Comparison of methods for detecting protein extracted from excess activated sludge

Abstract

The protein contents of hydrolyzed sludge supernatant are commonly determined with the Kjeldahl method, but this method suffers from complicated operations, long process times, and large quantities of chemicals consumed. In this paper, the Lowry, bicinchoninic acid (BCA), and Bradford methods were used to test the precision and spiked recovery of proteins from sludge supernatants 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 hydrolytic processes, the sludge protein values determined with the three tested methods were within 0.05 of each other, which met the experimental requirement for accuracy. Both the Lowry and BCA methods had recovery rates of 95-105%, while the Bradford method showed large deviations and was not highly reliable. The three protein determination methods showed significant differences with 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 deviations were analyzed based on the protein hydrolysate components and the mechanisms for the different detection methods. On the basis of these results, the BCA method was chosen as the most appropriate quantification method for use with sludge protein extraction, and it was used to analyze the protein contents extracted from residual sludge samples obtained from two sewage treatment plants. The reliability of the method was verified, and this lays a foundation for the extraction and reclamation of sludge proteins.