Bench Scale Comparison of Vacuum Evaporation Enhanced Anaerobic Digestion (IntensiCarbTM) with Thermal Hydrolysis

Thermal hydrolysis represents the state-of-the-art intensification for anaerobic sludge stabilization currently, providing high loading capacity, pathogen kill to Class A standard, reduced sludge viscosity and improved sludge dewaterability. Emerging intensification processes need to provide a similar or better range of benefits to drawbacks or provide similar levels at a lower lifecycle cost to enter the market effectively. The first step in this process is to understand the relative process performance of the two systems, treating sludges from the same facility.

A bench-scale pilot study was conducted at Western University to explore how the IntensiCarb™ process compares to thermal hydrolysis operated at their typical loading rates (Figure 1). For the IntensiCarb™ process this involved operating at a 20-day SRT and 5-day HRT and for thermal hydrolysis this represents operating at an SRT of 20 days, a maximum organic loading rate of <0.400 lb-VS/ft3-d and total ammonium-N concentration of < 3,000 mg-N/L. Thermal hydrolysis was conducted at 150°C for 30 minutes, prior to feeding to the digesters. The feed sludge to digestion was a mix of primary and secondary sludge from the Greenway Wastewater Treatment Plant, as characterized in Table 1.

Under the operating conditions outlined, above a variety of process metrics were evaluated to determine process performance and any relative differences between the two processes. Figure 2 compares the methane yield of the digesters (L-CH4/gCODfed) between the IC reactor and the THP system. What is apparent in the data is that during early operation the yield was comparable, but after approximately 1.75 SRTs there was a marked drop off in the methane production from the THP system, decreasing to 0.07 L-CH4/g-CODfed as compared to the IC system at 0.22 L-CH4/g-CODfed. As subsequently discussed, the drop in production was likely due to ammonia-N toxicity.

Figure 3, shows total ammonia-N concentration in the digesters. The IC digester operated between 800-1200 mg-N/L over the course of the evaluation period. The THP digester total ammonia-N increased to an average of 3,200 mg-N/L, above typical process limits. The THP digester was allowed to run for an additional 2 SRTs beyond the first indication of process deterioration. This was done to understand if the biomass, given sufficient time would acclimate to the ammonia. However, no improvement was realized. Specific methanogenic activity tests were conducted during this time to confirm that it was ammonia toxicity impacting the methanogenic population. SMA testing demonstrated consistent methane production from the IC digesters, while the THP sludge produced no gas, indicating acetoclastic methanogenesis was completely inhibited by ammonia. While the loading rate of the IC digester was high, it was protected by the removal of ammonia during the evaporation process. Figure 4 shows the partitioning of total ammonia-N between the digester and that removed in the condensate. Approximately 57% of the organic nitrogen converted to ammonium was removed in the condensate, keeping the concentration in the digester at tolerable levels.

Volatile acid data supported the observation that methanogenic activity was primarily inhibited by the elevated ammonia-N levels in the THP digester, as total VFA rose to concentrations greater the 8,000 mg/L as acetate (Figure 5). The data also appears to support prior observations regarding IC operations, that overtime there is a shift in the microbial community, as biogas production stabilized but VFA levels continued to decrease after 2.5 SRTs. Had the system fully stabilized, system performance should have been similar to prior SRTs of operation. The data to date indicate that for these high load systems, acclimation is critical to long-term performance. After several attempts at trying to recover the THP digestion system from ammonia toxicity it was decided the best course of action would be to reseed the digester with sludge from an operating thermal hydrolysis unit. Sludge was shipped overnight from the DCWater Blue Plains Advanced Wastewater Treatment Plant in Washington, DC which has been operating a CAMBI thermal hydrolysis process since 2014. By inoculating the THP digestion system with biomass acclimated to high load and high ammonia levels, a more representative comparative analysis could be conducted.

After approximately 20 days of operation the inoculated THP digester has shown strong performance and not exhibited the high VFA and low methane production its predecessor had, while being fed the same sludge from the Greenway facility. The results suggest that acclimation of biomass is increasingly critical for some digestion processes as loading rates increase beyond conventional norms.

This paper will present the complete data from both IC and THP trials, including the reseed reactor performance and ammonia stress trials investigating process robustness (data not shown). The final paper will present a direct comparison of process performance and an evaluation of the different impacts the two intensification strategies have on anaerobic digestion. This paper will provides a direct comparison of the current state-of-the-art digestion operations with a new and emerging technology demonstrating how different intensification strategies can address specific user needs and critical success factors.