Young, John Matthew; (1999) Enhanced bioprocessing of viable nematodes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Production of nematode-based pesticides involves the recovery of a viable nematode life stage known as an infective juvenile (IJ) from fermentation broth. The objective of this project was to identify large-scale methods for separating viable IJs from waste components of liquid nematode cultures. These investigations were conducted using cultures of three nematode species; Steinernema feltiae, Heterorhabditis megidis and Phasmarhabditis hermaphrodita. Culture composition was determined so that the separation problem could be defined. Waste components to be separated from IJs included non-IJ life stages (J1s, J2s, J4s, adults), dead nematodes, nematode debris, spent media and the nematodes' associated bacteria. To identify potential methods of separation, physical properties (solids content, IJ 'shear' stability, viscosity, component size, component density) of cultures were measured. Measurements indicated that a low 'shear' recovery process is required and that solid waste can be separated using differences in component size, density and settling rate as a basis. Predictions of IJ settling velocity, using property measurements, suggested that culture liquid should be separated by centrifugation as opposed to gravity settling. Separation characteristics were determined and verified using small-scale tests. Culture liquid could be separated by centrifugation but not by dead-end filtration. Solid waste was separated by flotation, settling, and sieving. Large-scale gravity settling, centrifugal settling and centrifugal flotation operations were evaluated using P. hermaphrodita. Centrifugal flotation was the most efficient operation. When culture solids were passed through a bowl centrifuge, after increasing continuous phase density using sucrose solution, solid waste was separated by flotation. There was no measurable loss of product. Separation efficiency was greater than that of comparable recovery procedures described in the literature. An integrated recovery process design is suggested, and involves centrifugal classification followed by centrifugal flotation. Density-adjusting medium used for flotation is recovered by filtration and evaporation and is then recycled.

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