Monitoring aerobic sludge digestion by online scanning fluorometry

With sludge samples from two wastewater treatment plants, batch experiments of aerobic sludge digestion were conducted under different dissolved oxygen (DO) and solids concentrations. A fluorometer capable of online excitation and emission scanning was used to monitor the digestion process. Three major fluorescence peaks were observed. The peak at excitation/emission maxima of 290/350 nm was attributed to the fluorescence of proteinaceous materials in the sludge, with tryptophan residues being the primary contributor. The sources for the other two peaks (at 370/430 nm and 430/510 nm) remain unknown. The well-known biological fluorescence from reduced nicotinamide adenine dinucleotides (NADH and NADPH), at excitation/emission maxima of 340/460 nm, was found very weak in the aerobic digestion systems studied. It was buried under the broad peak at 370/430 nm and was detectable only in the early stage of the experiment that had the highest solids loading (at 4.8%) and was operated under low DO (0.2-1.0 mg/L) conditions. On the other hand, the profile of the protein fluorescence (PF) correlated well with that of the volatile solids (VS) reduction in all the experiments. A semi-empirical exponential decay function was developed, which described well the profiles of both normalized VS and normalized PF. The feasibility of following the real-time performance of aerobic sludge digestion by monitoring PF was clearly demonstrated.