## 2 Methodology  
### Goal, Scope, Functional Unit, and Process Selection  
All LCA work completed in this research was consistent with International Organization for Standardization (ISO) 14040 guidance [9] and conducted using SimaPro software (PRe Consultants, version 9.3). The goal of these LCAs was to quantify the environmental impacts of each of the selected processes for producing biosolids and to identify the inputs within each process that most contributed to the suite of environmental impacts under study. This evaluation was performed by determining the percentage of impacts associated with fuel consumption, material inputs, infrastructure, and biogenic sources, as well as the composition of biogenic emissions. This study is a gate-to-gate evaluation, which begins after biosolids are generated in a conventional anaerobic digestion system and dewatered via a belt filter press (with the exception of TPAD) and ends after the biosolids are transported to a land application site (Figure 1). The processes 'upstream' of the system boundary are assumed to be similar for all five biosolids treatment processes according to the particular wastewater treatment system producing the biosolids. The land application of the resulting Class A biosolids would also generate similar impacts in each scenario. Therefore, these upstream and downstream processes were not included in the system boundary, keeping the focus of this study on the impacts of the decisions surrounding biosolids production. The functional unit used in this study is an annual production of 200 dry tons of finished Class A biosolids. This functional unit is typical of a small WRRF, which is the focus of this study. The five biosolids treatment processes illustrated in Figure 1 were selected for the following reasons: Composting and thermal stabilization (e.g., Direct Heat Drying) are among the most common conventional biosolids treatment technologies currently in use [2]. Although full-scale TPAD operations are much less common than Composting and thermal stabilization treatment systems, there is currently a lot of interest in TPAD because of its potential to improve the production of methane and destroy volatile solids within a small footprint relative to anaerobic digestion at a single temperature [10]. Composting is a conventional process that utilizes aerobic conditions to enhance the biological degradation of the biosolids organic matter. The biological degradation processes increase the temperature to the pasteurization range, destroy volatile solids, and increase the percent of total solids [11]. Direct Heat Drying can take many forms, but this study focused on the rotary dryer system. During Direct Heat Drying, hot gases are used to remove water from the biosolids, which increases the percentage of total solids to 90-95% [11]. There are numerous TPAD configurations. In this study, we chose to evaluate a two-stage system in which wastewater solids are treated via mesophilic digestion, followed by thermophilic digestion. The combination of mesophilic and thermophilic conditions  
Figure 1: System boundaries for each treatment process under study. Dashed lines represent system boundaries. decreases the solids retention time (SRT) and increases the efficiency of volatile suspended solids (VSS) destruction by 15-20% more than what can be accomplished with a single-stage mesophilic digester [11]. Based on the NEBRA study [2], the most common biosolids treatment process that falls in the LCLT category is long-term storage (also known as Lagoon Storage). Lagoon Storage holds biosolids for one or more years and allows the organic matter to decompose under a combination of aerobic and anaerobic conditions. This study specifically focused on a high solids sludge processing train (HSSPT) Lagoon Storage, where mechanical dewatering of biosolids occurs before being placed in the lagoons.