Improved Anaerobic Process Efficiency Using Mesophilic and Thermophilic Elutriated Phased Treatment

An innovative anaerobic digestion elutriated phased treatment (ADEPT) has been evaluated at mesophilic (M-ADEPT) $(35°C)$ and thermophilic (T-ADEPT) $(55°C)$ temperatures in which the organic loading rate (OLR) was increased until reactor failure $\left(\mathrm{pH}<5.5\right)\text{.}$ Single-stage continuously stirred tank reactors (CSTRs) at both temperatures were also operated as controls (M-CSTR for $35°C$ and T-CSTR for $55°C)\text{.}$ The T-CSTR failed at an OLR of 7.4 g volatile solid (VS)/L day and the M-CSTR at an OLR of 10 g VS/L day while the M-ADEPT continued until an OLR of 18 g VS/L day and the T-ADEPT reached an OLR of 24 g VS/L day before system failure. The T-CSTR produced the poorest effluent quality as manifested by high propionate concentrations (1,500–2,500 mg/L) while both M-ADEPT and T-ADEPT produced much better quality of effluent with propionate concentrations below 100 mg/L. Thus it appears that the T-ADEPT design may solve effluent quality problems associated with normally high propionate concentrations produced during thermophilic anaerobic digestion. Superior effluent quality, reduced reactor volume requirements, more stable methanogenesis due to the extended solids retention time, and uncoupling of the methanogen wasting from the refractory sludge wasting process resulted in stable and efficient processing at both temperatures for the innovative ADEPT design. Because the higher amounts of volatile fatty acids produced in the acid elutriation phase of the ADEPT system can be a favorable carbon source for biological nutrient removal in wastewater treatment plants, this positive aspect should be considered in future applications of the ADEPT system.