Incorporating hydrothermal liquefaction into wastewater treatment – Part I: Process optimization for energy recovery and evaluation of product distribution

Highlights

• Hydrothermal liquefaction (HTL) conditions were optimized for max energy recovery.

• Energy recovery was linearly and positively correlated to biocrude yield.

• HTL byproducts (aqueous and hydrochar) were minimized at the optimal conditions.

• High HTL reaction severities improved biocrude quality (more C and less heteroatoms).

• Elemental distribution (including metals) among HTL products was evaluated.

Abstract

The treatment of significant amounts of municipal sewage sludge requires novel and efficient technologies. This study evaluated hydrothermal liquefaction as a means to sustainably convert sludge waste into a renewable energy source – biocrude, which can mitigate both environmental and energy-related challenges. Response surface methodology was employed to investigate the effects of reaction temperature (290–360 °C) and residence time (0–30 min) on product yield and biocrude quality. Both the highest and the lowest reaction temperature or residence time had negative effects on biocrude yield and energy recovery (ER), while high reaction severities improved biocrude quality. Under optimized conditions (332 °C for 16.9 min), biocrude yield (48.9%, dry ash-free) and ER (70.8%) were maximized. Biocrude composition followed the order of N-heterocycles > O-heterocycles > hydrocarbons, while nitrogenous compounds reduced, and hydrocarbons increased with reaction temperature. More distillable fractions in biocrude were also produced at higher reaction severities. The possible reaction pathways of biocrude formation were discussed and updated to include catalytic effects on inherent metals and Brønsted (acidic and basic) sites. The high content of O (7.8–13.1%), N (4.4–4.9%), and TAN (48.6–63.6 mg KOH/g) suggested the necessity of biocrude upgrading. Separating and recycling trace metals (e.g., 497–656 mg/kg Fe) from biocrude are necessary to relieve upgrading challenges. C, N, and P were mostly distributed into HTL biocrude, aqueous, and hydrochar, respectively, allowing their recovery. Most metals were concentrated in hydrochar. The results contribute to the advancement of the state of the art in biorefinery, which will guide the design of full-scale HTL sludge treatment systems combining resource recovery.

Introduction

With increasing population and rapid urbanization, municipal wastewater treatment plants (WWTPs) generate a significant amount of wastewater solids (often called municipal sludge or sewage sludge), which require further treatment and disposal. Canada and the US reported an annual sludge production of >660000 dry tonnes (approximately 2.5 million wet tonnes) and 12.7 million dry tonnes (approximately 56 million wet tonnes), respectively [1], [2]. The known total sludge production rate from medium- and high-income countries was reported as 40 million dry tonnes per year [3]. Moreover, such a large amount is expected to keep rising due to population growth and increasingly stringent requirements for wastewater treatment. The current strategy of sludge disposal includes land application, incineration, and landfilling. More than half of total sludge in Canada is land applied after being stabilized (i.e., biosolids). The biological stabilization techniques (e.g., anaerobic digestion and composting) require a long sludge retention time (over 15 days), which are cost ineffective. Additionally, land application of biosolids is vulnerable to market fluctuations, such as high costs of transportation, agricultural land being saturated with nutrients, and public opposition due to the presence of pathogens and contaminants (e.g., hormones, pesticides, and heavy metals) [4]. Incinerating sludge requires significant expenditures and energy inputs [5]. Landfilling sludge is restricted for over-generating methane and wasting organic matters and nutrients [6]. Considering the growing sludge production and climate change concerns, alternate options for sustainable sludge treatment, such as waste-to-energy conversion, are demanded and have attracted extensive research.

Due to the high moisture content (almost 98%, by weight) and massive volume of raw sludge, its handling is challenging and costly. A novel technique, hydrothermal liquefaction (HTL), is considered one of the most promising technologies to address those challenges environmentally friendly, efficiently, and economically [7]. Municipal sludge is often seen as waste, but HTL converts sludge into biocrude that can be refined to a low-carbon fuel. HTL does not require drying processes but instead uses hot pressurized water as a reaction medium through a thermochemical process. This ability significantly reduces the energy input compared to other techniques that require dry feedstocks (e.g., incineration and pyrolysis) [8]. HTL can also efficiently decompose organic matter/pollutants and reduce the volume of residual solids by using elevated temperature (280–374 °C) and pressure (8–22 MPa) in a short period (typically < 30 min) [9]. After HTL, maximum C is converted into biocrude while most P is enriched in hydrochar to allow its recovery, representing a justifiable and sustainable operation for sludge treatment. Several studies also suggested that hydrothermal treatment especially HTL can remove most micropollutants [10], [11], [12]. In summary, HTL can be simultaneously used for waste-to-energy conversion, organic pollutants destruction, waste minimization, and sterilization for final disposal.

A successful bench-scale continuous-flow test at the Pacific Northwest National Laboratory (PNNL) has demonstrated that HTL is capable of handling municipal sludge, and the subsequent hydrotreating turned biocrude into liquid fuel [2]. A significant reduction (94–99%) of residual solids for disposal was also achieved. Such success shows a way to beneficially utilize waste and produce biofuel from clean and renewable resources, which addresses the global demand for low carbon energy. Currently, Metro Vancouver in British Columbia (BC, Canada) is leading the design of a pilot-scale HTL demonstration unit at the Annacis Island WWTP. However, several extensive literature reviews [3], [13], [14] identified many research gaps that need to be addressed before an HTL system can be successfully incorporated into a WWTP. First and foremost, the operational conditions of the HTL system need to be optimized for energy recovery and economically sustainable operation. The optimum HTL conditions are usually feedstock-specific due to the difference in biochemical compositions. Using model compounds, it has been identified that energy recovery (ER) in biocrude from HTL (350 °C) of different feedstocks followed the order of lipids (82.7–86.7%) > proteins (10.7–36.4%) > carbohydrates (8.3–13.7%) > lignin (2.5%) [15], [16]. Synergistic effects on biocrude yield and ER were also found from binary mixtures of proteins–carbohydrates, proteins–lipids, and carbohydrates–lignin due to the Maillard reaction, amides formation, and potential retro-aldol reactions favored by alkaline lignin, respectively [16], [17]. Therefore, using a feedstock balanced with various biochemical compounds can take advantage of those synergies for maximizing biocrude production and ER. In a typical WWTP, primary and secondary sludges are generated, which are rich in carbonates/lipids and protein, respectively [14]. Their mixture (mixed sludge) represents a balanced feedstock for HTL. On the other hand, the improvement by those synergies also depends on specific compositions of feedstock and HTL operational conditions [17], [18]. Therefore, process optimization is crucial for implementing HTL to a typical WWTP.

To date, only two studies used the response surface methodology (RSM) to optimize the conditions for HTL of primary sludge and co-HTL of waste activated sludge and sawdust regarding biocrude yield [19], [20]. To the best of the authors’ knowledge, optimization of HTL parameters aiming for biocrude ER from mixed sludge was never evaluated. This study was motivated to address the research gap and provide guidance in designing a pilot and subsequently a full-scale WWTP incorporating HTL sludge treatment. HTL reaction temperature and residence time have shown dominant effects on HTL products [13], [14], and thus they were investigated in this study. As RSM can effectively optimize experimental conditions with a minimum number of experimental runs, it was used to design the HTL batch experiments. Multiple response models were developed using RSM: Yield of HTL products (biocrude, hydrochar, aqueous, and gas) and biocrude quality parameters, such as carbon recovery (CR), higher heating value (HHV), energy recovery (ER), energy return on investment (EROI), and total acid number (TAN). The optimization results for maximized biocrude ER were validated with additional experiments. Besides, the effects of experimental factors on biocrude characteristics were investigated. Finally, the elemental distribution among HTL products was presented to guide their proper utilization.