Composting process of residual organic material generates considerable amounts of liquid leachate which contains high organic load. This waste stream can be considered as potential nutrient source to support microbial growth. In the present work, the utilization of compost leachate as fermentation substrate for Bacillus species production was studied. The physicochemical properties of the leachate and two co-substrates (residual yeast and whey permeate) were determined. The characterization of leachate showed that it is a potential source of carbon, but its nitrogen content may limit the bacterial growth. In order to determine a good recipe of culture medium for fermentation of individual strains of Bacillus species, leachate was added with yeast and whey permeate. Raw and diluted leachates with and without amendments were tested in shake-flask fermentation assays. Results showed that Bacillus sp. grew better in diluted leachate than in raw leachate. When co-substrates were added, the growth was improved and the sporulation rate also increased. Since the aim was to produce plant growth–promoting bacteria, one of the objectives of fermentation assays was the production of viable bacteria when Bacillus sp. arrives to soil as component of a fertilizer. For this reason, the obtention of sporulated Bacillus cells was desired. The highest sporulation rate was obtained with co-substrates, inducing more than 89% of vegetative cells to develop spores. This approach of leachate valorization will produce economical benefits reducing the volume of leachate waste to be treated, as well as contribute in a cost-effective production of biological amendments in a circular economy mode.

Combinatorial application of different dye removal methods with specific features can lead to a novel and robust decolorizing system. In this study the bacterial spore laccase and TiO₂ nanoparticles were co-entrapped to enhance dye degradation. The optimum entrapment conditions were achieved in the presence of alginate 2% (w/v) and Ca²⁺ (0.2M), Cu²⁺ (0.05M) and Zn²⁺ (0.25M) as matric polymer and counterions, respectively. Immobilized laccase showed a wide range of pH and temperature stability in comparison to the free spores. The entrapped degradation systems include single laccase, single TiO₂, laccase + TiO₂ (one-step remediation), TiO₂/laccase (two-step remediation), and laccase/TiO₂ (two-step remediation) that result to the 22%, 26% 45.6%, 47.6%, and 69.3% indigo carmine decolorization in 60 min. In the kinetic studies, the half-life of indigo carmine (25 mg/l) in the remediation processes containing laccase, TiO₂, laccase + TiO₂, TiO₂/laccase, and laccase/TiO₂ was calculated as 173, 138, 161, 115, and 57 min, respectively. The degradation products by co-entrapped system were not toxic against Sorghum vulgare. The results showed two-step decolorization by co-entrapped spore laccase and TiO₂ nanoparticles, including the pretreatment of dye by laccase, and then, treatment by TiO₂ has potential for degradation of indigo carmine.