Ismail, Amr

Amr Ismail is a doctoral student at Toronto Metropolitan University with a background in Structural Engineering and a masters degree in Environmental Engineering from Western University. His research models greenhouse gas emissions from sewer networks and wastewater treatment plants. Amr is also well-versed in resource recovery from organic wastes and wastewater biosolids, pretreatment technologies, and modelling wastewater treatment, water chemistry, and anaerobic digestion processes.

Titles from this speaker

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

Abstract

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.

This paper was presented at the WEF Residuals and Biosolids Conference, June 18-21, 2024.

SpeakerIsmail, Amr

Presentation time

09:00:00

09:30:00

Session time

08:30:00

10:00:00

SessionThermal Hydrolysis Process

Session number13

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicCo-digestion, thermal hydrolysis pretreatment, methane yields, Gompertz, kinetics, synergy

Author(s)

Ismail, Amr

Author(s)A. Ismail1, F. Kakar2, E. Elbeshbishy1, G. Nakhla3

Author affiliation(s)Toronto Metropolitan University 1; Brown & Caldwell 2; Toronto Metropolitan University 1; University of Western Ontario 3;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Jun 2024

DOI10.2175/193864718825159441

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count11

Description: Modelling the Impact of the Aerobic Sludge Age on Thermally Pretreated Wastewater...

Modelling the Impact of the Aerobic Sludge Age on Thermally Pretreated Wastewater Biosolids

Abstract

Introduction Background: The solids retention time (SRT) or sludge age is one of the most fundamental design parameters in the design of AS systems. As the sludge age of the AS system increases, the active fraction of the biomass decreases due to endogenous cell decay leaving cell residues or debris termed endogenous products [1], [2]. Various studies examining the biodegradability of waste activated sludge (WAS) concluded that endogenous products generated in the AS system are believed to be inert unbiodegradable particulate matter under both aerobic and anaerobic conditions [3], [4]. However, under starvation conditions, microorganisms can produce lytic enzymes that can degrade cell walls as a survival response [5]. Modelling the impact of thermal hydrolysis pretreatment (THP) has not been widely investigated, especially the impact of THP on the degradation kinetics or hydrolysis rates of different feedstocks, including mixtures of primary sludge (PS) and WAS. Additionally, due to the prevalence of sidestream treatment processes development for nutrient removal, the capacity of mainstream processes in handling thermal hydrolysis dewatering liquors has been largely ignored. Objectives: In this study, the impact of the aerobic sludge on the biosolids' composition and the performance of thermal hydrolysis is evaluated. The degradation kinetics of wastewater biosolids (i.e., PS and WAS) pre- and post-thermal hydrolysis are identified. This study also assesses the impact of thermal hydrolysis dewatering liquors on mainstream wastewater processes without sidestream treatment on effluent nitrogen concentrations and fractionation. Methodology BioWin 6.2 (EnviroSim Associates Ltd, Canada), was used to perform 18 scenarios of 3 full-scale plants at different sludge ages (i.e., 5, 10, 15, 20, 30, and 60 d), emulating the changes in the biosolids' characteristics at different sludge ages, the various THP induced conversions, their impact on downstream processes., and the impact of their conversion on methane production and biodegradability. Plant Configurations Figures 1--3 depict the schematic process of the models adopted throughout the study. Detailed influent wastewater characteristics are summarized in Table 1. Model Calibration Thermal Hydrolysis Unit Figure 4 depicts the conversion ratios of the different components of the pretreated biosolids. Kinetics For the 60-d sludge age models, an endogenous products decay rate of 0.012 d-1 was adopted from Lubello et al. [6] and Ramdani et al. [7] and integrated into the local kinetic model of the bioreactors to emulate the aerobic degradation of endogenous products at prolonged sludge ages (i.e., > 30 d). The hydrolysis rate for WAS of 0.21 d-1 was adopted from Abubakar et al. [13]. The hydrolysis rate for thermally pretreated WAS of 1.5 d-1 was adopted from Phothilangka et al. [9]. The hydrolysis rates of the raw and pretreated mixture of WAS and PS was identified through a series of simulations using the models depicted in Figures 1--3. Results and Discussion Simulations carried out on the mixed biosolids at higher aerobic sludge ages than 5 days using the hydrolysis rate attained from the baseline simulation (i.e., 0.73 d-1) were unsuccessful due to the model's reliance on the active biomass in the WAS for hydrolysis. Hence, as the active fraction of the biomass decreased, hydrolysis was hindered, and the biodegradability of the raw biosolids was grossly underestimated. The biodegradability of the PS and WAS had to be predicted separately to attain representative results. However, since an overexaggerated hydrolysis rate was required to predict the actual biodegradability of PS, the biodegradability of the PS was estimated from steady-state mesophilic AD data of Ikumi et al. [10] at SRTs of 10, 18, 25, 40, and 60 d by interpolation, and fixed to 39% at a 15 d SRT. The biodegradability of the biosolids mixture was calculated based on the weighted average biodegradability of the PS and the WAS, which depended on the sludge age of the AS system. The calculated biodegradability of the PS and WAS mixture was then matched with model simulations with hydrolysis rates ranging from 0.73 d-1 to 1.25 d-1. Changes in Characteristics The biosolids produced and processed by the plants decreased by 65% as the sludge age of the AS system increased from 5 d to 60 d. The production of endogenous products increased by 147% from 554 to 1371 kg COD/d due to cell decay. The impact of THP is reflected in the increase in the sCOD fractions from almost 0.2% to a range of 27% to 34%. The solubilization efficiencies and VSR in the MLE/MLE-MBR configurations decreased from 32% to 27% as the sludge age increased from 5 d to 60 d. However, the reduction in solubilization efficiencies was not as significant as that of the Carrousel configuration, which decreased from 37% to 28%. Methane Production, Yields, and Biodegradability Methane yields are depicted in Figure 5. The increase in the sludge age decreased methane production potential and the yields of the raw biosolids as the available biodegradable material decreased. The biodegradability of the raw biosolids mixture decreased from 50% to 32% as the sludge age increased from 5 d to 60 d, and from 18% to 12% as the sludge age of the Carrousel configuration increased from 20 d to 60 d. At the 5-d sludge age, the impact of THP was the lowest, increasing the methane yield by only 14%. At sludge ages 10 d and 15 d, the yield increase was 41% and 67%, respectively. Methane yields increased over twofold at the longer sludge ages, with increases of 65%, 62%, and 59% and 155%, 156%, and 186% at sludge ages of 20, 30, and 60 d. In the MLE-MBR and Carrousel configurations, respectively. The increase in methane yields in the Carrousel configuration resonates with the findings of Pinnekamp [11] who reported a 270% increase in the methane yield of anaerobically digested biosolids with THP at 180 °C at an SRT of 10 d. Recycled Nitrogen Table 2 summarizes the recycled nitrogen loads to the wastewater treatment plant and effluent nitrogen concentrations with and without THP. The recycled nitrogen loads decreased with the increase in sludge age, primarily due to the lower WAS:PS ratio. In the MLE configuration, the nitrogen loads recycled to the wastewater stream increased by 42% to 110%, with the least percentage increase at the 5-d sludge age, corresponding to the least improvement in biodegradability. At sludge ages of 15--30 d, the increase in the recycled nitrogen was relatively consistent at 108% to 110%, decreasing to 98% at 60 d, consistent with the improvements in anaerobic biodegradability, despite the reduction of the WAS fraction from 38% to 22% owing to the conversion of a larger fraction of endogenous products. In the Carrousel configuration, the increases in the recycled nitrogen loads were the highest across all scenarios ranging from 117% to 125%, despite the total COD being 47% to 65% less than the base model of a 5-d sludge age, due to the nature of the protein content of the WAS, making it susceptible to ammonification after thermal hydrolysis and AD [12], [13], which explains the higher nitrogen load at low sludge ages. Despite the significant increases in the recycled nitrogen loads, increases in effluent nitrogen concentrations ranged only from 6.9% to 13%, while maintaining the same effluent ammonia concentrations, demonstrating that conventional nitrogen removal systems are effectively capable of reducing recycled nitrogen loads without dedicated sidestream ammonia removal [14], agreeing with Lacroix et al. [15] who reported stable mainstream nitrogen removal as the recycled nitrogen load increased to 15% of the influent load at the Truckee Meadows Water Reclamation Facility in Reno, NV. Conclusions The increased biodegradability and enhanced methane production rate associated with THP stem directly from the conversion of endogenous products to simpler compounds favourable for microbial consumption, the fraction of which depends on the sludge age of the wastewater treatment system. Simulations demonstrated that AS systems operating at sludge ages of less than 10 d showed the least improvements in anaerobic biodegradability (i.e., 14%) due to the relatively high biodegradability of the raw biosolids. Even at sludge ages of 60 d where the biosolids might be considered aerobically stabilized, improvements in anaerobic biodegradability with thermal hydrolysis ranged from 59% and 186%.The improvements in methane yields were relatively constant at sludge ages > 10 d, despite the variation in the fractionation of the pretreated biosolids. Mainstream wastewater treatment can successfully handle the incremental nitrogen load from thermal hydrolysis without sidestream treatment and without compromising effluent ammonia requirements, as it constitutes only 2% to 13% of the influent nitrogen load.

This paper was presented at the WEF Residuals & Biosolids and Innovations in Treatment Technology Joint Conference, May 6-9, 2025.

SpeakerIsmail, Amr

Presentation time

16:25:00

16:45:00

Session time

15:45:00

16:45:00

SessionAdvances in Process Modeling: Aeration, Scaling, and Anaerobic Digestion Dynamics

Session number20

Session locationBaltimore Convention Center, Baltimore, Maryland, USA

TopicChemical Treatment, Modeling, Phosphorus Removal and Recovery, Specific Methanogenic Activity Test