Comparison of Methods for Detecting Protein Extracted from Excess Activated Sludge

Abstract

Currently, the protein content of hydrolyzed sludge supernatant is commonly determined by the Kjeldahl method, but this method suffers from complicated operation, a long time requirement and large chemical consumption. In this paper, the Lowry, bicinchoninic acid (BCA) and Bradfordmethods were used to test the precision and spiked recovery of protein from sludge supernatant hydrolyzed by alkaline-thermal hydrolysis (ATH), enzymatic hydrolysis (EH) and ultrasound-assisted enzymatic hydrolysis (UEH), and the results were compared with those obtained with the Kjeldahl method. For all the hydrolysis processes, the sludge protein values determined from the three tested methods were within 0.05 of each other, meeting the experimental requirements for accuracy. Both the Lowry and BCA methods had a recovery rate of 95%-105%, while the Bradford method showed a large deviation and was not highly reliable. The three protein determination methods had significant differences from the Kjeldahl method (P<0.05). However, the relative deviation between the Kjeldahl and BCA methods was the smallest (3%-5%), followed by those between the Kjeldahl and the Lowry (11%-21%) and Bradford methods (21%-90%), and the causes of the deviation were analyzed according to the protein hydrolysate components and the mechanisms of the different detection methods. On the basis of the above results, the BCA method was chosen as the most appropriate quantification method for use in sludge protein extraction, and it was used to analyze the protein content extracted from residual sludge from two sewage treatment plants. The reliability of the method was verified, which lays a foundation for the extraction and reclamation of sludge protein.