Application of Vacuum Extraction to Anaerobic Digestion for Process Intensification and Resource Recovery

The IntensiCarbTM process utilizes the decoupling of SRT and HRT, like recuperative thickening, to increase process capacity while simultaneously reducing the influence of toxic metabolic by products through extraction (ex. ammonia-N) and recover additional methane. This paper summarizes 3 years of research evaluating the efficacy of the process utilizing a side-by-side comparison of conventional mesophilic digestion with varying intensification factors (2x, 3x, 4x, 5x) in bench-scale trials conducted at Western University in London, Ontario digesting blend of primary and secondary sludge collected from the Greenway Wastewater Treatment Plant. Figure 1 provides a process flow diagram of the IntensiCarbTM process simulated in these tests. Figure 2, plots change in volatile solids reduction (VSr) in relation to hydraulic and solids retention times of IntensiCarb to explore its impact on the rate and extent of digestion. Based on this data it can be seen that the primary mechanism of process enhancement is the rate of digestion, rather than extent, being able to operate a higher loading rate, low HRT, with out significant increase in overall destruction at extended SRT. While some instability was observed in the higher loading rates it was transient during transition periods. With the move toward resource recovery, the energy yield of the digestion process is as critical as maximizing methane production per unit volume has potential long-term operating cost benefits as well as sustainability impacts. Figure 3, plots the impact of digester hydraulic retention time on the methane yield of the digester (m3-CH4/m3digester/d). Figure 3, clearly illustrates that with the IntensiCarbTM process the methane yield on an equivalent digester volume basis is more than 4.5 times higher than the conventional digester at 20 days SRT; 1.28 m3-CH4/m3digester/d verses 0.28 m3-CH4/m3digester/d, respectively. Furthermore, conventional mesophilic digestion typically does not operate, at scale, under 15 days of retention time. This is because below this level significant risk to process stability has been historically observed. The accumulation of ammonia has a self-limiting effect on anaerobic digestion. 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. This is significant in that the control of ammonia in the digester could support high loadings without the negative impacts of toxicity and reduce return stream impacts, often a concern with high rate anaerobic digestion processes. The data shows (Figure 4) that as the digester load increased with IntensiCarbTM, the overall total ammonia-N levels remained around 800 mg-N/L or less, while a more conventional system would be expected to be operating at 2,100 to >5,000 mg-N/L. This observation alleviates one of the primary limits to anaerobic digestion. High loadings and high ammonia concentrations have been attributed to shifts in digester populations, as outlined by Mah et al (2017) when evaluating thermal hydrolysis. With similarly high loads but significantly reduced ammonia-N levels, the population profile of several of the test digesters were evaluated and compared to the control. The relative abundance of bacteria and archaea (methanogens) depicted in Figure 6 indicates that the relative abundance of archaea increases with intensification factor, approximately 40% (Figure 5). Looking at the specific methanogenic population present, there is a shift that occurs between the conventional (Control) digester relative to the intensified digesters. As the intensification level increases the methanogenic population shifts from one dominated by the low-growth rate Methanosaeta to one dominated by high-growth rate Methanosarcina (Figure 6). Some have suggested that digesters dominated by Methanosarcina have a higher overall capacity and stability due to their significantly higher maximum specific growth rate. The opposite was reported for thermal hydrolysis based digestion (Mah et al (2017)). 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.