Several authors have pointed out the need to identify the optimum operating conditions (OCs) of autothermal thermophilic aerobic digestion (ATAD). This study proves the hypothesis that the OCs have the potential to substantially improve the energy efficiency and plant capacity of established ATAD systems. As ATAD is a semi-batch process, its energy efficiency has to be optimized via dynamic optimization (DO). This methodology requires an adequate mathematical model, and appropriate selection of optimization variables. The paper presents an improved mathematical ATAD model based on previous models found in the literature. A global sensitivity analysis (GSA) was performed in order to identify variables with significant influence upon energy efficiency and plant capacity, thus paving the way for the DO of ATAD systems. The results of the GSA show that reactor volume, reactor temperature, and aeration flowrate are significant variables, which is consistent with reported literature. The results of the GSA also show that both energy efficiency and plant capacity of ATAD systems can be substantially improved by altering reactor volume and OCs.
ATAD is an activated sludge process used in wastewater treatment with two goals: stabilization and pasteurization of the sludge. In this context, stabilization refers to the reduction of the volatile solids (VS) concentration of the sludge, while pasteurization refers to the elimination of pathogens through heat treatment. Comprehensive reviews on ATAD origin, design, and operation can be found in USEPA (1990), LaPara and Alleman (1999) and Layden et al., 2007a and Layden et al., 2007b.
As the name indicates, ATAD is an aerobic sludge treatment at thermophilic temperatures (45–65 °C). If the reactors are well insulated, the reaction becomes autothermal or self-heated, which means that no external energy is required to maintain thermophilic temperatures.
The fundamental way how ATAD works can be described as follows: Sludge containing organic matter, thermophilic and mesophilic (some of whom are pathogens) biomass is aerated in a well insulated reactor. Thermophiles grow on the expense of oxygen and organic matter (or VS), thus contributing to sludge stabilization. During their digestion, the thermophiles release vast amounts of metabolic energy, thus reaching thermophilic temperatures inside the reactors. The high reactors’ temperatures are lethal for pathogens, hence resulting in their elimination and, consequently, in sludge pasteurization.
Due to the high oxygen uptake rates (OURs) of thermophilic microorganisms, as high as 1000–2000 mg/l/h (LaPara & Alleman, 1999), ATAD has a relatively high energy requirement regarding the aeration of the reactors with 9–15 kWh/m3 (USEPA, 1990). ATAD is, therefore, an energy intensive process. In this study, only the energy used for aeration is considered. The typical plant capacity of conventional ATAD plants is 30–40 m3/day (USEPA, 1990).
Even though the development of ATAD technology started in the 1950s (LaPara & Alleman, 1999), its use remains nowadays relatively limited compared to other sludge treatments. Additionally, there are conflicting reports in current literature regarding the energy efficiency and cost effectiveness of ATAD technology which may have contributed to its limited use (Layden et al., 2007b).
An ATAD model consisting of a mass and energy balance with temperature-dependent kinetic parameters was presented in this paper. The two treatment goals (stabilization and pasteurization) were mathematically formulated and included into the model, thus allowing to exactly determine minimum reaction times necessary to comply with legal requirements. Two simulation studies were carried out so as to test model behaviour. A GSA was performed with the purpose of revealing the variables with strongest influence upon energy requirements and plant capacity. It has been shown that the use of SRCs and PCCs facilitates the selection procedure for variables that have more influence on model outputs. The current procedure enables to focus attention only on a few model input parameters for further studying. The results of the GSA showed that reactor volume, reactor temperature, aeration flowrate, initial concentration of slowly biodegradable substrate, and the COD-VS conversion factor are the significant variables. The fact that reactor volume, temperature, and flowrate are significant variables is consistent with reported literature. These three variables should be used to minimize the energy requirements of ATAD systems. It was also found that depending on reactor volume and OCs either stabilization or pasteurization become the limiting factor of the reaction, while the ATAD reaction tends to be limited by stabilization. This insight had not been reported in the literature before. Finally, it was found that both energy requirement and plant capacity can be significantly improved by altering the reactor volume and OCs.
