## 1 Introduction
### Anaerobic Digestion in WWTP  
Anaerobic digestion (AD) is the popular treatment process within WWTPs due to its proven efficiency to further reduce pollutant levels, yield a fairly stabilized sludge, substantially reduce sludge tonnage needing disposal, use of minimum input energy, and the production of biogas. Most WWTP ADs are actually treating waste sludges in the form of primary sludge and secondary sludge (i.e., waste activated sludge or WAS) [6]. Within an AD, anaerobic microbial consortia degrade the organic content within the input wet sludges under strict anaerobic conditions to produce a fairly stable solid (i.e., biosolids) and biogas. The anaerobic digestion process is based on four key microbial-based steps; hydrolysis, acidogenesis, acetogenesis, and methanogenesis which are shown in Figure 2. Hydrolysis is the first step where complex organic sludge constituents such as carbohydrates, proteins, and lipids are broken into smaller compounds such as fatty acids, amino acids, and glucose. This step is the rate limiting step since it takes the most amount of time due to the lysing of the recalcitrant cell walls. Following hydrolysis is acidogenesis where the acidogenic bacteria feed on sugars, fats, and amino acids to produce complex organic acids, alcohols, carbon dioxide, and ammonia. Next, acetogenic bacteria feed on the products of acidogenesis to produce low molecular weight fatty acids (such as acetic, lactic, and propionic acids), hydrogen, and carbon dioxide. Methanogenesis is the final step where methanogenic microbes, archaea (also known as methanogens), convert the organic acids, hydrogen, and carbon dioxide into biogas. Biogas in non-optimized AD systems typically produce a biogas with \(\sim\)55% methane (_v/v_) and \(\sim\)45% carbon dioxide (_v/v_). Optimized systems can produce a biogas with \(\sim\)80% methane and \(\sim\)20% carbon dioxide. The calorific value of biogas is generally 500-800 BTU/ft3 which is comparable to the calorific value of natural gas which typically ranges between 950-1150 BTU/ft3.  
Figure 1: Biosolids use and disposal (note: * denotes Class A sludges; source: US Wastewater Treatment3).  
Figure 2: Anaerobic digestion reaction processes.  
Anaerobic digestion can be conducted at 3 different temperature ranges: (1) psychrophilic (10-15 \({}^{\circ}\)C), (2) mesophilic (28-45 \({}^{\circ}\)C), and (3) thermophilic (>60 \({}^{\circ}\)C). Mesophilic AD is the most popular due to low energy requirements, the production of stable solids, and good biogas production. The metabolic activity of microorganisms is reduced with decreasing temperatures. Hence, temperature is an important parameter in the digestion process for maximum biological activity from microorganism leading to optimized pollutant removal and biogas production. For the same amount of organic pollutant removal, lower temperature kinetically slows down the hydrolysis process thus requiring longer hydraulic retention times (HRT) and solids retention times (SRT) compared to digestion at mesophilic temperatures (i.e., 35 \({}^{\circ}\)C) [7]. In addition, low temperature leads to less utilization of the substrate and changes the microbial community structure. Thermophilic temperature range produces higher biogas volumes; however, the high energy requirement (secondary heating often using the produced biogas) for the increased temperature increases the operation and management (O&M) costs. There also exists the potential for the formation of refractory compounds at very high temperatures.