Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 115

Medium particulates can be removed by sand, canister, or bead filtration (Figure A3.4). Sand filters (also called pressure sand filters or mixed‐bed filters) use various grades of gravel and sand (decreasing in size from the bottom up) to trap particulates. Water flows through the filter from the top down, with larger to smaller particulates being removed as it flows through. Water pressure builds as material gathers in the filter, plugging the channels of water flow. When the filters reach a predetermined pressure, water flow is reversed and the contents are flushed to municipal sewer outlets; this is known as a back‐flush or back‐wash. Canister filters contain a variety of filter media within cartridges or bags that are easy to remove for servicing. A cartridge may consist of pleated filter media. Bead filters use buoyant polyethylene beads that are electrostatically charged to attract negatively charged particulates as the water flows through them from the bottom up. Back‐washes can be driven by propeller or bubbles to dislodge the filtrate from the media and send it to municipal sewer outlets; this may be automated or done manually.

Figure A3.3 Sock filters (a) and a rotating drum filter (b).