IntensiCarbTM- A Novel Vacuum Evaporative Process for the Intensification of Anaerobic Digestion

Mesophilic anaerobic digestion is the most deployed biological sludge stabilization process at wastewater treatment plants in North America. It also plays a central role in energy and resource recovery from wastewater treatment plants. This paper explores the application of medium vacuum pressure (~100 mbar) operated in a side stream recirculating reactor as a mechanism for process intensification and resource recovery, utilizing the IntensiCarbTM process. Currently within the industry the primary means of process intensification are either through the elevation of operating temperature (ex. thermophilic digestion), pre-conditioning of sludges (ex. thermal hydrolysis) or separation of solids and hydraulic retention time (ex. recuperative thickening). The IntensiCarbTM process utilizes the decoupling of SRT and HRT to increase process capacity while simultaneously reducing the influence of toxic metabolic by products through extraction (ex. ammonia-N). This paper discusses the results of side-by-side comparison of conventional mesophilic digestion with varying intensification factors (2x, 3x, 4x) for bench-scale trials conducted at Western University in London, Ontario, based on the process flow provided in Figure 1. The intent of this experimental design was to focus on understanding the fundamental process changes that occur when vacuum evaporation is applied to a conventional mesophilic digester. In particular, the question being investigated is whether the evaporation process and the extraction of materials result in any fundamental changes in the operation of the digestion process or does it simply operate in a manner similar to recuperative thickening. Table 1 provides a summary of the operating conditions tested, with Intensification Factor 1 (IF1) representing the control and IF2, 3 and 4 representing varying degrees of decoupling of both SRT and HRT. All the digesters were fed a mix of primary and secondary sludge (Ave. TS = 2.9%, VS =2.18%) periodically collected from the Greenway Wastewater Treatment Plant in London, Ontario. The digesters were all operated at mesophilic temperatures (~35-37 °C). Across the range of intensification factors tested, in general, all processes demonstrated effective anaerobic digestion of the sludge, noting that the highest intensification factor (IF-4) there were episodic instability events (data not shown). This suggests that either the process is approaching its maximum extent of intensification, or the specific combination of HRT and SRT is not sustainable. A number of factors could be causing this observed instability including insufficient active fraction of biomass at the extended SRT, the biomass population is at an inflection point with shifting populations or an undefined stressor is impacting the population. Additional research will be required to explore the ultimate limits of intensification. What is apparent from the data in Table 1 is that in general IntensiCarbTM does not appear to significantly change the extent of digestion with the VSr ranging from 46-55 percent relative to the control at 50 percent. However, the rate of digestion is significantly enhanced by the increase in solids concentration, resulting in overall higher solids throughput rate and rate of methane generation increasing from 0.29 m3-CH4/m3-digester-day to a maximum of 1.08 m3-CH4/m3-digester-day (Figure 2). A challenge with most process intensification strategies deployed to date is the impact of metabolic byproducts on the process. Of particular interest is ammonia, as it is produced through the decomposition of proteins, which constitute a significant fraction of sludge materials. To date no technology has effectively managed ammonia-N in-situ, resulting in risks for process instability and or significant population shifts, as reported by Mah et al. (2017) for THP. The concentration of ammonia in digesters tested were very similar (Table 1). This is significant in that the control of ammonia in the digester could support high loadings without the negative impacts. Figure 3 simulates what solids concentrations sludge would need to be loaded at to a conventional digester to achieve the same degree of intensification and the resulting total ammonium concentration in the digester would be. What is apparent from the graph is that conditions similar to those observed in thermal hydrolysis would need to be achieved, but the resulting total ammonia concentrations would approach the current limits of those systems. Furthermore, the feed solids would need treatment to reduce the viscosity to make the material pumpable. Figure 4, presents the nitrogen balanced conducted across each system, showing significant removal of ammonium-N to the condensate of the evaporator from the digestate. Supporting the observation that the IntensiCarbTM process creates a unique condition where the negative impacts of ammonia can be directly controlled in a highly loaded digestion process. Furthermore, the condensate is a very clean liquid, devoid of solids making it an ideal solution for ammonia recovery and beneficial use; potentially eliminating the need for centrate treatment for nitrogen. The combination of reduced nitrogen load and stable operations suggest the peak capacity of the IntensiCarbTM process was not achieved, in these tests. Plotting the empirically derived peak loading conditions for common stabilization processes and the resulting average total solids load, at a volatile fraction of sludge equivalent to the one used in this study, it is possible to investigate potential peak loadings; assuming similar process limitations of peer processes. The IntensicarbTM process, as currently tested, would appear to have a similar capacity to thermophilic digestion (Figure 5) for loading. However, at the highest average loading condition, IF-3, no process instability was observed, suggesting further intensification could be possible and should be investigated. Overall, the IntensiCarbTM, based digestion process is early in development but is demonstrating promise and the novel application of vacuum is generating conditions within digesters that have not been evaluated to date. The combination of process intensification through decoupling SRT and HRT and the depression of ammonia-N concentrations would suggest the system may achieve further enhancement as optimal operating conditions are defined. Concurrent with the completion of these bench trials a pilot facility is being planned by the Great Lakes Water Authority to further vet this process. This pilot will investigate the scalability of the technology, introduce of real-world operating dynamics, further optimize operating conditions to define a design loading criterion and develop a techno-economic analysis. Vacuum enhanced anaerobic digestion represents a novel process for the intensification of anaerobic sludge stabilization. Potentially offering unique opportunities for resource recover, ammonia-N and dissolved methane, as well as providing a mechanism to alleviate the current limitations associated with anaerobic digestion, metabolic byproduct toxicity. If successful, this technology could provide the next step forward in resource recovery from sludges.