## 1 Introduction
### Solids Composition  
The treated non-gaseous products exiting an AD consist of liquid effluent and treated sludge streams [8]. Influent bulk solids concentrations into an AD at most WWTPs are in the 5% solids range which has the appearance and handling of a thick slurry that is pumpable. After digestion, the biosolids are typically dewatered to about 15 to 25% (_w/w_) solids with the liquid effluent either recycled back to the head of the WWTP or discharged depending on pollutant and pathogen levels. Dewatering of the slurries exiting an AD is often done using either a belt press or centrifuge [8]. Supernatant (liquid) pollutant levels exiting the AD are generally in the 500 mg/L 5-day biochemical oxygen demand or BOD5 range. The Wisconsin Department of Natural Resources (WDNR) quotes the US EPA as describing biosolids as \"primarily organic solid product yielded by municipal wastewater treatment processes that can be beneficially recycled\" as soil amendments (fertilizer and conditioners)\" [8]. As this description infers, biosolids has or should have a use beyond simple disposal. Without a use beyond production, biosolids and hence WWTPs cannot become truly sustainable. The sludge that exits the AD, once dewatered, is considered at that point to be the \"biosolid\" product. The sludge is often measured as total solids (TS) comprised of, an organic fraction also called volatile solids (VS), and an inorganic fraction or fixed solids [9]. Total Suspended Solids quantifies suspended solids (particulates) but not soluble solids while Volatile Suspended Solids (VSS) quantifies suspended organic solids that does not include soluble solids or inorganic solids. Other measurements such as Total Chemical Oxygen Demand (tCOD) which measures the total amount of oxygen equivalent as potassium permanganate required to oxidize the organic pollutant; Soluble COD (sCOD) which is same as tCOD but without oxidizing particulates (only soluble chemicals); and Total Organic Carbon (TOC) which measures the organic carbon present in wastewater (excluding inorganic carbon), are used to define the characteristics of the sludge. However, VS and TS are the most common measurements used to quantify and qualify sludges. Typical performance goals for AD systems at most US-based WTTPs are the >38% removal VS; 30% to 60% removal of TS; 85% to 99% pathogen removal; and biogas with a methane content >50% [8; 10]. The solids profile entering and exiting the AD are generally 75% volatile and less than 40% volatile, respectively, which indicates more VS removal (in other words, additional organics) could be achieved with further system optimization of a typical AD unit (there still generally remains 40% volatile fractions which tend to be fairly biodegradable). As stated above, once dewatered, the sludges exiting a typical AD range from 15% to 30% (_w/w_) solids content. At 15% solids, the material feels like a wet soil with 30% solids having the feel of a damp soil. At these higher solid concentrations, mechanical bulk handling, such as conveyance or bucket loaders, are often used for sludge transport. It is important to note that the primary composition of sludge inputs into an AD system at most WWTPs is wasted activated sludge which is mainly aerobic microbial cells. The aerobic microbial cells in the form of activated sludge are wasted to maintain a target cell density within the aerobic treatment phase of the plant (generally in the 1 to 5 g/L range). Sometimes, WWTPs will also have primary sludge inputs integrated within the waste activated sludge (aka. WAS or secondary sludge). Primary sludge is the sludge produced when the solids within the overall WWTP influent is settled thus it is composed mainly of feces, food wastes, bacterial cells, and light paper goods. Usually, WAS is the majority feed into an AD input sludge followed by primary sludge. These organic constituents of input sludge to an AD make up the bulk of the VS fraction.