Fate of perfluoroalkyl and polyfluoroalkyl substances (PFAS) through two full-scale wastewater sludge incinerators.

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are an emerging issue in wastewater treatment. High-temperature thermal processes, incineration being time-tested, offer the opportunity to destroy and change the composition of PFAS. The fate of PFAS has been documented through wastewater sludge incinerators, including a multiple hearth furnace (MHF) and a fluidized bed furnace (FBF). The dewatered wastewater sludge feedstock averaged 247- and 1280-µmol targeted PFAS per sample run in MHF and FBF feed, respectively. Stack emissions (reportable for all targeted PFAS from MHF only) averaged 5% of that value with shorter alkyl chain compounds comprising the majority of the targeted PFAS. Wet scrubber water streams accumulated nonpolar fluorinated organics from the furnace exhaust with an average of 0.740- and 0.114-mol F- per sample run, for the MHF and FBF, respectively. Simple alkane PFAS measured at the stack represented 0.5%-4.5% of the total estimated facility greenhouse gas emissions. PRACTITIONER POINTS: The MHF emitted six short chain PFAS from the stack, which were shorter alkyl chain compounds compared with sludge PFAS. The FBF did not consistently emit reportable PFAS from the stack, but contamination complicated the assessment. Five percent of the MHF sludge molar PFAS load was reported in the stack. MHF and FBF wet scrubber water streams accumulated nonpolar fluorinated organics from the furnace exhaust. Ultra-short volatile alkane PFAS measured at the stack represented 0.5%-4.5% of the estimated facility greenhouse gas emissions.

Novel free nitrous acid and ultrasonication pretreatment enhanced sludge biodegradability.

The main goal of this study was to investigate the novel combined Ultrasonication and Free Nitrous Acid (FNA) pretreatment on biodegradability and kinetics of thickened waste-activated sludge (TWAS). Partial factorial design with four levels of (0, 600, 1500, and 3000 KJ/Kg) for ultrasonication and 0, 0.7, 1.4, and 2.8  mg HNO2-N/L for FNA dose were examined creating 16 different combinations. Results revealed that combined pretreatment could significantly improve solubilization and solid destruction compared to solo pretreatments. The highest organic matter solubilization of 25.6% and volatile suspended solids destruction of 21.7% were observed when 2.8  mg HNO2-N/L and 1500 KJ/Kg were combined. Moreover, combining the pretreatments further enhanced biodegradability up to the highest percentage of 50.3% when pretreatment of 3000 KJ/Kg and 2.8  mg HNO2-N/L was applied. Also, the experimental data from a biochemical methane potential test was fitted well into First Order Kinetic and Modified Gompertz models, given that the coefficients of determination, R2, for models at all treatment levels were above 99%.

Differential impact of acidic and alkaline conditions on hydrothermal pretreatment, fermentation and anaerobic digestion of sludge.

Anaerobic digestion and fermentation processes in wastewater sludge treatment are limited by several factors, including the slow breakdown of complex organic matter and solubilization of solids. In this study, thermochemical pretreatment of thickened waste activated sludge using high temperature (>170 °C) was investigated to understand the impact of the pretreatment on the volatile fatty acids (VFA) production and its fractions during the fermentation process. Furthermore, the influence the thermochemical pretreatment on sludge disintegration and methane recovery was investigated. A range of acidic and alkaline conditions over the pH range of 4.5-10 was examined. Sludge (pH adjusted) was exposed to hydrothermal pretreatment (HTP) at a temperature of 170 °C for 30 min. Pretreated samples were then subjected to batch fermentation and methane potential tests which revealed that acidic and alkaline conditions resulted in increased sludge solubilization during HTP. Acidic conditions were associated with a higher VFA production yield of up to 185 mg chemical oxygen demand/g total chemical oxygen demand. Alkaline conditions led to a higher methane production yield where the maximum yield (276 mL CH4/g total chemical oxygen demandadded) was found to occur at pH 10. Therefore, alkaline sludge used for fermentation has shown technical and economic feasibility for sludge carbon recovery.

Impact of solid content on hydrothermal pretreatment of municipal sludge prior to fermentation and anaerobic digestion.

This study investigated the impact of the solid sludge content concentrations (SC) on hydrothermal pretreatment (HTP) before fermentation and anaerobic digestion. Five different SC of 3.5%, 7%, 10%, 12%, and 16% were investigated in two different scenarios. The first scenario entailed using only the pretreated samples as substrates, whereas in scenario two, the substrates included pretreated samples combined with the supernatant. Results revealed that the highest overall pCOD solubilization (considering HTP and fermentation) of 64% was achieved for the sample with 12% SC combined with supernatant. The maximum volatile fatty acids production of 2.8 g COD/L occurred with 10% SC without supernatant. The maximum methane yield of 291 mL CH4/g VSS added was attained at 7% SC without supernatant. Furthermore, the results indicated that increasing the SC beyond 7% in scenario 1 and 10% in scenario two led to a decrease in methane yield. Additionally, optimizing for all desired endpoints may be difficult, and there are limits on the increase in SC concerning methane production.

A proof-of-concept experimental study for vacuum-driven anaerobic biosolids fermentation using the IntensiCarb technology.

This study demonstrates the potential of an innovative anaerobic treatment technology for municipal biosolids (IntensiCarb), which relies on vacuum evaporation to decouple solids and hydraulic retention times (SRT and HRT). We present proof-of-concept experiments using primary sludge and thickened waste activated sludge (50-50 v/v mixture) as feed for fermentation and carbon upgrading with the IntensiCarb unit. IntensiCarb fully decoupled the HRT and SRT in continuously stirred anaerobic reactors (CSAR) to achieve two intensification factors, that is, 1.3 and 2, while keeping the SRT constant at 3 days (including in the control fermenter). The intensified CSARs were compared to a conventional control system to determine the yields of particulate hydrolysis, VFA production, and nitrogen partitioning between fermentate and condensate. The intensified CSAR operating at an intensification factor 2 achieved a 65% improvement in particulate solubilization. Almost 50% of total ammonia was extracted without pH adjustment, while carbon was retained in the fermentate. Based on these results, the IntensiCarb technology allows water resource recovery facilities to achieve a high degree of plant-wide intensification while partitioning nutrients into different streams and thickening solids. PRACTITIONER POINTS: The IntensiCarb reactor can decouple hydraulic (HRT) and solids (SRT) retention times in anaerobic systems while also increasing particulate hydrolysis and overall plant capacity. Using vacuum as driving force of the IntensiCarb technology, the system could achieve thickening, digestion, and partial dewatering in the same unit-thus eliminating the complexity of multi-stage biosolids treatment lines. The ability to partition nutrients between particulate, fermentate, and condensate assigns to the IntensiCarb unit a key role in recovery strategies for value-added products such as nitrogen, phosphorus, and carbon, which can be recovered separately and independently.

Combined hydrothermal and free nitrous acid, alkali and acid pretreatment for biomethane recovery from municipal sludge.

This study focused on investigating the effect of combined chemical and hydrothermal pretreatment (HTP) on the anaerobic digestibility of thickened waste activated sludge (TWAS). Three different combined pretreatment conditions of HTP + free nitrous acid (FNA), HTP + Acid, and HTP + Alkaline were applied to TWAS. To control and compare the effect of combined pretreatments and a single pretreatment, Acid, Alkaline, FNA and HTP pretreatments were applied done prior to AD. The results of this study revealed that combined pretreatments have higher potential to improve methane production yield and rate but not in the solubilization of COD. The highest methane yield of 275 mL CH4/g TCOD added was achieved for the combined pretreatment with FNA and HTP. HTP + FNA pretreatment was found to produce higher methane yields compared to the combination of other typical acid and alkaline reagents with hydrothermal pretreatment. Methane yields of 594, 527, and 544 L CH4/g VSS added, were achieved for HTP + FNA, HTP + ALK, and HTP + ACID pretreatments, respectively. The preliminary economic analysis showed that out of the combined pretreatment, only combining HTP with FNA is economically feasible.