Development of sludge based stable aqueous Bacillus thuringiensis formulations

This study focuses on development of aqueous flowable (suspension) formulations for Bacillus thuringiensis (Bt) based biopesticides from wastewater sludge. Different inerts like sorbitol, sodium monophosphate, sodium metabisulphite, sorbic acid, propionic acid, Tween-80, Triton X-100 and glycerol were tested for formulations. Five different formulations for non-hydrolyzed (NH) secondary sludges were tried and the best combination selected on the basis of various physical parameters like viscosity, particle size, suspendibility, entomotoxicity, and microbiological purity tests. F5 formulations (for secondary sludge) comprising sorbitol, sodium monophosphate and sodium metabisulphite gave better physical and biological characteristics with a small effect on entomotoxicity and spore concentration after 120 days at pH 6, 6.5 and temperatures 40 and 50°C and viscosity change at 40 and 50°C. The formulations were more stable at pH 4.0 to 5.0 and temperatures 4 to 30°C whereas at pH 6.0 and 6.5 and temperatures 40 and 50°C, there was degeneration of the product. Lower proteolytic activity and physical factors like ionic strength and surface group changes at pH 6 and 6.5 were responsible for the instability of the formulation.