The characterisation of the hydrodynamic vortex separator using residence time distribution analysis
The hydrodynamic vortex separator (HDVS) is currently employed at wastewater treatment works and in the sewerage system as a combined sewer overflow (CSO) for the separation of solids from an incoming waste stream. This project presents the first stage in developing and aiding the existing design methodology for the optimisation of kinetic processes within the HDVS. The kinetic process design methodology combines hydraulic and kinetic principles by using the true mixing regime characteristics of a system and batch reactor data to determine a kinetic processes efficiency. This project used residence time distribution (RTD) analysis to extensively characterise the mixing regime within a model and prototype HDVS. The HDVS was operated with and without a baseflow component and with and without the sludge hopper for a range of inlet flow rates and flow splits covering design flow rates for a number of existing applications. The RTD was obtained using a pulse tracer injection method and described using the complete range of data analysis techniques typical employed in RTD studies. This includes the axial dispersion model (ADM), tanks-in-series model (TISM), RTD indices and a RTD combined mathematical model. The combined model is configured to quantify the inactive flow behaviour within the HDVS i. e. stagnant and dead volumes. The HDVS has a complex imperfect plug-flow mixing regime. This non-ideal flow behaviour is associated with both dispersion and dead volumes and results in short-circuiting. At low flow rates the HDVS operating without a baseflow contains fluid elements which conduct flow slower than the mean velocity. At high flow rates the inactive flow behaviour is associated with dead volumes and subsequently short-circuiting. The flow rate at which this change in mixing characteristics occurs is termed the transition flow rate and is approximately 151/min and 901/min for the model and prototype HDVS respectively. At all flow rates above the transition flow rate the HDVS has a very stable mixing regime, which is associated with both the inactive flow behaviour and the plug-flow mixing characteristics. The ADM and TISM parameters increase as the flow rate decreases and therefore, the HDVS has improved plug-flow mixing characteristics and reduced dispersion at low flow rates. Removing the sludge hopper reduces the inactive flow behaviour and improves the plug-flow mixing characteristics. The inactive flow behaviour within the model HDVS operating with no baseflow occupies approximately 20-40% of the total volume and similarly for the prototype HDVS 5-25% and increases as the inlet flow rate increases. The inactive flow behaviour occupies a smaller fraction of the total volume and the plug-flow mixing characteristics are also improved as the HDVS is scaled-up in size. Hence, the scale-up of the HDVS will provide a mixing regime with less short-circuiting and improved plug-flow mixing characteristics and therefore, more conducive for certain kinetic processes and particularly chemical disinfection processes. The introduction of a baseflow component alters the total mixing regime within the HDVS. The baseflow component introduces an element of plug-flow mixing and subsequently the total plug-flow mixing characteristics of the HDVS operating with a baseflow component are greater than the HDVS operating without a baseflow. The baseflow component plug-flow mixing characteristics increase and the overflowcomponent decrease as the inlet flow rate increases. Short-circuiting of the baseflow and overflow component occurs as the inlet flow rate decreasesa nd increasesr espectively. Hence, there are different mixing regimes within the HDVS associated with the overflow and baseflow component. The HDVS operating with a baseflow component has improved plug-flow mixing characteristics when the sludge hopper is included. This project was also extended to include an experimental kinetic process analysis, by investigating the first-order decomposition of hydrogen peroxide (H202) using catalase. This was undertaken to compare the actual kinetic process performance within the HDVS to that estimated using the RTD. The H202 decomposition results showed that the design of the HDVS for kinetic processes can be achieved using only the RTD and relevant batch reactor data. This enables the HDVS to be optimised for kinetic process applications and eliminates the need for costly and time consuming pilot trials. The characterisation of the HDVS using RTD analysis creates scope for significant future research. This includes: alternative experimental RTD techniques, development of the RTD combined mathematical model to include a baseflow component and kinetic process principles, extensive kinetic process batch reactor investigations, application of both the hydraulic and kinetic data into chemical reactor design computer software and finally the scaling of the HDVS using the RTD and therefore the kinetic process optimisation. This work is a proactive response by practitioners and Hydro International Plc to pressure from the regulators and EU Directives, placing emphasis on the use of sophisticated treatment processes based on good scientific principles, to meet current and future stringent water quality standards.