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This study determined the optimal mixing ratio of food waste and livestock manure for efficient co-digestion of sewage sludge by applying the biochemical methane potential (BMP) test, Design Expert software, and continuous reactor operation. The BMP test of sewage sludge revealed a maximum methane yield of 334 mL CH4/g volatile solids (VS) at an organic loading rate (OLR) of 4 kg VS/(m3·d). For food waste, the maximum methane yield was 573 mL CH4/g VS at an OLR of 6 kg VS/(m3·d). Livestock manure showed the lowest methane yield. The BMP tests with various mixing ratios confirmed that a higher mixing ratio of food waste resulted in a higher methane yield, which showed improved biodegradability and an improved VS removal rate. The optimal mixing ratio of 2:1:1 for sewage sludge, food waste, and livestock manure was determined using Design Expert 10. Using continuous co-digestion reactor operation under an optimal mixing ratio, greater organic matter removal and methane yield was possible. The process stability of co-digestion of optimally mixed substrate was improved compared with that of operations with each substrate alone. Therefore, co-digestion could properly maintain the balance of each stage of anaerobic digestion reactions by complementing the characteristics of each substrate under a higher OLR.

Research on various security technologies has been actively underway to protect systems from attackers. However, attackers can secure enough time to reconnoiter and attack the target system owing to its static nature. This develops asymmetric warfare in which attackers outwit defenders. Moving target defense (MTD) technologies, which obfuscate the attack surface by modifying the main properties of the potential target system, have been gaining attention as an active cyber security technology. Particularly, network-based MTD (NMTD) technologies, which dynamically mutate the network configuration information, such as IP and ports of the potential target system, can dramatically increase the time required for an attacker to analyze the system. Therefore, this system defense technology has been actively researched. However, increasing the analysis complexity of the target system is limited in conventional NMTD because the variation of system properties (e.g., IP, port) that can be mutated is restricted by the system configuration environment. Therefore, there is a need for an MTD technique that effectively delays an attacker during the system analysis by increasing the variation of system properties. Additionally, in terms of practicality, minimizing the computational overhead arising by the MTD technology and solving the compatibility problem with existing communication protocols are critical issues that cannot be overlooked. In this study, we propose a technology called Ghost-MTD (gMTD). gMTD allows only the user who is aware of protocol mutation patterns to correctly communicate with the service modules of the server system through protocol mutation using the pre-shared one-time bit sequence. Otherwise, gMTD deceives the attackers who attempt to infiltrate the system by redirecting their messages to a decoy-hole module. The experimental results show that the proposed technology enables protocol mutation and validation with a very low performance overhead of only 3.28% to 4.97% using an m-bit (m ≥ 4) length one-time bit sequence and can be applied to real systems regardless of the specific communication protocols.

Biochemical methane potential tests and lab-scale continuous experiments were conducted to improve the yield and energy efficiency of anaerobic digestion through thermal hydrolysis pre-treatment. Methane generation, yield, and solubilization efficiency were evaluated through lab-scale tests. The pre-treated samples presented 50% biodegradability at 140 °C and 61.5% biodegradability at 165 °C. The increase in biodegradability was insignificant at 165 °C or higher temperature, and it was confirmed that the optimum conditions were achieved at 165 °C and 20 min of solubilization. The lab-scale continuous experiments confirmed that polymers were decomposed into low-molecular-weight compounds due to thermal hydrolysis, and pH decreased. NH4HCO3 produced by thermal hydrolysis acted as an alkali to enable a more stable operation compared to that before thermal hydrolysis. Total chemical oxygen demand as chromium (TCODCr) and soluble chemical oxygen demand as chromium (SCODCr) indicated 35.4% and 23.1% removal efficiency in terms of organic matter removal, respectively. Methane yield was approximately 0.35 kg m−3 at 2.0−4.0 kg (m3 d)−1 and 0.26 kg m−3 at 5.0 kg (m3 d)−1. The solubilization rate of 40.9% by thermal hydrolysis was confirmed through the lab-scale tests to determine its full-scale applicability.

Bacterial communities change in bulk solution of anaerobic digestion (AD) and bio-electrochemical anaerobic digestion reactors (BEAD) were monitored at each organic loading rate (OLR) to investigate the effect of voltage supply on bacterial species change in bulk solution. Chemical oxygen demand (COD) degradation and methane production from AD and BEAD reactors were also analyzed by gradually increasing food waste OLR. The BEAD reactor maintained stable COD removal and methane production at 6.0 kg/m3·d. The maximum OLR of AD reactor for optimal operation was 4.0 kg/m3·d. pH and alkalinity decline and volatile fatty acid (VFA) accumulation, which are the problem in high load anaerobic digestion of readily decomposable food wastes, were again the major factors destroying the optimal operation condition of the AD reactor at 6.0 kg/m3·d. Contrarily, the electrochemically activated dense communities of exoelectrogenic bacteria and VFA-oxidizing bacteria prevented VFAs from accumulating inside the BEAD reactor. This maintained stable pH and alkalinity conditions, ultimately contributing to stable methane production.