Kedudukan tangki biofilter
Bahan yang boleh digunakan sebagai substrat untuk bakteria melekat dan berkembang biak didalam sistem penapisan biologi tersebut. Seharusnya bahan media itu mempunyai jumlah luas permukaan per isipadu yang lebih tinggi. Ini bermaksud bagi sebiji bio media tersebut menyamai dengan permukaan sebesar 0.4 meter persegi. Didalam tangki biofilter yang berukuran 1 x 1 x 5 meter terdapat lebih kurang 6000 biji bio media yang menyamai 0.24 ha keluasan persegi
Bagaimana pun, untuk membina bio filter tidak semestinya anda menggunakan bio ball seperti diatas, anda boleh juga menggunakan kulit kerang atau pun batu karang mati sebagai ganti bio ball. Dari segi effisyen, batu karang mati adalah lebih baik, tetapi tidak digalakkan kerana keadaan ini akan menggalakkan banyak batu karang digunakan nanti sedangkan sumbernya adalah terhad. Oleh itu, penggunaan kulit kerang adalah digalakkan kerana ianya murah dan senang diperolehi khususnya di Pantai Barat Semenanjung. Pun begitu, hanya sebelah luar kulit kerang sahaja yang berfungsi kerana permukaannya yang kasar dan kesat.
Biofiltration:
The biological filter (biofilter) is the heart of the RAS. As the name implies, it is a living filter composed of a media (corrugated plastic sheets or beads or sand grains) upon which a film of bacteria grows. The bacteria provide the waste treatment by removing pollutants. The two primary water pollutants that need to be removed are (1) fish waste (toxic ammonia compounds) excreted into the water and (2) uneaten fish feed particles. The biofilter is the site where beneficial bacteria remove (detoxify) fish excretory products, primarily ammonia.
Ammonia and Nitrate Toxicity:
Ammonia and nitrite are toxic to fish. Ammonia in water occurs in two forms: ionized ammonium (NH4+) and unionized (free) ammonia (NH3). The latter, NH3, is highly toxic to fish in small concentrations and should be kept at levels below 0.05 mg/l. The total amount of NH3 and NH4 remain in proportion to one another for a given temperature and pH, and a decrease in one form will be compensated by conversion of the other. The amount of unionized ammonia in the water is directly proportional to the temperature and pH. As the temperature of pH increases, the amount of NH3 relative to NH4 also increases.
In addition to ammonia, nitrite (NO2) poisoning of fish also is an imminent danger in RAS. Nitrite levels should be kept below 0.5 mg/l. Brown blood disease (methemoglobinemia) occurs in cultured salmon and channel catfish when hemoglobin is oxidized by nitrite to form methemoglobin (a respiratory pigment of the blood that cannot transport oxygen). The disease can occur at nitrite concentrations of 0.5 mg/l or greater. As the name implies, the blood has a characteristic chocolate brown color. Adding salt (NaCl) at a rate of 1 pound per 120 gallons of water (a chloride to nitrite ratio of 16:1) will suppress this disease in soft water; a ratio of 3:1 is effective in hard water.
Calculating Ammonia Loading:
The amount of ammonia excreted into a tank depends on a number of variables including the species, sizes, and densities of fish stocked and environmental conditions (temperature, pH). Ammonia loading can be roughly estimated from the biomass (weight) of fish in the tank or it can be based on the weight of feed fed each day.
On the average about 25 mg (milligrams) of ammonia per day is produced for every 100 grams (3.5 ounces) of fish in the tank. Therefore, in a tank containing 1,000 striped bass fingerlings each weighing 75 g (75,000 g total fish weight), the daily ammonia load produced by all the fish would be 18,750 mg (18.8 g). To remedy excessively high ammonia levels, add freshwater, eliminate feeding or reduce the density of fish in the tank.
Ammonia loading also can be estimated based on the total amount of feed fed. For manufactured fish feed with standard protein levels of 30 to 40 percent, simply multiply the total weight of the feed (in grams) times 25. For example, if the fingerling stripers are fed 1 pound (454 grams) of pelleted feed per day, the amount of ammonia produced per tank would be about 11,350 mg per day.
Nitrification:
Ammonia is a poisonous waste product excreted by fish. Since fish cannot tolerate this poison, detoxifying ammonia is fundamental to good water quality, healthy fish, and high production.
Detoxification of ammonia occurs on the biofilter through the process of nitrification. Nitrification refers to the bacterial conversion of ammonia nitrogen (NH3) to less toxic NO2, and finally to non-toxic NO3. The process requires a suitable surface on which the bacteria an grow (biofilter media), pumping an continuous flow of tank water through the biofilter, and maintaining normal water temperatures and good water quality.
Two groups of aerobic (oxygen requiring), nitrifying bacteria are needed for this job. Nitrosomonas bacteria convert NH3 to NO2 (they oxidize toxic ammonia excreted by fish to less toxic nitrite), the Nitrobacter bacteria convert NO2 to NO3 (they oxidize toxic nitrite to largely nontoxic nitrate).
Nitrification is an aerobic process and requires oxygen. For every 1 milligram of ammonia converted about 5 milligrams of oxygen is consumed, and additional 5 milligrams of oxygen is required to satisfy the oxygen demand of the bacteria involved with this conversion. Therefore, tanks with large numbers of fish and heavy ammonia loads will require plenty of oxygen before and after the biofiltration process.
Nitrification is an acidifying process, but is most efficient when the pH is maintained between 7 and 8 and the water temperature is about 27-28 C. Acid water (less than pH 6.5) inhibits nitrification and should be avoided. Soft, acidic waters may require the addition of carbonates (calcium carbonate, sodium bicarbonate) to buffer the water. The addition of a salt as a therapeutic in striped bass as freshwater bacteria temporarily adjust to alteration in salinity.
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