Co-digestion and thermally hydrolyzed food waste and waste activated sludge

Executive summary: In this study, the impact of thermal hydrolysis pretreatment (THP) on the mono- and co-digestion of thickened waste activated sludge (TWAS) and food waste (FW) was carried out at temperatures of 150, 170, 190, and 210 C and volumetric ratios of 90:10, 70:30, and 50:50 in batch anaerobic tests. THP enhanced solubilization, reduced volatile suspended solids (VSS) and the particle size of TWAS as the temperature increased showing optimal improvements at 170 C. THP showed no significant impact on FW with only marginal improvements at 150 C. Increasing the temperature beyond 170 C for TWAS and 150 C for FW deteriorated anaerobic biodegradability forming refractory compounds. Co-digestion improved methane yields and kinetics as the contribution of FW increased. Co-digestion of thermally pretreated TWAS with FW improved the methane yields by 27% and kinetics by 29% at 170 C with no synergism. Co-digestion of thermally pretreated FW with TWAS improved methane yields by 15% and kinetics by 25% at 150 C, with improvements up to 21% in synergy. THP of the mixed feedstocks improved methane yields by 53% and kinetics by 92% at 170 C, with improvements becoming less pronounced with the increase in the volume of FW and temperature. Keywords: Co-digestion, thermal hydrolysis pretreatment, methane yields, Gompertz, kinetics, synergy 1. Introduction Background: Currently, the treatment of excess sludge produced from activated sludge treatment plants may account for more than 50% of the total operational cost .Anaerobic digestion (AD) of wastewater sludges results in reduction of sludge volumes, destruction of pathogenic organisms, stabilization of sludges, and production of valuable biogas. However, the application of anaerobic digestion to biological solids is associated with limitations in the hydrolysis stage. Despite the increasing interest in food waste prevention and recovery, very little food waste (FW) is recovered, mostly ending up in landfills. Reported advantages of THP are improved anaerobic degradation kinetics; increased biodegradability, volatile solids destruction, and methane production; solubilization of the macromolecular components of sludges (i.e., carbohydrates, proteins, and lipids); improved dewaterability; and higher digester organic loading rates. Considering that sewage sludge anaerobic digesters operate at relatively low organic loading rates ranging from 1.6 to 4.8 kg VSS/m d, with an unused capacity of almost 30%, and to alleviate operational upsets that arise from the mono-digestion of MSW and WAS, co-digestion becomes one of the most attractive solutions. Co-digestion promises a myriad of benefits such as process stability by supplemental alkalinity, trace elements, nutrients, enzymes; balanced C/N ratios; dilution of toxic compounds; diversion of food wastes from landfills; and the diversion of fat, oil, and grease from the wastewater collection infrastructure. Objectives: In this study, the impact of thermal hydrolysis on characteristic changes, anaerobic biodegradability, and kinetics of WAS and FW was evaluated at 150, 170, 190 and 210 C. Mixing ratios of 90:10, 70:30, and 50:50 (v/v) were adopted to identify the optimal pretreatment temperatures and mixing ratios for the thermally pretreated substrates with respect to anaerobic biodegradability and kinetics. The study also investigated the characteristic changes at elevated temperatures and their impact on subsequent anaerobic digestion and co-digestion, as well as the possible synergism through thermal hydrolysis and co-digestion. 2. Methodology Thickened waste activated sludge (TWAS) was collected from the rotary drum thickeners at Greenway Wastewater Treatment Centre (London, Canada). Anaerobically digested sludge was collected from a mesophilic anaerobic digester at the Water Pollution Control Plant (Stratford, Canada) fed primary sludge, and used as an inoculum for all biochemical methane potential (BMP) tests in this experiment. FW was collected from StormFisher (London, Canada). The design of each phase comprised pretreatment at 4 different temperatures (i.e., 150, 170, 190, and 210 C) and 3 mixing ratios for FW to TWAS (i.e., 10, 30, and 50%) on a volumetric basis. The volumes of both the substrate and the inoculum were calculated based on an inoculum to substrate ratio (ISR) of 2 g VSS/ g COD as recommended for most applications. All BMPs were carried out in duplicates at 37 ± 1 C by incubating the bottles in a water bath set at the designated temperature. 3. Results and Discussion TSS and VSS of the TWAS concentrations decreased as the THP temperature increased, resulting in solubilization efficiencies of 31%, 45%, 61%, and 62% at 150, 170, 190, and 210 C respectively. sCOD concentrations of the FW remained almost equal at all temperatures. The viscosity of the raw TWAS decreased sharply with the increase in temperature from 259 to 2 cP at 210 C. The mean particle size of the raw TWAS decreased from 167 µm to 106, 96, 57, and 42 µm at 150, 170, 190, and 210 C, respectively. The viscosity of the raw FW decreased with the increase in temperature from 670 cP to 270, 240, 220, and 45 cP at temperatures of 150, 170, 190, and 210 C respectively. The particle size distribution of the raw FW and pretreated FW at 150 C were bimodal, forming two peaks at 20 µm and 275 µm, and at 91 µm and 240 µm respectively. Samples pretreated at 170 C constituted a single peak at 91 µm, while samples pretreated at 190 C and 210 C peaked at 79 µm. Interestingly, the mean particle size of the FW increased with pretreatment from 106 µm to 111, 153, 109, and 114 µm at 150, 170, 190, and 210 C, respectively which is hypothesized to be due to the formation of chemical bonds catalyzed by temperature. Phase I The largest improvement in methane yields was in the 90:10 mixture at 170 C with a methane yield of 312 mL/g COD, 27% higher than the raw mixture. The highest methane yields from thermally pretreated TWAS were 234 and 250 mL/g COD at 150 C and 170 C respectively. Increasing the temperature to 190 C and 210 C detrimentally impacted methane yields. THP increased methane production rates of the raw TWAS from 147 mL/d to 199, 191 and 205 mL/d at 150, 170, and 190 C, respectively, while pretreatment at 210 C decreased it to 136 mL/d. Phase II The highest methane yield from thermally pretreated FW was 276 mL/g COD at 150 C. Increasing the temperature beyond 150 C decreased methane yields below that of raw FW. The 50:50 mixture produced the highest yield of 309 mL/g COD, 15% higher than the raw mixture. The increase in temperature to 170 C improved the methane yield by only 4%. THP had a more significant impact on the synergistic outcome of this phase with improvements ranging from 7% to 21%. At 150 C, a 17% synergistic increase in methane yields was observed for all mixtures. Despite the formation of refractory compounds at the higher temperatures (< 170 C), the dilution of these toxins with co-digestion compared to mono-digestion might have influenced improvements in both methane yields and kinetics. Phase III Methane yields improved in the 90:10 mixtures at all temperatures, with a maximum yield of 286 mL/g COD at 170 C, 52% higher than the raw mixture. However, increased solubilization was not reflected in increased biodegradability at the higher temperatures. The most pronounced improvement was in the 90:10 mixture where thermal hydrolysis at 170 C increased the methane production rate by 91% from 140 to 268 mL/d, beyond which kinetics deteriorated. 4. Conclusions THP showed a more significant impact on the characteristics and anaerobic biodegradability of TWAS than FW. The increase in the degree of solubilization, particle size, and VSS reduction with the intensification of the pretreatment temperature did not result in improved anaerobic biodegradability. The impact of thermal hydrolysis on co-digestion was inversely proportional to the biodegradability of the mixture showing the largest improvement of 53% in the methane yield and 92% in kinetics in the 90:10 mixtures relative to the raw mixture. Refractory compounds produced from thermally hydrolyzing FW seemed to be less inhibitory compared to those produced from TWAS and could be diluted with co-digestion, hence, synergistic improvements up to 21% in methane yields and 27% in kinetics were observed. The synergistic increase in methane yields observed with the co-digestion of thermally pretreated FW with TWAS decreased with the increase in the volumetric contribution of FW and pretreatment temperature, showing the largest improvements in the 90:10 mixtures owing to the larger biomass availability from the higher TWAS contribution.