As urbanization increased life drainage, lakes and marshes have become eutrophic. Consequently, blue-green algae multiplied terribly, which led to deterioration of the purity of water, so that the development of purification equipment, which is able to remove the algae rapidly, has been longed for. This time, magnetic separating continuous algae removal equipment with a superconducting magnet has been developed. It may lead to practical rapid and large volume purification equipment. Norihide Saho, Chief Researcher in Hitachi Mechanical Engineering Research Laboratory, said, "We have developed a structure which is able to remove blue-green algae continuously with setting magnetic filters outside of the superconducting magnet. We come close to the realization of a practical large volume removal equipment." Magnetic separation method has been so far applied to purification equipment in steelworks, which removes minute iron powder from large amount of recycled cooling water. When it is applied to removal of the algae, it requires a pretreatment. Because the algae are non-magnetic, we have to add a little magnetic powder and aggregater to the dirty lake water, and then mix and stir it, until the magnetic powder and the dirty particles aggregate together. So they form a soft mass, so-called "magnetic flock". A magnet separates and removes the magnetic flock from the dirty lake water to result in purification. However, the amount of the magnetic powder contained in the water is so little that a very strong magnetic field is necessary for attracting and capturing all the flocks in fast flow. Thus, a superconducting magnet has the advantage in the magnetic purification. In the equipment, magnetic filters, what are piles of magnetic wire gauze, are placed within a magnetic field at both ends of the electromagnet. The wires of the filter gather magnetic flux around themselves so that it generates very strong magnetic field gradient. Therefore, when pretreated dirty lake water flows through the filters, they attract and capture the magnetic flocks with strong magnetic force. The magnetic filters lose their magnetic force when they are moved outside of the magnetic field, so captured flocks can easily be cleaned from them. A trial-manufactured equipment consists of a pretreatment part, which makes magnetic flocks, a continuous magnetic separation part, which captures, separates, and cleans up the magnetic flocks, and an after treatment part, which dehydrates the flocks and collects the magnetic powder. The system is able to treat the removal of the algae consistently and continuously, and has capacity to treat 400m3 of lake water a day. A structure of the continuous magnetic separation part consists of twin-type superconducting magnets, which are directly cooled by refrigerators, a main magnetic filter placed between the magnets, which moves right and left, and a sub-magnetic filter, which rotates in leaked magnetic field of the lower magnet. The structure is designed so that the magnet can continuously separate and remove the magnetic flocks in exciting state. In other words, when half of the filter is in a magnetic field generated between the superconducting magnets, it capture magnetic flocks, whereas the other half is outside the magnetic field, and cleans the flocks from itself. Then filter moves, so the cleaned half is placed in the magnetic field in turn. Besides, the sub-magnetic filter is designed to capture magnetic flocks on the half of the filter in a magnetic field at the bottom of the lower magnet, and to clean them up on the other half outside the field. The sub-magnetic filter captures large-sized flocks in advance, which are dominant in magnetic flocks, so that it helps the main magnetic filter to lessen its load. This equipment has proved a capacity to continuously remove as much as 90% of the algae in lake water at 360m/h in tracheary (rate of treated water flowing through the filter), which is 10-100 times as fast as that of an ordinary filtration equipment. This result has made great progress in practicability for the algae removal equipment. |