Читать книгу Fundamentals of Aquatic Veterinary Medicine - Группа авторов - Страница 23

Recirculating systems can vary in complexity and design. Depending on the water source, varying types of water purification may be required or recommended to prevent pathogens at the point of entry as well as within the system itself. Mechanical filters are often used to improve water quality and reduce pathogen loads by removing particulate matter in the system. Recirculating systems that are considered self‐cleaning remove waste solids and provide some water purification prior to system reentry. Mechanical filter design based on water flow direction can be described as “downflow”, where the water to be processed passes by gravity or under pressure down through the media or “upflow”, where it is forced under pressure up through the media. The advantage of this upflow design is to allow sediment to fall away from the media, making it somewhat self‐cleaning (Tepper, 2000). Media used may be sand, polypropylene or fiberglass mesh or polypropylene brushes in downflow systems and polypropylene beads in upflow systems.

Prefilters can be used to separate settleable solids by rotating the water in an upflow direction. This strips off the heavier particulate, enabling more efficient water processing before entering the mechanical filter.

Industrial aquaculture facilities may use self‐cleaning tanks in recirculating systems to remove waste solids and provide some water purification using large rotating drum filters. Also, water sources with high total solids prior to system entry may also require large screen filtration to remove solids. Other large facilities may employ pressurized canisters of upflow design for this purpose (floating bead filters). These canisters are also often seen in home pond filtration, although downflow box filters with filter mesh for particulate removal are also used, where they are the most common type of filter for small aquasystems like freshwater and marine home aquaria. Sand canister filters are sometimes used due to their high efficiency at removing solids and low cost. They have the disadvantage of being hard to maintain over time due to “caking” of the sand media.