J. Oca, J. Ruiz, I. Masaló

Departament d'Enginyeria Agroalimentària i Biotecnologia (DEAB). Universitat Politècnica de Catalunya (UPC-BARCELONATECH) (Spain).

Email: joa.oca@upc.edu.

**Introduction**

Quick removal of biosolids in aquaculture facilities, and specially in recirculating aquaculture systems (RAS), is one of the most important step in waste management. Sedimentation dynamics of biosolids in an aquaculture tank will determine their accumulation at the bottom of the tank. If biosolids left undisturbed the resistance to resuspension increases, and leaching can take place, increasing the dissolved organic waste and complicating their management.

Masaló et al (2008) used an oscillating grid to study aquaculture biosolids resuspension and sedimentation at different turbulence levels (expressed as the root mean square of the velocity RMS). An oscillating grid consists of a grid that oscillates in a container. The turbulence generated depends on mesh size, amplitude and frequency of oscillation. The percentage of biosolids in the water column was quantified measuring the turbidity at each RMS. The limitation of the method was that a water sample was taken at each RMS, and consequently the water volume in the container was progressively reduced.

In this work the oscillating grid method has been improved by using a laser beam and image analysis techniques to quantify the percentage of biosolids suspended without removing water samples.

**Material and methods**

Biosolids were collected from a multivortex tank (Masaló and Oca, 2014) containing Sea bass (50.4±11.8g; Density 9.5 kg m^{-3}). Fish were feed daily at a ratio of 2.5% BW.

Collection of biosolids from the tank was made through a siphon tube to avoid biosolids disintegration and assure that faces were unaltered.

After biosolids collection resuspension (R) and sedimentation (S) dynamics was studied. In the resuspension trial the RMS were increased gradually every 20min. After total resuspension of biosolids, sedimentation trial was started, decreasing RMS gradually every 30min. Two replicates of the resuspension and sedimentation trials were made.

A horizontal red laser beam was projected along a transversal axis of the container at a height above the grid, and at each turbulence level three images were taken with a camera situated above the grid container. Each image was analyzed with a software for image analysis (Image J), which allows to obtain the red pixel intensity profile along an axis defined by the user (figure 1).

Assuming a uniform distribution of suspended biosolids, the decay per unit length of light reflected along the line analyzed (*dI/dx*) will be proportional to the light intensity (*I*) and to the concentration of biosolids suspended (*C*) (equation 1).

Eq. 1

Integrating the equation 1, we have:

Eq. 2

In equation 2 *b* and *k* are constants.

*k∙C* value is obtained from the pixel intensity profile (*I* versus *x*) by regression (figure 2(A)), and is used to obtain the percentage of biosolids in the water column at each turbulence level (RMS) according to equation 3.

Eq. 3

Where *(k∙C)*_{i} and *(k∙C)*_{max }are the exponent *k∙C* at a fixed RMS value, and when all the biosolids are in the water column (maximal RMS), respectively.

**Results**

The *k∙C* exponent increases with turbulence (figure 2(A)). Higher exponent values indicate higher decrease of red pixel intensity between reference point and any other point in the line analyzed.

Percentage of biosolids in the column increased with RMS in the resuspension trial, and decreases in the sedimentation trial (figure 2(B)), but with the same RMS during the sedimentation trial % of biosolids in the column were higher than in the resuspension trial (hysteresis).

**Discussion and conclusions**

The new method to quantify the biosolids resuspension and sedimentation reproduces the results obtained in a previous work by Masaló et al (2008). Image analysis presented advantages maintaining the same water volume along the experiment, since it is not necessary to withdrawn water samples out of the container.

The method can be used to analyze the influence of different culture conditions in sedimentation-resuspension dynamics of biosolids (e.g. specie, fish size, binders added to the feed, etc…).

**References **

Masaló, I., Guadayol, O., Peters, F., Oca, J. 2008. Analysis of sedimentation and resuspension processes of aquaculture biosolids using an oscillating grid <http://www.sciencedirect.com/science/article/pii/S014486090800006X>. Aquacultural Engineering 38, 135-144

Masaló, I., Oca, J. 2014. Hydrodynamics in a multivortex aquaculture tank: Effect of baffles and water inlet characteristics. Aquacultural Engineering 58, 69-76