7 Definitions Recirculating System: A production system that reconditions water to extend it’s reuseA technique that increases the value of a limited resourceAn artificial well that never ceases to flowA stable platform forming a reliable base for commercial production
8 Five Major Processes 1) Circulation 2) Clarification (Solids capture) 3) Biofiltration (BOD reduction/nitrification)4) Aeration (Oxygen replacement)5) Degasification (CO2 stripping)
9 The Five Major Processes in a Recirculating System Five Major Treatment ObjectivesThe Five Major Processes in a Recirculating SystemFeedingCirculationAerationFish TankDegasificationBiofiltrationSolids Capture
20 Impact of Solids on Recirculating Systems Solids CaptureImpact of Solids on Recirculating SystemsIncreased BOD load (organic solids) causes problems with BiofiltersIncreased system turbidity (fine particles)Gill damage in fish (fine particles)Aside from their direct physical impact, suspended solids generated in culture systems are highly organic. Most of the suspended solids generated in a recirculating system are feces, reflecting the undigested residuals of feed. Additional solids are generated as bacterial colonies (biofloc) grow from dissolved organic materials found in the water. Both feces and biofloc are mostly organic matter and are thus are subject to further breakdown by bacteria. Over two-thirds of the Biochemical Oxygen Demand (BOD) generated is attributed to suspended solids. It is not surprising, therefore, that clarification was the first wastewater treatment component added to extend water re-use and the first to be used to treat effluents. If allowed to remain in a culture system, solids encourage heavy bacterial growths that can clog (biofoul) a wide variety of treatment units. Excessive growths of bacteria can rapidly deplete oxygen in isolated pockets within a system. Heavy organic loads can inhibit critical nitrification processes in biofilters (Bovendeur et al., 1990). These localized, oxygen depleted zones can stimulate the development of a wide variety of anaerobic reactions that produce odor and contribute to off-flavor development. The bacteria will utilize large amounts of oxygen and produce TAN as they breakdown the solids. Additionally, these enriched conditions tend to cause population explosions of less desirable bacteria, leading to disease outbreaks.
21 Particle Size Distribution (microns) Solids CaptureParticle Size Distribution (microns)SettleableDissolvedColloidalSuspended
24 SEDIMENTATION INFLOW OUTFLOW Solids Capture Vh Vs Settling Zone Inlet ZoneOutlet ZoneSettling basins have been the clarifier of choice for a number of years. A settling basin is designed to provide an area of quiescent water where the solids can settle out into a cone, from which they can then be removed. More advanced configurations, tube settlers, use a media to shorten settling distances, reducing the size of the settling basin. Performing superbly on systems with high water replacement rates, settling provides poor control of fine suspended solids (< 80 microns in diameter), which tend to accumulate as replacement rates are reduced. This problem, coupled with labor issues, has prompted some to search for a more efficient, yet cost-effective approach.(Vs > Overflow Rateto settle)Sludge Zone
33 BiofiltrationThe classification of heterotrophic bacteria encompasses a great number of genera/species which share the common characteristics of extracting their nourishment from the breakdown (decay) of organic matter. Biochemical oxygen demand (BOD) is largely an indirect measure of the biodegradable organic material in water. Heterotrophic bacteria reduce BOD levels, consuming oxygen in the process. About 60 percent of the organic matter consumed is converted to bacterial biomass; whereas, the balance (40 percent) is converted to carbon dioxide, water, or ammonia. Heterotrophic bacteria grow very fast, capable of doubling their population every ten to fifteen minutes. If the BOD in the water being treated is very high (> 20 mg -O2/l), the heterotrophs will quickly dominate the bead bed, overgrowing the slower growing nitrifying bacteria and consuming tremendous amounts of oxygen.The second, yet more important, classification of bacteria is the nitrifying bacteria. These bacteria are specialists, extracting energy for growth from the chemical conversion of ammonia to nitrite and from nitrite to nitrate (Figure 3.6). Nitrate is a stable end product which, although a valuable nutrient for plants, displays little of the toxic impacts of ammonia and nitrite. Composed principally of two genera (Nitrosomonas and Nitrobacter), nitrifying bacteria are very slow growing and sensitive to a wide variety of water quality factors. It is not surprising that most bead filters used for biofiltration are managed to optimize conditions for nitrification.
45 Degasification Biofiltration Gas ExchangeFeedingCirculationAerationFish TankDegasificationBiofiltrationSolids Capture
46 Different Behavior Oxygen goes in easier than CO2 comes out Oxygen Gas ExchangeDifferent BehaviorOxygen21% of airSaturation 10 mg/lPoorly solubleTransfer H20 limitedCarbon Dioxide0.035% of airSaturation 0.5 mg/lHighly soluble50 mg/l +Transfer gas limitedOxygen goes in easier than CO2 comes out
47 No CO2 Removal Enrichment Devices Gas ExchangeNo CO2RemovalEnrichment DevicesCommonly used for large scale applicationsSelf-generates pressure ( psi)1 m3 liquid860 m3 gasDependent on local source of liquid oxygenNot impacted by power failure15-35cents/kg typicalLiquid OxygenTimmons and Lorsordo (1994) pg. 188
48 Air stones Paddle wheels Surface Agitators Spray nozzles Gas ExchangeAir stonesPaddle wheelsSurface AgitatorsSpray nozzlesPacked columnsAmbient Air –Oxygen Addition and Carbon Dioxide removal are balanced
49 Ambient Air Options Gas Exchange Carbon dioxide can be stripped from culture waters by the use of large amounts of air in traditional blown air systems usually consisting of a large rotary blower and banks of airstones. The most efficient approach identified so far involves the use of a packed column aerator (Colt and Bouck, 1984). A packed column degasifier consists of a tall column filled with a porous packing media. Water cascades down through the media as air is forced upwards. When operated with a high air to water ratio, the unit effectively strips excess carbon dioxide gas. Unfortunately, early units were subject to biofouling problems.
50 Circulation Fish Tank Biofiltration Degasification Circulation Feeding AerationCirculationFish TankBiofiltrationDegasificationSolids Capture
51 Airlift Pump Circulation Options Circulation Air Low head centrifugal or airlift pumps are the most common choices for recirculation of waters through the water treatment block in a recirculating system. Centrifugal pumps are extremely efficient at water movement, but, they must matched with the treatment components through their pump curve that relates flow deliver (liter per minute) with the expected headloss (meters of water). The best pumps are expensive but realize considerable savings over the long run generally drawing less than half the amperage of less expensive units commonly marketed for pool applications. Most recirculating systems operate well with head delivery pressures below 5 meters. Additional savings in energy can be realized by use of 220 or 440 V units. Airlift pumps consist in their simplest form of a partially submerged piece of pipe fitted with a air injection line near the bottom. When air is injected the density of the air/water mixture drops allowing water to enter from the bottom of the pipe eventually overflowing the above water lip of the pipe. Airlift pumps are capable of delivering large quantities of water, but, usually only at a few inches of lift. The lift is usually limited to about 30 percent of the submerged depth. Serially placed airlift pumps can lift water several feet but they are not particularly efficient. Airlift pumps are generally used only in systems specifically designed for their use. Their principle advantage is their substantial contribution to the reaeration needs of the system. Once their design variables are understood, they are easily constructed from standard PVC piping.Pump