Evaluation of enhanced digestion processes for treatment of municipal sludge and pathogen removal

The increasing demand for wastewater treatment has increased the generation of sewage sludge, thereby increasing the importance of treatment and disposal of sludge in a sustainable manner.;Two advanced digestion processes were operated for treatment of municipal sludge for this study: (i) Acid/gas phase digestion process consisting of acid phase digester (APD) at 55°C followed by mesophilic anaerobic gas phase digester (MAD) at 35°C (APD-MAD); and (ii) Dual Digestion (DD) process consisting of autothermal thermophilic aerobic digestion (ATAD) at 55°C followed by MAD at 35°C (ATAD-MAD). The pretreatment reactors, APD and ATAD were operated at HRTs (Hydraulic Retention Times) of 1.5 d, 2.5 d and 3.5 d. In addition to APD-MAD and ATAD-MAD processes, two more processes were evaluated. These processes were Enhanced Enzymic Hydrolysis (EEH-MAD) process which is a modification of acid/gas phase digestion; and ATAD followed by temperature phased anaerobic digestion (ATAD-TPAD) process which is a modification of dual digestion process.;The overall objective of this study was to compare acid/gas phase digestion and dual digestion, for treatment of municipal sludge in terms of volatile solids removal, biogas production and pathogen removal. This is the first side-by-side study reported which compares acid/gas phase digestion and dual digestion.;Best operating conditions for both APD and ATAD were observed for 2.5 d HRT with high VFA and sCOD production, and with the highest methane yield from the 7.5 d MAD. A comparison between the two processes was made for all of the six runs using biochemical methane potential (BMP) test in terms of overall efficiency of ultimate methane yield. When the two processes were compared with each other, no advantage of one process over another was observed using the BMP test results. When deciding between the two processes, the decision should be based on site-specific engineering analysis performed to evaluate whether it is sufficient to compensate the cost of aeration with relative autothermal heat produced for ATAD, compared to operating an APD process at 2.5 d HRT or lower.;The expected performance was not observed with the ATAD-TPAD process due to unstable operation of thermophilic digester of the TPAD (TPAD 55°C) with a low gas production for this digester affecting the overall gas production.;The effect of different OLRs (Organic Loading Rates) on inhibition of thermophilic digestion in ATAD-TPAD process was evaluated. The inhibition for high OLRs was attributed to high unionized acetic acid levels in the TPAD 55°C reactor (1.35 and 3.10 mmol/L for OLRs of 5.1 and 6.8 g VS/L/d, respectively) since unionized acetic acid is substrate as well as inhibitor for methanogenesis. On the other hand, for OLR of 3.4 g VS/L/d, increased VSr and increased methane yield was observed for the system.;Kinetic tests were performed to determine unionized acetic acid half saturation constant (Ks(HAc)) for different loading conditions. Ks(HAc) increased with increased solids level from 0.23 mg/L to 1.08 mg/L for total solids levels of 22.7 g/L and 78.0 g/L, respectively with a linear relationship between Ks(HAc) and sludge concentration in the digester. It was considered that after a certain point, the K s(HAc) would be high enough to cause high residual acetic acid, which explained acetic acid accumulation and resulted in inhibition of the digester.;Further research was conducted in order to understand inhibition due to high OLRs. Kinetic tests were conducted at total solids levels of 4%, 6% and 8%. For each solids level, tests were conducted for different free ammonia (NH3) concentrations of 25 mg/L as N, 50 mg/L as N, 75 mg/L as N and 100 mg/L as N. As NH3 concentration increased, both r max (maximum biomass production rate) and Ks(HAc) (half saturation constant for unionized acetic acid) decreased as a result of kinetic inhibition for all TS levels. Since rmax is dependent on active HAc degrading biomass concentration, Ks(HAc) is dependent on microbial growth rate as well, showing that Ks(HAc) is not a constant. Inhibition factors at fixed free ammonia concentrations are lower for lower TS levels. As a result, it can be concluded that the degree of methanogenic inhibition attributable to NH3 decreased as the TS level increased at equivalent NH3 concentrations.;Pathogen removal in sludge treatment is an important step. Many bacteria can enter the viable but nonculturable (VBNC) state when faced with stressed environmental conditions as observed for thermophilically treated sludge. Bacteria in VBNC state do not grow on standard culturing media. To investigate the nonculturable hypothesis, E. coli was enumerated using EPA Method 1680 and other culturing methods developed for investigation of non-culturable E. coli. Class A Biosolids requirements was achieved with E. coli density below 3 log10 for only 3.5 d HRT for ATAD-MAD process, with enumeration conducted with all the culturing methods investigated for this study. The time-temperature study shows the possible need for more conservative time-temperature requirements when compared to EPA time-temperature requirements even though further research is necessary in order to prove the new culturing methods are not giving false positives.