Introduction

As a reference for determining separation efficiency tests are usually run using yeast cells. Yeast cells are used as model particles in stead of animal cells since they are easy to obtain and handle.

Methods

To test the separation efficiency the medium is circulated through the BioSep using a circulation pump and a harvest pump. In some cases the harvest stream is also returned to the reactor to maintain the same cell concentration in the reactor.

The separation efficiency is calculated by dividing the amount of cells in the harvest stream by the amount of cells in the suspension

with C_h = cell concentration (g/L) in harvest and C_s = cell concentration (g/L) in suspension. The cell concentration is determined by measuring the optical density (OD) of samples from the harvest and the reactor. The optical density is then compared to a cell count. A calibration curve between the optical density and cell count determines the linearity between the optical density and amount of cells. After the cells are weighed and diluted the amount of cells per ml is also counted to determine the amount of cells per gram.

 Results

The separation efficiency is more than 99%, for a large range of flow rates and cell densities. At a cell density of 10 g/L, 95% separation efficiency was still reached at a harvest flow of 18 L/day for the 10L system.  

With the 10L BioSep operated at low cell densities and flow rates ranging at 2-4 L/day, the power output needed to achieve the optimal separation efficiency was found at a relatively low level of 3W for the 10L BioSep. However, at 20 g/L and 10 L/day the power output had to be increased to 8 W to achieve the optimal separation efficiency. The same tendency was observed for the 50L (4-10 W) and 250L (40-80 W).

Both, harvest pump and ultrasonic field are occasionally switched off with the integrated timer to help the cells sediment faster. When the field is switched off, the cells are no longer captured by the field and the agglomerated cells sediment back into the recirculation flow and are carried back into the reactor. Proper timing of on/off periods is especially important at higher flow rates and cell densities.