Valorisation of waste activated sludge for protease production by Bacillus licheniformis

Abstract

Waste activated sludge (WAS) is the main by-product of wastewater treatment plants. Its management is considered one of the major environmental challenges worldwide, due to the large volume generated, along with the presence of pathogens and hazardous substances. Nevertheless, the biopolymers present in its structure can make it a promising source of nutrients to obtain high value-added products. Therefore, this study is focused on the use of WAS as a renewable feedstock to produce proteases by Bacillus licheniformis. To this end, both thermal hydrolysis (TH) and wet oxidation (WO) were evaluated as strategies for the solubilization of the valuable compounds. The results revealed that an operation time of 90 min (at 140 °C) was optimal for both pretreatments, which allowed obtaining concentrations of carbohydrates, proteins and humic acids up to 2.7 ± 0.1 g/L, 5.3 ± 0.1 g/L and 5.1 ± 0.1 g/L, respectively. Regarding bioconversion processes, the initial pH of the medium significantly influenced the assimilation of the biopolymers, bacterial growth and enzyme biosynthesis. In this sense, a pH of 5.5 for the WAS pretreated by TH (WAS-TH) and 7.5 for the WAS pretreated by WO (WAS-WO) allowed maximizing the proteolytic activities, reaching values of 767 ± 2 U/mL and 838 ± 6 U/mL, respectively. Additionally, higher initial inoculation levels positively influenced the performance of the fermentative process. Consequently, a 30 % inoculum resulted in enzymatic activities of 828 ± 11 U/mL for WAS-TH and 903 ± 4 U/mL for WAS-WO. Partial purification of the medium with ammonium sulphate increased the enzymatic activity by 7 times for WAS-TH and 3 times for WAS-WO.