On-line Fluorescence Profile of Aerobic Sludge Digestion

Abstract

An online fluorometer designed for following intracellular NAD(P)H was used to monitor aerobic sludge digestion experiments. The fluorescence showed an initial rise to a high plateau, a sharp decline after staying at the plateau for 20-60 h, and a trailing very slow decrease. The characteristic fluorescence profile was shown to result mainly from the solids-associated fluorescence, after ruling out other factors such as pH, temperature, and supernatant fluorescence. The fluorescence profile was, however, not a mere result of the decreasing solids concentration. The varying sludge viability and population composition (e.g., the decay of heterotrophs and the increasing fraction of nitrifiers) played important roles. The fluorescence profile correlated well with the profile of the viable heterotrophic cell number concentration evaluated with TSB-agar plates. The initial increase of the number concentration was attributed to the growth of multiple small bacteria from the lysate of each large microorganism, which was demonstrated in the experiments with baker's yeast as the starting culture for digestion. The fluorescence profiles observed in the yeast experiments were similar to those in the sludge experiments. Responding to glucose additions and the switch from aerobic to anaerobic conditions, the yeast systems showed typical step increases of fluorescence as expected from the change of NAD(P)H level associated with heterotrophic metabolism. However, no such fluorescence responses were detectable in the sludge digestion systems. NAD(P)H were thus uncertain to be responsible for the online fluorescence observed. Nonetheless, the initial fluorescence plateau corresponded to the period of rapid digestion and, for the plant studied, the EPA regulation criteria of VSS reduction >38% and/or SOUR <1.5 mg of O(2) (g of TS)(-)(1) h(-)(1) were satisfied at the end of the plateau. The online fluorescence provides an effective means of monitoring the aerobic sludge digestion process.

Aerobic Sludge Digestion under Low Dissolved Oxygen Concentrations

“…The value of SOUR dropped rapidly initially as the heterotrophic population declined. However, the SOUR could persist at low levels for extended periods, presumably because of the respiration of the nitrifying population that survived much longer on the ammonia released from the decaying biomass (Li and Ju, 1999).…”

“…In addition, pH decreased from D1 to D3 (i.e., with the progress of the digestion). This might result from the combination of (1) decreasing ammonia generation because of less remaining proteinaceous materials, and (2) increasing net activity of nitrification (pHlowering) over denitrification (pH-raising) as the heterotrophic denitrifiers died off more rapidly than the autotrophic nitrifiers (Li and Ju, 1999).…”

Solid-phase fluorescence spectroscopy to characterize organic wastes

“…It enables the visualization of proteins (Mayer et al, 1999), humic substances (Sierra et al, 2005) and melanoidins . The fluorescence of these compounds were also detected by 3D-LPF spectroscopy in dissolved OM of farm wastes (Baker, 2002;Bilal et al, 2010;de Zarruk et al, 2007), landfill leachates (Huo et al, 2008) and SS (Li and Ju, 1999;Wang et al, 2009) as well as in particulate OM extracts of SS and compost (de Zarruk et al, 2007;Esparza-Soto and Westerhoff, 2001;Richard et al, 2009;Shao et al, 2009). In a simple spectrum acquisition, the 3D-LPF spectroscopy enables to detect all the fluorophores in a sample which can then be quantified using the fluorescence regional integration (FRI) approach (Chen et al, 2003).…”