Separation characteristics of liquid nematode cultures and the design of recovery operations

Production of nematode-based pesticides involves the recovery of a viable nematode life stage known as the infective juvenile (IJ) from fermentation broth. Waste components to be separated from the IJs include non-IJ life stages, dead nematodes, nematode debris, spent media, and the nematode's associated bacteria. This paper reports separation characteristics of liquid cultures and suspensions of the nematodes Phasmarhabditis hermaphrodita, Steinernema feltiae, and Heterorhabditis megidis measured at small scale. Separation characteristics were determined for dead-end filtration, gravity settling and flotation. Results were used to identify large-scale recovery procedures. Separation of culture liquid by dead-end filtration of the crude fermentation broth was not possible due to rapid blinding of filters. However, nematode-water suspensions prepared by gravity settling could be concentrated using this separation method. Settling tests indicated that IJs could be efficiently separated from culture liquid by centrifugation but not by gravity settling. Examination of the effects of nematode concentration indicated an optimum concentration for gravity settling that may entail modest dilution of the fermentation broth. Flocculation of insoluble spent media in suspensions of P. hermaphrodita prevented its separation from nematodes by gravity settling. However, attachment of air bubbles to spent media allowed removal by flotation. Finally, adjustment of continuous phase density using sucrose allowed separation of non-IJ life stages, dead nematodes, and discarded cuticles from the IJs by flotation. The efficiency of this separation decreased with increasing nematode-solute contact time.