Intensifying Anaerobic Sludge Fermentation Using The Novel IntensiCarbâ„¢ Technology: Municipal Application and Model Evaluation

Abstract
The paper presents the development of a novel technology and a model to simulate the IntensiCarbâ„¢ (IC) process, a patent-pending technology co-developed by USP Technologies and Brown and Caldwell which approaches process intensification through a nearly perfect and extremely efficient solid-liquid separation process enabled by vacuum evaporation at temperatures (20-60 oC) that are compatible with anaerobic biological processes. SumoTM was used to create a new process unit to simulate the physical and biological reactions occurring within the IC process. The model was used to simulate the operation of IC as a sludge fermenter (ICF) and compared against completely mixed fermenter (CMF) to generate volatile fatty acids from a mixture of primary and biological waste activated sludges. The hydrolysis kinetic parameters were modified to reflect the observed improvement in particulate substrate solubilization on mass basis of approximately 16%. The increase in hydrolysis rate could be related to changes in sludge physical characteristics such as viscosity and particle size distribution impacted by the nature of vacuum operation. To address this improvement, the authors used the Contois model and adjusted (Khyd), which is the half saturation constant of the biodegradable particulate COD (xbCOD) that is defined as the ratio of xbCOD to the active biomass (xbCOD/Xbio). Introduction Sludge fermentation is widely accepted to be the key design consideration for sustaining enhanced biological phosphorus removal at wastewater treatment plants by converting particulate substrate to readily biodegradable (hydrolysis), and consequently to VFA (fermentation) (Bernard, 2014). Fermenter design can be either integrated within the mainstream process configuration or conducted in dedicated reactors/separators. The major design elements for dedicated fermentation process include the ability to decouple the main process from the fermentation process, ability to control the solids retention time (SRT) in the fermenter, the ability to decouple SRT from hydraulic retention time (HRT), and the ability to extract VFA to feed the mainstream biological process. Decoupling of SRT and HRT is typically done through gravity-based separation, which suffers from poor effluent quality, difficulty to control SRT and the need for multiple process units. Vacuum evaporation is an established technology for biosolids dewatering (Gnida 2020), which provides a means to decouple the solids phase from the liquid and gas phases. This quality results in a number of benefits that set this technology apart from other conventional technologies. These include process intensification, pH controlled targeted extraction of value-added products such as volatile fatty acids (VFA) (Sonnleitner et al., 2012), effective sludge thickening in the same process unit, and odor containment and control. This paper discusses the application and modeling of the (IC) technology to intensify the fermentation process of municipal sludge based on a proof-of-concept study.
Materials and Methods
The study utilized a pilot scale setup of the ICF and a conventional CMF (Okoye et al., in preparation). The reactors were fed daily a 50:50 by volume mixture of primary and biological WAS from a nearby full-scale treatment plant. The fermenters were temperature controlled and were operated at 60 oC (Figure 1). For the ICF, sludge was wasted daily to maintain the respective HRTs (1.50-2.25 days), while SRT was fixed at 3.0 days. To determine the degree of process intensification, an Intensification Factor (IF) was calculated using the following equation: IF = SRT/HRT. This translates to IF of 1.0, 1.3, and 2.0 for the CMF, ICF at HRT of 2.25 days, and ICF at HRT of 1.5 days, respectively. A vacuum anaerobic digester model unit was created in Sumo21 (Dynamita, France). Principal components of the process include variable volume anaerobic digester, water extraction through the headspace, gas/liquid transfer model for stripping of ammonia, VFA, and other gases. Heating power is calculated using the targeted evaporation rate. The gas phase is considered saturated in terms of water vapor. The headspace pressure is determined using the water equilibrium curve of pressure and gas/liquid temperature (Sharp, 2001).
Results and Conclusions
Table 1 summarizes the operational conditions for each pilot reactor and the overall fermentation performance, after 10 days of operation, measured as % particulate COD solubilization [or % hydrolysis], % acidification to identify how much VFA was produced compared to soluble COD produced, and VFA yield compared to influent COD. The COD mass balances around the fermenter suggested some minor loss of COD with highest gap associated with the ICF, IF=2.0 reactor. This loss could be related to partial digestion at the elevated temperatures. Hydrolysis of particulate substrate was directly calculated from the mass loading reduction in xCOD between the feed and the wasted sludge. It was observed that approximately 16% improvement in %hydrolysis was achieved in the ICF, IF=2.0 over the CMF and ICF, IF=1.3. In addition, the VFA yield has significantly improved by approximately 60% for the ICF, IF=2.0. This improvement was only attainable in the model by adjusting the hydrolysis rate kinetics specifically to each reactor [Table 2]. Our team has chosen to adjust the Khyd parameter to address the changes observed in the overall hydrolysis performance in the ICF, IF=2.0. The rationale used here was that the affinity to the substrate may have increased due to physical changes induced by intense boiling under vacuum evaporation. One hypothesis, yet to be proven, is that the particle size distribution was altered and resulted in an increased surface area and accessibility by biomass. The change in hydrolysis rate with respect to the xbCOD at the modified Khyd is depicted in Figure 2. The fermentation rate was adjusted to achieve a good match with the observed data [Table 2]. Figure 3 shows the model output against observed VFA concentrations for all reactors. Figure 4 shows the model output against observed data for the ICF, IF=2.0.
This experimental study and model exercise showed that intensified fermentation using the vacuum-based IntensiCarbâ„¢ technology could simultaneously enhance hydrolysis and fermentation and thus enhance resource recovery as well as considerably reduce fermenter size compared to CMF fermenters. 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. The paper will include results (not shown) for a parallel study conducted at 45 oC using the same IF range.