Aerobic Sludge Digestion under Low Dissolved Oxygen Concentrations

Abstract

Low dissolved oxygen (DO) concentrations occur commonly in aerobic digesters treating thickened sludge, with benefits of smaller digester size, much reduced aeration cost, and higher digestion temperature (especially important for plants in colder areas). The effects of low DO concentrations on digestion kinetics were studied using the sludge from municipal wastewater treatment plants in Akron, Ohio, and Los Lunas, New Mexico. The experiments were conducted in both batch digestion and a mixed mode of continuous, fed-batch, and batch operations. The low DO condition was clearly advantageous in eliminating the need for pH control because of the simultaneous occurrence of nitrification and denitrification. However, when compared with fully aerobic (high DO) systems under constant pH control (rare in full-scale plants), low DO concentrations and a higher solids loading had a negative effect on the specific volatile solids (VS) digestion kinetics. Nonetheless, the overall (volumetric) digestion performance depends not only on the specific digestion kinetics, but also the solids concentration, pH, and digester temperature. All of the latter factors favor the low DO digestion of thickened sludge. The significant effect of temperature on low DO digestion was confirmed in the mixed-mode study with the Akron sludge. When compared with the well-known empirical correlation between VS reduction and the product (temperature 3 solids retention time), the experimental data followed the same trend, but were lower than the correlation predictions. The latter was attributed to the lower digestible VS in the Akron sludge, the slower digestion at low DO concentrations, or both. Through model simulation, the first-order decay constant (k d ) was estimated as 0.004 h 21 in the mixed-mode operations, much lower than those (0.011 to 0.029 h 21 ) obtained in batch digestion. The findings suggested that the interactions among sludges with different treatment ages may have a substantially negative effect on digestion kinetics. The use of multistage digesters, especially with small front-end reactors, may be advantageous in both ''process'' kinetics and ''biological reaction'' kinetics for sludge digestion. Water Environ. Res., 76, 453 (2004).

Application of phagotrophic algae in waste activated sludge conversion and stabilization

“…On the other hand, as described above, 42.4% VS reduction could be achieved in 1 day by the new sonication-phagotrophic process. The finding of relatively low VS reduction by aerobic digestion of the Akron WAS in this study was consistent with the results previously reported in the literature for the Akron WAS (Arunachalam et al, 2004). It is known that WAS may have low digestibility (Bougrier et al, 2005;Khanal et al, 2007).…”

“…Having an algal yield of 0.3 (g algae/g VS), the phagotrophic alga could consume the bacteria-sized VS (as substrate) at a specific rate of 8 d −1 (or 8 g VS/g algae-d), estimated by U sub = μ/Y = 2.4/0.3, where U sub is the substrate utilization rate and Y is the algal yield from consumed substrate. This is much faster than the specific VS decay rate (k d = 0.24-0.84 d −1 ) observed in conventional aerobic digestion (Arunachalam et al, 2004). The huge difference in VS consumption/decay rates explains why 42.4% VS reduction could be achieved in 1 day by the new phagotrophic algae-based process, while it would likely take more than 20 days through the conventional aerobic digestion.…”

“…The 2nd and 3rd batches of remaining solids were shown to also require shorter digestion, that is, 3 and 2 days, respectively, to meet the SOUR requirement. Aerobic sludge digestion is known for its associated energy cost for aeration (Arunachalam et al, 2004). For pilot and fullscale aerobic digestions, it was estimated that 1.74-2.07 (kg O 2 / kg VS destroyed ) is required and 2 (kg O 2 /kg VS destroyed ) is generally suggested (Wang et al, 2007).…”

Microaerophilic Digestion of Excess MBR Sludge

“…Microaerophilic digestion has been reported to have a slower specific degradation rate of VSS than aerobic digestion (Arunachalam et al, 2004). This effect can be overcome by increasing the total solids concentration of the digester to obtain a similar volumetric VSS degradation rate as observed in aerobic digesters.…”

“…The low dissolved oxygen (DO) condition is advantageous in eliminating the need for pH control due to simultaneous occurrence of nitrification and denitrification (Arunachalam et al, 2004). The low dissolved oxygen (DO) condition is advantageous in eliminating the need for pH control due to simultaneous occurrence of nitrification and denitrification (Arunachalam et al, 2004).…”