Energy efficiency improvement of wastewater treatment– case study of an autothermal thermophilic aerobic digestion process

Environmental and legislative constraints regarding sludge disposal, such as its application on agricultural land, stimulate research in this area. Since municipalities are responsible for sludge disposal and finance it from taxpayers’ money, cost is the major constraint in providing this service. Therefore, research in this area has to provide economically viable solutions. The Autothermal Thermophilic Aerobic Digestion (ATAD) process is one practical solution that enables the reuse of sludge in agriculture, though its high energy demand reduces its appeal. One of the main attractiveness of ATAD process is its stability and low sensitivity to changeable operating conditions. Our theory is that the cost efficiency of ATAD process could be enhanced by improving its energy performance and therefore operating cost attractiveness. Thus, the main objective of the research was to improve the energy efficiency of the ATAD sludge treatment process. The hypothesis of certain modifications applied to the ATAD process to increase its operational temperature can enhance its energy efficiency was investigated. This thesis answered the question whether the energy efficiency of the ATAD process can be improved by increasing the operating temperature in the reactors, e.g. reducing and/or eliminating the thermal shock. This research further deepened the understanding of the process and indicated areas of improvements for design and operation. It established, through analysis, redesign options and best operation conditions leading to a more energy efficient ATAD process. The dynamic mathematical model used is capable of predicting the rate of volatile solids degradation and the temperature of the sludge in the reactors, therefore the compliance with legislation (pasteurisation and stabilisation criteria) can be assessed. The applicability of the ATAD dynamic mathematical model on a full-scale was demonstrated. It was proven to give a more realistic picture of process performance than the current design guidelines. Recommendations for improving the existing process were proposed and verified by the ATAD mathematical model. Modifications made to the operational circumstances, such as changing the feeding pattern (feeding volume and frequency), changing the characteristics of the feed sludge, such as its concentration or temperature (achieved by heat integration) and changing the characteristics of the air supplied, such as its temperature; and combination of the above listed solutions were analysed. This project has proven that the design of the ATAD system could be further improved with the recommendation such as heat integration and changed feeding pattern. The results include: • More stable and steady operation. • The proposed improvements lead to reduced retention time, which also reduces the need for aeration and mixing energy – ultimately, this improves energy efficiency as it reduces the electricity needs by 25% for running the system for a certain throughput. A tool which can be utilized to evaluate the different ATAD systems has been provided. The model’s potential is not restricted to a tool for evaluation of process modifications from an energy efficiency perspective. It can be used along different objectives and at different stages of ATAD systems’ life span, such as design, operation and control; and even as a teaching tool. This research further deepened the understanding of the process and indicated areas of improvements for design and operation and also opens the doors for further research in this area. Improvements can lead to ATAD’s wider acceptance in the wastewater treatment field and its more extensive use in Ireland and around the globe.