Article

# Mathematical Model of a Foam Fractionator Used in Aquaculture

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## Abstract

Water recirculating systems are becoming more prevalent as wild fish supplies and fresh water sources become increasingly scarce. As water recirculating rates increase, the accumulation of fine and dissolved solids can degrade water quality and fish productivity. Foam fractionators have been used with some success to remove these solids, although an understanding of how to best operate such units is currently lacking. The foam enrichment process occurs as bubbles travel from the bottom of a fractionator column to the top of the water column. Factors affecting bubble enrichment include several operating or design parameters under operator control or influence, e.g, water pH, gas bubble size, airflow rates, and fractionator geometry. Predictive relationships were developed to describe these effects and are considered generally applicable to predict the foam enrichment process. Predicting the actual operation of a specific foam fractionator requires additional mathematical expressions to model the foam collection process and removal from the top of the water column. Here, the geometric characteristics and type of foam removal device or geometry of placement become important. Performance data in the literature was used in coajunction with a mathematical model that predicted foam enrichment to model the foam collection process for a typical foam fractionator design. This overall model can be used for foam fractionators that employ glass bonded air diffusers with an inverted funnel at the top of the fractionator column to concentrate and remove the foam being produced. An example problem is given to demonstrate the utility of the mathematical models. Since foam fractionators are typified by erratic performance, the mathematid models presented must be used with some caution and only viewed as providing estimates of average fractionator performance.

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... Micron-sized air bubbles may attach to the surface of surface-active particles and carry them to the free surface, forming a concentrated layer of foam that is then removed from the wastewater for separation. Skimmers are usually preferred as they are cost-effective and easy to use (Timmons et al., 1995;Blancheton et al., 2007;Suzuki et al., 2008;Brambilla et al., 2008;Roque d'orbcastel et al., 2009;Park et al., 2011). In rearing farms, foam fractionation allows the extraction of fine particles smaller than 30 m (Timmons, 1994;Chen et al., 1994). ...
... However, flotation is dependent on bubble diameter, concentration of the solids, air-to-water ratio, surface chemistry of the solids, and the surfactant concentration in the water (Summerfelt, 1999). Timmons et al. (1995), Brambilla et al. (2008) and Park et al. (2011) have shown that skimming efficiency is reduced with the addition of feed to rearing water. This is due to the lipid content of feed, which reduces the formation of foam. ...
... This is due to the lipid content of feed, which reduces the formation of foam. To limit this phenomenon, surface-active agents may be used to increase the formation of foam (Keyes and Stover, 1992;Timmons et al., 1995;Brambilla et al., 2008), but this is not recommended for the food industry. Skimmers are more often used in shellfish aquaculture where the presence of proteins and polysaccharides in the rearing water is high, allowing better foam fractionation (Muniain-Mujikaa et al., 2002). ...
... Solid particles greater than 100 m can be effectively removed with either gravitational or mechanical filters. Suspended solids less than 100 m in size are non-settleable and usually accumulate in intensive recirculating systems (Timmons et al., 1995). It is difficult to economically remove solids smaller than 50 m (Cripps and Bergheim, 2000). ...
... Foam fractionation is one of the most efficient and cost-effective ways to remove suspended and dissolved solids from the culture water (Chen et al., 1992;Timmons et al., 1995;Suzuki and Maruyama, 2002;Brambilla et al., 2008). Lomax (1976) reported that a combination of biofiltration and foam fractionation is the most cost-effective design for solids removal. ...
... This solids-removal process is also called protein skimming, flotation, or air stripping (Gregory and Zabel, 1990;Lawson, 1994). The solidsremoval efficiency is influenced by physical and chemical variables such as temperature, pH, bubble diameter, air superficial velocity, solids concentration, surface properties of the solids, surfactant concentration and salinity of the water (Timmons et al., 1995;Huguenin and Colt, 1989;Chen et al., 1992;Cripps and Bergheim, 2000). ...
Article
Foam fractionators, with and without additional ozonation, were evaluated for their effect on solids removal (suspended solids, SS; volatile suspended solids, VSS; dissolved organic carbon, DOC), particle size distribution of the foam in seawater recirculating systems during a 44-day experimental period. The effect of ozone on heterotrophic bacteria was also quantified in the entire system. Three separate but identical recirculating systems (4.5m3 system volume) with foam fractionators (300mm in diameter, 3m in height) were used in this study. One system (Control: CS) did not receive ozone, while the other two systems were ozonated at a rate of either 20g ozone/day (T 20) or 40g ozone/day (T 40) per kg of feed applied, respectively. A total of 107kg of black sea bream Acanthopagrus schlegeli (Bleeker) with an average weight of 334.5g was stocked into each system. Daily feeding rate was 1% of total body weight. The solids enrichment factor (EF=Cc/Ci, where Cc=concentration in foam condensate, Ci=concentration in inlet water) in T 40 was 10 times more dilute than the factors in CS and T 20. However, due to the higher volume of the foam (>10 times) in T 40, the removal rates of SS, VSS and DOC were the highest in T 40, but were not significantly different from T 20 (P>0.05). The mean particle sizes at the 90% cumulative removal point decreased with ozonation, 71.2±15.9, 57.9±10.2 and 48.0±10.2μm in CS, T 20 and T 40, respectively. The overall mean particle diameter of solids in the foam decreased as ozonation increased, with values of 29.4±4.4, 23.9±3.8 and 20.5±3.7μm in CS, T 20 and T 40. Numbers of heterotrophic bacteria in the inlet were 6.21±4.93×105CFU/mL, 0.29±0.19×105CFU/mL and 0.30±0.29×105CFU/mL in CS, T 20 and T 40, declining sharply with the addition of ozonation. As the number of the bacteria in the inlet decreased, the bacteria in the foam and the removal rate greatly decreased with increasing ozonation. However, EFs in T 20 (76.4) and T 40 (14.5) were higher than that in CS (12.2), and the T 20 showed significantly higher EF (P
... Al igual que en la mayoría de los dispositivos utilizados en acuicultura, existe una gran diversidad de modelos y criterios de diseño y operación de los fraccionadores de espuma (Weeks et al., 1992;Chen et al., 1994a;Timmons, 1994;Timmons et al., 1995). A pesar de esto, debido a la alta sensibilidad de las variables que afectan su funcionamiento y a las múltiples condiciones dadas en un sistema de cultivo, no se ha descrito una metodología o modelo matemático que prediga su desempeño real (Lawson, 1995). ...
... La velocidad de boyantez está en función del diámetro de la burbuja, la concentración de proteínas, y la velocidad superficial del gas (Timmons, 1994;Timmons et al., 1995). Timmons et al. (1993) correlacionó la velocidad de boyantez y la velocidad superficial del gas para una concentración de proteínas de 120 mg/l, normalmente encontrada en un sistema de recirculación. ...
Thesis
... Potentially, these interactions and implications can be further tested in other particle size ranges, also dealing with other particle chemical, physical, or electrostatic properties, such as the zeta potential (Tiller and O'Melia, 1993et al., 2013, 2014), foam fractionation (e.g. Timmons et al., 1995; Brambilla et al., 2008; Barrut et al., 2013), or flocculation (e.g. Avnimelech, 1999; Crab et al., 2007; Ritvo et al., 2003 ). ...
Chapter
The removal of waste from aquaculture facilities presents both technical and management problems. Aquaculture effluents are generally very diluted and expensive to treat, and effluent loads are a potential resource that should be recycled rather than wasted. A multitude of technologies are available for the treatment and recycling of fish farm wastes. This contribution provides information on methods for treating solids and, to some extent, hydrocolloid and dissolved wastes in aquaculture effluents. Solids can be removed using settling techniques, microscreen filters, granular media filters, membrane filters and by flotation. The two latter techniques are rarely applied in fish farming practice. The effectiveness of mechanical treatment depends on the particle size distribution and the ratio of particle-bound to dissolved wastes. The results of empirical studies show considerable variations in the size distribution of suspended solids and thus in the potential efficiency of mechanical treatments. The potentials and limitations of the different techniques with respect to variable farm characteristics are discussed. Potentially beneficial reuses of aquaculture sludge include agricultural application, composting, vermiculture, reed bed drying, and biogas production, all of which require a substantial pre-thickening of sludge water. The concentrations of potentially harmful chemicals in the thickened sludge are usually below regulatory limits for use on agricultural land.
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Article
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Recently, the pollution from pond aquaculture waste water has become more and more serious. In order to perfectly deal with such problem, the paper discussed the feasibility of using photochemical reactivity to remove pollutants through improving pond aquatic ecosystem from wastewater with the aim of water recycling and zero discharge. A photochemical treatment system (PTS) was designed that used photochemical reactivity to treat the polluted aquaculture wastewater. A spot observation method has been used in discussing the reactivity for the treatment of pond aquaculture wastewater with this system on a shrimp farm in Hainan province of China. The result showed that culture pond with PTS could realize progressive efficiency of pollutant removal without water exchange in a recirculating system, which could perfectly meet the need of environment protection and culture production.
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This study showed that particulate (i.e., physical) toxicity was responsible for rainbow trout deaths in bioassays with two separate solid wastes. This conclusion was based on: (1) fish necropsies which indicated physical damage to gills but no evidence of chemical damage to liver or kidney, (2) chemical analyses which indicated that levels of Priority Pollutants and other target compounds were too low to cause the observed toxicity, (3) structural and chemical analyses of the waste particles which showed that these consisted of inert materials, and (4) the use of centrifugation techniques to remove most of the suspended particulate material in bioassay tanks resulting in an elimination of most of the toxicity. The particles associated with the lethal effects were approximately 5 to 10 m in size. Regulatory testing of solid wastes must distinguish physical and chemical toxicity since disposal options can vary depending on the mode of toxicity. For instance, chemical toxicity raises concern regarding leaching through soils into groundwater, whereas if physical particles are responsible for toxicity, such leaching is not of concern.
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The results of an experimental study to determine the oxygen transfer properties of a 38 mm diameter airlift pump are presented. The effects of varying initial bubble size, flow rate, flow pattern, and water quality on oxygen transfer were examined. Pumping efficiencies of the airlift pump were found to be as high or higher than centrifugal pumps and oxygen transfer efficiencies as high or higher than fine bubble diffuser aeration systems. Generalized predictive equations for oxygen and nitrogen transfer and supersaturation in airlift pumping are presented
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Thesis (Ph. D.)--University of Maryland, 1976. Abstract. Vita. Includes bibliographical references (leaves 127-130). Photocopy. s
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Thesis (Ph. D.)--Cornell University, May, 1991. Includes bibliographical references.
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