Freshwater recirculation systems

Recirculating system - 60% System components solids filter tanks aeration

Recirculating system 90% System components solids filter tanks Aeration Degasser (CO2) biological filter buffering systems

Recirculating system 98% System components solids filter tanks Aeration Degasser (CO2) biological filter buffering systems Anaerobic biofiltration disinfection

System Components Primary Secondary Tanks Solids filter Biofilter Aeration Secondary Pumps Lighting Disinfection

Components of a Recirculating Aquaculture System continued 2. Biofiltration continued Engineered forms: Submerged Fluidized beds Emerged Trickling/RBC Combinations Submerged biofilters can either be up-, or down-flowing. Over time, submerged biofilters may block due to accumulation of sediments and must therefore, be flushed. Trickling biofilters are similar to submerged biofilters except that water trickles downwards over the media, keeping bacteria wet but not entirely submerged. Rotating biological contactors are circular media that turns like a wheel, thereby alternately bringing bacteria in contact with water and then air. They are cheap and also can act to oxygenate the water column while stripping gasses.

Pressurized downflow sand filter Solids Filtration Sources Fish wastes Uneaten food Types Settable Suspended Fine and dissolved Pressurized downflow sand filter

Solids Filtration Settable solids (gravity removal) Sediment trap Settling tank Hydrocyclone (swirl separator) Gravity Sump

Solids Removal Suspended Solids (non-gravity) Screen filtration Expandable granular media Downflow (fine sand) Upflow (course sand) Suction Screen Screen filters

Solids Filtration Fine and dissolved solids Principle Foam Fractionation Principle Particles attach to bubbles that rise through tube.

Biological Filtration Sources Fish metabolism Function Nitrification Oxidizes ammonia and nitrite to nitrate NO3- NH3 1½ O2 nitrosomonas nitrobacter NO2-

Types of Bio-filters Submerged filters Trickling filters Simple Inefficient Trickling filters Aerates Submerged filter

Oxygenation Non-pressurized DBC U-tube Downflow bubble contactor (DBC) Counter current diffusion column U-tube diffusers U-tube DBC

Degassing spray tower packed column packed column

Bicarbonate Drip Rational Purpose CO2 removal (<20 mg/L) Buffering from respiration Buffering nitrification is an acidifying process Purpose Adds alkalinity to water

Recirculating Aquaculture System Components continued 2. Biofiltration continued Ideal characteristics: Large surface area: volume ratios Long-lasting Cheap Light B-cell Kaldnes Commonly used media                                                                i.      Sand – cheap, high specific surface area, low void, heavy                                                              ii.      Gravel – cheap, low specific surface area, medium void, heavy                                                             iii.      Rock (limestone) – cheap, v. low specific surface area, low void, heavy                                                             iv.      Crushed oyster shell - cheap                                                               v.      Plastics (KMT, RBC) - expensive   198 m2/m3 152 m2/m3

Recirculating Aquaculture System Components continued 2. Biofiltration continued TAN NO2 NO3 time (days) mg l-1 40 15 The speed of maturation of the nitrosomonas and nitrobacter colonies can be decreased with addition of ammonia (1-5 mg/l as ammonium chjloride, hydroxide or phosphate) to the water column or by mixing with old media.

Particulate removal Removal by: Gravity separation Filtration Flotation 0.001 0.01 0.1 1 10 100 1000 supra colloidal settleable colloidal dissolved Particle size m Comprising uneaten feeds, fecal materials, mucus, bioflocs, fish scales, etc.

Most effective solids removal processes and particle range Particulate removal Most effective solids removal processes and particle range 10 25 50 75 100 fractionation sedimentation granular filters microscreens Particle size m cartridge filter Optional treatment Main treatment Pretreatment coarse screen

Recirculation systems

Biological filtration Biological filtration is the use bacteria to removal of waste metabolites that accumulate from keeping, feeding and growing fish in a closed recirculating system. After oxygen, it is the accumulation of ammonia - the waste metabolite from protein digestion - that is probably the most limiting factor affecting the success of operating closed recirculation systems.

Biological filtration The removal of ammonia (NH3) by oxidation to nitrite (NO2) and finally nitrate (NO3) is carried out by the bacteria Nitrosomonas sp. and Nitrobacter sp. respectively. This process is known as nitrification. Un-ionised ammonia and nitrite are both toxic to fish at relatively low concentrations whereas nitrate, the end product of nitrification, is relatively non-toxic at even quite high concentrations.

Biological filtration The rate of nitrification is optimal around 30°C. The particular size and type of filtration equipment needed will depend on a number of factors including, the type of fish, the biomass of fish, the amount of food fed the temperature of the water.