The IntensiCarbTM Technology as High-Rate Process for Anaerobic Biosolids Fermentation Intensification and Nutrient Recovery

The IntensiCarbTM Technology as High-Rate Process for Anaerobic Biosolids Fermentation Intensification and Nutrient Recovery Gholamreza Bahreini1, Masuduz Zaman1, George Nakhla1, Kati Bell2, Christopher Muller2, Jose Jimenez2, Ahmed Al-Omari2, Domenico Santoro3, Eunkyung Jang3, John Walton3, Frances Okoye4, Farokh Laqa Kakar4, Elsayed Elbeshbishy4, Ferenc Hazi5, Imre Takacs5, Sudhir Murthy6, Diego Rosso7,8* 1Department of Civil and Environmental Engineering, Western University, CANADA 2Brown and Caldwell, USA 3USP Technologies, CANADA 4Environmental Research for Resource Recovery Group, Department of Civil Engineering, Ryerson University, CANADA 5Dynamita, FRANCE 6NEWHub Corp, USA 7Civil & Environmental Engineering Department, University of California, Irvine, USA 8Water-Energy Nexus (WEX) Center, University of California, Irvine, USA *Corresponding author (e-mail: bidui@uci.edu) Abstract: The performance of an innovative vacuum-driven mesophilic anaerobic fermentation technology for the treatment of municipal biosolids was compared against a conventional fermenter. Each system was fed by a 50:50 v/v mixture of primary and thickened waste activated sludge (TWAS). The vacuum driven fermenter allowed for nearly complete decoupling of hydraulic retention time (HRT) and solids retention time (SRT). Two testing conditions with intensification factors (IF), defined as SRT/HRT, of 1.33, and 2.00, were tested. Results indicated that the vacuum-driven technology could simultaneously reduce the reactor size significantly (half of the control volume), while also enhancing process yields and nutrient partitioning between fermented solids, fermented liquid, and condensate. Specifically, relative to the conventional fermenter, the yields of volatile fatty acids for the vacuum-driven fermenter were enhanced by 29.5% (IF=1.33) and 73.8% (IF=2.00). Additionally, the hydrolysis yields increased substantially reaching 280 mg ˆ†sCOD gVSS-1 (IF: 1.33), and 321 mg ˆ†sCOD gVSS-1 (IF: 2.00), compared to 241 mg ˆ†sCOD gVSS-1 in the conventional fermenter. Specific denitrification rates using the fermentates as a carbon source were comparable with acetate. Keywords: Anaerobic Digestion; IntensiCarb Technology; Resource Recovery Introduction Sewage sludge management accounts for almost one-half of the total sewage treatment cost. Anaerobic digestion, one of the most widely used sludge stabilization and waste management techniques (Appels et al., 2008; Nguyen et al., 2021) is credited with additional energy and nutrient recovery. Contrarily, due to high solids content and complex structure, large digester volume and long SRTs are needed (Neumann et al., 2016). The vacuum-driven IntensiCarb (IC) technology (a trademark of USP Technologies) is a promising alternative to conventional anaerobic digestion of wastewater biosolids as it reduces digester volumes, transportation and energy consumption, while it increases the volatile solids removal (VSR) efficiency and facilitates separate recovery of ammonia, volatile fatty acids, and phosphorus. This process evaporates water under negative pressures and at low temperatures, which completely decouples HRT from SRT, allowing residual solids to be retained longer in the reactor, resulting in enhancement of hydrolysis and acidification of particulate organic matter. Application of vacuum pressure during IC operation also potentially can enhance the process through change in partial gas pressures in the reactors which affects dark fermentation metabolic pathways (Rajhi et al., 2016). The IC technology can effectively divert the recovered resources into separate streams by stripping almost 50% of ammonia in the condensate, while retaining the carbon (VFAs) in the fermented sludge. The concept of decoupling SRT/HRT through vacuum makes the technology more accessible to a broader range of utilities, including small municipalities as it increases the volumetric and solids loading per unit reactor volume. The evaluation of the impact of the vacuum-induced fermentation was one of the main objectives of this project. Additionally, assessment of the quality of the fermentate as supplemental carbon source for biological nutrient removal was also tested in this research. Material and methods A bench-scale IntensiCarb system (Figure 1) with operating volume of 3L was placed on a pre-calibrated induction heater connected to a vacuum pump and operated in parallel to a conventional fermenter (2L). Both ferementers were fed with 50/50 (on volumetric basis) primary and thickened waste activated sludge from the Greenway WRRF (London, ON). Operating temperature and pressure were maintained and monitored at 45 °C, and around 1015 mbar during fermentation. Vacuum was applied once per cycle intermittently at differential pressure of -900 mbar (equivalent temperature of 45 °C) for 4, and 10 hours for IC stage 1, and stage 2, respectively. System pressure and temperature of IC were continuously monitored during vacuum. The reactors were fed daily and fermented sludge was wasted daily to maintain the respective HRTs (1.50-2.25 days), while SRT was fixed at 3.0 days. Operations at conventional, IC (stage 1), and IC (stage 2) were continued until reaching to stabilized steady state condition (Δ YieldVFA ‰¤5%). Results and Discussion Cumulative VFA and soluble COD of the fermentates were plotted against influent VSS fed to the fermenters (Figure 2). Results indicate stabilized performance of the reactors with good correlations between cumulative parameters (R2>0.99) at steady state for all the tested conditions. The production yields calculated at different phases of the experiment are summarized in Table 1. As shown, operation of the reactor with the IC technology resulted in 29.5% and 73.8% increase in VFA yield (IF of 1.33, and 2.0), respectively, compared to the conventional fermenter. Additionally, the hydrolysis yield of the conventional fermentation, was enhanced by 16.2% and 33.2% through IC operation. VFA generation rate measured per unit biomass in the reactors were 28, 33, and 39 mg ΔVFA gVSSr-1 d-1 for the conventional, IC (IF: 1.33), and IC (IF: 2.00) reactors respectively. These values show the enhancement of the process in the IC reactors over the conventional fermenter (Figure 3). Thus, the IC technology not only increased volumetric and solids loadings from 0.34 (m3sludgem-3fermenter d-1), and 4.2 (kgVSS m-3fermenter d-1), to 0.44, and 0.67 (m3sludge m-3fermenterd-1), and 5.4, and 10.1 (kgVSS m-3fermenterd-1), for IC reactors operated at stage 1, and 2, respectively; resulting in processing of biosolids in significantly smaller fermenters; but also improved the VFA and SCOD yields as proven in this study (Table 1). In a typical WRRF processing 1 ton d-1 of mixed PS/TWAS, operation of an IC fermenter (IF:2.00, SRT:3 d) can reduce the fermenter size by 25% (from 3 to 2.25 m3), while increasing the VFA production by 73% (from 760 to 1330 g VFA d-1). Results from batch denitrification tests using fermentate and acetate as carbon source revealed biomass-specific maximum denitrification rates of 0.28, and 0.24 mg NO3-N mgVSSRAS-1 d-1 for acetate and fermentate, respectively, indicating a comparable denitrification potential of the fermentate as an ideal carbon source like acetate. Conclusions Intensified fermentation using the vacuum-based IntensiCarb technology could simultaneously enhance fermentation and resource recovery as well as reduce fermenter size compared to conventional fermenters. The IC fermentate showed comparable specific denitrification rates to acetate. The outcomes of this study clearly showed the tremendous potential of the vacuum-driven fermentation to reduce cost and enhance performance of the widely used anaerobic wastewater biosolids fermentation.