System Mix Ltd was awarded the contract to design, manufacture and install suitable mixing systems for four anaerobic digesters at Dwr Cymru Welsh Water wastewater treatment works located at the Cardiff and Afan in Wales. The award of this contract followed many months of design and development by Systems Mix in conjunction with the Vaughan Pump Company, who developed the patented Rotamix system.
The main design remit for the mixing systems was to provide more than 94% active volume mixing within the two sizes of digester proposed. Afan STW digesters required a working volume of 4,300 m3 with geometry of 17.8 m deep × 17.2 m in diameter and the Cardiff digesters required 6,500 m3 with geometry of 20.2 m deep × 20.2 m in diameter. All digesters were to be new build and constructed from concrete.
The feed to these digesters is hydrolysed sludge from the Cambi process which is fed at a minimum of 10% DSC (dried solid content). The requirements set by Welsh Water and by its contractors, Imtech Process, was to achieve a homogenous mix of the resulting digestate with a minimum of 94% active volume. System Mix and Vaughan, using CFD (Computational Fluid Dynamics) analysis, developed the design and provided data to all parties showing that the mixing systems offered would achieve this requirement.
Part of the design solution required by Welsh Water and Imtech was to have the internal mixing system in a format that would allow the installation and removal of mixing nozzles without needing to empty a digester, stop or interrupt the digestion process, or lose gas during this operation.A simple system
System Mix's ‘Drop-In' Mixing Lance system has a gas sleeve that seals with the top fluid level, allowing lance assemblies to be inserted and withdrawn without losing gas pressure. Lances are inserted through the roof of the digester and locate on docking cones permanently installed on the floor of the vessel. These docking cones locate each lance, stop deflection and ensure correct alignment for optimum mixing efficiency. Nozzles are strategically placed along the lance to create the Rotamix Dual Zone mixing action while the lower nozzles create rotation around and across the floor area and high level nozzles accelerate the upper sludge volume inducing a coriolis effect which drafts high level sludge back down to the central suction points. Sludge is drawn out through the suction point to the Vaughan Chopper pumps which are located outside the vessel at ground level. Each lance is powered by its own Chopper pump ensuring that there is always mixing, even when a unit is out for maintenance.
The pumps condition the sludge as it passes through the impeller cutter area and this reduces solid size as well as preventing reweave agglomeration of fibrous content. The continued conditioning of sludge enhances bio digestion as well as reducing the problems of rag mass downstream in the process.Developing digestion
Using Drop-In Lance mixing systems has been a significant development in digester mixing technology, with these lances allowing for easy removal and installation. This facility also offers greater flexibility for future enhancements by adding advance pre-digestion processes such as hydrolysed sludge treatment of pasteurisation as well as increases in dry solids loading, with little disruption to the biological process. Maintaining internals without needing to drain and enter the vessel removes the significant cost of disposing of high volumes of sludge.
Once the design of digesters was approved by Imtech Process, System Mix engineers produced 3D drawings of the systems and construction of all four digesters (two at Afan and two at Cardiff) were completed by Morgan Est and Imtech in early April 2010.
