Fate and Partitioning of Contaminants of Emerging Concern (CECs) during Hydrothermal Liquefaction of Wastewater Sludge

Objectives Hydrothermal liquefaction (HTL) is a promising technology which has many distinct advantages over existing wastewater sludge treatment processes (i.e., anaerobic digestion, composting, etc.) and other thermal treatment processes including lower reactor temperatures than pyrolysis, all while avoiding the need to dewater the feedstock.[1,2] This developing technology has the potential to completely eliminate the need for land application/landfilling of biosolids, while producing a new renewable energy stream while allowing for the recovery of essential plant nutrients (nitrogen and phosphorous) from the HTL aqueous and hydrochar phases. Despite the promise of HTL, little is known about the fate and partitioning of contaminants of emerging concern (CECs) during HTL. This includes the aqueous phase (i.e., remaining water which requires further treatment), and the biocrude/hydrochar streams. About 80% of the total sludge volume entering the HTL process exits in this aqueous stream which must be further treated at the wastewater treatment facility where CECs can potentially re-enter the process. Therefore, the objective of this study was to examine the fate and transport of multiple CECs through an HTL process to investigate their partitioning between phases and/or their removal. Methodology Wastewater sludge was obtained from a municipal secondary wastewater treatment facility in BC, Canada. Mixed primary (55%) and secondary (45%) v/v sludge was dewatered by centrifugation to 20% total solids (TS) without polymer addition. Five CECs, (including azithromycin (AZM), bezafibrate (BZF), carbamazepine (CBZ), diclofenac (DCF), and mefenamic acid (MFA)) were studied. Six runs total were performed at six different experimental conditions. The dewatered sludge was either not spiked (i.e., only environmental CEC levels were present), or was spiked, as shown in Fig. 1. A low and high level of spike, corresponding to concentrations of 200 and 500 µg of CEC/g TS, were selected. The dewatered sludge was then added to a 1-L Parr reactor vessel. The vessel was continuously mixed and heated to either 332°C (630°F) and held at temperature for 17 minutes or it was heated to 350°C (662°F) and held at temperature for 15 minutes. The pressure increased to 170 bar (2465 psi) during operation due to volatilization within the fixed-volume vessel. After cooling, the produced gas, liquid 'aqueous phase,' and the remaining solid phase (consisting of hydrochar and biocrude) was collected. The two components of the solid phase were analyzed together to eliminate the possibility of the CECs being lost to the extraction solvent. Isotope dilution CEC extraction and analysis methods (adapted from US EPA 1694 and another HTL study), were used to extract CECs from all matrices. [3,4,5] After extraction, CECs were analyzed via liquid chromatography with tandem mass spectrometry (LC-MS/MS). CEC concentrations prior to HTL in mixed sludge, dewatered sludge, and the resulting sludge dewatering centrate, along with HTL effluent solid and aqueous phases were analyzed to monitor CEC flux through the HTL process. Process influent and effluent was used to simulate a CEC mass balance across a model full-scale HTL train and for comparison to existing thermophilic anaerobic digestion processes. Findings The average yields of the HTL aqueous and solid products were 80% and 20% of the total mass, respectively, from the six runs. In past experiments within our group, the average output of the HTL process at these two reaction conditions (332°C/17 min and 350°C/15 min) yielded 82% aqueous, 3% hydrochar and 9% biocrude and 2% gas by mass. The amount of each CEC that would be recycled through the process back to the headworks in the mixed sludge dewatering liquids (prior to HTL) was determined based on the mass balance calculations of a full-scale system model. This represents the environmental CEC levels present. The mass balance showed that 24% of AZM, 40% of CBZ, 21% of DCF and 8% of MFA partitioned to the centrate during sludge dewatering. No BZF was returned as none was detected in the initial dewatered sludge. After HTL, except for trace (< 0.5 ppb) levels of AZM in four of the six scenarios, there were no detectable CECs in any of the HTL aqueous stream samples as shown in Figure 2. This indicates that any CECs remaining after HTL treatment are either destroyed or did not partition into the aqueous phase. Similar to the aqueous stream, the biocrude/hydrochar stream had lower levels of CECs remaining after the HTL process, as shown in Figure 3. Across the six scenarios, the three CECs (AZM, BZF and CBZ) were found to be <4 ppb. AZM was <0.3 ppb in each scenario indicating almost complete removal. BZF was only detected in the low spike scenarios, at 1.8 ppb for the 332°C condition and at 3 ppb for the 350°C condition. This again indicates an almost complete removal from the system. CBZ was detected in all six scenarios, although at very low (<1.5 ppb) concentrations. Based on these results, the percent removal was determined from the CECs concentration in the feedstock, the dewatered sludge. In the aqueous fraction, 100% removals can be seen for all five compounds investigated in all scenarios, except for the BZF in the 'No Spike' trials because there was none present in the input dewatered sludge (Figure 2). In the solid fraction, there was 100% removal for all three investigated compounds for the low and high spike treatments (Figure 3). However, in the 'No Spike' scenario, as in the aqueous, there was no BZF in the input dewatered sludge. Also, there was only a 65-70% removal for CBZ in that treatment. This is likely because of the low input concentration and the amount of noise inherent in this matrix during quantification. The combined results from the aqueous and solid fractions of the HTL products indicate the HTL process thermochemically converted almost all the environmentally present compounds in the sample matrix, as well as the externally spiked compounds to other end products. A fraction (8-40% depending on the CECs) of the CECs from the initial sludge are recycled as dewatering centrate to the headworks prior to HTL, for another cycle through the wastewater facility for further removal. The remaining portion that enters the HTL would achieve a nearly complete (>99.9%) removal. This suggests that incorporating an HTL process into a wastewater treatment facility will likely result in far lower levels of CECs discharged to the environment compared to an anaerobic digestion process which had limited success against refractory CECs investigated in a previous study.[6] Conclusions - This study indicates that HTL is an effective method for removing the studied recalcitrant pharmaceuticals from dewatered wastewater sludge. - In the aqueous product stream of the HTL process, trace amounts of AZM (<0.5 ppb) were detected in the three of the six scenarios. In the other scenarios, no AZM was detected, nor the other CECs examined. - In the HTL solids stream, the three CECs studied were detected in at least one scenario. However, the compounds were detected at low concentrations of <4 ppb. Learning Objectives 1.Hydrothermal liquefaction technology has the potential to remove contaminants of emerging concern from municipal wastewater sludge, reducing pollution in the environment. 2.In addition to removing trace contaminants, this process simultaneously converts wastewater sludge into a valuable renewable energy source. The remaining hydrochar reduces the volume of solid material for ultimate disposal by ≥ 97%.