## 1 Introduction  
Wastewater treatment plants (WWTP) are designed to treat municipal and industrial wastewater influents to lower the ecological risks associated with discharging the treated effluents into receiving water sources. Within these plants, the wastewater (WW) influent goes through a series of unit processes (mainly bio-based) to reduce the organic matter content, odor, and pathogen levels with the resulting end-materials exiting the plants being water effluent, biosolids, and biogas. Effluents are the treated water that are released after meeting applicable treatment standards. Biogas is the gaseous product produced during the anaerobic, methanogenic-based biodegradation of organic wastes and/or waste sludges. The gas is typically composed mainly of methane and carbon dioxide at levels in the 50-70% (_v/v_) and 30-50% (_v/v_) levels, respectively, with residual amounts of hydrogen, sulfur dioxide, and volatile organics. Biosolids are biologically digested sludges (end-process sludges), technically derived from either anaerobic or aerobic digestion processes that are typically dewatered to the 15 to 20% (_w/w_) range. In the context of this paper, only anaerobic digestion is considered. Over 7M dry tons of biosolids (the majority being anaerobic products) are produced annually in the United States [1]. With increasing urbanization due to population growth/densification, the amount of produced wastewaters is rising within a reduced area which makes the management of the biosolids one of the biggest concerns in WWTP industry. By the 2032, 56 million more people are expected to input into centralized urban-based WWTPs which is an overwhelming load for the WWTP industry to manage at its current capacity which is currently treating the generated wastewater from nearly 240 million Americans [2]. There are almost 14,700 WWTP facilities in the United States that produce approximately 33 billion gallons of WW effluents daily [3]. One key challenge at present is managing the solids that are being generated at the WWTPs. Biosolids contain low percent levels (_w/w_) of organic nitrogen and phosphorus and can be applied as fertilizers and soil amendments via land application. Other proven methods of handling biosolids are incineration, forestry application, and disposal in landfills [1]. The incineration of dried solids results in emission of greenhouse gases (GHG) which contributes to global warming. As seen in Figure 1 which shows the areas for biosolids use and disposal, agriculture is the mostly used area which requires the biosolids to be of high standard, free from pathogens, and rich in nitrogen and phosphorus. The second most popular choice for biosolids management is landfills which is not a sustainable option since available landfill volumes in the US are decreasing, thus becoming expensive as well as facing increasing regulatory and policy pressure to recycle or reduce wastes instead of landfilling. In addition to the unavailability of viable land, the cost to haul the dried solids is also of concern to WWTP managers. There is an average of $50 per dton tipping fee inclusive of hauling costs of the dried biosolids and landfill use fee which represents a significant expense to the WWTP. This also represents a waste of valuable carbon that potentially could have been used to make new, sustainable chemical products produced from urban bioreferieries. Table 1 shows the increase in tipping fees from 2018 to 2019 in different regions of the US which is expected to increase in the future [4]. The carbon footprint associated with the hauling of solids to landfill makes this option carbon-intensive and unsustainable. The disposal costs for biosolids generated at US WWTPs generally represent about 60% of the annual operational cost of the WWTP [5] and is a major reason to look into disposal/post-production use alternative which includes processes that can potentially reduce the generation of solids, yet still support biorefineries.