Magnetic separation of algae has been proposed in the past and is currently being investigated. The concept is to pass magnetized algal cells (or magnetic aggregates) past a magnet and remove them directly from the culture medium.

The original methods for magnetic separation used addition of magnetite and aluminum sulfate to the culture that are bound to the algal cells and were subse­quently removed with an electromagnet. This was very successful at neutral and slightly acid pH waters (79-94 % removal) but less effective with water at higher pH (55-64 %) (Bitton et al. 1975). This approach has the problem of pH sensitivity as well as the dependence on flocculant and added magnetic materials.

There are other current approaches for magnetic separation of algae. One company, Advantageous Systems LLC, specializes in functionalized nanomaterials and is marketing a system for the separation of algae. Public details of their methods are not available, but it appears that they have used functionalized nanomaterials added to the algal culture that is then passed through magnetic plates to separate the algae in a rapid, continuous, and efficient manner (http://www. adsalgae. com/). Another similar approach used chitosan-Fe3O4 nanoparticle composites to effec­tively (99 % efficiency) remove Chlorella sp. KR-1 from the culture medium using a permanent NdFeB magnet in laboratory scale (Lee et al. 2013).

Using molecular engineering, there are efforts underway to produce algae with improved iron uptake and storage as well as being more magnetically susceptible, so that they can be separated magnetically (Sayre and Postier 2013). These authors engineered iron uptake and storage proteins into Chlamydomonas reinhardtii and Auxenochlorella protothecoides and found improved iron utilization but, to this point, have been unable to definitively show improved magnetic moment (personal communication).

Magnetic separation systems that recognize only production strains of algae and harvest them specifically while excluding bacteria and other contaminants would be advantageous in that they could provide a clean separation of suitable feedstock for the downstream processing and a cleaner final product. However, addition of magnetic materials to the system would impact downstream processes as well as add OpEx for the materials used. Currently, there are no scaled-up systems for magnetically separating algae. However, large magnetic separators are already in use in the mining industry should technological and cost advantages be overcome.