Elsevier

Aquacultural Engineering

Volume 54, May 2013, Pages 16-21
Aquacultural Engineering

Foam fractionation efficiency of a vacuum airlift—Application to particulate matter removal in recirculating systems

https://doi.org/10.1016/j.aquaeng.2012.10.003Get rights and content

Abstract

The accumulation of particulate organic matter (POM) in recirculating aquaculture systems (RAS) has become an important issue with the intensification of finfish production. The objective of this study was to assess the foam fractionation efficiency of a vacuum airlift in different conditions (POM concentrations, airflow rates, bubble sizes, water renewal rates and feed addition). In sea water, the vacuum airlift allowed removing 20% of the initial POM concentration per hour (foam fractionation efficiency), corresponding to a 20.7-fold concentration factor between the tank and the foam. In rearing conditions, efficiency increased with decreasing water renewal rate or increasing POM concentration. An increase in airflow rate from 10 to 80 L min−1 in the vacuum airlift significantly decreased foam fractionation efficiency when feed was added to the water. The impact of feeding was only observed with high airflow rates where bubble coalescence occurred. Calculated POM production by fish ranged between 15.9 and 23.5 g h−1 and was equivalent to estimations based on feed conversion ratio (FCR). This indicated that all the POM produced was extracted by the vacuum airlift.

Highlights

► The foam fractionation efficiency and the concentration factor of a vacuum airlift were calculated in sea water and in rearing sea water. ► The influence of airflow and water renewal rates on foam fractionation efficiency was measured. ► In rearing conditions, differences in foam fractionation efficiency were observed before and after feeding. ► Particulate organic matter production by fish was calculated and found to be equivalent to estimations obtained using the feed conversion ratio (FCR).

Introduction

The presence and accumulation of particulate matter (faeces, uneaten feed, parasites, and bacterial flocs) in recirculating aquaculture system (RAS) can decrease water quality, which may increase the stress of reared organisms (Timmons, 1994, Cripps and Bergheim, 2000, Rubio et al., 2002, Sharrer et al., 2005). Although there is little information available on safe level of particulate matter concentration, studies have already shown that above a concentration of 80 mg L−1, salmonid growth is significantly slowed down in RAS (Piper et al., 1982, Laird and Needham, 1988). As this safe concentration level clearly depends on each fish species, the main concern associated with particulate matter accumulation is the increase in the biological oxygen demand and the development of heterotrophic bacteria (Timmons and Ebeling, 2010). It is therefore necessary to remove these particles and control the quality of water.

Several types of particle separators, or clarifiers, are commercially available for integration into intensive aquaculture treatment system. Solids separation technology can be divided into mechanical and gravitational methods, but their efficiency is affected by particle size. Average particle size depends on fish species and size, on the type of feed used and on the hydraulic regime in the rearing tank, but usually ranges between 3 and 300 μm. However, most particles are smaller than 30 μm (Cripps and Bergheim, 2000). The most popular method for mechanical particle separation involves the use of screens and rotating microscreens. Some problems encountered with this method include the difficulty to remove particles smaller than 50 μm and poor flow capacities due to the small pore sizes which means that most of the fine solids remain even after passing through biological filters (Timmons, 1994, Summerfelt, 2006). Drum filters with 60 μm screens allow around 50% of total particles to be eliminated with the other 50% usually being trapped in the biofilter (Cripps and Bergheim, 2000). The presence of particles reduces filter permeability and increases the growth of heterotrophic bacteria which oxidate organic matter. The consequences of this are (1) more frequent back-wash, (2) competition between autotrophic and heterotrophic bacteria for specific area and nutriments and (3) additional oxygenation (Blancheton, 2000, Blancheton et al., 2009). Use of drum filters with reduced porosity would entail higher energy costs, which is not viable for aquaculture applications. Sand filters are frequently used but they generate high head losses and require frequent maintenance (Summerfelt, 2006). Gravity sedimentation is also used as it is simple and highly energy-efficient (Rawat et al., 2011), however the process only works for large-sized and high density particles (Amaro et al., 2011, Chen et al., 2011). Sedimentation rates for particle sizes between 10 and 50 μm are slow and average settling velocity is below 1 m h−1 (Brambilla et al., 2008). Therefore, Particulate Organic Matter (POM) is generally first extracted by sedimentation of the larger particles (faeces and uneaten feed >100 μm) and then by mechanical filtration of the smaller particles (30–100 μm). Protein skimmers using foam fractionation can be used in addition to mechanical filtration to extract smaller particles and to relieve the mechanical filter in terms of efficiency and energy (Rubio et al., 2002, Sharrer et al., 2005, Summerfelt, 2006).

Foam fractionation is a water treatment technology that can be easily added to water reuse systems to directly remove dissolved and fine suspended solids. The process of foam fractionation, also known as flotation, protein skimming, or air stripping, has been widely described by Timmons (1994), Summerfelt (1999) and Brambilla et al. (2008). It consists of injecting fine air bubbles into wastewater. 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 μm (Timmons, 1994, Chen et al., 1994). Muniain-Mujikaa et al. (2002) and Suantika et al., 2001, Suantika et al., 2003 have shown that in rearing farms, only skimmers give rise to high quality water. The ability of skimmers to extract microparticles is also interesting in terms of biosecurity as they may be used to extract bacteria and viruses (Timmons, 1994, Suantika et al., 2001, Suzuki et al., 2008, Brambilla et al., 2008, Park et al., 2011). Other organisms such as toxic microalgae or parasites can also be extracted as they possess surface-active substances on their cell walls, which induces the formation of foam that may be collected (French et al., 2000, Teixeira and Rosa, 2006, Suzuki et al., 2008, Teixeira et al., 2010, Park et al., 2011).

Clarification of water by foam fractionation allows the reduction of UV irradiation for disinfection (Suzuki et al., 2008). Furthermore, injected micron-sized air bubbles also contribute to increasing aeration and CO2 stripping (Barrut et al., 2012).

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. 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). Soluble proteins induce the formation of foam because proteins migrate towards the water surface and concentrate while reducing surface tension. The more soluble a protein, the more foam is formed (Frénot and Vierling, 2002). It is also assumed that surface-active substances such as polysaccharides and proteins not only generate foam on the water surface, but also change the interface of solids from hydrophilic to hydrophobic, which facilitates their concentration in the foam (Suzuki et al., 2008).

The interest of vacuum flotation has been widely described in chemical engineering for solid–liquid separation, but there is no information concerning particulate removal in rearing water with the addition of vacuum on foam fractionation. The aim of this study was to evaluate the foam fractionation efficiency of a vacuum airlift for the removal of particulate matter from water and to study the effects of feed addition and water renewal on this efficiency.

Section snippets

Experimental set-up

The experimental set-up is described in Fig. 1. It comprised a 1000 L tank (1) open to the air and connected to a vacuum airlift provided by COLDEP® (2), composed of two concentric vertical transparent 6 m-long PVC pipes. The outer diameter (OD) of the internal pipe was 160 mm. The diameter of the external pipe was 315 mm (OD) along the first metre and 250 mm (OD) after the first metre and up to the top (Fig. 1). The top of the vacuum airlift was hermetically closed and connected to a vacuum pump

Foam fractionation efficiency on fragmented fish feed

After 4 h, POM concentration decreased by a factor of 4.2 in the tank and increased by a factor of 20.7 in the foam, for an extracted volume of 160 L, corresponding to a foam fractionation efficiency of 80% i.e. 20% per hour (Table 1).

Conversely to what is usually described in research literature (Timmons et al., 1995, Park et al., 2011), the addition of fish feed to water did not alter foam formation or decrease foam fractionation efficiency of the vacuum airlift.

Foam fractionation efficiency in rearing conditions

The amount of POM in the rearing

Conclusion

In sea water, the vacuum airlift provided a foam fractionation efficiency of 20% per hour and a concentration factor of 20.7 which were not altered by the addition of feed to water. In rearing conditions, efficiency increased with reduced water renewal rates, i.e. with increased POM concentrations. However, an increase in airflow rate from 10 to 80 L min−1 led to an important reduction in foam fractionation efficiency after feeding due to massive bubble coalescence. POM production by fish was

Acknowledgements

We would like to thank Pierre Bosc from ARDA and the Réunion Region as long as the French National Association for Research and Technology (ANRT) for their financial support of the project. This work was made possible thanks to the cooperation of François René (IFREMER). We also wish to thank Julien Jacquety from COLDEP® for all his assistance and hard work, and for providing the vacuum airlift.

Bertrand Barrut is a Ph.D. student at the University of Montpellier II. This paper is part of his Ph.D. thesis in aquaculture at the Marine Aquaculture Centre of ARDA (Reunion Island, France) and at the Aquaculture Centre of IFREMER (Palavas les Flots, France). “Study and modelling of an innovative water treatment process: the vacuum airlift”

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Bertrand Barrut is a Ph.D. student at the University of Montpellier II. This paper is part of his Ph.D. thesis in aquaculture at the Marine Aquaculture Centre of ARDA (Reunion Island, France) and at the Aquaculture Centre of IFREMER (Palavas les Flots, France). “Study and modelling of an innovative water treatment process: the vacuum airlift”

Jean Paul Blancheton is a senior scientist working in IFREMER, at the Aquaculture Research Station of Palavas les Flots, France. He is currently carrying out research on aquaculture production systems in the research teams ‘Laboratoire d’aquaculture du Languedoc-Roussillon’ and ‘Unité Mixte de Recherche UMR 5119′ Ecosym.

Jean-Yves Champagne is professor in hydraulic, working at INSA de Lyon, National Institute of Applied Sciences, in mechanical engineering. He make his research at LMFA, Fluids Mechanics and Acoustic Laboratory, Unité Mixte de Recherche du Centre National de la Recherche Scientifique, UMR CNRS 5509.

Alain Grasmick is professor in chemical engineering in “Polytech’ Montpellier”, the Engineering School of Montpellier University. He has studied intensive processes applied to water and wastewater treatment for more than 35 years.

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