Extraction, Characterization, And Optimization of Protein from Waste-Activated Sludge and Food Waste

Abstract

There is a growing interest in the recovery of valuable biomaterials from waste in line with reaping the benefits of a circular economy. Organic wastes, such as waste-activated sludge (WAS) and food waste (FW) contain substantial amount of protein that can be recovered for various applications. The animal feed industry faces unprecedented challenges and might be interested in such a product, as the high demand coupled with the high costs of feeds has put so much pressure on the available resources. This study evaluated the potential of waste-activated sludge and food waste as alternative sources of proteins for animal nutrition. Thermal alkaline and acid hydrolysis methods were used in the extraction of protein from FW and WAS. The possibility of enhancing protein yield and quality through the co-extraction of protein from WAS and FW was also investigated. Response surface methodology (RSM) was used to optimize the extraction process. Before extraction, WAS was purified by removing heavy metals using acid pretreatment. Also, the energy efficiency of the extraction process was investigated. It was established that FW had 21.5 g/100 g protein, while WAS had 19.9 g/100 g protein. The two extraction methods had superior extraction of protein from WAS as compared to FW. In addition, co-extraction of protein from FW and WAS did not significantly increase the yield. Furthermore, optimization using RSM showed that the optimal yield of 15.8 g/100 g was obtained at a pH of 13 and a temperature of 120 °C, close to the experimental yield of 16.6 g/100 g for WAS. Besides, the extraction process was most energy efficient at a temperature of 120 °C. Moreover, LC-MS analyses of the extracted protein showed that WAS had a good essential amino acid profile with threonine, lysine, leucine, methionine, and valine in concentrations of 3.3, 2.7, 1.8, 1.1, and 2.7 g/100 g, respectively. The adequate concentration of threonine pointed to the potential beneficiation of WAS as an animal feed supplement. In addition, the ratio of the first limiting amino acids (lysine to methionine) of 2.5 met the standards for application as an animal feed supplement. Given this favorable amino acid composition, WAS presents a viable option for use as a protein source in animal nutrition. This study is significant in the management of the growing stream of waste-activated sludge and food waste. Repurposing these wastes into animal feeds not only contributes to sustainable waste management practices but also reduces the environmental burden associated with their disposal.