Table of Contents • Executive Summary ----------------------------------------------------------------------1 • Introduction and Background ---------------------------------------------------------2 • Methodology ------------------------------------------------------------------------------3 • Project Plan --------------------------------------------------------------------------------4 • Reflective Discussion of the Findings -----------------------------------------------5 • Reference -----------------------------------------------------------------------------------6 Executive Summary The wastewater treatment process has played a significant role in the modern society because of increasing population and higher demand on water resource. During this process, primary digesters at Melbourne Western Water Treatment Plant anaerobic decomposition of the filtered city sewage into biogas and biosolid. The main component of the biogas is methane, a high flammable gas which can be used to generate electricity or further processed to replace fossil fuel; and biosolid is a complex mixture of high nutritious organic clay, which can be processed to use as livestock bedding and fertilizer. Both biogas and biosolid are renewable resources, which have huge economic and environmental potential for a more sustainable future. However, the mixing condition of the current digester in Western Water Treatment Plant is not desirable and operational cost is high. Thus, is important to improve biogas generation efficiency to increase the self-sufficiency of the plant and reduce the impact caused by suboptimal mixing for both neighborhood and environment. The methodology of approaches in this report is to compare four current types of digester models that have been created under three mixing methods, analysis design principles behind each model and hence find parameters which caused present suboptimal mixing condition. Anaerobic primary digesters will be modified by targeting parameters concluded above and a ANSYS CFD digester model will be established to verify improvements effectiveness. The principle of minimum changes is used through the modification process to reduce transformation cost and impacts of plant’s ordinary operation schedule. Associated improvement such as potential environmental enhancement, positive company image and healthier public relationship will also benefit by applied changes. After rigorous research, it was pin pointed that the ineffective distribution of the kinetic energy of mixing results in areas which are devoid of any motion (dead zones). These dead zones in the anaerobic digesters are the main hurdles restricting the methane generation. These zones thus have a direct influence on the power generation capability and the water processing capacity of the power plant. Furthermore, the Gas escapes from the current system and odour generated can have serious environment impacts. Thus, based on the main findings, our proposal aims to cover the major flaws in the current system namely • The ineffective distribution of mixing energy • High volume ratios of the “dead zones” • Ways to reduce the odour generation • Prevent gas leaks to the environment. Introduction and Background Anaerobic digestion has a strong effect in the modern society because its ability to transform wastes to renewable energy. Large amount of methane is produced in the digestion which can use to generate electricity to increase the self-sustainability; while high nutritious biosolid after digestion can be used as fertilizer to support local agricultural industry. The main objective is to increase the efficiency of the plant which will be achieved by enhancing the existing mixing conditions in the digester of the wastewater treatment plant. This will result in acquiring increased amount of treated water along with higher Bio-gas production rate. With the increasing demand of water resources; the dependency on the wastewater treatment plant has grown. Furthermore, the treatment plants are looking for improvements in meeting these requirements. The project not only takes into consideration the recycling of wastewater but also the environment sustainability through reduced use of fossil fuels. The use of methane as a fuel in energy production has helped to make the plant environment friendly. In addition to that, the plant is self-sufficient in terms of power and has a potential to produce it in excess. Our client i.e. Western Water had found that the overall efficiency of the plant had reduced in the subsequent years. The reason was the high organic substrate in the wastewater. Thus, the mixing conditions inside the plant found to be having flaws. The main set-back being the dead zones which indicates ineffective mixing. The requirements of our client mainly include improving the mixing conditions using innovative methods and hence produce an environment friendly and feasible project. The first task for the team is to collect information about the current mixing method at the Western water treatment plan. Further information was gathered about various mixing techniques used around the globe. Based on the information collected a deep dive was done to find out the shortcomings in the existing system and to arrive at the most effective methodology to eliminate these shortcomings in the existing system. The proposed concept evaluation will be done in comparison to the existing system and with approximate dimensions. Later the proposal with the design simulation will be presented to the western water treatment plant for approval. Methodology Problems exist in current system There are three common mixing approaches for anaerobic digester, which are internal gas sparging, external liquid recirculation and internal mechanical mixing. Despite of different mixing methods have been tried to improve the mixing condition in the primary digester through generations, current mixing conditions are still not ideal and the shortage of different models has been observed: • High ratio of inactive region in the tank • Frequent maintenance and cleaning schedule • Difficulty on system diagnose • High energy consumption • Potential environment impacts The cause for these shortages are mainly because current digester design is normally based on experience of the manufacturers, on-site operators rather than a definitive mixing theory (Dawson et al., 2000). It is important to understand the sludge rheological properties to optimize digester design and improve mixing conditions (Slatter, 2003). However, insufficient rheology data are available to analysis digested sludge and the non-Newtonian behavior for the sludge is still unclear (Baudez et al., 2011). Mori et al. (2006) suggested that sewage sludge is a complex non-Newtonian mixture, which feature with shear thinning characteristics. This characteristic allows sludge to change its viscosity during the mixing process, which makes sludge rheological properties vary based on location in the digester. For the sludge near the power input, high speed sludge, gas or turning mixing impeller dilute sludge near its contact region and thicken the sludge away from it. As the result, only the thin sludge near the input get properly mixed; thickened sludge get isolated from active digestion regions and accumulate at the bottom, which becomes “dead zone” . Tenney and Budzin (1972) claimed that more than a half of the volume in the digester is inactive during the digestion. Monteith and Stephenson (1981) further carried out that the dead zone consisted 77% of the digester volume, which is poorly mixed and become inactive during the digestion. It is also believed that high viscosity sludge deposition caused severe damage on the digester which make frequent maintenance and cleaning become essential to keep normal operation. Thus, increasing stirring or circulation rate is not effective to improve the mixing condition. Furthermore, less mixing is recommended during the anaerobic digestion for higher biogas production efficiency. Dague et al. (1970) and Stafford (1982) suggested higher mixing rate will only increase the gas production at the start and the total amount of gas generation by high-rate continuous mixer is lesser than the production made by mixing intermittently. To support this theory, Stroot et al. (2001) and Karim et al. (2005) discovered the minimal mixing can improve the distribution of microorganism population in the sludge and stabilize the unstable digesters, which improving the efficiency of the biogas generation as the result. Thus, minimal intermittent mixing is recommended for better digestion and higher biogas production. Modification and proposed approaches After a brief understanding about sludge characteristic, two principles on future implementation have been decided, which are • Improving the mixing energy distribution throughout the system • Use minimal intermittent mixing instead of continuous mixing Despite of two modelling design principles above, proposed modifications should also be cost effective to the company for a minimum long term operational cost. To maximum utilize the current devices, further modification should minimize the interruption on current digesters to reduce the initial investment cost and minimize affection on daily operation schedule. Meanwhile, modifications on extend the functioning time of current system need to be researched to increase biogas production. Lusk (1998) found most of the digesters in USA are required to shut down for maintenance after operation in a brief period. The frequent maintenance schedule considerably affect the production of the biogas and increase the operation cost of the company. Parameters on environment improvement, such as odor reduction should also include into the modification plan for a more positive company image and healthier public relationship. Project Plan Followed by modification principles above, siphon induced mixing system is proposed. The Inspiration of this idea came from the research Takuro et al published in 2013.Schematic of the research in figure 1. Figure 1: The demonstration of siphon induced mixing device used in Takuro et al.’s research in 2013. In the research, Takuro et al. (2013) compared the efficiency of biogas generation between siphon mixing reactor (SMR), unmixed reactor (UMR) and continuous mixing reactor (CMR) and results are listed below: • SMR improve the production of biogas compared to UMR and the yield is similar to amounts produced by CMR. • The flow pattern in SMR is more evenly distributed, which improve the sludge dispersion and reduced the sedimentation in the reactor as the result. • SMR has the longest duration of biogas production and highest efficiency in short hydraulic retention time. Advantages of using SMR meet the principle of our modification and low energy consumption is an expected outcome. Further modifications are done to suite the current equipment and sewage components composition. Schematic of proposed device is shown in figure 2: Figure 2: concept of proposed siphon flow mixing system for the project In the proposal, gas valves replace the U shape siphon. This preserves space to integrate current mixing device and increase system ability to digest high solid concentration sewage. Internal baffles are provided make multiple mixing sections to reduce the dead zones. The road map for the project is listed in table 1 and ANSYS FLUENT will be used for analysis. The CFD modeling procedure will be followed in three steps: • Basic design drawing of the proposed system (approximate dimensions) • Fluid flow pattern and mixing analysis using ANSYS • Modify the design parameters to optimize output Table 1: Group analysis schedule for the project Expected Outcome The following are the estimated improvements in the current project:- • Reduced energy consumption and improved energy dissipation • Improved flow pattern/ separate large lagoon tank to smaller regions to reduce the ratio of dead zones • Reduced possibility of layering and solid deposition • Improved efficiency of biogas generation • Modified current device as small as possible • Can be applied to both thin and thick sludge conditions The modelling and simulation software would prove handy in conducting and solving the flow analysis; thereby providing the optimal solution for the project. Reflective Discussion of the Findings The alternate approach of designing a completely different system altogether has helped in creating a broader scope for the project. In addition to that, the use of software tools for modelling and simulation will be a helpful approach in finding the solution to the query. The process for carrying out the modelling and simulation might be time consuming but it is more reliable, economical and efficient way of dealing with the issues regarding the problem. This is achieved by creating prototypes from the software and running tests to find the optimal solution. Through the use of ANSYS CFX software we will develop the model and find out the regions of inadequate mixing so as to improve the mixing efficiency inside the digester. To sum it up, this project has been valuable in terms of the knowledge and expertise gained from it in solving real world problems. References