The latest regulatory position statement from the Environment Agency reinforced their decision to close hazardous waste ‘mixing pits’, despite opposition from some companies within the industry.
As part of our continuing drive to meet and exceed legal requirements we had already decided to phase out mixing pits and set a new standard for treatment of wastes at our Cannock site.
Setting the standard
The site is a merchant waste treatment facility based in Staffordshire that handles liquid and solid hazardous wastes. On taking over the Cannock facility we commissioned a number of consultancies to work with the company’s development team, ahead of legislative requirements, to devise a reliable, fully enclosed means of treating a range of solid and liquid wastes.
When the UK Environment Agency announced a ban on mixing pits to take effect in July 2008, the plant concept was already well advanced but the team were faced with a tight schedule to resolve outstanding technical problems, finance and build the plant by the deadline.
State of the art
This development encompasses a cement stabilisation process with a supporting liquid treatment tank farm; a unique process design application to waste solidification and stabilisation. We experimented with various treatment options in order to arrive at a process which gave us confidence in the treated waste reliably meeting Waste Acceptance Criteria (WAC) at the company’s hazardous landfills.
Process parameters were determined using a pilot plant and experimenting with various types of small scale replicas of the mixing vessel. This testing lead to a design based on batch mixing which allows a high degree of control but ensures greater flexibility of operation.
Two stage process
The mixing system utilises a touch screen computer that automatically controls each feed of the process. Chemists identify, model and analyse suitable solid and sludgy wastes for each batch. These containers are then loaded onto the shredder feed conveyor belt. At the same time a measured quantity of a treated processed liquid waste is introduced into the operating mixer. As the mixer turns, measured quantities of cement and Air Pollution Control Residues (APCr), which are one of the ash types produced by incinerators, are loaded through solid flow meters into the mixer. This process is one of only a handful of plants in the UK that utilises solid flow meters.
As the cement, APCr and aqueous waste are mixing the solid or sludgy hazardous wastes are introduced. The pre-loaded containers are passed up and into a shredder which is capable of processing IBC’s, drums or combinations. The shredder is specially designed to guarantee a shredding size no greater than 150mm to both improve homogeneity of the mix and prevent large objects entering conveyors or the mixer. After all material has been thoroughly mixed the discharge valve is opened and the resultant stabilised residue is guided onto a conveyor which discharges it into a waiting trailer for onward disposal at hazardous landfill.
Up and running
The new mixing plant is now up and running allowing us to process a greater range of difficult hazardous wastes, increasing our production efficiency and helping our chemists to handle wastes in a more controlled, safe and compliant manner. The tank farm is well underway in both design and tendering for construction. The delivery of this will be towards the end of 2008 and once in position will significantly expand Augean’s waste handling capabilities.