Special Issue Survey of Cetaceans in Captive Care

6. Life Support Systems 357from coal, granulometry 0.8 to 1.6 mm) can helpremove additional dead organic matter and reduceturbidity. These filters are backwashed regularly toremove accumulated particles and maintain filtrationcapacity. This is done after emptying the tankof its water and reversing the flow inside the tank.To increase cleaning efficiency, the substrate canbe decompressed and agitated by air scour throughthe injection of air from below. High-rate pressuresand filters do not sustain a stable microbial populationthat can act as a biological filter, althoughsome microorganisms might survive backwashesand help to break down dissolved substances in aminimal way. This is why a sterilisation processhas to be added to the mechanical filtration.Flocculation—Flocculation is a useful additionalprocess used in substrate filtration. A flocculantis used to coagulate small particles intobigger ones—the flocs—which will be moreefficiently trapped in the filtering substrate and,thus, reduce oxidant demand (Anonymous, 1999;Gregory, 1989; Spotte, 1991). The flocculant mostcommonly used is aluminium sulphate or alum,but polyaluminium chloride and ferric chlorideare also used. The flocculent and water should bemixed properly; thus, a mixing chamber is recommended(Boness, 1996).Figure 6.4. Filters for large whale poolsProtein Skimmers/Foam Fractionators—Anothermechanical system to remove organic matter andreduce nitrogen compounds (e.g., ammonia, nitrite,nitrate)—a protein skimmer or foam fractionator—is now commonly installed in marine mammalfacilities. This dispersed-air flotation system usuallyis combined with an oxidising system; thus, italso could be presented in the “Chemical Filtration”section. The polluted water is mixed with an air/ozone mixture through an injector and pumped intothe reaction chamber. Here, vortexing causes turbulenceand intimate mixing of the water and airbubbles. Proteins with a lower specific weight thanwater are the first to flow upwards through the internalriser. Solids become attached to gas bubblesalready loaded with proteins and also flow upward.Pollutants are concentrated into foam, which thenis skimmed, leaving clean water to flow througha different outlet (Shuran Sea Water Equipment:Aquaflotor Technical Guide; van der Toorn, 1987).This system usually is installed as a complementto sand filters and is combined with ozone asan alternative to other means of disinfection orsterilisation.Figure 6.2. Steel tower sand filters (Photograph from I. Smit)Figure 6.3. Horizontal steel filtersFigure 6.5. Protein skimmers with ozone injection(Photograph from I. Smit)