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

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.