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
Producto del fuerte crecimiento de la acuicultura en los últimos años, la recirculación del agua ha surgido como una alternativa tecnológica capaz de responder acertadamente a las problemáticas del sector productivo relacionadas con la disponibilidad hídrica, selección de sitios de cultivo, y prevención de enfermedades, entre otras. No obstante, los niveles de producción en estos sistemas están limitados principalmente por la calidad del agua. Esto ha promovido el desarrollo y la aplicación de diversos mecanismos de tratamiento destinados a la desinfección y el control de la calidad del agua. Tal es el caso del ozono, un agente oxidante altamente reactivo que posee propiedades que lo convierten en un eficaz tratamiento para ser usado en los sistemas de recirculación. En el presente estudio se evaluaron las propiedades y los efectos del ozono en un sistema de recirculación para el cultivo de juveniles de turbot. Con este propósito, se desarrollaron dos etapas. La primera consistió en evaluar comparativamente tres dispositivos de absorción de gases: un tubo–U, un inyector venturi y un cono aerador. En la segunda etapa se implementó el sistema de cultivo y se evaluó su funcionamiento. En la primera etapa, la elección del dispositivo más apropiado para el sistema de cultivo se basó en la evaluación conjunta de tres criterios fundamentales: funcionalidad, adaptabilidad y eficiencia de absorción de oxígeno. Tras el análisis cualitativo de los dos primeros criterios, y del análisis cuantitativo de los resultados de la eficiencia de absorción, se encontró que el inyector venturi y el cono aerador exhibían una cierta desventaja en comparación con la eficiencia de absorción de oxígeno presentada por el tubo–U (±10%). Sin embargo, se concluyó que el uso de un inyector venturi asociado a un fraccionador de espuma presentaba mayores ventajas comparativas en términos de funcionalidad y adaptabilidad respecto de los demás dispositivos de absorción. De este modo, en la segunda etapa se implementó un sistema de recirculación con aplicación de ozono para el cultivo de juveniles de turbot en estanques tipo shallow raceways, incluyendo un fraccionador de espuma con inyección venturi para la absorción de ozono. Se evaluó el crecimiento y la supervivencia de los juveniles de turbot mediante muestreos semanales de peso y el registro periódico del número de peces muertos, respectivamente. El crecimiento registrado durante los 24 días que duró esta experiencia se evaluó a través de la Tasa de Crecimiento Instantánea y del Indice de Conversión. Además, se evaluó la calidad del agua mediante muestreos periódicos de las variables físicas, químicas y biológicas críticas del sistema. Finalmente, se analizó el desempeño de cada uno de los dispositivos involucrados en el sistema mediante el análisis de los procesos respectivos. Los resultados obtenidos indican que la Tasa de Crecimiento Instantánea de los juveniles concuerda con los valores encontrados normalmente en sistemas de recirculación para esta especie. Sin embargo, el Indice de Conversión reveló una alta eficiencia en la utilización del alimento respecto a otros estudios realizados en turbot para este tipo de sistemas. Dada la estabilidad de la mayoría de las variables ambientales y el escaso manejo de los peces, la supervivencia al final del periodo alcanzó un 98%, con una tasa de mortalidad diaria menor a 0,01. A pesar de los buenos resultados encontrados en la evaluación individual del biofiltro, la capacidad de nitrificación de este dispositivo fue insuficiente debido a que no se dispuso de un filtro mecánico adecuado para las características de este sistema. Esto se tradujo en un aumento en la concentración de compuestos nitrogenados, los que alcanzaron niveles cercanos al límite de tolerancia para esta especie. Los valores del resto de las variables muestreadas a lo largo de la experiencia se mantuvieron dentro de los rangos normales. La ozonación del agua a través de la inyección venturi del fraccionador de espuma permitió mantener aguas de excelente claridad y buenas condiciones sanitarias, aunque debido a la alta demanda de ozono presentada por el sistema, las concentraciones residuales fueron insuficientes para lograr un desempeño más eficiente en su globalidad. No se observaron efectos negativos derivados del uso del ozono, en los peces o en el funcionamiento general del sistema. La concentración de oxígeno disuelto y la saturación de gases en el sistema se mantuvieron controladas a cabalidad por la columna de empaque, lo que sumado al buen desempeño hidráulico de los shallow raceways posibilitó una distribución homogénea de los peces a lo largo y ancho de los tanques. Los resultados de este estudio sugieren que el uso del ozono en el tratamiento de aguas de un sistema de recirculación es efectivo en la medida que la remoción de sólidos funcione correctamente. De este modo, el efecto sinérgico de la biofiltración y ozonación podría constituir una herramienta de grandes proyecciones en acuicultura.
... 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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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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